What type of instruments are used as force calibration standards? (FAQ - Force)
Machines capable of undertaking force calibrations are known as force standard machines and they may be categorised as either primary or secondary. Primary standards in force measurement are machines whose uncertainty can be verified through physical principles directly to the fundamental base units of mass, length, and time. Secondary standards are machines which can reliably reproduce forces and can be compared to primary standards by the use of a force transfer standard which is a calibrated force transducer, frequently a strain gauge load cell.
Various types of force standard machines and their characteristics are given in the table below. The instrumentation and procedures used during the calibration must be formally shown to be capable providing properly traceable results.
Types of force standard machine
| Machine type | Principle of operation | Uncertainty attainable ±% |
Category |
| Deadweight | A known mass is suspended in the Earth’s gravitational field and generates a force on the support | 0.001 | Primary or secondary |
| Hydraulic amplification | A small deadweight machine applies a force to a piston-cylinder assembly and the pressure thus generated is applied to a larger piston-cylinder assembly | 0.02 | Secondary |
| Lever amplification | A small deadweight machine applies a force to a set of levers which amplify the force | 0.02 | Secondary |
| Strain-gauged hydraulic | The force applied to an instrument is reacted against by strain-gauged columns in the machine's framework | 0.05. | Secondary |
| Reference force transducer | A force transfer standard is placed in series with the instrument to be calibrated (typically in a materials testing machine) | 0.05. | Secondary |