How to test RTD

RTD Means RESISTANCE TEMPERATURE DETECTOR.   Working:- The resistance of certain metals changes with temperature change.
RTD1
RTD Materials:-
1. PLATINUM
2. NICKEL
3. COPPER are generally used.
Temperature Range: –200 to 650o c.
RTD with wheatstone bridge circuit
The changes in resistance caused by the changes in temperature are detected by  the wheatstone  bridge circuit. The temperature sensing element inside a well along with the bridge circuit forms the temperature  measuring  system. The sensing element (B) is made up of a material having high temperature coefficient of resistance.  Below circuit a,b,c are made up of material whose resistance is practically constant, under temperature change.
RDT2
When the bridge is balanced.
ac=b(P+B+Q)
Now if resistance of (B) changes the bridge gets unbalanced and the (G) Galvanometer shows deflection, which can be calibrated on a suitable temperature.
RTD CHARACTERISTICS:-
RDT3
The characteristics of RTD represents the positive temperature coefficient of resistance.  Two wire systems are used only when lead wire resistance can be kept at minimum it can be changed for more accuracy is required.
R1(a+x+b)=R2R3
i.e., RI=R2, R3=a+x+b.
TESTING RTD WITH DIGITAL MULTIMETER:-
Digital Multimeter

  • Select DMM Selector switch to Resistance mode (Auto Range) DMM Means Digital MultiMeter.
  • Connect DMM + ve probe to one end of RTD
  • Connect DMM – ve probe to another end of RTD
  • See the DMM Display reading in normal RESISTANCE range.
  • Switch on the soldering Iron (20W) at near the RTD.

VERIFICATION:

  1. The DMM reading shows when increases temperature by the heater, the Resistance of the RTD is decreases or decreases with polarity of the DMM.
  2. For accuracy measurement need a Bridge Circuit.

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