 Tare device

A tare device establishes a zero indication on a weighing scale while a load remains on its load receptor. An additive tare device preserves the weighing range for net loads; a subtractive tare device reduces that range.^[ref]

For example, place an empty $0.40\mathrm{kg}$ container on the load receptor and operate the tare device. The display reads zero even though the scale still supports the container. Adding $1.10\mathrm{kg}$ of material gives gross mass $1.50\mathrm{kg}$ and, ideally, an indicated net mass of $1.10\mathrm{kg}$. Removing both material and container without clearing the tare would ideally give $-0.40\mathrm{kg}$. This negative indication is consistent with the changed reference.

The cancellation can be seen in a linear model. If the unrounded indication before tare is $r(m)=am+b$, and the supported container has tare mass $T$, the indicated result after adding net mass $N$ isThe additive offset $b$ and the container contribution cancel. A response scale factor $a\ne 1$ remains, so taring alone does not establish calibration across the load range.

The container's physical load remains on the scale after taring. Consequently the available maximum weighing capacity depends on the kind of tare device, even when two instruments show the same zero. A preset tare device performs a related numerical subtraction using an entered or recalled value, rather than necessarily measuring this particular empty container.