not recording proper values

[triumph]

Hello,

I'm using Signal Express, with a 9171/9205 module. I'm trying to record a Reference load cell, I've calibrated and configured, and this records and displays

accurately, when placing weights on it. Now the load cell under test, I just want to record in millivolts, no scaling, just the raw voltage. Basically, I'm just setting up a back to back

calibration. My issue is when I change the samples rate / average to anything other than 10/1k, I get almost a 5 millivolt offset, I'm new to lab view /signal express, and didn't think this would be a task to set up 2 channels, does anybody have any suggestions?

Thanks

[Dennis_Knutson]

You should ask the moderator to move your question to the SignalExpress board. This is the LabVIEW board and very few people here usr SignalExpress.

[triumph]

will do

[M0535]

Hi triumph,

How are your inputs connected to your 9205? One possible reason for the offset is ghosting between the inputs. If you can, try and place the inputs for the second load cell as far away as possible from the reference load cell lines on the 9205 to prevent one from interfering with the other.

Just to clarify, on the software side, during the calibration, if you change the settings from a 10Hz sample rate with 1k data points being averaged, you are getting the offset, but if you leave the settings at those, you don't see the offset?

[triumph]

Justin,

thanks for the response, I'll answer as much as I can. Here is my setup, my reference load cell is being getting excitation and amp. by a Ectron amp, with adjustable gain and zero, I have 25 lb calib weights, and have adjusted the amp for .500 for every 25 lbs, ie. 0 lbs = 0.00 volts, 25 lbs = .500, 50 lbs =1.00 volts. My calibration for that load cell is slope 50 y=0, prescaled /volts and scaled/lbs, pretty simple. My 2nd load cell, I just want to read actual raw voltage, which is in millivolts, I've read it with my meter, so I know what the readings should be. My current setting for sample to read and Rate is 5 and 10. which only gives me 10 samples per second. I look at the data in excell, and the lbs readings for the reference are correct, and the millivolt readings are correct for the 2nd load cell. But i want a faster sample rate with data, whenever I change either of the samples to read or Rate, my offset goes up with what I'd call noise. 

My goal with this setup is to introduce Labview/Ni where I work, and to use this simple usb device to do back to back calibration/verifications. I've used other data acquisition systems and there software is much easier to set up a 2 channel acquisition system. I do know Labview is a powerful software, and overkill for what I need, but I thought signal Express was suppose to be easier. 

I will move the 2nd load cell further away and see what that does, but any suggestion would be greatful

Thanks

[triumph]

Justin,

OK I went back through and after reading some information, started all over, and I think my problem was when I was calibrating the channel/sensors I left the sample to read at 1000, I did it again at 100, and my load cell that is amplified, reads correctly and so does my string pot (non amplified), and reading at 1000 hz. I'll end up slowing it down to probably to 250 - 500 for the macros to run.

But my next question is I want to add a load cell, and read raw output voltage with no particular scaling, which is in millivolts (0-20mV) at full scale, I'm using a 9205 module, and it supposudely has a +/-200mv range. How do I select this for a particular channel? or is it to far in the low end to read properly/

any ideas?

thx

[M0535]

Hi triumph,

I'm glad you were able to resolve your original sampling/voltage offset issue!

You definitely can read the voltages properly, but because of the limitations of the accuracy of the 9205 you would only be able to get 127 steps between 0 V and 20 mV. You can set the range by using the DAQmx Channel property node as seen in this KnowledgeBase article: http://digital.ni.com/public.nsf/websearch/2B0317259B06D96386256EE80048F67A?OpenDocument