Force calibration
Force is realised with known masses in the local gravity field and transferred to the field with load cells. The big errors in practice are alignment, creep and temperature, not the standard.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Dead-weight force standard machine | 1 N to about 1 MN | 0.002 to 0.01 % | Primary or national standard |
| Reference load cell or force transducer | 50 N to several MN | 0.02 to 0.05 % of reading | Transfer standard for calibration labs |
| Working load cell | 1 N to MN | 0.02 to 0.1 % of full scale | Weighing systems and test rigs |
| Proving ring | 1 kN to MN | 0.1 to 0.3 % | Checking testing machines |
| Materials testing machine | To MN | ISO 7500-1 class 0.5, 1, 2 or 3: ±0.5 to ±3 % | Tensile, compression and fatigue testing |
| Hand-held force gauge | 1 N to 1 kN | 0.2 to 1 % of full scale | Spring, switch and product testing |
Tension and compression are different measurements: calibrate in the direction of use. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
ISO 7500-1 testing machine classes
| Class | Limit on relative error of indicated force | Limit on repeatability |
|---|---|---|
| 0.5 | ±0.5 % | 0.5 % |
| 1 | ±1.0 % | 1.0 % |
| 2 | ±2.0 % | 2.0 % |
| 3 | ±3.0 % | 3.0 % |
ISO 376 classes of force-proving instruments (00, 0.5, 1, 2) also specify limits on reproducibility, repeatability, zero, interpolation and creep, and a lowest usable force that depends on the resolution (500 to 4000 times the resolution). Read the standard for those limits.
Reference standards and traceability
- Dead-weight machines. Force from mass, local gravity and air buoyancy, F = m g (1 - ρair/ρmass), so traceable to mass, length and time.
- Force transducers. Reference load cells are calibrated against the standard machine and classified by ISO 376 as class 00, 0.5, 1 or 2.
- Testing machines. ISO 7500-1 and ASTM E4 describe how to verify a testing machine against a reference.
- Load cells in weighing. OIML R 60 covers load cells used in weighing instruments.
- Quality system. ISO/IEC 17025, scope listing force, range and direction.
How it is calibrated
Typical procedure
- Let the amplifier and load cell warm up, and keep the temperature steady.
- Mount the transducer square and centred, with the right adapters and couplings.
- Preload the system to maximum force a few times, with a short hold, so the first reading is repeatable.
- Apply at least eight steps up to the maximum, increasing and then decreasing.
- Rotate the transducer to a few positions, for example 0°, 120° and 240°, and repeat.
- Record zero before and after, calculate the error and the repeatability, and report the class and uncertainty.
Where it goes wrong
- Misalignment and side loads. Bending moments add errors the standard machine does not show.
- Creep and hysteresis. Readings drift under a constant load and differ going down.
- Zero not returning. A load cell that does not return to zero has been overloaded or is shifting.
- Local gravity. Using 9.80665 m/s² when your site is 9.79 is a 0.2 % error.
- Temperature. Sensitivity and zero drift with temperature.
What goes in the uncertainty budget
- Reference machine or transducer uncertainty.
- Alignment, side loads and adapters.
- Repeatability and reproducibility in rotation.
- Creep, hysteresis and zero return.
- Temperature, resolution and the amplifier.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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