A force transducer, dynamometer, load shackle or hydraulic jack gives a figure in newtons, kilonewtons, kilograms or tonnes. But how do you know that figure is correct? Through use, overloading, temperature and ageing of strain gauges and electronics, a force measuring instrument can drift unnoticed. A force calibration establishes how large that deviation is and how reliable the measurement still is.
In this article you can read what a force calibration involves exactly, how the measurement is carried out on a tensile or compression test bench, which standards play a role and what you will find on the certificate. How we do this in practice is described on the service page force calibration in tension and compression.
Calibrating is comparing
Calibrating does not mean adjusting, but comparing. During a force calibration, your measuring instrument is placed in the same line of force as a reference force transducer (transfer standard) whose deviation and measurement uncertainty are known. Both experience exactly the same force. The difference between the indication of your instrument and the value of the reference is the deviation. If the instrument is subsequently corrected, that is called adjustment; it is always followed by a new measurement.
The reference transducer is itself calibrated traceably, via an unbroken chain to the national force standard. As a result, the result on your certificate is also traceable. The chain ultimately ends at mass and gravity: a force standard is realised with accurately known masses under the local gravitational acceleration. How mass, weight and force are related is explained in mass, weight and force: what is the difference?
Tension and compression
Force has a direction. A load shackle, crane scale or tension force gauge is loaded in tension: the instrument is stretched. A hydraulic jack, cylinder or compression transducer under a press is loaded in compression. Many force transducers and dynamometers can do both. Because the deformation of the measuring body in tension differs from that in compression, each direction is calibrated separately. On a tensile test bench the instrument is suspended between two crossheads with tension fittings or a shackle; on a compression test bench it stands between two compression platens.
The range of the bench determines what is possible. In our force laboratory we calibrate in tension up to 500 kN (approx. 51 t) and in compression up to 4,000 kN (approx. 408 t). For each calibration we select the reference transducer that matches the range of your instrument, so that the measurement uncertainty remains as small as possible.
Load steps up and down
A calibration consists of a series of load steps, evenly distributed over the range of the instrument. A simple check uses three steps, an extensive calibration eight or more. The force is increased step by step to the maximum value and then reduced in the same steps. This produces, for each measuring point, a value at increasing and at decreasing load.
From that series follow the most important characteristics of the instrument:
- the deviation per measuring point relative to the reference;
- the hysteresis: the difference between increasing and decreasing loading at the same force;
- the repeatability: the spread between repeated series;
- the zero return after unloading.
More about these terms can be found in hysteresis and repeatability in force measurement. Before the actual measurement, the instrument is preloaded several times to the maximum force, so that the measuring body has settled and the measurement is representative of its use.
Transducer or complete measuring chain
A force transducer delivers an electrical signal, usually in mV/V. Only the amplifier or the display turns this into a force. You can have the transducer calibrated on its own (the signal is then measured with a precision amplifier) or the complete measuring chain: transducer plus the readout you work with in practice. For most industrial applications the measuring chain is the most logical choice, because the certificate then applies to the figure you actually read. When each option is appropriate is explained in load cell calibration: sensor or complete measuring chain?
Which standards apply?
Three documents keep coming up in force calibration:
- ISO 376 describes the calibration and classification of force measuring instruments used to verify tensile and compression test benches. It is a demanding procedure with several mounting positions and a classification into classes 00, 0.5, 1 and 2. See ISO 376 explained.
- ISO 7500-1 covers the verification of the force measuring system of uniaxial tensile and compression testing machines themselves. This requires a reference instrument according to ISO 376.
- EURAMET cg-4 is the European guideline for determining the measurement uncertainty of force measurements. It is used to calculate the uncertainty on the certificate.
Not every calibration is automatically an ISO 376 calibration. A dynamometer from lifting applications or a force transducer from a press usually requires a calibration of the measuring chain in the direction in which you measure, with a clear certificate. An ISO 376 calibration is needed if you use the instrument itself to verify machines.
What does the certificate state?
A good calibration certificate for a force measuring instrument states at least:
- the identification of the instrument and, for a measuring chain, of the readout;
- the direction (tension, compression or both) and the unit used;
- all measuring points with the reference value, the indication and the deviation;
- the measurement uncertainty, calculated according to EURAMET cg-4;
- the reference transducer used and its traceability;
- the environmental conditions during the measurement;
- where applicable the class according to ISO 376, and after adjustment the values before and after.
What you then do with those figures, for example apply a correction or monitor a tolerance, depends on your application. How to read a certificate is explained in calibration certificate explained.
How often to calibrate?
No law prescribes a fixed interval for force measuring instruments; your quality system determines the interval. Twelve months is a common starting point, shorter with heavy use and immediately after an overload, fall or repair. More on this in how often should a force gauge be calibrated?
In summary
Force calibration is the comparison of your instrument with a traceable reference transducer in the same line of force, in tension and/or compression, over a series of load steps up and down. The result is a certificate with deviation, hysteresis, repeatability and measurement uncertainty, with which you demonstrate that your force measurements are reliable. Would you like to have a force transducer, dynamometer, load shackle or jack calibrated? See the options at force calibration in tension and compression.