Error in Strain Measurement

[Controls_Engineer]

I am using SCXI-1520 and SCXI 1314 to measure strain from 4 strain gauges. These gauges are attached to two cylinders. I am measuring higher values of strain then I am expecting. The strain values measured are higher than the actual strain the cylinders can withstand. So definitely something is wrong in the measurement. Also when the pressure is reduced to zero the strain value reduces but does not become zero. Please suggest if there is a problem with the gauges or the data acquisition system.

[AlanArmstead]

How are you measuring your strain guages? Are you using a LabVIEW program, or testing in Measurement & Automation Explorer(MAX)? I would suggest setting up a DAQmx Global Channel in MAX, and testing your strain guages there. To set up your channel, right-click on the Data Neighborhood folder, select Create New>>DAQmx Global Channel>>Analog Input>>Strain, and follow the wizzard. Make sure you have the correct values for guage factor, guage resistance, excitation, etc...
Some other info that might be helpful: What version of NI-DAQ do you have? What DAQ device is controlling the SCXI chassis.
-Alan A.

[Logan_K]

It is quite common for strain gages to have offset errors or gain errors due to irregularities in the strain gage material, and the mounting of the strain gages. If your errors are relatively small, then you most likely just need to calibrate your strain gage for offset errors and gain errors. When you create a DAQmx global strain channel or a Traditional DAQ virtual strain channel in MAX, you will have the option to do a null and/or shunt calibration of your strain gage. The null calibration will eliminate small offset errors, and doesn't require any additional connections. The shunt calibration will calibrate out any gain or slope errors, but requires an additional connection to SCA on the 1314 terminal block (read the SCXI-1520 user's manual for more information on this connection). If these calibrations don't eliminate the errors that you are seeing, then you should double-check your settings in MAX to make sure that they match your strain gage.

Logan Kunitz
SCXI Product Support Engineer
R&D, National Instruments