 Zero-setting device

A zero-setting device makes a weighing scale indicate zero with no load on its load receptor.^[ref] A tare device instead establishes a zero indication with a load present. This difference identifies which physical reference the instrument is being asked to use.

For an illustrative linear response, write the unrounded mass indication aswhere $m$ is the applied mass, $a$ is the response scale factor and $b$ is an additive offset. An ideal zero adjustment subtracts the unloaded indication $r(0)=b$, givingIf $a=1.01$ and $b=0.03\mathrm{kg}$, a true $1\mathrm{kg}$ load initially indicates $1.04\mathrm{kg}$ and indicates $1.01\mathrm{kg}$ after this zero adjustment. The $0.03\mathrm{kg}$ offset has disappeared, while the proportional measurement error remains. Correct zero does not establish correct response at other loads; a calibration using known nonzero loads tests that additional question.

A practical observation follows directly from the model: record the empty indication, apply a known load, remove it and check the empty indication again. Agreement at zero tests the reference return, while agreement under load also tests the response away from zero. Instrument-specific zero-setting limits still govern actual operation; the algebra describes an ideal offset correction rather than a replacement for those limits.