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To make my question simple and easy to understand I am giving an example below. I have an RTD (platinum 100) which change resistance with increase in temperature, for checking that rtd is showing actual temperature I can check the ohms by connecting a multimeter and change its setting to ohms and it will show us the resistance of rtd. now if we connect the rtd to a temperature transmitter or controller of which input is rtd's resistance and output is mA and we try to check the resistance of rtd while connected to transmitter/controller the output of transmitter rises to maximum which is 20mA. I want to know why is this.

I know we should not do like this but I just want to know for learning purpose.

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An ohmmeter applies a current to the component under test, and measures the resulting voltage (or applies a voltage and measures the current). Other components in parallel with the the component being measured, or voltage or current appied by another circuit, will make the ohmmeter measurement invalid.

The RTD measurement circuit will also apply a voltage or current to the RTD, so the two measurements will interfere with each other, and any readings you obtain from either the ohmmeter or the RTD measurement circuit will be invalid.

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RTD to current transmitters are calibrated to avoid resistance errors from cables. They are current sources which when converted to voltage at Rx, may use any length cable. (within range)

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