There are three main options available for calibrating a force measuring system and establishing its measurement uncertainties.

• leave the force transducer in its permanently installed position and use a transfer standard to carry out the calibrations
• calibrate the force transducer prior to installation and remove as required for further calibrations
• calibrate the force transducer prior to its permanent installation in the force measurement system but don't recalibrate it during the life of the installation.

Permanently installed transducer with transfer standard

The preferred option for a system which requires a low level of uncertainty (ie high accuracy) is to permanently install the force transducer and then use a transfer standard to provide regular calibrations. Prior to installation the permanent force transducer should be calibrated over its full operational range, replicating where possible the anticipated operational conditions including the range, direction, rate of change, and duration of applied force, as well as temperature and simulation of a typical working cycle. This initial calibration, which may have been carried out by the manufacturer prior to delivery, is used to determine creep, non-return to zero, hysteresis, and non-linearity. 

At suitable intervals during the life of the system, a transfer standard is installed in the rig in series with the permanent device and the outputs are compared over a range of applied forces. These calibrations ensure that the uncertainty level is maintained, and also assist in identifying any changes or problems to the installation or transducer. The use of automated calibrations may be appropriate when trying to meet low measurement uncertainties and/or tight calibration time-scales. Automated calibration may more accurately reflect the operational conditions of the force measurement transducer and hence can reduce the system's measurement uncertainty.

Removable force transducer

If the force transducer can be removed from the installation then, in addition to its initial calibration, it may be removed periodically for re-calibration against a suitable standard. This method gives an accurate calibration to the transducer, but may not take into account losses in the installation such as friction, lever ratios, and so on. The associated instrumentation may be calibrated with the transducer or separately against another standard.

Permanently installed force transducer without periodic calibration

A permanently installed force transducer without periodic calibration may be considered when both removal and in-situ calibration are impractical. The initial calibration should establish a level of measurement uncertainty well within the system requirements to accommodate drift and deterioration in the force transducer during its operational life. Some confidence in the ongoing operational uncertainty may be achieved by periodically performing a shunt calibration and checking one or more well-defined operating points including zero applied force.

Note: Although this may be a practical solution and indeed inevitable in some circumstances, traceability cannot legitimately be claimed for the force measurement results so obtained - even if the initial calibration was against a traceable standard.