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
Thermistor resistance and temperature
14), 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 data logger example:
Therm109
(T109_C,1,U1,U10,20000,
60
,1.0,0.0)
CR800-series and CR3000 data loggers example:
Therm109
(T109_C,1,1,1,20000,
_60Hz
,1.0,0.0)
8.4 Long cable lengths
Long cable lengths (>50 ft) may require longer than normal analog measurement settling times.
Enter a longer settling time in the
SettlingTime
parameter of the
Therm109()
instruction.
109SS Temperature Probe
9
