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Drift and stability

The change in an instrument’s reading over time, with no corresponding change in the quantity being measured.

Drift is the continuous or gradual change in an instrument’s reading over time while the value of the measured quantity stays put. The instrument can therefore give a different reading today from the one it gave at its last calibration, even though it is measuring the same standard under the same conditions.

Stability is the instrument’s ability to hold its metrological properties constant as time passes. The smaller the drift over a given interval, the more stable the instrument is considered to be.

How it shows up

Drift can take more than one form.

Zero drift

The zero reading itself moves. An unloaded balance, for instance, may read 0.3 g instead of zero after some time has passed.

Sensitivity drift

The sensitivity or the calibration curve changes, so the error grows by a different amount at each point of the measuring range.

Drift of a micrometer reading over six months Reading plotted against time. The horizontal dashed line marks the value of the gauge block standard, 50.000 mm, which stays constant. The micrometer reading rises gradually: 50.002 mm at calibration, 50.004 mm at three months and 50.006 mm at six months. The total change is 0.004 mm. Standard 50.000 mm 50.002 50.004 50.006 0.004 mm calibration 3 months 6 months Time Instrument reading
Fig. 1. The standard stays where it is, the reading moves. Three successive checks show that the deviation does not appear all at once· it builds up gradually.
Worked example

A micrometer is checked at intervals against the same 50.000 mm gauge block, under the same conditions:

At calibration  50.002 mm
Three months   50.004 mm
Six months     50.006 mm

The total change of 0.004 mm is evidence of drift, provided the standard and the measuring conditions stayed the same. The three successive readings also reveal its character: the deviation does not appear suddenly, it grows steadily.

Where it comes from

Drift may come from ageing components, wear, mechanical parts working loose, changes in electronic components, or the release of internal stresses. Changes in temperature and humidity can shift the reading too. To separate genuine drift from an environmental effect, though, the comparisons have to be made under the same specified conditions.

How it is caught

Drift is not the same thing as random error. Random error scatters the readings· drift shows up as a trend over time. It is caught through periodic calibration, intermediate checks, and plotting the results to watch where they are heading.

Watch out

Re-zeroing can hide a zero drift for a while, but it proves nothing about whether the instrument is still correct across its whole range. That is why stability is assessed with standards at more than one point of the range.

Sources: JCGM, “International Vocabulary of Metrology, VIM” (3rd ed.), §§4.19 and 4.21· N.V. Raghavendra, L. Krishnamurthy, “Engineering Metrology and Measurements” (Oxford University Press), §§12.2.5 and 12.2.6· T.G. Beckwith, R.D. Marangoni, J.H. Lienhard V, “Mechanical Measurements” (Pearson, 6th ed., 2006), ch. 2, calibration drift and environmental effects.

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