Measuring 4-20mA; TC-08 & Single-Channel Terminal Board

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Measuring 4-20mA; TC-08 & Single-Channel Terminal Board

Post by MartinOBeirne »

I am trying to measure various temperatures(using k-type thermocouples) and a flow rate through the single channel terminal board. The flow meter is a Georg Fischer 2551 Magmeter with a 4-20mA output. My problem is, the voltage from the flow meter is 24VDC and this appears to be disrupting the thermocouple readings when i plug the terminal board into the TC-08.

I am new to data logging and especially 4-20mA loop circuits so any help would be greatly appreciated.

Many thanks,

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Joined: Mon Aug 11, 2014 11:14 am

Re: Measuring 4-20mA; TC-08 & Single-Channel Terminal Board

Post by Gerry »

Hi Martin,

The question is what are you doing with the reference voltage (Gnd) from the excitation source (the supply for the flow meter)?

If any voltage that you're applying to the TC-08 terminal board exceeds +/-7V then the channel is pulled out of common range of the TC-08, which means all of the channels will start reading incorrectly. Also, if there is a difference in ground potentials between the source and any input or between any 2 inputs of more than +/-7V then the same problem occurs. You need to apply the 24V to the flow meter and then have the output from the flow meter and the ground from the excitation voltage applied to the terminal board. You also need to ensure that the Magmeter has a good earth ground (as mentioned on page 3 of the manual here: ... ument.html) .

This should solve the problem but if you are still seeing the issue then you need to establish what is happening between the output of the Excitation source and the power to it (i.e. if it is a power supply with floating outputs then you should use a PSU where the Gnd output has a direct earth ground connection).


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