Every measuring instrument has an input and an output. On a load cell the input is force and the output is an electrical signal. Calibration is the work of checking — and where necessary correcting — that the output corresponds to the input across the range you actually use. An instrument that has never been calibrated is not a precise instrument; it is an unknown one.
What calibration actually means
The word gets used three ways, and it helps to know which one is meant:
As adjustment. Bringing an instrument back to the manufacturer’s stated specification.
As documentation. Producing the report or certificate that shows the end user the instrument conforms.
As measurement. To an engineer, establishing the relationship between the true value of the quantity and what the instrument displays.
In practice it is done by comparing the instrument’s reading against a reference — a certified weight, a standard instrument, a simulator. That reference is itself periodically sent to a calibration laboratory and checked against national standards, which is what makes the whole chain mean anything.
Any correction made must leave the deviation inside the permitted tolerance — the small margin of error the application can live with.
Why instruments need it, and how often
Everything ages. Electrical and electronic components lose stability and drift away from the specification they were built to. Ordinary handling affects it; rough handling can put an instrument out of specification while it still looks perfectly sound from outside.
Recalibration is mandatory after any repair or maintenance work, to confirm the instrument still matches its reference data.
The general rule
At least once a year. The manufacturer performs the first calibration; after that it falls to the end user or back to the manufacturer.
When to do it more often
Critical applications, harsh environments, or wherever the reading determines a price. Frequency follows how the instrument is used, not the calendar alone.
Range, span and the terms people mix up
| Term | Meaning |
| Calibration range | The region between the limits within which the quantity is measured, expressed by the lower range value (LRV) and the upper range value (URV). |
| Span | The difference between them: span = URV − LRV. Calibrate a weighbridge over 0–40 tf and the span is 40 tf. |
| Instrument range | What the instrument is capable of, as printed on its label — for example 0–80 tf, output 2 mV/V. |
| Ranging | Setting the upper and lower values so the instrument responds with the sensitivity you want. A 0–10 tf reference set to give 0 mV/V at zero and 0.25 mV/V at 10 tf has been ranged. |
Do not confuse instrument range with calibration range
They are different things. An instrument capable of 0–80 tf can perfectly well be calibrated over 0–40 tf, and in that case 0–40 tf is the calibration range. Only when you calibrate across the whole capability do the two coincide.
Zero and span adjustment
Adjusting both zero and span lets you set the instrument to any range within the manufacturer’s limits. On analog instruments the two interact: changing the span almost always shifts the zero point as well, so the work becomes iterative — set one, re-check the other, repeat.
Smart transmitters do not have this problem. Zero and span are independent, and each can be set without disturbing the other.
Calibrators
A calibrator is whatever provides the known reference. The form depends entirely on what is being calibrated:
Reference masses
Certified test weights for weighing equipment. Temperature baths serve the same purpose for RTDs and thermocouples.
Signal sources
Generate a known voltage, current or frequency. Feed it into the instrument and adjust until the display matches. A simulator does the same by imitating a sensor output — a load cell simulator is exactly this.
Pressure standards
Supply a defined pressure for checking pressure instruments, usually acting as the pressure source too.
Calibration records
The record exists so the instrument’s history is not lost, and so drift over time can be seen rather than guessed at. A record should show:
- As-found data — readings taken before any adjustment
- Date of calibration
- As-left data — readings after adjustment
- Name or signature of the technician
- Date the next calibration falls due
Both sets of readings are taken at the same points across the range — typically 0%, 25%, 50%, 75% and 100%. Comparing as-found against the previous as-left is what shows you how fast the instrument is drifting, and therefore whether the calibration interval is right.
Metrology note
KALA Technical Notes
Where a scale is used for trade, calibration and legal verification are separate obligations — calibrating a bridge does not make it legally verified, and a verified bridge still drifts between inspections. Tell us how your scale is used and we will set out what each one requires.
● Test weights, load cell simulators and calibration support for industrial weighing.