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					Aquatrac Instruments
                                                 pH Temperature Compensation
Application Notes


1. OBJECTIVE

Uses thermal sensor inputs ‘A’ or ‘B’ to temperature compensate a pH sensor


2. OVERVIEW

As temperature decreases, pH increases.
Controller inputs ‘A’ and ‘B’ measure temperature.
Usually the temperature is used to compensate a conductivity sensor. However it may
also be used to temperature compensate a pH sensor.

This Product Advisory describes how to compensate a pH sensor connected to
controller input ‘C’ using a temperature connected to controller input ‘A’ where
temperature is measured in degrees F.

The METHOD demonstrates the derivation of the compensation value and may be used
on any pH sensor connected to any input.

3. METHOD

Thermal compensation is entered as a percent per degree F after the sensor Gain &
Offset are applied to the sensor raw millivolt measurement, making compensation
simple to calculate since we can use the actual pH and not the sensor millivolts.

Temperature compensation %/F differs with sensor type so let’s use an example:
EXAMPLE 1: At 80F, pH = 7.6 At 45F, pH = 9.0
             Over 35F, pH varies -1.6 pH
             As an equation: 7.6pH x Z = 9.0 pH,
             where Z = a constant raised to the power of the temperature difference.
              Solving for Z = ( 9.0 / 7.6) ^ -1/35 (The negative root of the pH ratio)
                          Z = 0.9952
             %Thermal Compensation / F = (Z-1) x 100 = - 0.48


Verifying Temperature Effect on pH
1. Log the pH value over the typical system operating temperature range without
   compensating the pH and without pH control ON.
2. Plot the controller data and note both the pH variation and the temperature variation.
3. Note that changes in conductivity, which may effect alkalinity and ORP may also
   effect pH.
4. Use the difference in pH over the observed temperature to calculate the %Thermal
   Compensation / F co-efficient.




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Aquatrac Instruments
                                                     pH Temperature Compensation
Application Notes


Typical Compensation Coefficient
The previous example defines a very high %Thermal Compensation / F value.
A typical, theoretical compensation value would be:

EXAMPLE 2: At 77F, pH = 7 At 122F, pH = 6.61
           Over 45F, pH varies 0.39 pH
           As an equation: 7pH x Z = 6.61pH,
           where Z = a constant raised to the power of the temperature difference.
           Solving for Z = ( 7.0/ 6.61) ^ -1/45 (The negative root of the pH ratio)
                       Z = 0.99873
           %Thermal Compensation / F = (Z-1) x 100 = - 0.127


WARNING: Compensation Coefficients greater then -0.5 should be used with
          caution since a 10F changes will cause a pH change of 5%.


Using HyperTerminal to Set Compensation
Use the PA command to set Thermal sensor ‘A’ to compensate the pH sensor at ‘C’
using the Compensation Factor calculated in-
 EXAMPLE1:
 Key PAC,,,,,TA-.48 & Enter      Note that there are 5 commas between C & T
Note that the compensation value has been set to 0.9952 which will reduce the pH by
0.48% per degree F

EXAMPLE2: Typical Compensation
 Key PAC,,,,,TA-.13 & Enter      Note that there are 5 commas between C & T
Note that the compensation value has been set to 0.9987 which will reduce the pH by
0.13% per degree F


4. VERIFICATION

You can verify thermal compensation by changing the temperature.

Note the current pH & temporarily change the temperature by single point calibrating the
thermal sensor at ‘A’ to a 10 degree lower temperature:

If you are using the typical thermal compensation of EXAMPLE2 you will see a pH of
8.000 fall to a pH of 7.897 after lowering the temperature.

Correction = pH x Compensation Factor ^ 10 = 8.000 x 0.9987 ^ 10
                    ( where ^ = raise to the power of )

If you are observing on the LCD you will see rounding to the selected display resolution.



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