Campbell Scientific CS547A-L Conductivity/Temperature Probe and A547A Interface - Manual (Page 24)

CS547A Conductivity and Temperature Probe and A547 Interface
9. Deriving a Temperature Compensation Coefficient
1. Place the CS547A in a sample of the solution to be measured. Bring the
sample and the probe to 25°C.
2. Enter the example program from Section 5.2 in the datalogger and record
C
t
at 25°C from Location 3. This number will be C
25
in the formula in
Step 4.
3. Bring the solution and the probe to a temperature (t) near the temperature
at which field measurements will be made. This temperature will be t (in
°C) in the formula. Record C
t
at the new temperature from Location 3.
This number will be C in the formula in Step 4.
4. Calculate the temperature coefficient (TC) using the following formula.
TC
C C
t C
C = ∗
−
− ∗
° 100
25
25
25
( )
( )
%/ =
Enter TC in the appropriate location (nnn) as shown in the program segment in
Section 5.2 .
10. Therm107 / P11 Instruction Details
Understanding the details in this section is not necessary for general operation
of the CS547A probe with CSI's dataloggers.
The Therm107 instruction (or P11 in Edlog) outputs a precise 2 VAC
excitation (4 V with the 21X) and measures the voltage drop due to the sensor
resistance. The thermistor resistance changes with temperature. The
instruction calculates the ratio of voltage measured to excitation voltage
(Vs/Vx) which is related to resistance, as shown below:
Vs/Vx = 1000/(Rs+249000+1000)
where Rs is the resistance of the thermistor.
See the measurement section of the datalogger manual for more information on
bridge measurements.
Temperature is calculated using a fifth order polynomial equation correlating
Vs/Vx with temperature. The polynomial coefficients are given in Table 10-2.
The polynomial input is (Vs/Vx)*800. Resistance and datalogger output at
several temperatures are shown in Table 10-1.
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