Shunts to convert current readings to voltage?
 

I have had a few people advise me that I can use a resistor as a shunt and measure voltage instead of current for 4-20 mA output devices. This is extremely desirable for me as I have 2 4-20 mA output instruments (dP xmitter and mag flowmeter) that I'd like to set up. Can you guys point me towards a link/reference/explanation of how to do this exactly? I'm assuming it somehow relates to ohm's law but please be gentle as I'm a lowly experimental chemist and not an electronics master of any sort.

I'm actually writing up a lab now where we did this to evaluate amplifier performance (EE is such a fun major!). I've never worked with flowmeters, but maybe I can help.

Basically resistors follow ohm's law. So V = IR or in words, if you put your DMM across the two terminals of the resistor, the voltage you see is the current through it times the it's rated resistance.

Their are a few tricks here. First is that the resistor will change the circuit a little. So you need to choose a resistor size that won't change the current reading too much. Generally you do this by selecting a smallish resistor, but not one too small that the reading is difficult to make.

Second, you need to actually measure the resistor. And you need to make sure that you don't heat it up too much so that its resistance changes during use.

About the devices you're measureing, they probably change their resistance to indicate the reading, right? Sort of like an RTD or thermistor. So you give them a voltage and measure the current? I've never used flowmeters, so I don't know how they work.

Nothing spacific for your question. Maybe Groth or Since87 can expand. A few links I came up in Google:

http://jet-server.commtest.co.nz/kb2/11624.htm
http://www.eaelec.com/tips1.htm

Probably not alot of help.

EDIT: looks like redleader is on the ball tonight, beat me to it. :D

Maxim is useful.

Helpful.

http://www.datel.com/data/meters/dms-an20.pdf

Basically as redleader states put a resistor in series and measure the voltage across it. A 250ohm will give 1-5V, if the transmitter can handle the voltage required to generate 20mA through this shunt.
The method suggested by Maxim in the link above is nicer though, if you feel like making a proper circuit.

Thanks guys it all makes sense now. That doesn't look too bad at all.

How are you measuring the voltage pHaestus? Curious about the evolution of your setup, are you using a DAQ? Labview?

Labview with an HP scanner to change channels and a 5.5 digit HP DMM to handle the voltage readings. Still in the beginning stages of setting it up unfortunately. I've had NO time since our son got older and my tenure track got accelerated. :/

Yep, that's the way to do it.

If you find a source for a cheap but accurate shunt, let me know, ok? ;)

(I'm dedicating two 8840As w/GPIB for those measurements)