Control Loop Current Measurement

Install spade terminals as standard. Put a meter across both screws in the terminal, and open the spade.

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"i connected my fluke across the circuit on the i/o. i had my varying mA analogue signal alright but there was invalid measure on the DCS Console. can i have some explanation on that?"

Most DCS/PLC 4-20ma analog inputs will have 250 Ohms between the + and - terminals. When you put your meter (in ma reading mode) across the circuit, you were putting another low resistance in parallel with that resistor.

"what best way can i measure current in a live loop."

Instead of trying to read the current directly, try reading the voltage across those same terminals. To start find an unused input and read the resistance from + to - terminals. Ten dollars says it is 250 Ohms. Use the resistance you read in the equation E=I*R (rearrange to I=E/R) to determine the ma from the voltage.

How ironic; I gave a class on this today to some instrument techs.

To read a 4-20mA signal
1) Use a Fluke 771 meter which is a clamp-on. Easiest non-intrusive method to read a current, but you have to possess one to be able to use it. It's not cheap and there's no Chinese knock-off.

2) Put a diode in series in the loop, forward biased so that it conducts under normal operation. When you put an ampmeter/ammeter/multi-meter-in-current-measuring-mode across the diode, the low resistance of the meter's input is effectively a short circuit compared with the 0.7V or so voltage drop across the diode. The meter's voltage drop be in the mV range, so the loop current takes the path of least resistance and flows through the meter.

You can buy terminal blocks with a diode installed already.

Industrial pressure transmitters do exactly this. They have a diode behind a pair of "test terminals" to allow the current loop signal to be read without breaking the primary loop signal. Just put a milliammeter across the test terminals and voila, get a loop current reading. Doing so does not divert any current from the rest of loop, everything else works as it should.

3) Put a precision resistor in series in the loop and measure the voltage drop with a voltmeter. There are standard precision resistor values. 10 ohms generates a 40mV - 200mV drop; 250 ohms generates a 1.0-5.0Vdc drop. The lower the resistance, the lower the burden on the loop power supply. Too high a resistance can limit the current in the loop if the power supply doesn't source sufficient voltage.

The problem for most people is associating the voltage drop to an equivalent milliamp current level, unless you do it every day.

4) Open the loop (remove a wire) and insert a milliampmeter and all that breaking the circuit involves.

2nd part: loop current monitoring

1) Crabtree (GA) mentioned this early on in this thread, but I want to expand on it.

Use a terminal block with a fuse holder. Connect a milliammeter 'across the fuse' (across the terminals), then remove the fuse from the circuit so that the loop current runs through the ammeter.

2) Check to see if the device indicates the loop current

a) Most industrial transmitters with a local indicator display will usually display the loop current in a local indicating display

b) A loop controller's milliamp current output is proportional over the output range of 0-100%. The bottom display shows the output in percent.

Same practical problem here as with voltage drop across a resistor - understanding the relationship of 0-100% percent to 4-20mA. 15% of 4-20mA = 6.40mA

c) Smart valve positioners can display the SP (the input signal). The screen shot below shows actual position in the top display and the input signal as 2 digits (in percent) in the bottom display (AUT is for automatic mode) 36% of 4-20mA = 9.76mA

3) Since HART inherently uses a DC current loop for its carrier, use HART, if available.

That's if
- the field instrument is HART
- and HART is enabled
- and the loop has 250 ohms of resistance

then use a HART modem or a HART communicator to read the loop current.

The displays below show the On-line mode for a 375 or 475 HART handheld communicator showing the loop current.

All the 1.000 values in the 475 screen shot are clearly bogus, probably because Rosemount had too many people calling support asking why their current value was different from the one in the manual.

4) Put a loop powered digital display in series in the loop and configure it to display loop current:

The meter above is not displaying current, but any loop powered meter can be configured to display 4.00 - 20.00 over the range of the input.

Specifically for 1794-IExx Flex I-O, the input resistance for 4-20mA is 238 ohms:

If you put a DC voltmeter across any pair of red and blue terminals, you'll read the voltage drop across the 238 ohm resistance, where 4mA = 0.952V, 12.0mA = 2.856V, 20mA = 4.76V