small voltage readings with NI 9215

[scm923]

I currently am using the cDAQ 9172 Data Acquisition board with several analog and digital input/output modules. I am trying to use the 9215 Analog Input module to measure voltages from a simple anemometer. The readout range for the anemometer is 0 -1 V. However, for our use, we are analyzing low velocities which will give no more than 0.2 V.

When the anemometer is attached to the 9215 module it will not register any voltage reading unless it is given a very high velocity (and thus a higher voltage readout). We have tested the anemometer with a simple oscilloscope and it easily recorded the low voltages at the low flow rates, so the problem is not with the anemometer itself.

So these are my questions:

Is it possible that the 9215 is not sensitive enough for the < 0.2 V reading?

Is the problem also that the cDAQ 9172 also contains 6 other I/O modules and is housed in a electrical cabinet and the signal is lost in the noise/interference?

Is there anything that I can try in LabView itself to increase the sensitivity?

Many thanks to anyone who can provide some insight.

[JÞB]

SCM-

 from pg 17 of the 9215 manual

""With BNC .......................................All inputs must remain within ±10.2 V of the average AI– inputs""

 

Your symptoms seem to indicate that the average AI input of all channels is masking the lower voltages from the anemometer.  Have you tried disconnecting the other AI chanels to see if this is the case?

Jeff 

[scm923]

Jeff-

Thanks for the response. I also was thinking the same thing - to try it with an empty module and/or empty DAQ board.

I did forget to mention that we are using the 'screw terminal' version of the NI 9215, and not the BNC one, if that makes a difference.

Also, the module has a 16-bit resolution. So according to our calculations it should be able to read as low as 0.0003 V.

I will let you know how it goes... 

[scm923]

It appears we are still having problems. We tried the anemometers with an empty DAQ module and an empty DAQ board. Neither seems to help the voltage signal.

The anemometers were also tested with a spyder box, and seemed to function fine. The spyder box was able to read the small voltages at the low velocities. This means, of course, that the anemometers are working correctly. But for some reason the DAQ board is unable to read the low voltages.

Again, according to the specifications, the modules should have more than enough resolution and sensitivity to read between 0.1 to 0.3 V, which is the range within which we would most commonly be operating.

I am not sure what else to try. My only other idea is to give the module a known voltage input between 0.1 and 0.3V, probably from a simple voltmeter. If the DAQ can read it, then the problem is with the connection between the anemometers and the module. If it can't read the signal, then it is probably that the module/board is/are the problem.

But I am of course open to any and all ideas and suggestions...

[KyleBryson]

Hi scm,

Could you post some more info about the anemometers you are using?  Also, have you made sure that your module is referenced to ground properly?  The 9215 front end is isolated.

 Check the 9215 manual for tips on how to connect different sensors.  What is connected to the COM terminal on your module?

Regards,
Kyle

[scm923]

Here is the operation manual for the anemometer. http://www.schiltknecht.com/uploads/media/MiniAir6.pdf

I will check on Monday about the COM connection. (Our equipment is off-site, so I can not investigate more until next week). I am 99.9% sure that the ground is referenced properly. The cDAQ board is hardwired into a large control box that we are using to control and interface with a number of sensors, motors, etc. And since it was done by a professional, I will assume that everything is grounded properly. But, again, I can recheck that on Monday.

One of the strange things that I find with this situation is that it is not a problem with any of the other analog and digital I/O's. But maybe that is because they are reading out higher voltages, rather than the very low range that the anemometers operate under.

Thanks for your help, Kyle. I will post an update on Monday with more info. 

[scm923]

I have checked the module this morning, and it is grounded properly. The COM terminal is wired to the ground and to the ground wire for the anemometer. Everything with regards to the anemometer seems to be wired properly. I received one suggestion that advised to try the module with a known low voltage input, from a voltmeter, for example. If the module can detect the low voltage from the voltmeter, then perhaps the problem is in the anemometer wiring. But, again, this seems counterintuitive as the anemometer works just fine with the spyder box. I hope to try the voltmeter this week. Otherwise, I am running out of ideas and options.

Regards,

scm

[scm923 · Accepted solution]

Problem solved!

Apparently it was the wiring after all. I do not know what the problem was, but it is fixed and everything is well.

Thanks to everyone for their suggestions and help!