Campbell Scientific 43347 RTD Temperature Probe and 43502 Aspirated Radiation Shield - Manual (Page 24)

43347 RTD Temperature Probe, 43502 and 41003-5 Radiation Shields
Where: ΔT = self heating in °C
Ix = current excitation
RRTD = 1000 Ω RTD resistance
θ = 0.05°C/mW self heating coefficient
Solving the above equation for Ix:
Ix = (ΔT / RRTD θ)^1/2
To keep self-heating errors below 0.002 °C, the maximum current Ix is:
Ix = (.002 °C / (1000 Ω *.05 °C / .001W)) ^1/2
Ix = 200uA
The best resolution is obtained when the excitation is large enough to cause the
signal voltage to fill the measurement full scale range (the possible ranges are
±5000, 1000, 200, 50 and 20 mV).
The maximum voltage would be at the high temperature or highest resistance
of the RTD. At +40°C, a 1000 Ω RTD with α = 3.75 Ω/°C is about 1150 ohms.
Using Ohm’s law to determine the voltage across the RTD at 40°C.
V = Ix R
Using an Ix value of 200uA, the voltage is:
V = 200uA * 1150 ohms
V= 230mV
This is just over the ±200 mV input voltage range of the CR3000.
For a maximum voltage of 200 mV, the current Ix is:
Ix = 200mV/1150 ohms
Ix ~170uA
6.3.2 Reducing Measurement Noise
AC power lines, pumps, and motors can be the source of electrical noise. If the
43347 probe or datalogger is located in an electrically noisy environment, the
measurement should be made with the 60 or 50 Hz rejection options.
Offsets in the measurement circuitry may be reduced by reversing the current
excitation (RevEx), and reversing the differential analog inputs (RevDiff), as
shown in the program examples in Section 6.2, Datalogger Programming.
7. Maintenance
Inspect and clean the shield and probe periodically to maintain optimum
performance. When the shield becomes coated with a film of dirt, wash it with
mild soap and warm water. Use alcohol to remove oil film. Do not use any
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