Model 107 Temperature Probe
9
excitation at the Vx line and measures
the voltage drop across the 1 kΩ resistor
at the Vs line.
The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to
thermistor resistance (Rs), and the 1 kΩ ohm and 249 kΩ
fixed resistors as
described in the following equations:
Vs/Vx = 1000 / (Rs + 249000
Ω
+ 1000
Ω
)
Solving for Rs:
Rs + 250000 Ω = 1000 • (Vx/Vs)
Rs = 1000
•
(Vx/Vs) –
250000
The relationship of Rs to temperature is tabulated in Appendix
(p. C-1)
, but is calculated by
Therm107()
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
= 8.271111E–4
B
1
= 2.088020E–4
C
1
= 8.059200E–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. This noise can usually be filtered
out.
The following code snip examples filter 60 Hz noise.
CR6-series datalogger example:
Therm107
(
T107_C,1,U1,U10,0,
60
,1.0,0.0
)
CR800-series and CR3000 dataloggers example:
Therm107
(
T107_C,1,1,Vx1,0,
_60Hz
,1.0,0.0
)
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 the 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
Scientific suggests doubling the settling time every 50 ft. The following code
snip examples increase settling time by 20000 µs by placing
20000
as the
argument in the
SettlingTime
parameter:
