Calculated net value ($N_{\mathrm{calc}}$)

A calculated net value is the difference between a measured weighing value, gross or net, and a preset tare value. The key distinction is the origin of the subtracted quantity: it is entered or recalled numerically rather than necessarily being a freshly measured tare for that individual container. OIML R76-1:2006 defines these terms separately from an ordinary net indication after operation of a tare device.^[ref]

Write the arithmetic aswhere $PT$ is the stored preset tare value. Here the weighing values are expressed as mass estimates, for example in kilograms; they are not forces expressed in newtons. The instrument determines mass using gravity under its measurement model.^[ref]

For a chosen packaging example, let a loaded container give $2.480\mathrm{kg}$ and let the stored nominal container tare be $0.180\mathrm{kg}$. The calculated net value is then $2.300\mathrm{kg}$. If this particular empty container actually has mass $0.195\mathrm{kg}$, its contents have ideal net mass $2.285\mathrm{kg}$. The calculated indication overestimates that value by $0.015\mathrm{kg}$. The subtraction has been performed correctly; the discrepancy arises because a nominal tare was substituted for an individual one.

More generally, let the true contents' mass be $m$, the actual container mass be $t$, and the measured gross value be $m+t+e$, where $e$ is measurement error. ThenRepeating weighings can assess variation in $e$, but it does not by itself remove an incorrectly chosen $PT$. A useful check therefore weighs representative empty containers as well as repeating loaded measurements. Report the stored tare and what it represents along with the result. This makes the physical assumptions behind a calculated net value visible rather than treating a precise-looking display as evidence that every container has the nominal tare mass.