= Repeatability condition of measurement
A <repeatability condition of measurement> belongs to a specified set of conditions used to assess <measurement repeatability>. The VIM set includes a common <measurement procedure>, operator, <measuring system>, operating conditions and location, with repeated measurements of the same or similar objects over a short period.<International vocabulary of metrology by JCGM>[VIM3 entry 2.20, definition and note 1]{ref} A condition is therefore meaningful relative to the whole declared set, rather than being a property supplied by the word repeat alone.
For a physical length experiment, specify one operator, one <differential length comparator>, a reference block, the contact orientation, the remove-and-replace cycle, the settling rule and a short session after thermal equilibration. Keep the <measurement calibration> fixed during that series. Record every completed measurement, including the elapsed time and block temperature. Reading the unchanged display repeatedly and replacing the block between readings are different <measurement procedures>, even if the displayed numbers happen to coincide.
An original thermal calculation explains why the operating conditions matter. Suppose a metal reference block has length $L_0=100\,\mathrm{mm}$ at $T_0$, with stipulated constant linear <thermal expansion> coefficient $\alpha=12\times10^{-6}\,\mathrm{K^{-1}}$. In the small-expansion model,
$$
L(T)-L_0\simeq\alpha L_0(T-T_0).
$$
A temperature rise of $2\,\mathrm K$ changes its length by $0.0024\,\mathrm{mm}=2.4\,\mathrm{\mu m}$. If the intended <measurand> is length at $T_0$, that change requires an appropriate correction; it should not be silently interpreted as random comparator scatter. This calculation holds other thermal effects fixed and does not model expansion of the comparator itself.
The same room and a short elapsed time do not prove that the relevant conditions were stable. Record observed deviations and assess their effect. Deliberately varying operators over several days at the same location leads to a study of <intermediate measurement precision>; varying locations and <measuring systems> leads toward <measurement reproducibility>.
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