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4.3 RTD Output (PT100)
The RTD (Resistance Temperature Detector) is a 3-wire 100 Ohm Platinum Resistance
thermometer (PT100), built into the meter cap of the flow meter. The RTD is directly
mounted to the PCB of the flowmeter, which is then encapsulated into the meter cap using
a thermally conductive epoxy potting compound, ensuring quick thermal transfer from the
process liquid to the temperature sensor.
The RTD has standardised characteristics according to IEC 60751, with a temperature
coefficient of +3850 ppm/K (between 0
o
C and 100
o
C), and is a tolerance class B (F0.3)
device.
The RTD may be used in either a 2-wire or a 3-wire system; each have their own benefits,
however for best accuracy it is recommended that a 3-wire system is used. When using the
RTD in a 3-wire system the third wire is used to calculate the average resistance of the
cable between the readout instrument and the RTD, this allows the readout instrument to
subtract the resistance of the cable from the resistance of the RTD itself – giving a much
more accurate measurement. The downsides of a 3-wire system is a small amount of extra
cost.
For installations which cannot accept the extra cost of the additional wire in the cabling
plus the more expensive receiving instrument, a 2-wire system can provide a reasonably
accurate temperature measurement when cable lengths are less than 10 metres.
To avoid self-heating of the temperature sensor, the measurement current should be kept
between 0.1mA ~ 1mA; this will limit the self-heating effect to less than 0.1
o
C and will
maintain the accuracy of the device. Self-heating errors are proportional to the DC power at
the RTD and are approximately equal to 0.8
o
C / mW.
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