Instruction manual

Model 109SS Temperature Probe
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, Conversion
of Thermistor Resistance or Voltage Ratio to Temperature, 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
TM
.
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. Depending on the datalogger
model, this noise can usually be filtered out.
The following code snips filter 60 Hz noise by placing the _60Hz argument in
the Integ/Fnotch parameter (in bold type).
For CR6 datalogger:
Therm109(T109_C,1,U1,U10,20000,_60Hz,1.0,0.0)
For CR800, CR1000, CR3000, and CR5000 dataloggers:
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 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. The following
code snips increase settling time by 20000 µs by placing 20000 as the
argument in the SettlingTime parameter:
9