Correct, cheap, and mature platinum resistance temperature detectors (PRTDs) with an working vary extending from the cryogenic to the incendiary are a gold (no! platinum!) customary for temperature measurement.
Equally, the 4 to twenty mA analog present loop is a legacy, however nonetheless fashionable, noise- and wiring-resistance-tolerant interconnection methodology with good built-in fault detection and transmitter “phantom-power” options.
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Determine 1 combines them in a easy, low cost, and cheerful temperature sensor utilizing simply eight off-the-shelf (OTS) components, counting the PRTD. Right here’s the way it works.
Determine 1 PRTD present loop sensor with Ix = 500 µA fixed present excitation.
Ix = 2.5v/R2, PRTD resistance = R1(Io/Ix – 1)
R1 and R2 are 0.1% tolerance (ideally)
The important thing to measurement accuracy is the two.50-V LM4040x25 shunt reference, out there with accuracy grade suffixes of 0.1% (x = A), 0.2% (B), 0.5% (C), and 1% (D). The “B” grade is constant (simply barely) with a temperature measurement accuracy of ±0.5oC.
R1 and R2 ought to have related precision. R2 throttles the two.5 V to offer Ix = 2.5/R2 = 500 µA excitation to T1. As a result of A1 constantly servos the Io output present to carry pin3 = pin4 = LM4040 anode, the two.5 V throughout R2 is held fixed, subsequently Ix is likewise.
Thus, the voltage throughout output sense resistor R1 is pressured to Vr1 = Ix(Rprtd) and Io = Ix(Rprtd/R1 + 1). This makes Io/Ix = Rprtd/R1 + 1 and Rprtd/R1 = Io/Ix – 1 for Rprtd = R1(Io/Ix – 1).
Wrapping all of it up with a bow: Rprtd = R1(Io/(2.5/R2) – 1). Be aware that lodging of various Rprtd resistance (and subsequently temperature) ranges is an easy matter of selecting totally different R1 and/or R2 values.
Conversion of the Io studying to Rprtd is a straightforward chore in software program, and the step from there to temperature isn’t a lot worse, due to Callendar Van Dusen math.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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