Change Sig Exp graph/input time to min or hr?

[SF_SigExp]

SigExp 2.0 w/ USB DAQmx A/D 9215.  All Sig Exp configurations and graphs seem locked on using seconds as the time domain.  How to change to minutes or hours?   Experiments typically 1 point per 3 minutes, over 24 hours.  Entering 0.0055555.... as the frequency (Hz) isn't friendly and leads to long-duration rounding error.

[Ed W]

Hi,

Thank you for posting to the NI forums.  For this type of application, there isn’t an easy way around this issue.  Like you mentioned in your post, Signal Express does give access to an x-scale multiplier (right-click on the data view >> Properties >> Scales >> X-Scale >> Multiplier).  But because your frequency is not a rational number, your scale will begin to show misleading or incorrect values. 

There are a couple of options to remedy this situation.  The first is to simply change your sampling period to a rational number.  If you decrease your sampling period, you would collect more data, and you wouldn’t have a problem with viewing it.  The other is to use LabVIEW – the options for graphs and charts are much more flexible.

Please post back if you have further questions.

Ed W.

Applications Engineer

National Instruments

[SF_SigExp]

Ed,

Is there a processing trick such as this ......   acquire at 1 Hz, but then do a processing or analyzing step that selects just the points at a defined interval (eg, the 3 min interval) and saves that data?

Thanks, Steve

[Ed W]

Hi Steve,

What you’re describing is called resampling.  You can do this by adding a Processing step >> Subset and Resampling.  This will allow you to resample your data at whatever sample period you need.

Regards,

Ed W.

Applications Engineer

National Instruments

[--Chris]

If you are acquiring a single value you can configure the DAQmx Acquire Step to use SW Timed Single point.  At which point you can enter an update rate in seconds.  In your case entering 180 seconds will give you an accurate 1 sample every 3 minutes.

[SF_SigExp]

"proud"

That is the best solution, although it is still a little awkward using seconds over 24hr runs, using "1 sample (on demand)" in Timing Settings (Acquisition Mode) and Sample Period = 180 (sec) works for me.   It also puts the entire run on a single screen/view, which other approaches had problems with.

Thanks!