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View Poll Results: What is your 'go-to' if you have the option
0-10Vdc 3 6.67%
0-20mA 0 0%
4-20mA 41 91.11%
Does not matter 1 2.22%
Voters: 45. You may not vote on this poll

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Old March 15th, 2018, 10:32 PM   #16
ASF
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Originally Posted by keithkyll View Post
You can measure Ohms at the field end if you can disconnect the wires. If not, then measure voltage, and assume a 250 Ohm load. If the input impedance is different, readings will be half or double what's expected.
You can only do that if you have a wire connected to the analog input common/negative in the field. If it's a two-wire loop-powered transmitter, all you have is a 24VDC from your DC P/S and the analog input positive, so you can't measure the reistance of your analog input. You can't measure voltage either - you'll get a small voltage drop across the transmitter (that is used to loop power it), but it will not correspond to the 4-20mA reading in any way.

Again, this sort of discussion is exactly the sort of discussion students should be having! Have them test a 0-10V input and see how easy it is, then have them go through the process of working out how much harder it is to do it with a 4-20mA signal in the field
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Old March 16th, 2018, 10:43 AM   #17
Tom Jenkins
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Originally Posted by ASF View Post
Again, this sort of discussion is exactly the sort of discussion students should be having! Have them test a 0-10V input and see how easy it is, then have them go through the process of working out how much harder it is to do it with a 4-20mA signal in the field
This is why I always use disconnect terminal blocks for field connections on 4-20 mA loops. Set your probes, disconnect, read, reconnect, all done.
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