Gear
Instrument drift
Tapes stretch, ruler ends wear, printed scales fade; an instrument checked a year ago is not the instrument you hold now, so re-check before a careful session.
Guides on Gear: Every instrument, and what each costs you, Everything you can wrap, and what each reads, Verifying the thing you measure with
Yes, instruments drift: a tape stretches, a ruler's end wears and printed marks fade, so a check done when you bought it stops being true with time. None of the drift is dramatic on its own, and all of it moves in the same direction: away from the reading you calibrated against.
What changes with time and handling
A fibreglass or vinyl tape stretches gradually with repeated tension, and each stretch is permanent - the tape does not spring back overnight the way it might feel like it should. A rigid ruler's end, the part pressed hardest and most often against bone, wears fractionally with use, rounding a square edge that started sharp. Printed graduations fade with handling and light exposure, thinning the lines until the boundary between one millimetre and the next gets genuinely harder to place, not just harder to see.
None of these changes announces itself. A tape does not feel different to hold after it has stretched half a millimetre over a 20 cm span, and a ruler end does not look worn until you compare it closely to a fresh one.
Why "I checked it before" stops being true
Metrology has a name for this: the International Vocabulary of Metrology (JCGM 200:2012) defines instrumental drift as "continuous or incremental change over time in indication" caused by changes in the instrument itself. An instrument checked against a bank card or a trusted ruler when you first got it was accurate then. That check is a statement about the instrument on that day, not a permanent property of it, in the same way a measurement without its method attached stops being data after enough time passes - the conditions that made it trustworthy do not carry forward automatically. A year of normal storage and occasional use is enough for a cheap tape to have stretched detectably and a ruler end to have taken enough small knocks to matter.
The fix is cheap and takes a minute
Before a session where the number matters - a comparison to a past reading, or a figure you intend to write down and keep - recheck the instrument rather than trusting the last check. The same card-and-ruler comparison used the first time works again here, and it costs nothing beyond doing it. Professional programmes treat checking as routine rather than a one-off: the CDC's NHANES Anthropometry Procedures Manual (2021) has staff check the stadiometer against a 100-cm metal calibration rod at the start of each stand and daily. This matters more for a tape than a ruler, because stretch accumulates with use while a ruler's scale, absent physical damage, does not drift on its own. Storage affects accuracy; day-to-day handling affects something else again - instrument hygiene and how a mark is made on the skin is the companion concern worth reading alongside drift.
An instrument you re-check stays comparable to your own past readings, which is the only thing this measurement was ever really being compared to. It has no equivalent on the photograph side of this territory: a rating from Rate Cock, an estimate from an AI model, a score built by reading an image, or a human judge's opinion does not stretch in a drawer, because none of them was a physical instrument to begin with.