Most textile laboratories have a calibration folder. Fewer have a system that tells them, on any given Tuesday, whether the tensile tester is still reading correctly. The difference between the two is the subject of this article.
The reference document is ISO/IEC 17025:2017, the general requirements for the competence of testing and calibration laboratories. You do not need to be accredited to use its logic; it is simply the most widely recognised description of how to keep measurement equipment under control.
Four words that are used as if they meant the same thing
| Term | What it is | Who usually does it |
|---|---|---|
| Calibration | Comparing the instrument's reading with a reference of known value, and stating the difference and its uncertainty | A competent calibration laboratory |
| Adjustment | Changing the instrument so that it reads closer to the reference | The calibrator or the manufacturer |
| Verification | Deciding whether the instrument meets a specified requirement — for example a tolerance in a test standard | The laboratory, using calibration results |
| Intermediate check | A simpler check, between calibrations, that the instrument has not drifted | The laboratory itself |
A certificate that says "calibrated" without telling you whether the instrument meets the tolerance your test method requires is only half the job. The verification — comparing the result against the requirement — is yours.
What ISO/IEC 17025 asks for
The equipment clauses of the standard are short and worth reading in full. In summary:
- equipment must be capable of the accuracy or measurement uncertainty needed for a valid result (6.4.5);
- measuring equipment must be calibrated when its accuracy affects the validity of results, or when calibration is needed to establish metrological traceability (6.4.6);
- the laboratory must have a calibration programme that is reviewed and adjusted as necessary (6.4.7);
- when intermediate checks are necessary to maintain confidence, they are carried out according to a procedure (6.4.10);
- equipment that is overloaded, mishandled or gives questionable results is taken out of service, and the effect on earlier results is examined (6.4.9);
- records are kept for each item of equipment (6.4.13).
Intermediate checks also appear in clause 7.7.1 among the ways a laboratory monitors the validity of its results.
A textile instrument is several instruments
A tensile tester, a wash-fastness tester or a Martindale is not calibrated as a single object. It is a set of measured quantities, each with its own reference:
| Instrument | Quantities that affect the result |
|---|---|
| Tensile / tear tester | Force, extension or crosshead travel, speed |
| Wash-fastness tester, washing machine | Bath temperature, time, rotation, liquor volume |
| Martindale, crockmeter | Mass of loading pieces, cycle count, stroke |
| Conditioning room or cabinet | Temperature and relative humidity |
| Balances | Mass |
| Flammability cabinets | Time, flame height, gas flow |
| Light cabinet | Light source condition and hours in use |
For each quantity, write down three things: the tolerance the test method requires, how it is calibrated, and how it is checked in between. For tensile testers, the force-measuring system is normally verified to ISO 7500-1, which classifies the machine by its accuracy; check that the class on the certificate is good enough for the methods you run.
Designing intermediate checks that people will actually do
An intermediate check is useful only if it is quick enough to be done when it should be. Some practical patterns:
- Mass: a set of check weights kept beside each balance, weighed at the start of the day or of a series. Record the reading, not a tick.
- Force: a reference weight hung in the grips or a check load cell, at one or two points in the range you use most.
- Temperature: a reference thermometer in the wash bath, oven or conditioning cabinet, compared with the instrument display.
- Humidity: an independent data logger in the conditioning room, compared periodically with a reference instrument such as a psychrometer.
- Time and counts: a stopwatch against the instrument timer; a manual count against the cycle counter over a short run.
- Loading pieces: weigh Martindale and crockmeter loads on a calibrated balance; they get dropped, chipped and swapped.
- Whole method: an in-house control fabric tested at intervals, with results kept on a control chart. This catches problems no single check finds — a worn abradant, a changed operator habit, a new lot of consumables.
Set an acceptance limit for every check before you start, and write down what happens when it is exceeded.
Setting and changing intervals
There is no universal calibration interval. Start with the manufacturer's recommendation, the requirements of the standards you run and your customers' expectations. Then use your own data: an instrument whose intermediate checks never move can justify a longer interval; one that drifts needs a shorter one. ISO/IEC 17025 explicitly expects the programme to be reviewed and adjusted.
Reading a calibration certificate
Before you file it, check:
- Range — were the calibration points inside the range you actually use?
- As found / as left — was the instrument adjusted? If so, what did it read before adjustment? That is what your results were based on.
- Uncertainty — is it small enough compared with the tolerance you need?
- Traceability — does the certificate state the reference and its traceability?
- Conformity — has someone in your laboratory compared the results with the method tolerance and signed the decision?
When something is out
If a calibration or intermediate check shows the instrument outside its limits, stop using it, label it, and go back through the results produced since the last good check. Decide whether they are affected and whether customers need to be told. This is uncomfortable, and it is exactly why frequent, simple checks are worth more than an annual certificate alone.
Grey scales and other visual references deserve the same treatment: they fade and get handled, and they should be inspected and replaced on a plan.
We build our instruments in our own workshop in Istanbul and supply the technical data you need to define calibration points and check procedures for each of them.