How do I measure multiple resistance values with a DAQ and a DMM?

[drhim91]

I am currently working in a lab wherein I must measure the resistance changes in a strain gauge. Currently, I've only managed to set it up to a DMM and measure the resistance of one circuit at a time. However, I would like to set it up so that I can monitor at least three different resistance values at once. I have a DAQ, if need, but I just can't figure out how to get the DMM input function to give out three separate values. Please help. Thank you.

[crossrulz]

Resistance is measured by supplying a current and measuring the output voltage.  A DMM can't do multiple resistance measurements without some switching.

A trick I've done before is supply a voltage (like 5V) through a set resistor and then through the variable resistor (strain guage in this case).  You can measure the voltage across the strain gauge with the DAQ.  Do some math with the voltage divider to turn the voltage into a resistance.  Repeat this circuit for each strain guage (obviously using different DAQ channels).

[billko]

---

@crossrulz wrote:

Resistance is measured by supplying a current and measuring the output voltage.  A DMM can't do multiple resistance measurements without some switching.

A trick I've done before is supply a voltage (like 5V) through a set resistor and then through the variable resistor (strain guage in this case).  You can measure the voltage across the strain gauge with the DAQ.  Do some math with the voltage divider to turn the voltage into a resistance.  Repeat this circuit for each strain guage (obviously using different DAQ channels).

---

One thing the OP did not mention is what type of DMM.  Does it have multiple channels?

[drhim91]

Ah yes, I forgot to mention that. The DMM I have is the NI USB-4065 DMM. However, being rather unfamiliar with the DMM model in general, I'm not sure if that has multiple channels. From what I see, there are two black ports which I take to be ground ports and the three red ports, that I assume can be used to complete the circuit. Currently, the lab I work in have the two ports on the left plugged in and we use it to measure resistance. As for the DAQ, we have the 6289 version. But seein as the last time I used a DAQ was in my lab session in college, it would be best if you considered me as a complete newbie in both regards.

[crossrulz]

If you look closely at the DMM, it is clearly marked at having 1 port.  The extra set of banana jacks are for sense wires (when doing 4-wire resistance measurements) or current measurements.

Use my suggestion about supplying a voltage and measuring the divided voltage using the DAQ.

[drhim91]

Alright, then, that sounds like a plan. However, I don't understand as to why I should send it throught a set resistor and then the strain gauge. Since I can control the voltage and the current outage from the DAQ, wouldn't it be more efficient to just measure the voltage difference across the strain gauge? I'm kinda new to the whole circuitry thing.

[Dennis_Knutson]

But you DON'T have control of the current. You can only set an output voltage. That is why you need the known resistor.

[drhim91]

Oooooohhh.......I see. Well.....well then. In that case, thank you very much!