Gain error with shunt calibration

Hello,
I have a problem with shunt calibration of bridge based displacement transducers. The shunt calibration routine sets gain factors that give errors around 2% in the measured displacement. I have also found that the shunt calibration gain factors run out of bounds without a preceding offset null.
The hardware is a cDAQ-9188 with 9237 modules. The gauges have four wires, two input wires that I have connected to EX+ with jumper to RS+ and EX- with jumper to RS-/SC-, and two output wires that I have connected to AI+ with jumper to SC+ and AI-. LabVIEW is used with a VI shown in the attached figure.
Does anyone know why the shunt calibration behaves like that or what I do wrong?
/Emil
Attachments:
labview.png ‏19 KB

Hi Anton,
Setting the gain manually from a known displacement is not an option as some of the transducers are mounted so that they cannot be tested with gauge blocks.
Setting the DAQ Assistant Express VI or Create Channel VI to strain mode gives the same shunt factors as the general bridge mode.
I have done additional measurements to outrule any chance of coincidence with the sensing wires. With jumpers from EX+ to RS+ and from EX- to RS- on the first four of ten transducers the shunt factors are:
1.0046; 1.0246; 1.0347; 1.0246; 0.9649; 0.9811; 0.9600; 0.9720; 0.9591; 0.9793
with jumpers only on the first two transducers the shunt factors are:
1.0044; 1.0247; 0.9867; 0.9729; 0.9648; 0.9811; 0.9599; 0.9721; 0.9589; 0.9791
and finally for all transducers without jumpers the factors are:
0.9624; 0.9715; 0.9868; 0.9728; 0.9647; 0.9809; 0.9600; 0.9721; 0.9592; 0.9792
The conclusion is that the shunt factors are wrong but stable if the wiring is unchanged. Jumpers between EX and RS terminals effects the shunt factors. The shunt factors are between 0.04 and 0.06 larger with jumpers.
Should the RS terminals be connected for gauges without an extra sensing wire? Should there be a difference with and without the jumpers? Do you suggest anything special to look into regarding the EX wiring?
/Emil

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