The temperature to which a measured value is referred, fixed at 20 °C for dimensional work so that a drawing dimension means the part's size at 20 °C. Measurements made at other temperatures need thermal expansion corrections, and the uncertainty in the part's temperature and expansion coefficient often dominates dimensional budgets. Steel grows about 11.5 µm per metre per °C, so a 1 °C error on a 500 mm part is nearly 6 µm.
Shop floor's at 26 degrees, either correct back to reference temperature or let the part soak in the lab.
Also heard as
- standard reference temperature
- 20 degree rule
Related terms
Influence quantity
A condition that is not the thing being measured but still changes the reading, such as temperature or humidity.
Correction
The amount added to a raw reading to cancel a known, repeatable error, equal to that error with its sign flipped.
Soak
Letting an instrument, artifact or part sit in the lab environment until it reaches thermal equilibrium.
Jo blocks
Shop name for gauge blocks, precision steel or ceramic blocks wrung together to build reference lengths.