Dimensional limits in IEC 60061 are defined at a reference temperature of 20 °C. The thermal expansion coefficient of steel is around 11 to 12 parts per million per kelvin, and brass is around 19. On a 27 mm cap, a 10 K difference between the gauge and the part produces a relative change of roughly 0.005 mm to 0.008 mm, which is the same order as the tolerance it is being compared against.
Bringing a pallet of caps into the inspection room and gauging them immediately measures the difference between warehouse and room temperature, not the cap. Parts need to soak until they reach room temperature; for a small cap in still air that is tens of minutes, for a full carton it is hours. The gauge needs the same treatment, which is why gauges should live in the inspection room rather than travel to the line.
A gauge held in a warm hand for a minute changes temperature measurably. Use a holder or handle the gauge by its non-functional surfaces, and let it rest between batches. On a high volume line the cumulative effect is that the gauge drifts warm through the shift and the first parts of the next shift are judged differently from the last parts of the previous one.
If the room genuinely cannot be held at 20 °C, record the actual temperature and treat the result as indicative rather than conforming. A report that states the temperature is auditable; one that assumes 20 °C is not.
