Campbell Scientific CS547 Conductivity and Temperature Probe and A547 Interface - Manual (Page 19)

CS547 Conductivity and Temperature Probe and A547 Interface
15
9. Deriving a Temperature Compensation Coefficient
1. Place the CS547 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 5.4.
10. Instruction 11 Details
Understanding the details in this section are not necessary for general operation
of the CS547 probe with CSI's dataloggers.
Instruction 11 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. Instruction 11 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.
Instruction 11 then calculates temperature 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.