247 CONDUCTIVITY AND TEMPERATURE PROBES
9
9. DERIVING A TEMPERATURE
COMPENSATION COEFFICIENT
1. Place the 247 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.
(C - C
25
)
TC = 100 *
_____________
= % / °C
(t - 25) * C
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 247
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.
TABLE 10-1. Temperature , Resistance, and
Datalogger Output
0.00 351017 -0.06
2.00 315288 1.96
4.00 283558 3.99
6.00 255337 6.02
8.00 230210 8.04
10.00 207807 10.06
12.00 187803 12.07
14.00 169924 14.06
16.00 153923 16.05
18.00 139588 18.02
20.00 126729 19.99
22.00 115179 21.97
24.00 104796 23.95
26.00 95449 25.94
28.00 87026 27.93
30.00 79428 29.95
32.00 72567 31.97
34.00 66365 33.99
36.00 60752 36.02
38.00 55668 38.05
40.00 51058 40.07
42.00 46873 42.07
44.00 43071 44.05
46.00 39613 46.00
48.00 36465 47.91
50.00 33598 49.77
52.00 30983 51.59
54.00 28595 53.35
56.00 26413 55.05
58.00 24419 56.70
60.00 22593 58.28
TABLE 10-2. Polynomial Coefficients
COEFFICIENT VALUE
C0 -53.4601
C1 9.08067
C2 -8.32569 x 10
-01
C3 5.22829 x 10
-02
C4 -1.67234 x 10
-03
C5 2.21098 x 10
-05
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