"Waveforms have different dt values"

[mysticfree]

I'm trying to set up limit testing for a system that outputs a voltage based an input pressure (psi).  Unfortunately, I keep getting the error "Waveforms have different dt values" from the "Limit Testing Frequency.vi" module.   My setup is as follows:
The upper and lower limits values are arrays of psi,voltage based on the formula:
        Voltage=A*psi +B, where A and B are constants for the respective upper and lower limits.  The Voltage is computed at psi values of [10, 20, ..., 90, 100]. Each of these arrays are feed into "Create Continuous Mask.vi".  dx for these modules = (100-10)/(10-1) and x = 10

I need to compare those limits against data that is acquired from a DAQ. The data consists of 125 points that follow the form of:
     Voltage=0.038055*psi+0.574909
The pressure follows an expotential curve similar to:
      psi = 110*(1-exp(-0.05*(data point)))
These two are combined into an array of (psi, voltage) and feed into a "Create Continuous Mask.vi" module with settings of dx = (110-0)/(126-1) and x = 10

I can graph all three curves (upper, lower, acquired data) without a problem.  Is the "Waveforms have different dt values" problem because the "test data vs pressure" is curved while the "limits vs pressure" is linear?  Is there anyway around this?

I'm using Labview 8.5 Developer Suite on Windows XP.

Thank you for your help,
Ron

[RavensFan]

I would highly recommend posting whatever code you have.  It is difficult to follow your written description without having the code to compare it to.  Are these functions taking waveform datatypes?  If so, have you undbundled the dT value from them to see how they compare?  How do these waveforms get generated?

[mysticfree]

Sorry about that.  I have attached a modified version of my VI (I removed the DAQ portion from the case statement).  Maybe I just misunderstand how the limit VIs work.  I thought it was a simple matter of putting in XY data and having the VIs line everything up to make their pass/fail assessment.  Is this not the case?

[mysticfree]

Hallaluah! I was finally able to see the error in my ways and corrected the problem! 

I am using the "Limit Testing Frequency.vi" and "Create Continuous Mask.vi" modules for Limt/Mask testing.  I thought that these modules could interpret data arranged as an X,Y array, and easily overlay the limits and my readings to determine pass/fail.   Not the case!  My limits were values taken at 10, 20, 30, and 40 psi (increments of 10 on the X scale).  My readings were taken at 10, 11, 12, ... 39, 40 psi (increments of 1 on the X scale).  Not matter what I put into the "dx" parameter of the "Create Continuous Mask.vi", I kept getting "Waveforms have different dt values" error from the "Limit Testing Frequency.vi".   The solution was to take of limit data (increments of 10), run it into the "Interpolate 1D.vi" and generate data based on increments of 1 psi to match the spacing of my readings.

I don't understand the function of the "dx" parameter but apparently it has nothing to do with allowing X,Y data to be overlaid easier in the limit/mask VIs.  I hope that this entry helps someone.