Subtractive tare device

A subtractive tare device is a tare device whose operation reduces the weighing range available for net loads.^[ref] Its zero display accounts for a load already present, while that load continues to occupy part of the instrument's capacity.

Consider a chosen model with maximum weighing capacity $\mathrm{Max}=10\mathrm{kg}$ and no additive tare capacity. If a container of tare mass $T$ is supported on the load receptor, the admissible net mass $N$ in this model satisfiesWith $T=2.5\mathrm{kg}$, the largest net load in this capacity budget is $7.5\mathrm{kg}$. Adding $8\mathrm{kg}$ after taring would produce gross mass $10.5\mathrm{kg}$, even though the desired net indication is below $10\mathrm{kg}$.

Changing to a $0.8\mathrm{kg}$ container would leave $9.2\mathrm{kg}$ available for net mass in the same model. This is a physical consequence of reducing the supported tare mass, rather than a benefit obtainable by repeatedly resetting the display.

An additive tare device is defined by retaining the net-load weighing range. Therefore the equation above is a model for subtractive tare and a shared gross-load bound, rather than a universal rule that every scale's printed $\mathrm{Max}$ includes every possible tare allowance. The chosen container, allowed tare range and instrument capacity must describe the same load configuration.