Values of strain (full bridge) using Labview vs. Vishay strain box are different.

[kmac]

When measuring strain with my old Vishay strain box, all the equasions workout. With my Labview SCC2345 half bridge values checkout but full bridge values are off by a factor of three.


I measured the voltage strained and unstrained using a meter and the Lab view full bridge signal conditioning device setup to display voltage. Strained with 10lbs voltage = 4.26mv unstrained effectively 0.

Using the equations in the NI manual

Voltage ratio = Vstrained - Vunstrained/Vex 4.26mv/2.5v = 1.704m

strain E = -Voltage ratio / GF -1.704m/2.115 = -805u

-800u is exactly what Labview measures in Strain.

The Vishay box measures 3.1mE strained with
10lbs ... voltage = 4.26mv


But the Vishay box EX voltage is 2.0 vs 2.5v Ex Labview is using. I tried using the external EX from the Vishay box to provide the EX and have Labview measure the strain, same result 800uE.

I called NI so far they don't have an answer. It may be an algorithm issue in the Labview software. My incedent number is 565591.

[Matthew C]

Hello,

Thank you for contacting National Instruments.

Check that the Vishay and/or LabVIEW strain values are not being scaled with a custom scale.

Also, are you using the same strain gauge in both tests?

Your system may also need calibration.

Matthew C
Applications Engineer
National Instruments

[kmac]

Hi Matt,

There is no custom scale I am using an Express VI for Full bridge strain. Same strain gauge/setup for both instruments. I don't believe its a calibration issue, when I display strain as a voltage in Labview it is the same value as the Vishay box is recieving for a voltage. It just not processing the conversion to strain correctly.

[Matthew C]

Hello,

I apologize for the delayed response.

What SCC module(s) are you using with your SC-2345? Different modules are designed to work with different strain gauge applications (i.e. full bridge, half bridge, etc.).

Matthew C
Applications Engineer
National Instruments

[kmac]

Matt,

I am using the SCC-SG04 for full Bridge, the SCC-SG03 for half Bridge.

Kevin

[Matthew C]

Hello,

I apologize again for the delayed response.

I noticed that your phone support service request for this same issue has been resolved. Are you still having problems with this issue?

Matthew C
Applications Engineer
National Instruments

[kmac]

Nothing has been resolved I am still looking for an answer.