= Calculated net value
{title2=$N_{\rm calc}$}
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.<Non-automatic weighing instruments, Part 1>[T.5.2.2, T.5.3.1 and T.5.3.2, printed page 16]{ref}
Write the arithmetic as
$$
N_{\rm calc}=W_{\rm measured}-PT,
$$
where $PT$ 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.<Non-automatic weighing instruments, Part 1>[T.1.1, printed page 5; T.5.3.2, printed page 16]{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. Then
$$
N_{\rm calc}-m=(t-PT)+e.
$$
Repeating 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>.
Back to article page
