Using SCC SG01 should I null and shunt calibrate or do a multipoint calibration?

[StevenO]

Hello,

I am using a SC 2345 Signal Conditioning Carrier to read strain through SCC-SG01 and SCC-SG02 modules.  I am connected to a 6036E PCI card connected to windows XP.  I am running LabVIEW 8.2.  That said, I have been having issues with calibrating the strain gauges.  I created a program to null and shunt calibrate each channel, but you can also do a multi point calibration in max.  Which is best?  Should I do both?

I also have found the nulling pot on the module to be very touching and "drift" during measurements.  I can usually get within +/-.00005 volts on the null.  Is this close enough to zero?  Also, how do I prevent drift.  Yesterday I read a correction factor of 1.018 on one gauge and today I read 0.988.

Thanks,

Steven O'Hara

Test Engineer

[Raajit_Lall]

Steven,

Does this happen when you are trying to null the voltage offset of the module?  To do this create a task in Measurement and Automation Explorer and rotate the trimming potentiometer wiper with a screw driver till you see 0.0.  Could you please try this for different values of input limits in your test panels.   The steps for this are given in the  user manual on page 18 which you can access here.  Please also refer to the Using Shunt Calibration to Correct for Gain Error section of the user manual linked above.

Could you also please elaborate on what you mean when you say that the nulling pot is touching in drifts?  Does this happen after you have gone through the entire process of calibration and try to do it again?  Also, what is the input limits that you are setting when you are getting within +/- .00005 volts on the null.

Looking forward to your reply.

Regards,

[StevenO]

Raajit,

the +/- 0.00005 is how close I've gotten to null the gauge.  Nulling to 0.0 doesn't seem to be a fine enough precision.  That said, when I null to a finer precision I've noticed the null drifting over time.  What I mean to say is, after I have nulled the gage, measured the correction factor (from shunt calibration) and begin to collect data, I notice an offset when the gauge is not stressed.  If I go back to check the null of an unstressed gage, it is usually off.  This doesn't occur on all the channels, but certainly creates questionable data.  This is the drifting that I was having problems with.

The other question or concern about the nulling process is how sensitive (touchy) the trimming potentiometer ("nulling pot") is.  The slightest turn can really cause the readings to fluctuate.

Thanks,

Steven