Full Bridge Strain Gage reads 25% expected value

[eroberts@water.ca.gov]

SCXI-1000, 1520, 1314.  MAX v 4.7.  Specimen is a column-type load cell, 2 active and 2 compensating gages.  Full Bridge Type III strain selected as measurement.  Other parameters are: range = 10m to -10m, GF = 2.115, Gage Res = 350, Vx = 10v, no scale, 100 hz filter.  An Offset Null & Shunt Cal were performed and appeared normal.  A press was used to apply various force values, the strain readout using MAX was always approximately 25% of expected value.  This was verified by 2 independent Vishay P3500 strain indicators.

This equipment has worked fine previously with 1/4 bridge strain gages, this is our first attempt using it to measure a full-bridge compensating strain gage.

[enderWiggin10]

Hi eroberts,

Are you attempting to read this using a task in MAX, and express VI, or low-level DAQmx calls in LabVIEW?  Further, are you creating a Strain task, voltage task, or custom votlage with excitation?  Another thing to check would be that the setup is in accordance with the diagram for Full Bridge III here:

Are you using internal excitation or external?  What have you specified for your poisson ratio?  Have you verified the connection diagram to the 1520?  This is listed on page 2-8 of the specifications.

Perhaps you could try a custom voltage with excitation task to verify the voltage is being read correctly as well.  This would allow you to test the bridge voltage with a handheld DMM to verify the voltage is brought in correctly, and could help to isolate software from hardware.

Best,

[eroberts@water.ca.gov]

Hi, thank you for the response.  I am reading this task in MAX, it is a strain task, the excitation is internal 2.0v. poisson ratio is 0.3.  The connection is correct.  One piece of information I would like to have:  In the MAX task, there is a shunt calibration.  My SCXI-1520 has a 100k resistor in both SCA & SCB slots.  What equivalent micro-strain does the 100k resistor represent?  I am reading approximately 635 micro-strain "simulated".  Yet charts from other sources indicate a 100k resistor should simulate approx 1300 micro-strain.  Any ideas on this? 

[enderWiggin10]

Hi eroberts,

I am assuming that you are performing a shunt calibration in MAX, is this correct?  If not, you can create a task in MAX and then calibrate accordingly by selecting the Device tab next to the Settings tab and choose Strain Calibration. 

When this is done, you select the value for the shunt resistor, as well as the leg of the bridge it will be imposed upon.  It will then tell you what the simulated strain would be with the shunt resistor in the Gain Adjustment columns.  This will be compared with the measured strain, and then the err% is reported.

If not, you can calculate this using the bridge resistance formula.  The leg with shunt cal will then have an equivalent resistance with the 100kOhm in parallel. 

You can then calculate the equivalent strain from the strain equation for full bridge: 

Let me know if this helps,