Preset tare device

A preset tare device subtracts a preset tare value from a measured gross or net indication and displays the calculated result.^[ref] A stored container value can therefore be reused without weighing each empty container individually.

Suppose a container and its contents have measured gross mass $G=2.30\mathrm{kg}$ and the entered preset tare value is $PT=0.30\mathrm{kg}$. Subtraction gives a calculated net mass of $2.00\mathrm{kg}$. If this particular container actually has tare mass $T=0.34\mathrm{kg}$, however, its contents have mass $1.96\mathrm{kg}$. Even an otherwise exact gross measurement would then overestimate the contents by $0.04\mathrm{kg}$.

The error follows without any instrument-specific assumption:A mismatch in the entered container value passes directly into the result. Reusing one value for nominally identical containers is therefore an assumption about their tare masses, rather than a new measurement of each container.

The starting indication also matters. If an ordinary tare device has already removed a $0.30\mathrm{kg}$ container contribution, the displayed $2.00\mathrm{kg}$ is already net for that reference. Subtracting the same $0.30\mathrm{kg}$ again would give $1.70\mathrm{kg}$. A deliberate second subtraction can represent another distinct component, but applying the same container correction twice is a different calculation from removing its tare mass once.