Campbell Scientific 43347 RTD Temperature Probe and 43502 Aspirated Radiation Shield - Manual (Page 23)

43347 RTD Temperature Probe, 43502 and 41003-5 Radiation Shields
Const RTD_Ro = 1000.00 'This is the actual RTD resistance for this sensor at 0.0°C
'Define Data Tables
DataTable(PRT_Data,1,1000)
DataInterval(0,10,Min,1)
Average (1,RTD_C,IEEE4,False)
Sample (1,43502_Tach,FP2)
Endtable
'Main Program
BeginProg
Scan(3,Sec,10,0)
'Measure the 43347-IX Probe
Resistance (RTD_Res,1,mV200,1,Ix1,1,170,True,True,0,_60Hz,1,0)
'Convert RTD resistance to temperature
RTD_RsRo = (RTD_Res / RTD_Ro)
PRT (RTD_C,1,RTD_RsRo,1.0267,0.0)
'Measure the 43502 tachometer output
PulseCount (Tach_Hz,1,11,0,1,1.0,0)
CallTable PRT_Data
Next Scan
EndProg
6.3 Resistance Measurement Instruction Details
The Resistance instruction applies a switched current excitation to the 43347
probe, and makes two differential voltage measurements. The first differential
voltage measurement is made across the RTD; the second is made across a
precision 1000 Ω resistor in the CR3000 current excitation circuitry. The
measurement result (X) = Vs/Ix = RTD resistance in ohms, where Vs is the
measured voltage and Ix is the excitation current.
The maximum excitation current is ±2.5 mA. The parameters for the excitation
current, measurement range, differential channel, and options to reverse the
excitation current and switch the differential inputs are configurable, as
discussed in the following sections.
6.3.1 Determining the Excitation Current
Current passing through the RTD causes heating within the RTD, which is
referred to as “self-heating”, resulting in a measurement error. To minimize
self-heating errors, use the minimum current that will still give the desired
resolution. The best resolution is obtained when the excitation is large enough
to cause the signal voltage to fill the measurement range.
The following example determines an excitation current that keeps self-heating
effects below 0.002°C in still air.
Self heating can be expressed as
ΔT = (Ix 2 RRTD) θ
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