Campbell Scientific 109SS Temperature Probe - Manual (Page 16)

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 fixed resistor as described in the
following equation:
Vs/Vx = 24900 / (Rs + 24900)
Solving for Rs:
Rs + 24900 = 24900 • (Vx/Vs)
Rs = 24900 • ((Vx/Vs) - 1)
TABLE 8-1, 109SS Measurement Details, and TABLE 8-2 , 109SS Temperature
Calculation, describe how measurement results Vs/Vx and Rs are converted to
temperature by Campbell Scientific dataloggers.
TABLE 8-1. 109SS-Measurement Details
Datalogger
Model
Measurement
Instruction
Excite
mV
Voltage
Input
Range
Result Scaling
Equation
Applied to
Scaled Result
CR200(X)
Series
CR800
CR1000
CR3000
CR5000
CRBasic
Therm109()
Steinhart-Hart
(automatically
applied)
CR500
CR510
CR10
CR10X
Edlog
AC Half
Bridge (P5)
2500 2500 mV Vs/Vx ln(Rs)
by 1E–3
Steinhart-Hart
(use
Polynomial
(P55))
21X
CR7(X)
CR23X
Edlog
AC Half
Bridge (P5)
5000 5000 mV Vs/Vx ln(Rs)
by 1E–3
Steinhart-Hart
(use
Polynomial
(P55))
TABLE 8-2. 109SS Temperature Calculation
CRBasic Dataloggers 1
Therm109() instruction measures the ratio Vs/Vx, calculates the thermistor resistance
Rs, and converts Rs to temperature using the Steinhart-Hart equation 2 :
T = 1 / (A + (B • ln(Rs))) + (C • ((ln(Rs))) ^ 3) – 273.15
where:
T = temperature in degrees Celsius
A = 1.129241E–3
B = 2.341077E–4
C = 8.775468E–8
Edlog Dataloggers 3
AC Half Bridge (P5) instruction measures the ratio Vs/Vx. Mathematical instructions
calculate and pre-scale ln(Rs) by 1E–3. This creates adequate resolution for
Polynomial (P55) instruction to apply the Steinhart-Hart equation with a fifth-order
polynomial. The inverse of the result, Tk, is found with the Z=1/X (P42) instruction:
1/Tk = C0 + C1•X + C2•X^2 + C3•X^3 + C4•X^4 + C5•X^5
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