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Humidity Measurement in Industrial Gases Wednesday, 11th October 2006 National Physical Laboratory
Hampton Road, Teddington, Middx TW11 0LW
Programme 09:30 Registration
10:00 Welcome to NPL
10:05 Water Vapour Measurement in HCl Graham Leggett (NPL) Gordon Ferrier (Air Products)
10:35 Humidity Measurements in Compressed Gases Thomas Hübert (BAM)
11:05 Analyser Development for Fast Diagnostics of Plant Conditions Paul Stockwell (IMA)
11:25 Moisture in Medical Gas: The New HTM02 Regulations Nick Malby (Michell Instruments)
11:55 Discussion
12.25 Lunch
13:30 Users View on Humidity Measurement Nick Bates (National Grid, formerly Transco)
14:00 Materials Performance in the Presence of Moisture Alan Turnbull (NPL)
14:30 NPL's Facility for Humidity Calibration at Elevated Pressures and Non-air Gases Stephanie Bell (NPL)
15:00 Break 15:15 Round table discussion on NPL’s three year research plan and meeting industrial needs
16:30 Close
Trace Water Vapour Measurements and Calibration
including Gas Matrix Effects
Tom Gardiner, Graham Leggett, Analytical Science Team,
National Physical Laboratory
Humidity Measurements in Industrial Gases11th October 2006
Summary
• NPL’s Trace Water Vapour Facility• Comparison of trace water vapour sensors• Infared spectroscopic (CRDS) measurements of water
vapour, including matrix effects• Measurements of trace water vapour in HCl.• Conclusions
Requirement for Trace Water Vapour Calibration
• Increasingly challenging purity specifications for gases used in high-tech manufacturing processes.
• Specifications based on gas concentrations (gravimetric traceability) rather than humidity scales (thermal traceability).
• Analytical Science group at NPL has extensive experience in the preparation of static and dynamic gas standards.
• Comparability with humidity standards is a crucial element of facility validation.
NPL Trace Water Calibration Facility
Flow mixing system using mass flow controllers and
critical orifices
Heated getter unit to provide ultra-high purity source gas
Multi-port outlet to enable simultaneous sensor
evaluation
Micro-balance providing on-line weighing of magnetically-suspended permeation tube
Dynamic Flow Dilution System
Temperature control unit
Output valve
Input valve
MFC (diluent)
MFC (WV source)
Vent o/pvalve
WV source outputSonic flow
nozzles
Source control valves
WV source input
Microbalance for On-line Weighing of Permeation Tube
Automated balance 0-20 g load, 3 µg reproducibility
Magnetic suspension of load
Permeation tube mounted in temperature-controlled
cell. Customised EP-stainless steel cell for water
vapour system
Facility Performance
• Zero-offset level measured using spectroscopic techniques to be less than 1 ppbv (limited by spectrometer).
• Flow switching and dynamic dilution systems validated to better than 0.2%.
• Uncertainties in mass emission rate typically 0.3% over one hour.
• Evaluation of the facility using a calibrated frost point hygrometer have demonstrated good (~1%) agreement between frost points of –67oC and –90oC (4 ppmv to 30 ppbv).
Water Sensor Evaluation
• A group of 13 trace moisture instruments have been tested, in order to provide general information on the performance of different sensor types.• The following data shows examples of the tests of response accuracy, time, linearity, and hysteresis.• Other tests were carried out to assess long-term stability, ambient temperature response, and effects of exposure to ultra-dry (sub-10 ppbv) conditions.• The study enables some general conclusions to be drawn for the different measurement methods and their suitability in different applications.
Assessment of Water Vapour Sensors
0
100
200
300
400
500
600
S1 S2 C1 C2 E1 E2 P1 P3 P4 P5 P6 P7
Tim
e / m
inut
es
90% (mins)10% (mins)
Response times for a upward step change of 300 to 850 nmol/mol
Response time, linearity and hysteresis results from water vapour sensor assessment.
Instrument S1 S2 C1 C2 E1 E2 P1 P3 P4 P5 P7
Type CRDS Laser absorption
Hysteresis (at 300
nmol/mol)0.017 0.282 0.009 0.034 0.217 0.2 0.082 0.139 0.27 0.391 0.007
Linearity (R2)
0.9998 0.9829 0.9993 0.9985 0.9513 0.9614 0.9641 0.9877 0.9894 0.9817 0.9875
Frost-point hygrometers Electrolytic Probes Capacitance Sensors
CRDS Measurements of Trace Water Vapour
• A CRDS instrument (Tiger Optics Lasertrace) is being used within NPL’s trace water vapour facility
• CRDS instrument selected as an on-line sensor due to its high sensitivity, fast response and good linearity.
• Currently assessing absolute accuracy of CRDS measurement through comparisons against gravimetric and thermal (frost-point) humidity standards.
• Issues affecting CRDS accuracy include laser stability, absorption and desorption from cell and sample line walls, Tau0 (empty cell) values, gas temperature and pressure, traceability of spectroscopic parameters, and matrix effects.
Infrared Spectroscopic Measurements of Gases
• Infared spectroscopic measurements work by using light to measurement the optical absoprtion ‘fingerprint’ of the target species.
• In the infared region the spectroscopic fingerprint of a particular species depends upon the rotational and vibrational modes of the molecule.
• The shape of the absorption features are effected by the ambient conditions and interactions with surrounding molecules.
Spectroscopic Measurement Methods
Direct Absorption Spectroscopy
Source DetectorResonant cavity with HR mirrorsMeasurement volume of length L
( ) ( )( )⎟
⎟⎠
⎞⎜⎜⎝
⎛−=
λλλα
0
logIILN
Cavity Ringdown Spectroscopy (CRDS)
0
0
Time
Sign
al
Expontential signal decay with a period of Tau (the ring down time)
Source Detector
Resonant cavity with HR mirrors
( ) ( ) ( )⎟⎠⎞⎜
⎝⎛ −= λτλτλα 0
111c
N
Absorption lineshapes
• Natural linewidth – due to Heisenburg’s Uncertainty Principle, and driven by the electron lifetime of the initial and final states (∆ti and ∆tf).
• Doppler linewidth – due to the Doppler shifts from the thermal motion of the molecules, determined by temperature (T) and molecular mass (m).
• Pressure linewidth – due to perturbation of the molecule by collisions with other molecules and close encounters with molecular electric fields. Determined by the molecular density (n) and the collision cross-section (σ).
⎟⎠⎞
⎜⎝⎛
∆+∆≈∆fi ttc
112
2
πλλ
mkT
c22λλ =∆
mkTn
c22
πσλλ =∆
Effect of Matrix Gas on Spectroscopic Measurements
• Pressure linewidth dominates for infrared measurements at or above atmospheric pressure.
• Overall pressure linewidth is a combination of the pressure linewidths for each gas present, including the target gas itself (self-broadened linewidth).
• Molecules with significant electric fields (polar molecules such as water vapour) have stronger interactions, leading to broader lines.
• Level of effect is not necessarily the same for each absorption line as it depends on the electron transition involved.
• So the overall lineshape is a complex relationship involving the gas mixture present, the temperature of the system, and the absorption line being measured.
Water Vapour in HCl – Measurements at Air Products Crewe Facility
BIP N
2
VLSI HC
l
P PTiger Optics
MichellHygrometer
Ventto scrubberVacuum
Panel supplied by Air Products
Flow meter
Measurements of Water Vapour in HClCRDS Scan of Water Vapour in Nitrogen
-1.E-02
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
7177 7179 7181 7183 7185 7187
Wavelength (nm)
modelmeasured
CRDS Scan of Water Vapour in HCl
0.E+00
1.E-02
2.E-02
3.E-02
7177 7179 7181 7183 7185 7187
Wavelength (nm)
modelmeasured
• Spectral scans of several water vapour lines made by changing wavelength of laser in CRDS.
• Comparison of scans in nitrogen and HCl matrices shows the broadening in HCl.
• This is due to the stronger interaction between the HCl and H2O molecules.
CRDS Scan Fitting SensitivitiesLinewidth sensitivity
-3
-2
-1
0
1
2
3
-5 -3 -1 1 3 5
Change in linewidth (%)
Cha
nge
in c
once
ntra
tion
(%)
3.0E-03
3.5E-03
4.0E-03
4.5E-03
5.0E-03
fit R
MS
(arb
. uni
ts)
% change inconcentrationRMS
X-axis stretch sensitivity (tuning rate)
-2
0
2
4
6
8
-5 -4 -3 -2 -1 0 1 2 3 4 5
X-axis stretch (%)
Cha
nge
in c
once
ntra
tion
(%)
0.0E+00
5.0E-03
1.0E-02
1.5E-02
2.0E-02
2.5E-02
Fit R
MS
(arb
. uni
ts)
% change inconcentrationRMS
Shift sensitivity
0.0
0.1
0.2
0.3
0.4
0.5
0.6
-4 -3 -2 -1 0 1 2 3 4Frequency shift (% of Voigt FWHM)
Cha
nge
in c
once
ntra
tion
(%)
3.0E-03
6.0E-03
9.0E-03
1.2E-02
Fit R
MS
(arb
. uni
ts)
% change inconcentrationRMS
TemperatureEffects line strength, line width and gas density
-2.475 Almost linear relationship
Pressure Effects linewidth, line position, gas density 0.526 Almost linear
relationship
Linewidth
Due to spectroscopic errors or uncertainty in matrix effects
-0.475 Slight curvature
Stretch Due to uncertainty in laser tuning rate 0.935 positive
curvature
Shift Due to uncertainty in laser temperature
0.04 for a 1% FWHM shift 0.13 for a 3% FWHM shift
minimum at optimum value
Parameter Notes on possible causes and effects
Sensitivity (%conc./%parameter)
Sensitivity behaviour
Analysis of Water Vapour in HCl Results
• Initial estimate of the relative linewidths was taken from the self-broadened linewidths
• A broadening factor was then applied to all of the linewidths, and optimised to give the best fit to the scan.
• A width parameter of [0.575 (+/-0.005) * self-broadened width] gave the best fit to the HCl scan.
• The width of the strong line at 7181.156 cm-1 increased by a factor of 2.78 when switching between nitrogen and hydrogen chloride matrices.
• This compares very well with the value of 2.76 measured by Vorsaet al.– Quantitative absorption spectroscopy of residual water vapor in high-
purity gases: pressure broadening of the 1.39253-µm H2O transition by N2, HCl, HBr, Cl2, and O2’; V. Vorsa et al; Applied Optics, 44(4), 611-619; Feb 2005
Conclusions – Calibration Facility
• Validation of trace water sensors requires careful design of thecalibration source, with particular attention to gas handling.
• A new trace water vapour calibration facility has been developedcapable of generating controlled amounts of water vapour down to10 ppb (and up to 4 ppm+)
• In addition to evaluating measurement accuracy, the facility can be used to assess other aspects of sensor performance, including– Linearity– Response time– Hysteresis– Long term stability
Conclusions – Spectroscopic Measurements
• Spectroscopic measurements provide an useful, non-contact measurement method for trace water vapour, with high sensitivity, linearity and response time.
• Accurate spectroscopic measurements require a good understand of how the absorption features are effected by the measurement conditions.
• This is particularly true for measurements in different matrix gases, where there can be significant changes to the spectroscopy.
• As with all trace water measurement techniques, the sample handling is a crucial part of the measurement method. This is particularly the case if there is strong interaction between the water vapour and the matrix gas.
Acknowledgements
• Gordon Ferrier; Air Products• Kevin Cleaver and Keith Waterfield; BOC• Wen-Bin Yan and Calvin Krusen; Tiger Optics• Kevin Lehmann; University of Virginia• Stephanie Bell and Marc Stevens; NPL Humidity Group
11/10/06 Humidity Measurements in Compressed Gases 1
HUMIDITY MEASUREMENTS IN COMPRESSED GASES
Thomas Hübert
Bundesanstalt für Materialforschung und -prüfung,
D-12203 Berlin, Germany
11/10/06 Humidity Measurements in Compressed Gases 2
What is
?Bundesanstalt fürMaterialforschung und -prüfung
Federal Institute forMaterials Research and Testing
Safety and reliability in chemical and materials technologies
History: 1870 Preußische Königliche Mechan.-Techn. Versuchsanstalt1920 Chemisch-Technische Reichsanstalt1969 Senior Federal authority
Staff: 1579 permanent, temporary, apprentices and trainees Organisation: 9 specialised departments, 35 divisionsBudget: 96.4 Mio€ federal funds
17.7 Mio€ research projects and fees for testing
11/10/06 Humidity Measurements in Compressed Gases 3
1. INTRODUCTION MOTIVATION
Water in compressed gasses :
Demands from Standards: • Ph. Eur. “European Pharmakopoe” 4.07/1238.• ISO 8573-1 “Compressed Air – Contaminates and Purity Classes”.• EN 12021 „Respiratory protective devices – Compressed air for breathing apparatus“
Stimulation of corrosionLeaching of lubricants and acceleration of wearDestruction of moving unitsFormation of aggressive substances (acids)Ice formation and disruptionGrowth of fungi and bacteria
11/10/06 Humidity Measurements in Compressed Gases 4
1. INTRODUCTION
OUTLINE
1. INTRODUCTION
2. THEORY
3. SENSOR TESTING
4. RESULTS
5. SUMMARY
11/10/06 Humidity Measurements in Compressed Gases 5
2. THEORETICAL APPROACH
van der Waals equation 1873
Redlich-Kwong-Soave 1972
Virial equation
TRnnbVV
anp ⋅⋅=−⎟⎟⎠
⎞⎜⎜⎝
⎛+ )(2
2
All formulas are approximations !
interaction (attraction) of gas moleculs
volume of moleculs
Gas mixture (Dalton law):
othersOHNOwetair ppppP +++= 222
Z - compressibilitygas law (real) TRZnVp ⋅⋅⋅=⋅
gas law (ideal) TRnVp ⋅⋅=⋅
11/10/06 Humidity Measurements in Compressed Gases 6
2. THEORETICAL APPROACH : HUMIDITY MEASURES
100),('
'⋅=
tpee
w
No. measure symbol unit equations related to e
1 Water vapour pressure of pure phase or air
e, e´ Pa
2 Water vapour saturation pressure above water or ice of pure phase or air
ew, ew’ei, ei´
Pa
3 Dew point or frost point temperature
td, tf °C
4 Relative humidity related to water or ice
Uw, Ui %
5 Water vapour density , absolute humidity
dV kg/m3
6 Mixing ratio r kg/kg
7 Volume content of water vapour wv m3/m3
8 Water vapour molar fraction x mol/mol∑∑ +
=+
=iiv
v
pee
nnnx
''98,621ep
er−
⋅=
100,
⋅⎥⎦
⎤⎢⎣
⎡=
tpvw
vw x
xU
vvv
v xn
npe
VVw ====
∑'
),('' dw tpee =
Te
ZVmd
mix
vv
'121667.0 ⋅⋅==
Humidity measurement in gases: determination of water vapour concentration
11/10/06 Humidity Measurements in Compressed Gases 7
4. SENSOR TESTING : CALCULATIONS
No. software supplier name source
1 Thunder Scientific, USA HumiCalc www.thunderscientific.com
2 Michell Instr. UK HumiCalc www.michell.co.uk
3 E+E Electronik, Austria HumCalc www.epluse.at
4 PTB, Dr. Mackrodt hygdat
5 LAB-EL Laboratory Electronics, Poland
LAB-EL Humidity Calculator
www.label.com.pl
6 ThermExel PsychroSi www.thermexcel.com
7 National Weather Service Forecast Weather Calculator www.srh.noaa.gov
8 Australian Bureau of Meteorology Humidity Calculator www.bom.gov.au
rdUvtee d ↔↔↔↔↔ '
11/10/06 Humidity Measurements in Compressed Gases 8
2. THEORETICAL APPROACH
⎟⎟⎠
⎞⎜⎜⎝
⎛+⋅
⋅==dw
dwdw tb
tatete exp2.611)()(
-45 > t> +60°Cpure water phase :aw= 17.62, bw = 243.12 K
Magnus
Sonntag (-100.0 < t < +100.0 °C, water)
TTTTTew ln433502.210673952.110711193.2635794.169385.6096)(ln 2521 +⋅⋅+⋅⋅−+⋅−= −−−
TT
TTTTew
log5459673.61014452093.0
1041764768.01048640239.03914993.11058002206.0)(log37
24114
+⋅⋅−
⋅+⋅⋅−+⋅⋅−=−
−−−
Hyland and Wexler
Temperature dependence of water saturation pressure
11/10/06 Humidity Measurements in Compressed Gases 9
2. THEORETICAL APPROACH: REAL GAS
⎥⎦
⎤⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛−+⎟
⎠⎞
⎜⎝⎛ −= 11exp),(
w
w
eP
Peptf βα
∑=
−=4
1
)1(
i
iitAα ∑
=
−=4
1
)1(expi
iitBβ
1+⋅⋅= PTgf h
Concept of enhancement factorGoff 1949
Hebestreit 1988
Water vapour (saturation) pressure depends onTemperatureGas pressureNature of gas
www efe ⋅='
)(),('),(
TZtpZ
xxtpf
w
wgasw ⋅=
calculation of f (Greenspan, Wylie):
11/10/06 Humidity Measurements in Compressed Gases 10
2. THEORETICAL APPROACH
Enhancement factors
t/°C p/MPa p/MPa p/MPa
0,1 1 10
0 1.008 1.076 1.761
20 1.005 1.055 1.548
40 1.004 1.040 1.403
t/°C p/MPa p/MPa p/MPa
0,1 1 10
0 1.005 1.005 1.511
20 1.004 1.039 1.392
40 1.003 1.031 1,306
air methane
Calculated according from uncertainty ~10%
Hebestreit 1988
1+⋅⋅= pTgf h
11/10/06 Humidity Measurements in Compressed Gases 11
2. THEORETICAL APPROACHGas pressure influence on water vapor amount
Water solubility in air is nearly constant up to 300 bar, pressure increase results in condensation
isochorisobar
“A wet sponge being squeezed”
11/10/06 Humidity Measurements in Compressed Gases 12
2. THEORETICAL APPROACH
Pressure dew point (PDP)
derived from the Magnus equation:
Gas pressure influence on water vapor amount
aaaaddd P
PPtftePtfte 1,111 ),()(),()( ⋅=
aww
dw
aw
dwd
d
pp
batb
pp
atbt
t1
1
1
ln1
ln
⋅⋅+
−
⋅+
+=
11/10/06 Humidity Measurements in Compressed Gases 13
2. THEORETICAL APPROACH
Compression from normal pressure to 8 bar results in increase of dew pointfrom -8 to 20 °C.
Expansion of air of dew point of 10 °C form 36 bar to 5 bar results in decrease
of dew point to - 23 °C.
Gas pressure influence on dew point
2
1
1
2
11/10/06 Humidity Measurements in Compressed Gases 14
3. SENSOR TESTING: Calibration
Definition: The set of operations which establish, under specific conditions, the relationship between values indicated by a measuring instrument, measuring system or material measure, and the corresponding known values of a measurand. (VIM-6.13)
Calibration function
Analytic function
)(SGc =
)(cFS =
Output y Measuring Instrument Input x
11/10/06 Humidity Measurements in Compressed Gases 15
3. SENSOR TESTING : TEST FACILIY
11/10/06 Humidity Measurements in Compressed Gases 16
3. SENSOR TESTING : TEST FACILITY
11/10/06 Humidity Measurements in Compressed Gases 17
3. SENSOR TESTING : EXPERIMENTAL RANGE
humidity range range of measurement uncertainty conditions
ambient 1 ≤U ≤ 99 % (U - rel. humidity)-40 ≤ td ≤ +40 °C (td –dew point temperature)
0.5 to 2 %≥0.2 °C (td)
-40 to +100 °C, ±0.3 K
trace 0.02≤ v ≤ 2000 µl/l (ppmv)v – volume amount-100 ≤ tf ≤ -14 °C(tf –frost point temperature)
0.02 to 35 µl/l
≥ 0.2 to 2 °C
1 bar, room temperature
high 50 ≤ f ≤ 300 g/m3
40 ≤ td ≤ 80 °C
5.5 to 10 g/m3
≥0.3°C
70 to 180°C0.3 K1 bar
pressure -60 ≤ td ≤ +60 °C 0.1 ≤ f ≤ 120 g/m3
0.2 to 1 °C0.005 to 3 g/m3
1 to 350 bar
11/10/06 Humidity Measurements in Compressed Gases 18
• Accreditation
competence of a lab for testing and calibration (DIN EN ISO 17025).
• Certification
conformity of a product(DIN EN 17025, DIN ISO 9000).
3. SENSOR TESTING : QUALIFICATION
11/10/06 Humidity Measurements in Compressed Gases 19
4. RESULTS: METHODS FOR HUMIDIY MEASUREMENTS
No Method Principle Measured signal
Gas pressure range
Humidity range
Manufacturer(selection)
1 Two-pressure generator
Mixing of gas streams Pressure/volume
1…10 bar td -90…+90°C NPL, NIST, PTBE+E electronics
2 Mechanical Change of length length ? r.h. 10…100 %. Galltec, Brown Boveri-Kent
3 Psychrometer Cooling of wet air stream
temperature ? td -20…100 °C r.h. 5…100 %
BARTEC
4 Gravimetric Absorption of water Mass/ gas volume
? td -60 …+60 °Cr.h. 0…100 %
NPL
5 Condensation Water of ice formation on a mirror
Light intensity/temperature
1…300 bar td -90…+60 °C Michell Instr., MBW, General Eastern,Mini: CIS, IL-Metronic
6 Conductivity Change of conductivity
Voltage, impedance
r.h. 0…100 % Novasina
7a Capacity Change of permittivity of polymer
capacitance 1-15 bar r.h. 0…100 %td -60…40 °C
TESTO, CS-Messtechnik,Vaisala, Rotronic
7b Capacity Change of permittivity of oxide (alumina)
capacitance 1…350 bar -100…+20 °C Panametrics, Endres& Hauser, Michell Instr., Alphamoisture
8 Acoustic Wave Adsorption changes wave expansion
frequency ? td -100… Du Pont, Beckmann, AAA
9 Spectroscopic CRD,Absorption of water
absorption 1..250 bar 0.2 ppb…5 ppm-80…+30 °C
Tiger Optics, BARTEC
10 Electrolytic Water electrolyse current 1…200 bar 1…5000 ppm MEECO, DKS
11/10/06 Humidity Measurements in Compressed Gases 20
4. RESULTS
Comparison of precision dew-point hygrometers
frost point S4000TRS (°C, ist) -53,19
corrected frost point S4000TRS
(°C)*-53,12
frost point MBW 373 (°C) -53,09
difference +0,03 K
frost point S4000TRScalc. from 1 bar
-39,13 °C (6,035 bar)
frost point MBW 373 -40,00°C (6,035 bar)
difference -0,87 K
1 bar
6.035 bar
11/10/06 Humidity Measurements in Compressed Gases 21
4. RESULTS : TESTING OF NEW HUMIDITY SENORS
CCO, CCC and LiCl dew-point sensors
11/10/06 Humidity Measurements in Compressed Gases 22
3. RESULTS : HUMIDITY SENORS
Detection principle of CCO, CCC and LiCl dew-point sensors
Saturation vapor pressure above water and LiCl solution
Pres
sure
in P
a
Temperature in °C
LiCl
water
11/10/06 Humidity Measurements in Compressed Gases 23
4. RESULTS : DEW POINT SENSOR
Deviation from Reference <0.5 K
CCO Sensor
11/10/06 Humidity Measurements in Compressed Gases 24
4. RESULTS : DEW POINT SENSOR
Deviation from Reference <1 K
CCC Sensor
11/10/06 Humidity Measurements in Compressed Gases 25
4. RESULTS : DEW POINT SENSOR
Deviation from Reference <1 K
LiCl Sensor
11/10/06 Humidity Measurements in Compressed Gases 26
4. RESULTS : HUMIDITY SENORS
Polymer and Oxide Sensor
semiconductingn-type Fe2O3
Polymer Sensor
Pt100
11/10/06 Humidity Measurements in Compressed Gases 27
4. RESULTS : HUMIDITY SENORS
Polymer and Oxide Sensor
beaeZ −⋅=)(
0 20 40 60 80 100
1300
1350
1400
1450
1500
1550
1600
1650
1700 H2OK2SO4K2NO3
(NH4)2SO4
NaClKJ
K2CO3
t=25°C
CaCl2
LiCl
Polymer Sensor 1006
increasing humidity decreasing humidity linear Fit
sens
or c
apac
ity (p
F) b
ei 1
0 kH
z
relative humidity (%)
exponential decreaselinear increase
.).%( hrbapF ⋅+=
11/10/06 Humidity Measurements in Compressed Gases 28
4. RESULTSOxide Sensor
0 2 4 6 8 10
2
-10-5
05 10152025
Sen
sor S
igna
l (V)
Dew-Point Temperature (°C
)
Gas Pressure (MPa)
nitrogen
11/10/06 Humidity Measurements in Compressed Gases 29
4. RESULTSOxide Sensor
methane
11/10/06 Humidity Measurements in Compressed Gases 30
4. RESULTS
Gas pressure influence on oxide sensor signal
0 500 1000 1500 2000 2500 3000 3500 4000
1,0
1,5
2,0
2,5
3,0
3,5
4,0 MPa
0,1 1 4 7 10
sens
or s
igna
l (V)
water vapor pressure e*f (Pa)
0 2 4 6 8 10
0,0002
0,0004
0,0006
0,0008
0,0010
0,0012
0,0014
B
deka_sensor_CH4a_graph5, 21.3.06
sens
itivi
ty (V
/Pa)
gas pressure (MPa)
∑=
+= n
iii
E
PK1
1
1θ
Langmuir Isotherm for adsorption of several gases on surface
fraction of empty sites: Decrease in adsorption sites for water
1E-4 1E-3
1,0
1,5
2,0
2,5
3,0
3,5
0,1 MPa 10 MPa
Sens
or s
igna
l (V
)
Molanteil
11/10/06 Humidity Measurements in Compressed Gases 31
4. RESULTSPolymer Sensor
Deviation from Reference <2 K
11/10/06 Humidity Measurements in Compressed Gases 32
4. RESULTS
Gas pressure influence on polymer sensor signal
pRHbpRHb
RHRHpRHRHp
oooop )(1
)(),(+
⋅∆−=∆−= ∞
formal fit with Langmuir-like mode of gas sorptionLuijten 1998
11/10/06 Humidity Measurements in Compressed Gases 33
4. RESULTS
Example for calibration at increased pressure
Frost point S4000TRS (°C)* -64,80 -19,78 -14,65
Frost point at 7 bar absolute pressure (°C)** -50,36 + 2,35 + 9,31
Reading frost point sensor (°C) - 56
Deviation of the sensor from calculated value of S4000TRS
(K / 7bar abs. pressure)
- 5,64
Adjustment of reading of sensor (K) + 6
Reading Frost point of Sensors (°C)
after correction - 50 + 3 + 10
Deviation of sensor after adjustment (K) + 0,36 + 0,65 + 0,69
*NPL- Calibration Certificate No. 44436 (02.10.03)** Software „HumiCalc“, Copyright 1993 Thunder Scientific Corporation, Vers. 1.21w
Polymer sensorsource: TESTO
11/10/06 Humidity Measurements in Compressed Gases 34
4. RESULTS
References
http/www.drucklufttechnik.de/english
[1] Ph. Eur. “European Pharmakopoe” 4.07/1238.[2] ISO 8573-1 “Compressed Air – Contaminates and Purity Classes”.[3] EN 12021 „Respiratory protective devices – Compressed air for breathing apparatus“, 1998.[4] ASTM D 1142-95, “Standard Test Method fort he Water Vapor Content of Gaseous fuels by
Measurement of Dew-Point Temperature“, ASTM International, USA 2000.[5] St. Bell, St. Boyes, “An assessment of experimental data that underpin formulae
for water vapour enhancement factor”, NPL 2001 online.[6] J. A. Goff, “Standardization of Thermodynamic Properties of Moist Air”,
Heating, Piping and Air Cond. 21 (1949), S. 118.[7] L. Greenspan, “Functional Equations for the Enhancement Factors for CO2-free Moist Air”,
J. Research NBS, A. Physics and Chemistry, vol. 80A, 41-44, 1976. [8] R. G. Wylie and R. Fisher, “Molecular Interaction of Water Vapor and Air”,
J. Chem. Eng. Data 41, p. 133-142, 1996.[9] A. Hebestreit, „Messung der Wasserdampftaupunkt-temperatur in Hochdruckgasleitungen“,
msr, 31, 403-405, 1988..[10] C.C.M. Luijten, M.E.H. Dongen, L.E. Stormbom, “Pressure influence in capacitance humidity
measurement”, Sens. & Act. B 49, 279-282, 1998.
11/10/06 Humidity Measurements in Compressed Gases 35
5. SUMMARY
5 Sensors (CCO, CCC, LiCl, Polymer, MOX) tested at -15 (-60) to 20°C
Deviations from reference <2 K
Application in compressed gases up to 10 MPa.
Calibration in pressure range of 0.1 to 10MPa
Gas pressure influence due to adsorption model
1. INTRODUCTION humidity in compressed gases
2. THEORY real gas behaviour: f(p,t)
3. SENSOR TESTING dewpoint and capacitive hygrometer
4. RESULTS under pressure uncertainty ~ 2 K
Comparison of 2 precision hygrometer with deviation 0.03 K (1 bar)
11/10/06 Humidity Measurements in Compressed Gases 36
5. SUMMARY
Over two thirds of the earth’s surface is covered with water, 97.2% of which is contained in the five oceans. The Antartic ice shild, containing 90% of all fresh water on the planet, is visible at the bottom. Atmospheric water vapor can be seen as clouds, contributing to the earth's albedo.
tanks to my co-workers:
Ulrich BanachHeidi LorenzKarin KeilGerd HenningAndreas LorekDirk KleinJürgen Raesch
Improving gas analysis
Simulating Plant Conditions
An examination of instrument response to brief moisture ingress
Improving gas analysis
Plant conditions
Most process applications require switching between gas supplies of various qualities. When using gases in some processes it is important that components like catalysts are not exposed to greater than 2 PPMv
Improving gas analysis
Plant conditions
Industrial process with using gas switching systems there are lengths of pipeline that contain stationary gas at some point during the process
Stationary gas will wet up
Improving gas analysis
Plant conditions
Moisture levels will increase in gases within pipe systems with zero flow– Dalton's Law of Partial Pressures explains
the reason that “dry” high pressure gases have a lower water vapour pressure than the surrounding ambient air at low pressure
– Some pipe walls are porous or will out-gas moisture content
– Seals and joints leak
Improving gas analysis
Consider Partial Pressures
Atmospheric
N2 = 78% = 0.78 BarA
O2 = 21% = 0.21 BarA
H2O = +6 oC DP = 10 mBarA
Compressed Gas
N2 = 78% = 7.8 BarA
O2 = 21% = 2.1BarA
H2O = -60 oC DP = 0.01 mBar
Improving gas analysis
Plant conditions
It is possible that brief “slugs” of moist gas will enter the system as valves are opened or when purging the static gas from the pipework system
This would lead to a rapid wetting of the system followed by a rapid drying once a dry gas flow is re-established
Improving gas analysis
Gas Rig
What would be the response of moisture analyser systems under these rapidly changing conditions?
A rig was designed to reproduce the "worst case" situation and challenge two types of moisture analyser
Improving gas analysis
Gas Rig
Improving gas analysis
Test Procedure
To simulate the sample system a 2m length of ¼ tube was installed between the moisture generator and the analysers on testTwo systems were tested– An aluminium oxide transmitter– A G6 Tunable diode laser system
Improving gas analysis
Test Procedure
The analysers were installed in series to avoid preferential flows. – The laser did not change response when
placed in front of the aluminium oxide sensor
– The aluminium oxide performed slightly better when placed in ahead of the laser
A flow rate of 2.5 l/min was used
Improving gas analysis
Test Procedure
To simulate a brief "slug" of moisture into a pipework system a 0.5 Sec. exposure to moist gas was used
A flow rate of 2.5 L/Min
After the initial dry-down the test was repeated every 10 minutes
Improving gas analysis
Dry Down
The aluminium oxide sensor and system showed good dry down response reaching 90% in 5 min
The laser analyser and system had a 90% step change in 38 Sec
0
50
100
150
200
250
300
350
400
450
14:30 15:00 15:30 16:00 16:30 17:00
G6
ALU
Improving gas analysis
ResultsReproducing plant failure
0
50
100
150
200
250
300
350
400
450
14:4
0:00
14:5
0:00
15:0
0:00
15:1
0:00
15:2
0:00
15:3
0:00
15:4
0:00
15:5
0:00
16:0
0:00
16:1
0:00
16:2
0:00
16:3
0:00
16:4
0:00
Time
PPM
v
G6 Laser Aluminium Oxide
Improving gas analysis
Conclusion
Things CAN happen quickly in dry gas systemsIf this was a real system the user would not be aware that there is an issue to be addressedThe speed of response of the laser system allows better visibility of process conditions
Improving gas analysis
Further Work
What is the effect of adding a particular component to the response of the pressure reduction sample system?– Regulators– Valves– Filters– Flow meters
Does temperature play a role?
Materials Performance in the Presence of Moisture
Alan TurnbullCorrosion and Electrochemistry Group
What does moisture do to materials
Corrosion and corrosion assisted fracture of metals
Accelerated fatigue cracking
Polymer/composite/glass degradation
Degradation of stone work, concrete
Cracking of wood
Condensing media for electrochemical catalytic activity in fuel cells
In-Plane TensionUnidirectional Laminates
Continuous UD glass fibre-reinforced laminate
Typical tensile failure
Stress RuptureStress RuptureEE--glass Fibresglass Fibres
E-glass fibres are sensitive to moisture and handlingUTS = 3.5 GPa (virgin) and 2.0 GPa (post-processing)
Effect of moisture on glass fibres: polymer composites &optical fibres
Leaching of alkali oxides (sodium and potassium oxide) from the fibre surface.
— Si —O —R + H2O → — Si —OH + R+ + OH-
Formation of surface micro-cracks ⇒ stress concentrators.
Permanent loss of strength (even after drying).
Accelerated metal fatigue
Moisture can significantly enhance the crack growth rate associated with cyclic loading of metals (fatigue)
For “water-free” fatigue testingdewpoint temperature can bevery low, e.g for some Al alloys itcan be below –50 °C.
(Courtesy of J Petit)
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(da/dN)cr
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nce
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Adso
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∆Keff
Hydro
gen
assista
nce
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e II
Adso
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ance
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∆Keff
Hydro
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assista
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Adso
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(da/dN)cr
Fatigue of silicon MEMS in humid environment
High stresses induce a thickening of the amorphous surface SiO2oxide layer at stress concentrations such as notches: stress/moisture-assisted cracking of this oxide layer results in fracture of the beam
Corrosion in condensing moisture
Corrosion occurs when liquid layer or droplets form on the surface to enable electrochemical reaction – wetting of surface enhanced by hygroscopic salts; corrosion rate influenced by material and by composition of atmosphere.
Critical relative humidity
Humidity at which no corrosion of metal in question takes placeAffected by capillary condensation and by nature of salts formed on surface
RH OF CONDENSATION ON A POLLUTED SURFACE AT 20ºC
Polluting salt
Na2SO4 (NH4)2SO4
NaCl CaCl2
Calcium chloride is a component of sea salt.
RH of condensation
93% 81% 78% 35%
So, a surface polluted with sea salt is wet at all RH > 35%
Topical issue - nuclear waste containment
Electronic components
D
BA
C
• 400 x 200µm pattern• AuNi finish
Contaminants
Contaminant Chemistry ConcentrationAnionic Surfactant Laurybenzolsulfonsaeure (LABS) 1%
Solvent flux Adipic, succinic, glutaric acid & rosin
1.7%
Exposure to moisture at temperature
More corroded
Water base Acrylic (2)
Epoxy Water base Acrylic (1)-Spray
Water base Acrylic (1)
Polyurethane Fluoroacrylate SiliconeSolvent base Acrylic
Moisture in gases – corrosion and fracture
Gas pipelineo Top-of-the-line corrosiono Bottom-of-the-line corrosion
Anhydrous ammonia cargo tank
Gas cylinders
Top of line corrosion in wet gas pipelines(courtesy of IFE)
Water chemistry in condensing water will be very different from the bulk water phase:
Condensing water will have low pH (from CO2) and high corrosivityCorrosion inhibitors will not be present in the condensing waterSalts from formation water not present in top of lineCorrosion products accumulate rapidly in the condensing water
TLC problems in the field(courtesy of IFE)
Corrosion when water condensation rate is above 0.15 to 0.25 g/m2s
Presence of organic acid (e.g. acetic) in the gas
Top of line corrosion reported in several gas pipelines in South East Asia
Excessive cooling – no insulation, flowing river water
Moisture in natural gas pipeline:bottom-of-the-line
At 5:26 a.m. on August 19, 2000, an explosion occurred on one ofthree adjacent large natural gas pipelines near Carlsbad, New Mexico. Southern California. Twelve people, including five children, died as a result of the explosion. The explosion left an 86 feet long crater.
Moisture in natural gas pipeline
Moisture in natural gas pipeline
Moisture in natural gas pipeline
Moisture in natural gas pipeline
Moisture in natural gas pipeline
$2.52m civil penalty: “…….This included failure to communicate to appropriate personnel when excessive water content was in the gas stream……”
Low gas velocity allowed water to separate from gas and collect in pipeline
Dip in pipeline at location → stagnant pool
Micro-organisms accumulated and with CO2, H2S, O2 and Cl- - very aggressive combination for corrosion
Configuration at location prevented use of pigs to “sweep” liquid away
No internal corrosion checks done at susceptible location
Fracture occurred due to localised wall thinning and exceedance of critical stress for fracture at that location
Moisture in anhydrous ammonia – a beneficial effect at the right level?
On August 22, 2003, anhydrous ammonia was being transferred from the storage tank to thecargo tank
While the cargo tank was still being loaded, its front head cracked open, releasing vapour
Moisture in anhydrous ammonia – a beneficial effect at the right level
Through-wall crack caused by stress corrosion cracking – not a new problem forstorage of anhydrous ammonia. Reason for failure was that production of theammonia had become so improved that water levels were below recommendedsafe level – 0.2% by weight. Procedures require addition of water for Q&T steelbut not followed
Internal corrosion of gas cylinders
Explosions of CO2 gas cylinders due to corrosion and stresscorrosion cracking is major problem (in UK, 10 explosions in 18 months to May 2002).
Solely due to moisture contamination – liquid backfeed and rainwater ingress main problem
Teaspoonful of water (5 g) is enough to destroy a cylinder
Internal corrosion of steel gas cylinders
CO2 + H2O = H+ + HCO3-
Acidic pH, high stress, susceptible steel→corrosion and cracking
Residual pressure valves can limit ingress of water but in absence of these, reliable method of detecting small amounts of water required.
Internal corrosion of gas cylinders
Other failures include HF, HBr….. – combination of corrosion and high pressure
Summary
Materials exposed to humid environments, whether general atmospheric or industrial gases, can degrade by a range of mechanisms depending on the material and its functionality
Failures can be catastrophic
Awareness of the potential for a problem supported by reliable measurement and monitoring with rigorously imposed protocols is essential
Complacency is a major concern
Humidity Calibration for Elevated Pressures and Non-air Gases
Stephanie Bell (NPL)
THERMAL MEASUREMENT AWARENESS NETWORK (TMAN)Humidity Measurement in Industrial Gases
NPL, 11 October 2006
The issues with gases
The issues with humidity calibration and sensors in different gas environments: some are “physical”– Thermal properties of a gas affected by pressure
(number density)– Heat capacity– Thermal conductivity, convective and other heat
transfer– If heat exchange with sensors is significant, differs
between calibration and use, how valid is the calibration?
– Does water vapour diffusion vary with carrier gas conditions?
The issues with gases
Plus a set of more “chemical” issues:– Some gases corrode or permanently degrade
sensors– (we don’t tend to want to calibrate in those gases!)– Other gases temporarily affect sensors
(interferences)– Some gases react with water vapour, making
humidity measurements “ambiguous”
• We can begin to judge whether these effects are significant for users if we– Have a capability to make tests/calibrations under
“conditions of use”– Contrast these with performance in “traditional”
calibration conditions– Try to infer something for extremes (what happens at
10 bar → what at 100 bar)– Compare unalike sensing methods against each
other
Our study• We looked at user needs
– partly via contacts – partly speaking direct to
users
• Who wants what?
Gas Sub-atmospheric
Near 1 bar >1 bar to 20 bar
20-40 bar
40-70 bar 70-200 bar
Air / Nitrogen
Environmental “altitude” testing
Calibrations available
Large interest Compressed air
Natural Gas
Some applications
Large number of applications
Some applications
CO2 Power industry
Argon Military applications – leak testing
Military applications – leak testing
SF6 Switchgear
H2 Hydrogen fuel cells
HCl Some process applications
User priorities
• Compressed air up to 10 bar- - dew points (at pressure) -75 °C to +20 °C
• Natural gas- initially near 1 bar – key threshold near 50 ppm water content (~dew/frost point near –45 °C )
• Inert or less reactive (e.g CO2)- - initially near 1 bar, later to 20 or 40 bar. Frost point range down to about –75 °C (~ 1 ppm).
• Devices to be calibrated- mainly capacitive, condensation,spectroscopic types
• But sometimes users want to reduce a process gas to atmosphere then interpret for process pressure– Relies on calculations– Humidity pressure conversion in air relatively easy– Conversion in natural gas less so (gas non ideality)
e.g. conversion of units in natural gas ASTM 1142, ISO 18453
• Project will give some attention to this
What other NMIs do• Germany – BAM – work up to 3 MPa (30 bar)
– ISHM 2006 poster (dependencies on pressure for some sensor types)
• Netherlands NMi/VSL – to look at pressure and/or non-air gas (natural gas) …at early stage
• Austria E+E Elektronik (appointed national humidity standard) calibration at pressures up to 10 bar
• “Two-pressure” generators in many calibration labs but don’t provide calibrations “at pressure”.
Others in UK – past – e.g.– Domnick Hunter humidity generator – to 17 bar– Michell/Air products work 1998 (nitrogen &
tetrafluoromethane)– Sira humidity work at pressure– Any others we should know about currently ?!
• “Standard”… “facility”… or “capability” at NPL?• Prior recommendation (NMS Thermal Working Party)
Objectives
• Construct a facility for humidity calibration in air at pressures initially up to 10 bar (perhaps higher later)
• Next, adapt this for non-air gases according to user priorities
• Range nominally 1% water vapour down to at least 50_ppm, possibly to 1 ppm (i.e. dew point +20 °C to at least –50 °C, perhaps –75 °C)
• Uncertainty less critical to most users – target 5 to 10 times that of established humidity standards at NPL
• For rapid and cost-effective development, facility initially based on modified commercial two-pressure generator
• Gas blending approach, in certain cases
• Resulting realisation will be either dew point or other units (e.g. mole fraction) as appropriate
• Also caters for nominally ambient-pressure instruments that need high input pressures
• Sub-atmospheric pressures possibly in future
• An outline design exists • gas supply, compressor, filter and dryer stages;
two-pressure generator; pressure and flow controls; and test instrument manifold
• Considerations of materials compatibility; gas purification and (e.g. for natural gas) composition; gas disposal/recovery and safety
Steps
• Consultation/design study - completed• Initial development - pressure capability – to Mar 07• Initial verification - pressure capability – to Mar 07• In 2007-10 programme
– Complete the verification of facility for calibrations of humidity sensors at elevated pressures
– Extend to enable calibrations in gases other than air
Generator
• Thunder 3900 as a starting point
• Modified with valve on outlet to control pressure
• Work on flow-pressure control configurations
Collaboration - instruments
New team member
• Wepawadee Pothinual - PhD student from Brunel University and NIMT (Thai national metrology institute)
Gas pressure issues
If thermal properties (heat exchange) an issue• Obviously consider gas flow• If pressure ↑
– heat capacity per volume ↑– (heat capacity per mole unchanged)– thermal conduction ↑
If water vapour diffusion an issue• If pressure ↑, vapour diffusion rate ↓
Experimental approach
Natural gas composition
Component Range (mole %)
Methane 87.0 - 96.0Ethane 1.8 - 5.1Propane 0.1 - 1.5iso – Butane 0.01 - 0.3normal – Butane 0.01 - 0.3iso – Pentane trace - 0.14normal – Pentane trace - 0.04Hexanes plus trace - 0.06Nitrogen 1.3 - 5.6Carbon Dioxide 0.1 - 1.0Oxygen 0.01 - 0.1Hydrogen trace - 0.02
(and in “sour gas” H2S,)
http://www.uniongas.com/aboutus/aboutng/composition.asp
Natural gas composition
Component Range (mole %)
Methane 87.0 - 96.0Ethane 1.8 - 5.1Propane 0.1 - 1.5iso – Butane 0.01 - 0.3normal – Butane 0.01 - 0.3iso – Pentane trace - 0.14normal – Pentane trace - 0.04Hexanes plus trace - 0.06Nitrogen 1.3 - 5.6Carbon Dioxide 0.1 - 1.0Oxygen 0.01 - 0.1Hydrogen trace - 0.02
(and in “sour gas” H2S CO2)
http://www.uniongas.com/aboutus/aboutng/composition.asp
Where matrix gas components can condense, saturation/condensation humidity processes need caution
More on gases data
Approximate molar heat capacities at constant pressure as Cp/R (near ambient T)
Ar 2.5 CH4 4.3Cl2 4.1 CO2 4.5H2 3.4 H2O 4.0He 2.5 NH3 4.3N2 3.5 HCl 3.5O2 3.5 C2H6 (ethane) 6.3
R = 8.314 51 J · K −1· mol −1
http://www.kayelaby.npl.co.uk/chemistry/3_10/3_10_1.html
Gas non-ideality
Recommended