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ON-LINE ELECTRODE IMPEDANCEMEASUREMENT
Appl ication Data SheetADS 43-017/rev.B
August 2004Theory
MEASUREMENT
The Model 3081 and 4081 pH/ORP Smart Two-WireTransmitters and
Model 54e pH/ORP Analyzer includea diagnostic impedance
measurement. This measure-ment is made between the glass pH
electrode and ref-erence electrode, and a solution ground or, in
thecase of the pH sensor without a solution ground,between the
glass pH electrode and the referenceelectrode. In the latter case,
the impedance of the ref-erence electrode is small enough (a
fraction of amegohm) to be negligible in comparison to the glasspH
electrode, which can exhibit an impedance in the
gigaohm range (109 ohms). The Model 3081 can mea-sure glass pH
electrode impedances from 0 to > 2gigaohm with a specified
accuracy of 10% of readingor 2 megohm, whichever is larger. When
using on-line measurements, the described effects in the chartsthat
follow.
SOLUTION CONDUCTIVITY EFFECTS
The conductivity of the process solution can have aneffect on
the measured electrode impedance basedupon the solution
conductivity and the geometry of theparticular pH sensor being
used. The reason behindthis effect is apparent when one considers
that theimpedance measuring circuit consists of not only
theelectrode(s) and/or solution ground, but also the resis-tance
due to the process solution surrounding them.
The contribution of the process solution resistance tothe
overall measured impedance is inversely propor-tional to the
solution conductivity. The electrode(s)and/or solution ground used
for the impedance mea-surement form the electrodes of a
conductivity cell,whose cell constant will depend upon the
particularsurface areas, distances of separation, and orientationof
the electrodes.
For example, studies were done using the Model 381+sensor in
samples ranging from 1.0 to 10,000 S/cm.
The measured impedance was plotted against theinverse of the
conductivity to yield a straight line with aslope equivalent to an
apparent cell constant of3,018 cm-1. For all but the low
resistivity electrodes,with impedance less than 100 megohms, the
contribu-tion of solution conductivity was negligible above
200 S/cm. See Charts 1 & 2.The distance between the glass pH
electrode and thesolution pH electrode and the solution ground in
aModel 385+ is about 3/4 inch (1.9 cm). Decreasingthis distance to
1/8 inch, by adding an extension to thesolution ground, decreases
the apparent cell constantand the conductivity effect by 90%.
In practice the effect of solution conductivity on a par-ticular
sensor can be checked by simply immersing itin a series of
solutions with conductivity values span-ning the expected
conductivity range on-line. If theglass pH electrode and solution
ground or referenceelectrode are within an inch of one another and
the
conductivity is 500 S/cm or greater, any effect ofconductivity
should not be noticed.
Chart 1. Glass Impedance vs. Log Conductivity
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ON-LINE VERSUS LABORATORY IMPEDANCEMEASUREMENTS
When glass electrode impedance is measured byimposing a voltage,
the glass electrode polarizes inproportion to the applied voltage.
This polarizationsets up an opposing voltage, which increases
theapparent electrode resistance, and can represent asubstantial
fraction of the measured impedance. Thefull polarization effect can
take up to one minute todevelop.
The typical laboratory measurement of impedanceoften involves an
applied voltage of 50 volts or more,which is applied until a stable
impedance reading isreached. This implies that the pH electrode is
fullypolarized when the impedance reading is taken. Incontrast, the
Model 3081 pH/ORP and the Model 54epH/ORP applies a lower voltage
for a much shorterduration.As a resul t, the Model 3081
pH/ORPandthe Model 54e pH/ORP impedance reading, while
repeatable, will indicate a lower value than the typ-ical
laboratory measurement.
Chart 2. Glass Impedance vs. Conductiv ity
TEMPERATURE DEPENDENCE OF GLASS pHELECTRODE IMPEDANCE
While the reference electrode impedance is indepen-dent of
temperature, the glass pH electrode impedanceis strongly
temperature dependent, decreasing byroughly one-half for every 10C
increase in tempera-ture. See Chart 3. While the temperature
dependence
of a glass electrode generally follows the ArrheniusEquation,
which is an exponential equation, there aresubtle differences in
glass electrodes, which can intro-duce errors when a general
equation is applied to tem-perature compensation. A software
routine in the menuof the analyzers under the mnemonic IMPCF allows
acorrection to be made to the temperature compensa-tion, once the
pH sensor has reached the process oper-ating temperature. This will
result in a temperaturecompensated impedance accurate to 10%.
2 Rosemount Analytical
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Chart 3. GPLR Glass Impedance vs. Temperature
Model 54e pH/ORP MicroprocessorAnalyzer
NEMA 4X (IP65) weather-proof, corrosion-resistant enclosure.
Automatic temperaturecompensation.
Dual isolated current outputs.
Three process alarms with programmable logic, a timer
function,and a fourth relay for fault conditions.
Automatic buffer recognition with stored buffertemperature
curves.
Advanced on-line sensor diagnostics.
INSTRUMENTATION
Model 5081 pH/ORP SmartTwo-Wire MicroprocessorTransmitters
Hand-held infrared remotecontrol link activates all
thetransmitters functions.
Large custom LCD display.
NEMA 4X (IP65) weatherproof, corrosion-resistantenclosure.
Comprehensive pH glass and reference diagnos-tics.
Non-volatile EEPROM memory to hold data inevent of power
failure.
HART or FOUNDATION Fieldbus options.Model 1055 Conductivity
Analyzer
Multi-parameter instrument.
Measures your choice of conductivity, percent(%) concentration,
resistivity, pH, dissolved oxy-gen, chlorine, ozone, temperature,
and flow.
Up to two (2) sensor inputs and two (2)
4-20 mA outputs, plus alarms. Easy to use.
Multi-language.
Emerson Process Management
Liquid Division2400 Barranca Parkway
Irvine, CA 92606 USA
Tel: (949) 757-8500
Fax: (949) 474-7250
http://www.raihome.com
Rosemount Analytical Inc. 2004
