United StatesDepartment ofAgriculture

Forest Service

ForestProductsLaboratory

GeneralTechnicalReportFPL-GTR-6

ElectricMoisture Metersfor WoodWilliam L. James

Abstract Contents

Electric moisture meters for wood measure electricconductance (resistance) or dielectric properties, whichvary fairly consistently with moisture content when it isless than 30 percent. The two major classes of electricmoisture meters are the conductance (resistance) typeand the dielectric type. Conductance-t ype meters usepenetrating electrodes that measure in a small volume,so moisture gradients may be deduced by repeatedmeasurements at increasing depths. Dielectric-typemeters use surface electrodes that do not puncture thewood surface, and can measure the moisture content ofrelatively dry wood. This paper describes these majortypes of meter and includes detailed information on thekinds of electrodes used with each type.

Readings of moisture meters are affected by suchfactors as wood species and temperature, chemicaltreatments, moisture distribution, and operator skill.This paper contains information for correcting forthese factors and for minimizing potential errors.

Keywords: Moisture measurement, moisture meters,electric properties

Revised June 1988

James, William L. Electric moisture meters for wood. Gen. Tech.Rep. FPL-GTR-6. Madison, WI: U.S. Department of Agriculture,Forest Service, Forest Products Laboratory; 1988. 17 p.

A limited number of free copies of this publication are available tothe public from the Forest Products Laboratory, One GiffordPinchot Drive, Madison, WI 53705-2398. Laboratory publications aresent to over 1,000 libraries in the United States and elsewhere.

The Laboratory is maintained in cooperation with the University ofWisconsin.

P a g e

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Electric Properties of Wood . . . . . . . . . . . . . . . . . . 3

Conductance-Type Moisture Meters . . . . . . . . . . . 6

Dielectric-Type Moisture Meters . . . . . . . . . . . . . . 8

Accuracy of Moisture Meters . . . . . . . . . . . . . . . . . 11

Maintenance of Moisture Meters . . . . . . . . . . . . . . 15

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

The use of trade or firm names in this publication is for readerinformation and does not imply endorsement by the U.S. Departmentof Agriculture of any product or service.

ElectricMoisture Metersfor Wood

William L. James, PhysicistForest Products Laboratory, Madison, WI

Introduction

Moisture content affects most of the importantproperties of wood, and it can vary widely dependingon the environment and history of the wood. Effectiveuse of wood and wood-base materials, therefore,requires efficient and reliable methods of measuringwood moisture.

For relating wood properties to moisture content, themoisture content is defined as the weight of the watercontained in the wood expressed as a percentage of theovendry weight of the wood. Thus, the moisturecontent may range from zero for ovendry wood to over100 percent when the water in the wood weighs morethan the wood substance.

The two basic methods of determining wood moisturecontent are the ovendrying method and the distillationmethod. The ovendrying method is generally acceptedfor basic laboratory work and as a standard forcalibrating other methods (American Society forTesting and Materials (ASTM) 1968). This methodinvolves simply weighing the specimen before and afterovendrying to constant weight at 103° C. Because thequantities used for defining moisture content aremeasured directly, this method is the most preciseknown. However, it is physically impossible to removeall the hydroscopic water by heating without pyrolyzingthe wood. For this reason, constant weight is defined asno further weight loss when a specimen is weighed toan accuracy of ± 0.2 percent at 2-hour intervals.

If wood has been treated or impregnated withchemicals, ovendrying moisture measurements may beinaccurate. An impregnant that is volatile at oventemperatures will evaporate during ovendrying, and the

resulting weight loss can be misinterpreted as due toevaporated water. An impregnant that is nonvolatilewill remain in the wood and so increase the apparentovendry weight of the wood. For treated wood, thedistillation method may be more accurate thanovendrying (ASTM 1968) for measuring moisturecontent. In this method, water is removed from thewood specimen in a closed system, in which the water iscollected and measured directly. Any extraneousorganic materials in the wood are dissolved out of thespecimen by an organic solvent during the waterextraction process. The distillation method is also moreaccurate than the ovendry method for some species thatnaturally contain large amounts of volatile materialsother than water.

The disadvantages of these two basic methods ofdetermining wood moisture are that they are timeconsuming (the drying period alone is at least 24 h),they require expensive apparatus and considerable skillin manipulation, and they destroy the specimen. Theseproblems have prompted the search for other simplerand faster methods for measuring moisture content.

Some early methods, tried as substitutes for theovendrying method, used humidity sensors or indicatorsto estimate wood moisture content from the humidityat the surface or inside the wood. Others usedchemicals to extract water from a pulverized woodspecimen, using various means to determine the amountof water adsorbed by the extracting chemical. Thepotential value of electric conductance as a moistureindicator became evident when Suits and Dunlap (1931)studied the dependence of electric conductance of woodon its moisture content.

A conductance-type (resistance-type) moisture meterdiffers from an ordinary ohmmeter only in theunusually high values of resistance (low conductance)that must be measured when checking wood withmoisture content below about 10 percent. First attemptsto develop a portable instrument capable of measuringsuch low conductance began in the late 1920’s and ledto the “blinker-type” meter. This device consisted of aneon lamp in parallel with a high-quality capacitor thatwas charged through the wood specimen as a seriesconductor. When the capacitor voltage reached thefiring voltage of the lamp, the lamp conducted briefly,thereby discharging the capacitor and starting theprocess over again. The time required to charge thecapacitor increased as the series conductance decreased,so the rate of flashing of the neon lamp indicated theelectric conductance of the wood.

After the blinker-type meter, a high-resistancevacuum-tube bridge was developed that led to themodern direct-reading conductance-type moisturemeters (Davies 1937). These instruments are basicallyconductance bridge circuits, using a wide range ofstandard resistors and a high-resistance electronicvoltmeter to measure the bridge output. At about thesame time as direct-reading conductance-type metersappeared on the market, dielectric-type meters weredeveloped. These types of meter operate on therelationship between the dielectric properties andmoisture content of wood.

To date, three types of electric moisture meter, eachbased on the fundamental relationship betweenmoisture content and a different electric property, havebeen developed: the conductance-type (resistance-type),which uses the relationship between moisture contentand direct current conductance; the power-loss type,which uses the relationship between moisture contentand the dielectric loss factor of the wood; and thecapacitance type, which uses the relationship betweenmoisture content and the dielectric constant of thewood. The latter two types of meter are classed asdielectric types. Meters that use the relationshipbetween moisture content and electric conductance havebeen referred to traditionally as “resistance-type”meters. However, it is conductance that increases withincreasing moisture content, so wood technologists arebeginning to use the more descriptive term“conductance-type” for these meters. Conductance issimply the reciprocal of resistance.

At present, there are several manufacturers of portableconductance-type meters and one manufacturer ofportable power-loss-type meters in the United States.No purely capacitance-type portable meter is currentlybeing marketed although one portable meter is, ineffect, a combination of capacitance and power-losstypes. This meter is called the capacitive-admittance-type meter.

In addition to portable meters, stationary meters areavailable that monitor the moisture content of lumbermoving along a conveyor and mark or eject individualpieces that are outside of moisture specifications. Bothconductance-type and dielectric-type automaticmoisture meters are available; all use nonpenetratingelectrodes. Typical systems are shown in figure 1.Systems are also available for monitoring the dryingprogress of lumber in a dry kiln. Such systems arebased on the conductance principle (use of permanentlyinstalled penetrating electrodes) or on the alternating-current impedance of the lumber (use of electrodesplaced in the kiln loads between lumber courses).

In the discussion that follows, the electric propertiesthat provide the basis for electric moisture meters andthe operating procedures for each type of meter will bedescribed in more detail.

Figure 1 — Typical installation of nonportablemoisture meter for monitoring the moisturecontent of material moving along a conveyor.(M 143 064)

2

Electric Properties of Wood

The electric properties of wood that are important forelectric moisture meters are direct current conductance(resistance) and the dielectric constant and loss tangent(power factor).

Direct Current Conductance

Electric conductance is the property of a material thatpermits the flow of electric current through thematerial. The magnitude of conductance determines thecurrent flow with a given potential difference orelectromotive force across the conductor. In most cases,the magnitudes of conductance (or resistance),electromotive force, and current are related by a simpleproportion known as Ohm’s law.

Effect of Moisture Content

The direct-current conductance (resistance) of woodvaries greatly with moisture content below fibersaturation. As the moisture content decreases fromfiber saturation (about 30 pct moisture, based on thedry weight of the wood) to the ovendry condition, theconductance decreases by a factor of over 10 million(table 1). In this range of moisture content, a roughlylinear relationship exists between the logarithm ofconductance and the logarithm of moisture content. Atmoisture content levels beyond fiber saturation, theelectrical conductance correlates very poorly withmoisture content. Increasing the moisture content fromfiber saturation to complete filling of the capillarystructure of the wood with free water typically increasesthe conductance by a factor of 50 or less. This is trueeven though most species at complete saturationcontain well over 100 percent moisture content based onthe ovendry weight of the wood. Typical values ofresistance of Douglas-fir, measured at roomtemperature using a moisture meter electrode, are22,400 megohms at 7 percent moisture content and0.46 megohm at 25 percent moisture content.Corresponding data for other species and levels ofmoisture content are given in table 1. Values ofconductance in microsiemens are reciprocals of thetable data.

Effect of Temperature

The electric conductance of wood increases as thetemperature of the wood increases (Davidson 1958;James 1968). This is opposite to the effect oftemperature on resistance in metals and suggests that inwood the mechanism of conduction is by charge

carriers whose number or mobility is increased bythermal activity. Thus, the conduction of current bywood is likely to be at least in part ionic. For moisturelevels above approximately 10 percent, the conductanceof wood is roughly doubled for each increase intemperature of 10 °C.

Effect of Grain Angle

The conductance of wood parallel to the grain is aboutdouble that of conductance perpendicular to the grain.Ratios of conductance perpendicular to the grain, inrelation to the longitudinal value, are about 0.055 forradial and 0.50 for tangential.

Dielectric Constant and Power Factor

The dielectric constant of a material is defined as theratio of the capacitance of a capacitor, using thematerial as the dielectric, to the capacitance of thesame capacitor with a vacuum (in practical usage, air)as the dielectric. In principle, the dielectric constant is ameasure of how much electric potential energy (dipolemoment per unit volume) is stored in the material whenit is placed in a given electric field.

The dielectric power factor is a measure of the rate ofdissipation of electric energy as heat within a dielectricmaterial. When a dielectric material is placed in aconstant electric field, it absorbs a certain amount ofenergy from the field and stores it as potential electricenergy. With a perfect dielectric material, this energy iscompletely recoverable when the field is removed. Inpractice, however, some energy is lost in thestore-recover cycle; this energy appears as heat withinthe dielectric material.

If the dielectric material is in an electric field that isoscillating at a constant frequency, the dielectricmaterial will absorb and dissipate from the field powerproportional to the product of the frequency and theloss factor. The power absorbed does not necessarilyincrease linearly or even monotonically with increasingfrequency, however, because the loss factor may varyconsiderably with frequency (James 1974).

Effect of Moisture Content

Dielectric constant increases with increasing moisturecontent. The effect of moisture content is much greateras the frequency of the applied field decreases(James 1974). There is a roughly linear relationship

3

Tabl

e 1

– Th

e av

erag

e el

ectri

cal r

esis

tanc

e al

ong

the

grai

n of

sev

eral

spe

cies

of

woo

d at

diff

eren

t lev

els

of m

oist

ure

cont

ent1

Moi

sture

con

tent

(pct

)Sp

ecies

of w

ood

78

910

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

Resis

tanc

e (Μ

Ω)(Μ

Ω)C

onife

rsB

aldc

ypre

ssD

ougl

as-f

ir (

coas

t reg

ion)

Fir,

Cal

ifor

nia

red

Fir,

whi

teH

emlo

ck, e

aste

rnH

emlo

ck, w

este

rnL

arch

, wes

tern

Pine

, jac

kPi

ne, l

ongl

eaf

Pine

, red

Pine

, whi

tePi

ne, p

onde

rosa

Pine

, sho

rtle

afPi

ne, s

ugar

Red

woo

dSp

ruce

, bla

ckSp

ruce

, Sitk

a

Har

dwoo

dsA

sh, b

lack

Ash

, w

hite

2

Asp

en, b

igto

oth

Bas

swoo

d2

Bir

ch2

Bir

ch, p

aper

Elm

, Am

eric

anH

icko

ry2

Kha

ya3

Mag

nolia

2

Mah

ogan

y (S

wie

teni

a)M

aple

, sug

arO

ak, n

orth

ern

red4

Oak

, whi

tePh

ilipp

ine

mah

ogan

y(S

hore

a Sp

p.)

Swee

tgum

Tup

elo,

bla

ck4

Wal

nut,

blac

kY

ello

w-p

opla

r4

12,6

003,

980

1,41

022

,400

4,78

0

1,66

031

,600

6,76

0 2,

000

57,6

00

15

,850

3,9

80—

120,

000

20,

000

22,9

005,

620

2,04

039

,800

11

,200

3,

980

450,

000

52,

000

9,5

002

5,0

00

8

,70

0

3,1

60

700,

000

100

,000

1

7,00

020

,900

5,62

0 2,

090

39,8

008,

910

3,31

043

,600

11

,750

3,

720

22

,90

0

5,2

50

1

,66

02

2,4

00

4

,68

0

1,5

50

700,

000

90,0

00 1

6,00

022

,400

5,89

0 2,

140

630

630

725

1,12

04,

300

850

1,44

52,

800

1,32

04,

300

850

1,41

01,

350

645

615

4,30

083

0

265

265

315

415

1,30

040

056

01,

000

575

1,30

040

564

556

028

025

01,

400

365

14,0

0012

,000

300,

000

36,3

0087

,000

200,

000

18,2

00—

44,6

0043

,700

20,9

0072

,400

14,4

0017

,400

2,30

02,

190

24,0

001,

740

19,9

5024

,000

2,00

031

,600

16,2

0012

,600

6,76

013

,800

4,79

03,

550

600

200

8569

025

010

54,

000

1,10

036

047

018

0

85

4,47

0 1,

290

470

5,00

0

1,4

0055

035

011

0 45

2,19

034

011

56,

310

2,75

0 1,

260

5,01

0 2,

040

910

2,29

087

038

03,

160

690

250

1,59

063

026

51,

100

415

170

2,89

069

022

0 80

35

38,0

006,

460

2,09

081

534

531

,700

12

,600

5,

020

1,82

072

551

,300

9,77

0 2,

630

890

355

24,0

008,

320

3,17

0 1,

260

525

120

6033

18

.6

11.2

7.

14.

6 3.

09

1.78

1.

26

0.91

0.

66

0.51

0.

4212

0 60

33

18.6

11

.2

7.1

4.6

3.09

2.

14

1.51

1.

10.7

9.6

0.4

615

0 83

48

28.8

18

.2

11.8

7.6

5.01

3.

31

2.29

1.

58

1.15

.83

.63

180

8346

26

.9

16.6

11

.06.

6 4.

47

3.02

2.

14

1.55

1.

12.8

6.6

245

0 20

010

0 45

.0

25.0

14

.08.

8 5.

40

3.50

2.

30

1.60

1.

10.7

8.5

718

5 98

51

28.2

16

.2

10.0

6.0

3.89

2.

52

1.58

1.

05.7

2.5

1.3

725

012

063

33

.9

19.9

12

.37.

6 5.

02

3.39

2.

29

1.62

1.

20.8

7.6

644

0 21

011

0 60

.0

35.0

21

.0

13.0

8.

80

5.80

3.

80

2.60

1.

80

1.30

.98

270

135

74

41.7

24

.0

14.4

8.9

5.76

3.

72

2.46

1.

66

1.15

.79

.60

470

210

100

52.0

28

.0

16.0

10

.0

6.80

4.

40

2.80

1.

90

1.20

.91

.67

200

102

58

33.1

19

.9

12.3

7.9

5.01

3.

31

2.19

1.

51

1.05

.74

.52

300

150

81

44.7

25

.1

14.8

9.1

5.62

3.

55

2.34

1.

62

1.15

.87

.69

255

130

69

38.9

22

.4

13.8

8.7

5.76

3.

80

2.63

1.

82

1.29

.93

.66

140

7644

25

.7

15.9

10

.06.

6 4.

36

3,02

2.

09

1.48

1.

05.7

5.5

610

0 45

22

12.6

7.

2 4.

73.

2 2.

29

1.74

1.

32

1.05

.85

.71

.60

580

250

120

68.0

38

.0

23.0

14

.0

9.60

6.

30

4.30

3.

00

2.10

1.

40

1.00

165

8344

25

.1

15.5

9.

86.

3 4.

27

3.02

2.

14

1.58

1.

17.9

1.7

1

40

20

106.

0 3.

4 2.

1 1.

3.9

0.6

0.4

2.3

2.2

5.2

0.1

755

28

14

8.

3 5.

0 3.

22.

0 1.

32.8

9.6

3.5

0 .4

4.4

0.4

015

0 60

30

16.0

8.

6 5.

03.

1 2.

00

1.30

.88

.61

.43

.33

.26

45

27

16

9.6

6.2

4.1

2.8

1.86

1.

32.9

3.6

9.5

1.3

9.3

120

0 96

53

30.2

18

.2

11.5

7.6

5.13

3.

55

2.51

1.

78

1.32

.95

.70

230

110

57

30.0

17

.0

10.0

6.0

4.00

2.

50

1.70

1.

10.8

1.5

9.4

320

12

7

3.9

2.3

1.5

1.0

.66

.48

.42

.40

.40

.40

.40

50

21

11

6.3

3.7

2.3

1.5

1.00

.71

.52

.44

.40

.40

.40

630

340

180

105.

0 60

.2

35.5

21

.9

14.1

0 9.

33

6.16

4.

17

2.82

1.

99

1.44

435

205

105

56.2

29

.5

16.2

9.1

5.25

3.

09

1.86

1.

17.7

4.5

0.3

218

0 85

43

22.4

12

.3

7.2

4.4

2.69

1.

66

1.07

.72

.49

.35

.26

105

5329

16

.6

10.2

6.

84.

5 3.

16

2.24

1.

62

1.23

.98

.75

.60

125

6332

18

.2

11.3

7.

34.

6 3.

02

2.09

1.

45.9

5.8

0.6

3.5

080

42

22

12.6

7.

2 4.

32.

7 1.

70

1.15

.79

.60

.49

.44

.41

15

9 5

2.8

1.7

1.1

.7.4

5.3

0.2

1.1

6.1

2 .0

9.0

716

0 81

45

25.7

15

.1

9.3

6.0

3.98

2.

63

1.78

1.

26.8

7.6

3.4

627

512

058

27

.6

13.0

6.

93.

7 2.

19

1.38

.95

.63

.46

.33

.25

155

7841

22

.4

12.9

7.

34.

9 3.

16

2.14

1.

48

1.02

.72

.51

.38

250

140

76

43.7

25

.2

14.5

8.7

5.76

3.

81

2.64

1.

91

1.39

1.

10.8

5

1 R

esis

tanc

e m

easu

red

in m

egoh

ms

at 8

0 °F

bet

wee

n tw

o pa

irs

of n

eedl

e el

ectr

odes

spa

ced

1-1/

4 in

ches

apa

rt a

nd d

rive

n to

a d

epth

of

5/16

inc

h. T

he r

ecip

roca

ls o

f th

ese

data

are

cond

ucta

nce

in m

icro

sie

men

s.2 E

xact

sp

ecie

s un

know

n.3 K

now

n in

the

tra

de a

s “A

fric

an m

ahog

any.

”4 T

he v

alue

s fo

r th

is s

peci

es w

ere

calc

ulat

ed f

rom

mea

sure

men

ts o

n ve

neer

.

between moisture content and logarithm of thedielectric constant at all frequencies, but the slope ofthe relationship increases as the frequency decreases(James 1974).

The power factor usually increases with increasingmoisture content, but the reverse is true at somecombinations of moisture content, temperature, andfrequency. Power factor is a nonlinear function ofmoisture, temperature, and frequency, and exhibitsmaximum and minimum values at variouscombinations of these variables.

Effect of Density

The dielectric constant of wood increases nearly linearlywith increasing density, although a slightly concaveupward trend is apparent as the moisture content of thewood increases.

The power factor of ovendry wood increases rapidlywith increasing density up to a density of about25 pounds per cubic foot; above this density the powerfactor increases only slightly. At higher moisture levels,the power factor-density relationship becomes slightlyconcave upward.

The loss factor, since it is the product of two quantitiesthat increase with density, also increases with density.

Effect of Temperature

The dielectric constant of wood increases withincreasing temperature, except at very high moisturecontent where it is erratic and possibly reversed; thereversal could be related to the lowering of thefiber-saturation point at higher temperatures(James 1974). The increase in dielectric constant withincreasing temperature indicates the presence ofthermally activated mechanisms of polarization. Thesemechanisms probably are interracial polarization, wherethe external electric field causes ionic charge carriers toaccumulate at internal discontinuities in the wood, andfixed dipole polarization, associated with orientation ofpolar cellulose molecules caused by the external field.Other mechanisms of polarization exist in wood, suchas induced molecular dipole moment, but areinfluenced only slightly by temperature.

As stated earlier, the power factor is not a simplefunction of temperature but may increase or decreaseas the temperature increases, depending on frequencyand moisture content.

Effect of Species

Most wood species with approximately equal densityhave similar electric properties, but there areexceptions. For example, silk-oak (Grevillea robusta)apparently contains unusually large concentrations ofsoluble salts or other electrolytes and has very unusualelectric properties.

5

Conductance-Type Moisture Meters

Portable conductance-type moisture meters arebattery-operated, wide-range ohmmeters. Most modelshave a direct reading meter, calibrated in percentmoisture content for one species and with correctiontables for other species. A portable conductance meteris illustrated in figure 2.

To measure the electrical conductance of a woodspecimen, the specimen must be arranged as an elementin an electrical circuit. This requires electrical contactwith the wood at two points, using a method of contactthat produces consistent and meaningful results. Twotypes of electrode can be used: surface-contact orpin-type electrodes.

Surface-Contact Electrodes

Surface-contact electrodes are not generally usable withconductance-type meters, except possibly on thinveneer, because of the conductance gradients associatedwith wood drying under normal conditions. Withsurface contacts, the generally drier and hence vastlymore resistive surface dominates the measuredconductance, and only readings of the dry surface arepossible. In general, surface electrodes on oppositesides of a flat specimen indicate the moisture content ofthe driest layer of wood between the electrodes.

Pin-Type Electrodes

Electrodes that penetrate the wood have greaterapplicability than surface-contact electrodes. Pin-typeelectrodes are widely used because of their simplicity.The simplest of these penetrating electrodes have polesconsisting of nail-like pins that are driven into thewood. Electrodes that are screwed into the wood are inlimited use.

When pin-type electrodes are driven into wood, theobserved conductance is almost exclusively in the thinshell of wood that is in immediate contact with the pin.Neither the conductivity of the wood between the polesof the electrode nor the distance between the poles hasany significant effect on the reading of aconductance-type moisture meter.

Pin-type electrodes are driven into the specimen fromone side, so the measured conductance is nominally ina plane parallel to the surface of the specimen and notacross its thickness. Thus, the flow of electric current isparallel to the planes of nearly equal moisture content,

Figure 2—A portable resistance-type moisturemeter, with an electrode of two 1-inch pins.(M 870 131)

assuming no gross anomalies in moisture distribution.Because of the steep conductance gradients in wooddrying under normal conditions, practically all thecurrent flows through the wettest layer of wood that isin contact with both poles of the electrode. If themoisture content of the wettest wood in contact withone pole of the electrode differs from the moisturecontent of the wettest wood in contact with the otherpole, the drier of the two loci will limit the current andthus be responsible for the reading. Thus, it isimportant to emphasize that the reading of aconductance-type moisture meter will be related to thewettest wood that contacts both poles of the electrode.Usually this wood is located right at the tips of thepenetrating electrodes. This characteristic ofconductance meters enables estimation of moisturecontent gradients since readings can be taken atvarious depths.

Wood of rectangular cross section that has been dryingat a fairly constant rate and is drier throughout thanfiber saturation will have a moisture distribution suchthat, at a depth of one-fourth to one-fifth of itsthickness, the moisture content will be nearly equal tothe average for the entire cross section. This is referred

6

to as the “one-fourth to one-fifth thickness rule.” Thecorresponding depth for circular cross sections is aboutone-sixth of the diameter. Electrode pins shouldtherefore reach these depths in order to indicate averagemoisture content (ASTM 1968).

Conductance data for calibrating conductance-typemoisture meters have been obtained with the currentflowing parallel to the grain. Therefore, when usingmeters of this type, the electrode should be oriented sothat the current flows parallel to the grain. If thereadings drift, take the reading immediately after theelectrode is driven into the specimen.

Four-Pin Electrode

One type of moisture meter electrode uses four steelphonograph needles (pins) that extend about8 millimeters (5/16 in) beyond their mounting chucks.Each pole of the electrode uses two pins. The poles areabout 3 centimeters (1-1/8 in) apart, and the two pinscomprising each pole are about 1.5 centimeters (1/2 in)apart. The electrode is composed of a plastic material,combining good mechanical strength and high electricalinsulating value. A handle is attached for driving andextracting the electrode. The pin length is aboutone-fifth of the thickness of nominal 38-millimeter(2-in) dimension lumber, so the pin is suited forindicating the average moisture content for thisthickness. The average moisture content of thinnermaterial may be read by driving the pins to less thantheir full depth. Although most data for calibratingconductance moisture meters have been obtained usingthe four-pin electrode, its use in the field has declinedin recent years, probably because of the greaterconvenience and versatility of the two-pin electrode.

Two-Pin Electrode

Measuring the average moisture content of materialthicker than nominal 38 millimeter (2-in dimensionlumber) requires a pin longer than 8 millimeters. Mostmeter manufacturers fill this need with a two-pinelectrode: each pin comprises one pole of the electrodeand is about 25 millimeters long (1-in). To achieve thenecessary strength, these pins are larger in diameterthan the pins used in the four-pin electrode. Two pinsare used instead of four to permit the larger pins to bedriven and extracted more easily. A two-pin electrode isillustrated in figure 2; this electrode features a centralretractable probe that moves a scale for indicating thedepth of penetration of the pins.

Despite the larger diameter and consequent largercontact area of the pins used in the two-pin electrode,readings using this electrode are consistently lower(about 1/2 to 1 pct) than readings made using thefour-pin electrode (James 1961). Apparently, doublingthe contact area of a single pin is substantially lesseffective in increasing the net conductance thanduplicating the contact area with a second pin. Thus,when using any two-pin electrode, a correction of1 percent moisture content should be added when theindicated moisture content exceeds 15 percent, andwhen the meter is calibrated for a four-pin electrode.

At least one manufacturer of conductance-typemoisture meters offers a two-pin electrode with pinsabout 7.5 centimeters (3 in) long for use on poles,bridge timbers, and other large material.

Insulated-Pin Electrode

Some manufacturers offer electrodes with 25- or75-millimeter (1 or 3 in) pins that are covered by atough insulating resin except at the tip. These electrodesare useful when testing lumber with a high superficialmoisture content, such as that caused by rain or dew.Superficial films of high moisture are rarely detrimentalto the usefulness of the lumber. However, if the pinelectrodes are not insulated, a conductance-typemoisture meter would indicate the high surface moisturecontent and could cause the lumber to be rejected.Therefore, a valid estimate of average moisture contentcan be obtained by using insulated pins driven to theproper depth.

Even insulated pins cannot be used successfully onlumber that has free water on the surface because thewater may follow the pins as they penetrate the wood,resulting in a very high, misleading moisture reading.

Veneer Electrode

The electrode used with a conductance-type meter onveneer consists of a large number of needles, eachabout 3 millimeters (1/8 in) long, which are arrangedinto two groups; each group is one pole of theelectrode. Normal calibration factors are assumed to bevalid for this type of electrode.

Substitute Electrodes

When the pins of the available electrode are too shortto measure either the average or the core moisture

7

Dielectric-Type Moisture Meters

content of a specimen with large cross section, twonails may be substituted for electrode pins. The nailsshould be driven to the proper depth and about thesame distance apart as the pins on the standardelectrode. The reading then can be obtained bytouching the standard electrode pins to the nailheads. Itshould be emphasized, however, that the reading is notinfluenced by the distance between the nails.

When using two nails, as when using any two-pinelectrode, a correction of 1 percent should be addedwhen the indicated moisture content is over 15 percent.

Permanent Electrodes

It is sometimes necessary to monitor the moisturecontent of a specimen of wood in a relativelyinaccessible place. A simple and convenient way to dothis is to drive electrode pins into the specimen, leavethem in place, and bring wires attached to theelectrodes out to the moisture meter. This procedure isused in some dry kiln monitors. This method has aserious problem, however, in that the permanentelectrodes give erratic and unreliable data after a fewhours, especially when the moisture content of thespecimen is in the upper range of readability of themeter. However, when the moisture content of thespecimen is no greater than about 15 percent,permanent electrodes are as reliable as freshlyimplanted electrodes.

Useful Range of Conductance-Type Meters

The useful range of conductance-type moisture metersis from about 7 to about 30 percent moisture content;only approximate qualitative readings may be obtainedon wood with over 30 percent moisture content. Thelower limit results from the difficulty in measuring thevery small conductance (high resistance) involved, andthe upper limit results from the fact that conductance isonly a weak and erratic function of moisture contentgreater than fiber saturation.

The two basic types of dielectric moisture meter are thecapacitance and power-loss meters. A third type,usually referred to as the capacitive-admittance type, isessentially a combination of capacitance and power-losstypes. In the trade, all dielectric meters are sometimesreferred to as “capacity-type” or “radiofrequency-type”meters; neither term directly applies to dielectric metersin general.

Capacitance Type

Capacitance-type moisture meters use the relationshipbetween moisture content and dielectric constant. Thewood specimen is penetrated by the electric fieldassociated with the capacitor of the frequency-determining circuit of an oscillator when the electrodeof the meter contacts the wood. The frequency of theoscillator is changed according to the effect of thespecimen on the capacitance of this capacitor or, inother words, according to the dielectric constant of thespecimen. A frequency discriminator generates a signal,read on a meter, proportional to the changes infrequency. Using the relation between dielectricconstant and moisture, the meter can be calibrated toread moisture content. Because of technical problemsand high cost, this type of meter is not at present beingmanufactured commercially.

Power-Loss Type

Power-loss type meters use the relation betweenmoisture content and loss factor. The wood specimen ispenetrated by the electric field radiating from anelectrode that is coupled to a low-power oscillator inthe meter. Power absorbed by the specimen loads theoscillator and reduces its amplitude of oscillation,which is in turn indicated by the meter dial. Since theloss factor depends on moisture content, the meter dialcan be related to percent moisture.

A portable power-loss type moisture meter is illustratedin figure 3.

8

Electrodes

Figure 3—A dielectric power-loss type moisturemeter. (ZM 118 516)

Capacitive Admittance Type

The electrode of this meter is a capacitive element in aresistance-capacitance bridge circuit. When a woodspecimen contacts the electrode, its capacitance andlosses (admittance) are increased so that the bridge isunbalanced in proportion to the dielectric constantand loss factor of the specimen. The meter dial readsthe amount of bridge imbalance, which can be relatedto the moisture content of the wood specimen causingthe imbalance.

A portable capacitive admittance meter is shown infigure 4.

Electrodes for dielectric-type meters are nonpenetrating.They vary in design according to particular applicationsbut are not interchangeable for use with oneinstrument, as are electrodes for conductance meters.The electrode of a dielectric meter is an integral part ofthe instrument.

Rough Lumber Electrode

On portable meters, this electrode consists of a numberof short, spring-loaded rods with the exposed endsrounded and mounted in a circular plastic plate about7.5 centimeters (3 in) in diameter. As the electrode ispressed into contact with the surface of the specimen,the spring-loaded rods are pushed into their mountingsockets in the plastic plate. Because of the restrainingaction of the springs, each rod maintains firm contactwith the specimen surface. Thus, the rods adjust toirregularities in the surface, making the calibrationof the meter nearly independent of the shape ofthe surface.

A modification of the rough lumber electrode consistsof a single spring-loaded metal disk, about25 millimeters (1 in) in diameter, surrounded by acircle of smaller but similarly spring-loaded metal disks.This arrangement is mounted on a plastic plate about9 centimeters (3-1/2 in) in diameter.

The electric field from rough lumber electrodespenetrates about 2 centimeters (3/4 in) into thespecimen, so that specimens up to about 4 centimetersthick (1- 1/2 in) may be used. However, when anysurface-contact electrode is used with a dielectric meter,the surface layers of the specimen have a predominanteffect on the meter readings, simply because the electricfield is stronger near the electrode. This effect may bevery pronounced when readings are made on wood withtypical residual drying gradients. High moisture contentmaterial as little as one-eighth inch below the specimensurface may not be properly represented in the meterreading (Mackay 1976).

Smooth Surface Electrode

The quadrant-type electrode gives slightly higherprecision but is usable only on smooth, plane surfaces.This electrode consists of the four quadrants of a75-millimeter disk, separated slightly and independently

9

General Purpose Electrode

Figure 4—A dielectric capacitive admittance-type moisture meter. (M 141 438; M 141 439)

free to move slightly, mounted on a plastic plate. Thefield penetration of this electrode is about25 millimeters.

Veneer Electrode

This electrode consists of several concentric rings, all inone plane, mounted on a plastic plate about75 millimeters in diameter. The field penetration of thiselectrode is about 2 millimeters. When measuring themoisture content of material thinner than 3 millimeters(1/8 in), it may be important to consider the materialon the other side of the specimen. If the backupmaterial is metal or a high-loss dielectric material, thereading of the moisture meter will probably be toohigh. It is best to use a low-loss backup material, suchas rigid polystyrene foam.

Thick-Specimen Electrode

The electrode for thick specimens consists of a ring ofspring-loaded metallic disks surrounding a somewhatlarger single disk in the center. This electrode is similarto the electrode described earlier for rough lumber butis scaled up in size, so the field penetrates about50 millimeters (about 2 in). We again emphasize,however, that the meter registers a much smallerresponse to material 2 inches from the electrode than tomaterial near or in contact with the electrode.

This electrode consists of a circular disk that in use isseparated from the specimen by 2 or 3 millimeters oflow-loss insulation. This separation makes the surfacecondition of the specimen less influential, so theelectrode is usable on either rough or smooth surfaces.

Useful Range of Dielectric Moisture Meters

Dielectric moisture meters will read down to zeromoisture, although the precision of the meters isreduced at very low moisture levels. Presently availablepower-loss meters have an upper range limit of about15 to 30 percent moisture, depending on the woodspecies. The most precise operating range of thecapacitive admittance meter is under 30 percentmoisture content, but this meter can give qualitativereadings at moisture levels greater than 30 percent.

10

Accuracy of Moisture Meters

The accuracy of an electric moisture meter in goodcondition is never limited by the ability of the meter torespond precisely to the fundamental electrical propertyof wood on which its calibration is based, nor by theprecision to which the dial can be read. The accuracyof a meter is limited by the influence of factors otherthan moisture content on the readings of the meter,insofar as these factors are unknown or not properlytaken into account.

One such factor is the calibration of the meter. This isusually considered the responsibility of themanufacturer, and the user accepts the calibration datasupplied with the meter. The accuracy of calibration,especially in regard to sampling and specimen control,is usually unknown to the user. Unless the user iswilling to run an involved calibration procedure(James 1961, 1964), the influence of this factor isuncontrollable.

However, some factors that affect meter readings canbe controlled by the user. The principal factors otherthan moisture that affect the readings of electricmoisture meters are adequacy of sample specimencharacteristics (species, density, moisture distribution,thickness, and temperature), electrode contact, graindirection, chemicals in the wood, weather conditions,and skill of operator.

Adequacy of Sample

If all individual pieces in a lot of lumber were at thesame moisture content and moisture meters gave thesame readings on all wood at the same moisturecontent, the moisture content of the entire lot could bedetermined by a single reading. But the moisturecontent of any lot varies from piece to piece, themoisture content of every piece varies from point topoint throughout the piece, and moisture meters givevarious readings on different specimens even at thesame actual moisture content. For these reasons, anaccurate estimate of the average moisture content of thelot requires more than one reading. A reading could bemade on every piece in the lot, but usually the sameinformation can be obtained for less cost by makingreadings on a smaller number of pieces, i.e., on asample, properly selected from the lot.

Proper sampling entails selecting specimen pieces insuch a way that in total they represent the lot withoutbias, and selecting a sufficient number of specimens to

reduce the influence of variability to an acceptable level(Fell and Hill 1980; Freese 1967).

Selection of Specimens

If the sample is unbiased, the average of readings onthe sample will differ from the average of all possiblereadings on the lot only because of chance, not becauseof some consistent effect of the sampling method. Thiscondition can be assured if the individual specimens areselected from the lot at random; thus, every piece in thelot has an equal chance of being in the sample.

A lot of lumber frequently consists of a number ofsubunits, such as kiln loads or forklift packages. Underthese conditions, an unbiased sample usually can beselected most easily by a process called stratifiedrandom sampling (Freese 1967). The lot is divided intoany number of roughly equal parts on the basis ofsome index of similarity (such as same kiln load), andtwo or more specimens are randomly drawn from eachpart. If the parts are not equal, the number ofspecimens from each part should be proportional to thenumber of individuals in the part. Again, the basicrequirement of stratified random sampling is that allindividual pieces have the same probability of beingincluded in the sample.

Number of Specimens

The number of specimens required for the sampledepends on the level of accuracy desired and thevariability of the data.

Although the level of accuracy is arbitrary, a reasonableand practical goal is to obtain a probability of 0.95 thatthe sample average is within 1.0 percentage point of theaverage of all possible readings on the lot. Acceptingthis, the required sample size depends only on thevariability of the data.

In general, the variability of the data is unknown andcan be estimated only from readings on a sample. Theproblem therefore is to draw an unbiased sample ofarbitrary size and to determine from the observedvariability of this sample whether more specimens areneeded. Because statistically ideal computation ofvariability and sample size is too cumbersome forroutine inspection of lumber, I suggest the followingprocedure for approximating sample size:

11

(1) Draw an unbiased sample of about 20 specimensand record one reading from each; take all readings50 centimeters (approximately 2 ft) or more from theend of the piece.

(2) Find the “range” by subtracting the smallest readingfrom the largest, square this difference, and divide itby 4.

The resulting number is an estimate of the requiredsample size (Freese 1967). If this number is largerthan 20, additional specimens are necessary to meetthe desired standard of accuracy. If stratifiedsampling is used, this method may slightly overestimatethe variance, so the estimate of sample size willbe conservative.

If the lot to be inspected is quite small, it may be lesscostly to read every individual piece in the lot than tobe concerned about proper sampling. If the lot is onlya few pieces, two or more readings should be made atrandom on each piece. The same basic rules of samplesize apply even to readings on a single specimen, exceptthat using range as an estimate of variability is ratherunreliable for small sample sizes. On the other hand,concern about sample size on a single specimen israther academic, inasmuch as more readings thanstatistically required may be obtained in less time thanit takes to estimate the required sample size.

Specimen Characteristics

The characteristics that affect meter readings arespecies, density, moisture distribution, thickness,and temperature.

Species

At a given moisture content, both the conductance anddielectric properties of wood depend on species (James1961, 1964). In regard to conductance, species differ instructure and electrolyte concentration; in regard todielectric properties, species vary with these samefactors and also with density. Consequently, correctionsfor species should be made when the necessarycorrection data are available.

If species corrections are not available, conductancemeters may be used for approximate readings becausespecies corrections are usually less than 2 percent,especially for native North American-grown species.Dielectric meters may also be used by assuming the

calibration for a species with a density similar to thatof the specimens being tested, but the results will bereliable only as rough approximations. When a singlespecies correlation is applied to several species in acommercial group, the readings may be biased and lessprecise than if each species were considered separately.

Density

The readings of conductance meters are practicallyindependent of specimen density. In contrast, thereadings of dielectric meters are affected by the densityof the specimens. A substantial part of the speciescorrections for dielectric meters is actually a correctionfor density. The species correction must be related tothe average density of the species; any single moisturedetermination will be in error to the extent that thedensity of the specimen deviates from the average forits species (or more precisely, the average of the sampleused for calibration of the meter). The American firmthat manufactures power-loss meters provides twospecies corrections for some widely used species, onefor high-density specimens and the other forlow-density specimens. However, even if specimendensity could be determined easily and reasonablyaccurately, available information on the effect ofdensity on electric properties is inadequate to permitmore than an approximate correction to be made.

Moisture Distribution

High surface moisture, such as from rain or dew, formsa surface layer of high conductance and dielectricconstant and loss factor. In general, superficialmoisture causes excessively high readings in electricmoisture meters of any type.

The average moisture content of a specimen with highsuperficial moisture may be read using a conductance-type meter equipped with an insulated pin electrode. Iffree water is standing on the surface, however, falsereadings are likely even with insulated pins.

Uneven moisture distribution along the length or widthof a specimen may also result in meter readings thatare grossly different from the true average. For thisreason it is advisable, when individual readings areimportant, to make more than one determination on agiven specimen.

12

Because of accelerated end-grain drying near the endsof specimens, moisture meter readings should be takenno closer than 50 centimeters from the end or at thelengthwise midpoint of the specimen, whichever isclosest to the end of the specimen.

Moisture gradients in wood that is drying may differgreatly from the expected form. Consequently, readingsof conductance meters at one-fourth to one-fifth of thewood’s thickness may differ greatly from the averagemoisture content of the cross section. This situationmay be recognized by reading the meter as the electrodepins are driven progressively into the specimen.Deviation from a smooth increase in reading withdeeper penetration or a reading over 30 percentmoisture content near the center suggests that “theone-fourth to one-fifth thickness rule” cannot beconsidered reliable (Skaar 1964).

Irregular drying gradients have unpredictable effects onthe readings of dielectric meters, as the reading is theintegrated effect of all the specimen material penetratedby the field. The moisture content of the materialnearest the electrode has a predominant effect on thereading; in extreme cases (such as wet surfacesmentioned earlier), the reading could differ greatlyfrom the average moisture content.

Thickness

The problem of specimen thickness is related to that ofmoisture distribution or gradient. If the specimen has auniform moisture content, excessive thickness does notaffect the accuracy of the meter reading. Whenspecimens are drying, moisture gradients must be takeninto account. To obtain a valid reading of averagemoisture content, the depth of the reading must berelated to the thickness of the specimen. Thus, it isnecessary for the pins of conductance meter electrodesto be long enough to reach one-fifth to one-fourth ofthe thickness of the specimen, and the field fromdielectric meter electrodes should penetrate essentiallyundiminished to the middle of the specimen (which isnever actually achieved in practice). With both types ofmeter, the electrode should be selected to match thespecimen thickness as nearly as possible.

If dielectric meters are used on specimens that aremuch thinner than those used to calibrate the meter,the readings will be too small. As mentioned earlier,

the material behind thin specimens may also affectthe reading.

Temperature

As the temperature of wood increases so does itselectrical conductance, and vice versa (Davidson 1958;James 1968, 1974; Keylwerth and Noack 1956).Temperature corrections should be made when using aconductance-type meter on specimens that are warmerthan 90 °F or cooler than 70 °F. The amount ofcorrection depends on both temperature and moisturecontent, so it is best to determine the correction from achart (fig. 5). If a chart is not available, a roughcorrection can be made by subtracting 1 percentmoisture content from the reading for every 20 °F thespecimen temperature is above the calibrationtemperature specified by the manufacturer, and adding1 percent for every 20 °F the specimen temperature isbelow the calibration temperature.

The effect of temperature on power loss and capacitiveadmittance is more complicated than its effect onconductance, so temperature corrections for thesemeters are not as simple as for conduct ante meters.Temperature corrections for power-loss and capacitiveadmittance meters can be made using charts, such asshown in figures 6 through 8, or special tables thatprovide readings corrected for temperature (Bramhalland Salamon 1972).

When using any type of electric moisture meter, themeter indication should first be corrected fortemperature, and then the established roomtemperature-species corrections or calibration factorsshould be applied.

Temperature of the lumber also affects the calibrationof in-line systems for monitoring moisture of lumbermoving along conveyors. The limit settings shouldbe adjusted for the temperature of the woodbeing monitored.

Other Factors

In addition to specimen characteristics, meter readingsare influenced by the contact of the electrodes with thespecimen, orientation of the electrodes relative tospecimen grain, treatments used to preserve specimensas well as adhesives and finishes, weather conditions,and skill of the operator.

13

Figure 5— Temperature corrections for reading of conductance-type moisture meters, based oncombined data from several investigators. Find meter reading on vertical left margin, followhorizontally to vertical line corresponding to the temperature of the wood, and interpolate correctedreading from family of curves. Example: lf meter indicated 18 percent on wood at 120 °F, thecorrected reading would be 14 percent. This chart is based on a calibration temperature of 70 °F. Forother calibration temperatures near 70 °F, adequate corrections can reobtained simply by shiftingthe temperature scale so that the true calibration temperature coincides with 70 °F on thetemperature scale. (M 76476 F)

Electrode Contact

For accurate readings, the electrode pins of theconductance meter must be driven to their proper depthinto sound wood, and the surface electrodes of thedielectric meter must be pressed firmly against thespecimen. When a dielectric meter is used, the specimenshould be larger than the electrode; on all sides of themeter, the amount of wood to spare should at leastequal the specimen thickness. With all meters, onlysound wood free of decay, knots, and other defectsshould be in actual contact with the meter electrode.

Grain Direction

Grain direction has no effect on the readings ofdielectric meters because the electrodes are symmetrical.With conductance meters, however, the electrode shouldbe oriented whenever possible so the current flowsparallel to the grain. At moisture levels below about15 percent, the effect of grain direction is negligible. Atmoisture levels above 20 percent, readings across thegrain may occasionally be as much as 2 percent lowerin moisture content than readings parallel to grain.

14

Chemical Treatments, Adhesives, and Finishes

Wood that has been treated with salts for preservativeor fire-retarding purposes or that has been in prolongedcontact with seawater will generally have higherconductance, dielectric constant, and loss factor thanuntreated wood with the same moisture content.Consequently, electric moisture meter readings ontreated wood will generally be too high. The error isusually negligible when the wood is below about8 percent moisture content and increases withincreasing moisture content. At moisture levels above10 to 15 percent, the error grows rapidly anderratically, making correction impossible. Oilborneorganic preservatives, such as creosote andpentachlorophenol, do not significantly affect thereadings of electric moisture meters (James 1966).

Some types of glue used in plywood are electricconductors and may therefore affect the readings ofelectric moisture meters (Bell and Krueger 1949). Theeffect of plywood gluelines on the reading of aconductance meter may be determined by observing the

Maintenance of Moisture Meters

meter reading as the electrode pins are driven into andthen through the first ply. If the the reading increasesabruptly as the pins contact the glueline, readings onthat plywood will be unreliable. If no such effect isnoted, the glue will not affect the readings.

Finishes rarely affect the readings of electric moisturemeters. If it is suspected that a resin or metallic finishmay be electrically conductive, the reading may beobtained using a conductance meter with insulated pins.The conductivity of the finish may be checked bypricking the finish film with electrode pins; a highmoisture reading indicates a conductive finish, and noreading indicates a nonconductive finish.

Doors, tabletops, or other panel products that include ametal lamination to provide resistance to heat, fire, orx-rays are likely to give false readings with electricmoisture meters.

Weather Conditions

If electric moisture meters are used in foggy or rainyweather or are moved from cool surroundings intowarmer, more humid surroundings, films of moisturemay form on parts of the meter. These films may thenprovide leakage paths that seriously affect theoperation of the meter. Usually these conditions maybe recognized by difficulty in adjusting or balancingthe meter, erratic or unstable zero settings, or noresponse from the meter when taking readings onmaterial at low moisture levels.

Skill of Operator

Electric moisture meters are relatively easy to operate.However, the accuracy and reliability of the readings dodepend on the care exercised by the operator.Important considerations are careful adjustment of themeter controls, proper application of necessarycorrection factors, proper application of the electrode,attention to the condition of the instrument, andcorrect reading of the meter. Finally, the operatorshould select specimen material carefully to achieve thereal objectives of the moisture measurements efficiently.

The principal item of maintenance is replacement ofdefective or exhausted components of the instrument.Recalibration is rarely needed, especially withconductance meters, but the calibrations shouldnevertheless be checked periodically using standardssupplied by the meter manufacturers. Dielectric metersare usually provided with a built-in standard forchecking calibration.

Most portable electric moisture meters are powered byself-contained batteries. Dry batteries commonly willlast for 6 months to a year with average use, but theyshould be replaced when adjustment controls must beset near the limits of their travel in order to adjust themeter correctly. Rechargeable batteries should becharged routinely. Vacuum tubes used in the oldermoisture meters are operated far below their ratedpower and will normally give years of service.Occasional replacements may be required. Electroniccomponents other than tubes and batteries mayoccasionally fail and should be replaced only by acompetent technician. The pins of conductance meterelectrodes necessarily receive hard usage, and it is notuncommon for them to bend or break in use. It isadvisable to have spare pins and the installation tools inthe meter case. The electrode of any type of moisturemeter should be kept clean to assure accuracy of thereadings. The meter should always be handled carefullybecause rough handling can damage such fragilecomponents as the meter movement or vacuum tubes.

15

Figure 6—Approximate temperature correctionsfor readings of power-loss-type moisture meters;data taken using a Moisture Register model L.Locate the point whose coordinates are theobserved scale reading and the specimentemperature, and trace back parallel to thecurves to the calibration temperature of themeter (usually 80 °F). The vertical coordinatehere is the corrected scale reading, which isthen converted to moisture content using theusual species conversion tables. Solid lines arefor the meter itself at room temperature; dottedlines are for the meter at the same temperatureas the specimens. (M 134 523)

Figure 8—Approximate temperature correctionsfor capacitive admittance meter; data takenusing a “Sentry” hand meter with calibrationsetting of 15 or less. Chart is used as indicatedfor figure 6. (M 134 530)

Figure 7—Approximate temperature correctionsfor capacitive admittance meter; data takenusing a “Sentry” hand meter with calibrationsetting of 20 or greater. Chart is used asindicated for figure 6. (M 134 527)

16

Literature Cited

American Society for Testing and Materials. 1968.Methods of measuring moisture content of wood.ASTM Standard D 2016. Philadelphia, PA.

Bell, Earl; Krueger, Norman. 1949. Effect of plywoodglue lines on the accuracy of moisture meterindications. In: Proceedings, Forest ProductsResearch Society Vol. 3, p. 85.

Bramhall, G.; Salamon, M. 1972. Combinedspecies-temperature correction tables for moisturemeters. Rep. VP-X-103. Vancouver, B. C.: WesternForest Products Laboratory.

Davidson, R. W. 1958. Effect of temperature on theresistance of wood. Forest Products Journal. 8(5):160.

Davies, I. I. 1937. A wide range vacuum-tuberesistance bridger. Instruments. 10:261.

Fell, J.; Hill, J. L. 1980. Sampling levels for hardwoodkiln drying control. Forest Products Journal. 30(3):32-36.

Freese, Frank. 1967. Elementary statistical methods forforesters. Agric. Handb. 317. Washington, DC: U.S.Department of Agriculture.

James, William L. 1961. Calibration of electricmoisture meter for jack and red pine, black spruce,paper birch, black ash, eastern hemlock, andbigtooth aspen. FPL Rep. 2208. Madison, WI: U.S.Department of Agriculture, Forest Service, ForestProducts Laboratory.

James, William L. 1966. Effects of wood preservativeson electric moisture-meter readings. Res. NoteFPL-0106. Madison, WI: U.S. Department ofAgriculture, Forest Service, Forest ProductsLaboratory.

James, William L. 1968. Effect of temperature onreadings of electric moisture meters. Forest ProductsJournal. 18(10): 23.

James, William L. 1974. The dielectric properties ofwood and hardboard; variation with temperature,frequency, moisture content, and grain orientation.Res. Pap. FPL 245. Madison, WI: U.S. Departmentof Agriculture, Forest Service, Forest ProductsLaboratory.

Keylwerth, R.; Noack, D. 1956. Uber den einflushohrer temperature auf die electrischeholzfeuchtigkeitsmessung nach demwiderstandsprinzip. [On the influence of highertemperatures on the electric measurement of woodmoisture content using the resistance principle.] Holzals Rob- und Werkstoff. 14(5): 162.

Mackay, J.F.G. 1976. Effect of moisture gradients onthe accuracy of power-loss moisture meters. ForestProducts Journal. 26(3): 49-52.

Skaar, C. 1964. Some factors involved in the electricaldetermination of moisture gradients in wood. ForestProducts Journal. 14(6): 239-243.

Suits, C. G.; Dunlap, M. E. 1931. Determination ofthe moisture content of wood by electrical means.General Electric Review; p. 706.

James, William L. 1964. Calibration of electricmoisture meters for some wood species grown inHawaii. Res. Note FPL-061. Madison, WI: U.S.Department of Agriculture, Forest Service, ForestProducts Laboratory.

2.5-6/88

17