measuring mA on 4 wire instrument

is it possible to measure mA on a 4 wire analog o/p of a transmitter, in series without breaking the loop, as in a 2 wire loop with a diode in series?

Dont understand the diode thing.

But to measure a 4-20mA signal without interrupting the loop, connect the meter parallel over a place where the loop can be conveniently opened. When meter is in place, open the loop to force the signal through the meter, read the value from the meter, then close the loop again.
We sometime have knife terminals for just this purpose.

good u possible give me a schematic of what you are saying, because by connecting in parallel, the meter will draw the mA signal down, n u dont want that.

i just did a test. connect a 250ohm resistor in series, resulting in a 1-5vdc signal across the resistor which is directly proportional to the 4-20mA signal being used by the analog card, but this should all be done during the commissioning stages...

I use the Fluke 771 Milliamp Process Clamp Meter to do this.

http://www.fluke.com/Fluke/usen/Cla...mp-Process-Clamp-Meter.htm?PID=69415&trck=771

I am assuming that you want to do this for testing or calibration purposes

yeah that's a very useful test instrument to have. but for those that don't im sure the method that i mentioned in my previous reply, would work just as well.

Perhaps you would find this interesting.

http://iamechatronics.com/notes/general-engineering/267-troubleshooting-current-loops

Stu....

@sthompson.yeah i did thank you much appreciated.

sthompson said:

Perhaps you would find this interesting.

http://iamechatronics.com/notes/general-engineering/267-troubleshooting-current-loops

Stu....

Click to expand...

Nice article. I had never seen the diode thing before. I generally will just do a quick voltage measurement across the PLC input. They are generally 250ohm loads and I can get an snap measurement that is good enough for most troubleshooting.

Also, there is no reason the diode method shouldn't work for a 4 wire instrument too as long as it is placed in the signal loop.

JesperMP said:

Dont understand the diode thing.

But to measure a 4-20mA signal without interrupting the loop, connect the meter parallel over a place where the loop can be conveniently opened. When meter is in place, open the loop to force the signal through the meter, read the value from the meter, then close the loop again.
We sometime have knife terminals for just this purpose.

Click to expand...

i always do that or use the new mini clampmeter from fluke but break it last week

is like putting a serial resistor but the resistor is your metter...If nothing is in place to get the signal opened with a meter in place, you would need to strip wires and connect the meter before opening the loop (For exemple on terminal strip...you connect the metter on 1 side and strip the wire on the other side without disconnecting it, connect the other lead of the meter on the striped wire...the disconnect the striped wire from the terminal block so your meter is in serie with the loop and it never got broken....just put it in place after...
It can be tricky if you loose the connection with the meter you will open the loop but the only way to avoid that is to design the panel with reading points (Knife terminal)

I have the fluke 772 I think.Expensive but well worth the cost for quick troubleshooting.