A load cell or force transducer does not itself measure a force in newtons. The measuring body deforms under load, strain gauges convert that deformation into a small electrical voltage, usually expressed in millivolts per volt of excitation voltage (mV/V), and only the amplifier or the display converts that into a force or a weight. During a calibration you therefore have to choose: do you have only the transducer calibrated, or the whole chain from transducer to readout? Both are possible, but they mean different things. How we calibrate force transducers and load cells is explained on the service page.
What is a measuring chain?
A force measuring chain consists of three links:
- the transducer: the measuring body with strain gauges, which outputs an mV/V signal;
- the amplifier or indicator: powers the transducer, measures the signal and converts it into a unit;
- the readout: the display, the software or the PLC on which you read the figure.
Each link contributes to the deviation. The transducer has its own sensitivity, linearity and hysteresis; the amplifier has its own accuracy, resolution and temperature behaviour; the setting of the readout determines whether the whole is correct. A certificate only applies to what has actually been measured.
Option 1: calibrating the transducer on its own
In a stand-alone calibration, the transducer is connected on the force calibration bench to a precision amplifier belonging to the laboratory. The result is a table of force against mV/V, often with an interpolation equation. This has advantages:
- the result is independent of your own electronics and is therefore easy to compare with the factory specifications;
- you can use the transducer with different readouts, provided you enter the sensitivity correctly;
- for reference transducers according to ISO 376 this is a common approach, in combination with a precision amplifier.
The disadvantage: the certificate says nothing about what your own display indicates. An error in the amplifier, an incorrectly entered sensitivity or a deviating excitation voltage remains invisible.
Option 2: calibrating the complete measuring chain
For a calibration of the measuring chain you send in the transducer, cable and readout together. On the bench, the force from the reference transducer is compared with the figure on your display, in the unit you use in practice: N, kN, kg or t. For most industrial applications this is the most logical choice, because:
- the certificate applies to the figure you actually read;
- errors in the amplifier, settings and cable are included;
- your auditor sees one certificate that matches the instrument as it is in use.
The downside: the calibration is tied to that combination. If you replace the indicator, the chain must be recalibrated. Therefore record the serial number of both transducer and readout on the certificate and in your measuring equipment register.
When to choose which option?
| Situation | Advice |
|---|---|
| Dynamometer, crane scale or force gauge with fixed display | Complete measuring chain; transducer and display are inseparable |
| Load cell in a press, assembly line or dosing system with its own indicator | Complete measuring chain, with the indicator from the installation |
| Reference transducer for verifying tensile and compression test benches (ISO 376) | Measuring chain with the associated precision amplifier; for a stand-alone calibration, calibrate the amplifier separately |
| Transducer used with varying readouts | Transducer on its own in mV/V, plus a calibrated amplifier |
| Weighing system with several load cells and a junction box | Calibrate the complete weighing system on site with weights, see scale calibration |
What do you send in?
For a calibration of the measuring chain you need: the transducer, the original cable, the indicator or amplifier including power supply, and preferably the mounting hardware such as tension fittings, compression pads or a shackle with which the instrument is normally loaded. Also let us know in which direction you measure (tension, compression or both), in which unit you want the result and whether you need a particular number of measuring points or a class. That way the calibration matches your application.
Practical example: load cell in a press
A machine builder uses a 200 kN compression transducer under a press to monitor the pressing force of bearings. The transducer is connected to a PLC input via a separate amplifier. If only the transducer is calibrated, the quality department knows that the mV/V signal is correct, but not whether the amplifier scales it correctly and whether the PLC displays the right unit. By sending in transducer, cable and amplifier together and taking the value at the PLC input as the readout, the certificate applies to the pressing force the operator sees. If the amplifier is permanently built into the machine and cannot be sent in, the alternative route is: calibrate the transducer on its own in mV/V and have the amplifier calibrated electrically, after which the two certificates together cover the chain.
The same applies to a load cell under a silo or in a dosing system, with one difference: as soon as several load cells jointly determine a mass via a junction box, a calibration with weights on site is the appropriate route, because then the load distribution and the mechanical installation are also taken into account.
Shunt calibration is not a calibration
Many indicators have a shunt or cal function: a fixed resistor simulates a signal from the transducer. That is a useful check of the electronics and the cable, but not a calibration of the force. The shunt says nothing about the measuring body, the mounting or the hysteresis. Use it as an interim check, not as a replacement for a calibration against a reference transducer.
In summary
Preferably have a load cell calibrated as a complete measuring chain, so that the certificate applies to the figure you read. Choose a stand-alone calibration in mV/V if the transducer is used with varying readouts or serves as a reference, and then calibrate the amplifier as well. In doubt? State your application when requesting a quote and we will advise which approach suits. See calibrating a force transducer or load cell.