Campbell 109, Instruction Manual, Page: 17 / 30

Model 109 Temperature Probe
9
FIGURE 8-1 , 109 Thermistor Probe Schematic ,
Therm109()
applies 2500 mV
excitation at the Vx line and measures the voltage drop across the 24.9 k Ω
resistor at the Vs line.
The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to
thermistor resistance (Rs) and the 24.9 kΩ bridge resistor as described in the
following equations:
Vs/Vx = 24900 Ω / (Rs + 24900 Ω )
Solving for Rs:
Rs + 24900 Ω = 24900 Ω • (Vx/Vs)
Rs = 24900 Ω • ((Vx/Vs) – 1)
The relationship of Rs to temperature is tabulated in Appendix C , Thermistor
Resistance and Temperature
(p. C-1)
, but is calculated by Therm109() using the
Steinhart-Hart equation, described as follows:
T
c
= (1 / (A + B • ln (R
s
) + C • (ln (R
s
)) 3 )) – 273.15
where:
T
c
= temperature in degrees Celsius (°C)
A
1
= 1.129241E–3
B 1 = 2.341077E–4
C 1 = 8.775468E–8
1
Coefficients provided by Measurement Specialties™.
8.3 Electrically Noisy Environments
EMF noise emanating from the ac mains power grid can be a significant source
of measurement error. 60 Hz noise is common in the United States. 50 Hz
noise is common in Europe and other regions. This noise can usually be filtered
out.
The following code snip examples filter 60 Hz noise.
CR6-series datalogger example:
Therm109 ( T109_C,1,U1,U10,20000, 60 ,1.0,0.0 )
CR800-series and CR3000 dataloggers example:
Therm109 ( T109_C,1,1,1,20000, _60Hz ,1.0,0.0 )
An integration parameter is not available for CR200(X)-series dataloggers.
8.4 Long Cable Lengths
Long cable lengths (>50 ft) may require longer than normal analog
measurement settling times. Settling times are increased by adding a
measurement delay to a datalogger program.
The 60 Hz and 50 Hz integration options include a 3 ms settling time; longer
settling times can be entered into the Settling Time parameter. Campbell