GlossaryKind of term: Standard and conventionReference temperature (20 °C)
What is the reference temperature?
The reference temperature is the temperature at which drawing dimensions and tolerances apply: 20 °C under ISO 1, unless the drawing states another.
That temperature applies to the whole geometrical specification, form and location requirements included: the drawing describes the workpiece as it is at that temperature. It refers to the temperature of the workpiece, not of the room. Measuring at another temperature is possible, but the measured size can then differ, depending on the difference in expansion between workpiece and gauge. You then either correct for it or include the deviation in the measurement uncertainty.
What does the standard say about the reference temperature?
ISO 1 fixes 20 °C as the standard reference temperature for geometrical product specification; the current edition is ISO 1:2022, which replaced ISO 1:2016. The standard fixes the temperature of the specification, not the temperature at which you must measure. If a drawing explicitly states another temperature, that is a deliberate deviation for that part. How temperature deviations lead to systematic errors and measurement uncertainty is covered by the technical report ISO/TR 16015. A correction is only as good as the known expansion coefficients and the measured temperatures of workpiece and gauge.
How does the reference temperature work in practice?
Whether temperature matters depends on length, material, temperature difference and tolerance. For example, 100 mm of steel at 5 °C above 20 °C is about 6 µm longer, aluminium about twice that. On a loose tolerance that disappears; on an IT6 or IT7 dimension it can be a large share of the tolerance. Measure steel with a steel gauge at the same temperature and the expansions largely cancel; measure aluminium or plastic with a steel gauge and they do not. A part warm off the machine needs time to reach temperature. With plastics such as PA, moisture uptake changes the size as well. Without an agreement on measuring temperature, shop and customer can find different sizes for the same part.
What is the reference temperature often confused with?
The reference temperature is not the temperature of the shop or the inspection room: that is an ambient condition, the reference is an agreement on what the dimension means. An inspection room at 20 °C only helps once workpiece and gauge have reached that temperature too. Nor is it thermal expansion: expansion is the physical effect, the reference temperature the point at which the dimension is agreed.
Frequently asked questions
Why is 20 °C the temperature for drawing dimensions?
Because ISO 1 sets it as the standard reference temperature for geometrical specifications. Every dimension then refers to the same state of the workpiece, wherever and whenever it is measured.
Do I have to measure every part in an inspection room?
Not necessarily. Whether temperature counts depends on length, material, temperature difference, tolerance and gauge. For tight dimensions on aluminium or plastic, measuring at temperature, or correcting, usually belongs in the route.
How long should a part be left to reach temperature?
Until workpiece and gauge are at the temperature of the inspection room. How long that takes depends on mass, material, wall thickness and surroundings: a thin-walled aluminium part in minutes, a large steel block sometimes hours.