Campbell Scientific 108-L Temperature Probe - Manual (Page 17)

Model 108 Temperature Probe
Vs/Vx = 1000 / (Rs + 40000 Ω + 1000 Ω)
Solving for Rs:
Rs + 41000 Ω = 1000 • (Vx/Vs)
Rs = 1000 • (Vx/Vs) – 41000 Ω
TABLE 8-1, 108 Measurement Details, and TABLE 8-2, 108 Temperature
Calculation, describe how measurement results Vs/Vx and Rs are converted to
temperature by Campbell Scientific dataloggers.
TABLE 8-1. 108 Measurement Details
Datalogger
Model
Measurement
Instruction
Excite
mV
mV
Input
Range Result Scaling
Equation
Applied to
Scaled Result
CR800
CR1000
CR3000
CR5000
CRBasic
Therm108()
Steinhart-Hart
(automatically
applied)
CR500
CR510
CR10
CR10X
Edlog
AC Half
Bridge (P5)
1000 25 Vs/Vx Multiply
by 200‡
5th order
polynomial
(use
Polynomial
(P55))
21X
CR7(X)
CR23X
Edlog
AC Half
Bridge (P5)
2000 50 Vs/Vx Multiply
by 200‡
5th order
polynomial
(use
Polynomial
(P55))
†Fixed series resistance is subtracted before applying Steinhart-Hart.
‡ Multiplier of 200 scales Vs/Vx for the polynomial fit.
TABLE 8-2. 108 Temperature Calculation
CRBasic Dataloggers 1
Therm108() 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 = 8.271111E–4
B = 2.088020E–4
C = 8.059200E–8
Edlog Dataloggers 3
AC Half Bridge (P5) instruction measures the ratio Vs/Vx. Polynomial (P55)
instruction converts the measurement result Vs/Vx * 200 to temperature using a 5 th
order polynomial:
T = C0 + C1•X + C2•X^2 + C3•X^3 + C4•X^4 + C5•X^5
where:
T = temperature in Celsius
X = (Vs/Vx) • 200
C0 = –26.97
C1 = 69.635
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