Using a Shunt Calibration Resistor
Since a strain gage load cell is a passive electrical device, there exists a simple, yet effective, method for checking the calibration of a load cell system in the field or when a means of applying actual forces is unavailable. Inducing an electrical imbalance in the cells bridge circuit will simulate the bridge imbalance caused by the application of actual forces to the load cell. Then the system gain may be adjusted so that the system output signal or display indicates a known force on the load cell.
Be careful not to use Shunt Calibration as a substitute for actual force calibration of a system. Shunt Calibration merely supplies a known signal to the signal conditioning unit, in order to check its gain or span adjustment.
On the Calibration Certificate for each Low Profile load cell, Interface routinely supplies the value of the equivalent force resulting from connecting a specified shunt calibration resistor across one leg of the bridge. For other types of cells, Interface will supply shunt calibration values on special request.
Shunt Calibration Connections
The standard connections used by Interface for tension and compression shunt calibration are specified on the Calibration Certification for each load cell.
It is important that the standard connection be used, although a similar (but not equal) output would result from connecting to the opposite leg of the bridge. Shunt calibration is relatively insensitive to small changes in temperature, although the calibration is precisely correct only at the Lab Standard conditions noted on the load cells Calibration Certificate.
The following values of shunt resistors will cause an output of approximately 73% of Rated Output for the load cell types indicated when connected across the specified load cell terminals.
|For 4 mV/V cells:||RS-100-30K (30,000 ohms, ±0.01%)|
|For 3 mV/V cells:||RS-100-40K (40,000 ohms, ±0.01%)|
|For 2 mV/V cells:||RS-100-60K (60,000 ohms, ±0.01%)|
|For 1 mV/V cells:||RS-100-120K (120,000 ohms, ±0.01%)|
To perform a shunt calibration, use the following procedure:
- Remove or stabilize all forces on the load cell.
- Adjust the display or indicator ZERO to read exactly zero.
- Connect the shunt calibration resistor to the terminals specified on the Calibration Certificate, and adjust the SPAN or GAIN until the display reads the force value stated on the Certificate.
- Repeat the procedure to insure a valid calibration.