= Tare mass
{title2=$m_{\mathrm t}$}
{wiki=Tare_weight}
= Tare weight
{synonym}
<Tare mass> is the specified container or packaging <mass> excluded when reporting <net mass>. It is also called <tare weight> in weighing terminology. The intended boundary matters: a jar alone and that jar with its lid have different <tare masses>. A <tare device> adjusts a <weighing scale>'s indication with a load present, whereas a <zero-setting device> establishes zero with the <load receptor> unloaded.<Non-automatic weighing instruments, Part 1>[OIML R76-1:2006, T.2.7.2 and T.2.7.4, printed p11; T.5.2.2–T.5.2.3, printed p16]{ref}
A simple demonstration separates the reading from the physical load. Choose an empty container of <mass> $m_{\mathrm t}=0.120\,\mathrm{kg}$ and an ideal <weighing scale> that initially reads zero. After the container is placed on the <load receptor>, operate the <tare device>. Its displayed value becomes zero while the container's <mass> remains $0.120\,\mathrm{kg}$. In a uniform <gravitational field> with chosen magnitude $g=9.81\,\mathrm{m\,s^{-2}}$, its <gravitational weight> is still
$$
W=m_{\mathrm t}g=1.1772\,\mathrm N\approx1.18\,\mathrm N.
$$
The supporting <force> remains present after the display adjustment.
In an ideal unit-gain <measurement model>, the tared indication for a complete load of <mass> $m$ is $I(m)=m-m_{\mathrm t}$. Adding $0.300\,\mathrm{kg}$ of contents gives <gross mass> $0.420\,\mathrm{kg}$ and indication $0.300\,\mathrm{kg}$. Removing the complete load while retaining that reference gives indication $-0.120\,\mathrm{kg}$. The negative indication expresses the difference from the stored reference; the removed container still has positive <mass>.
A <preset tare device> instead uses an introduced <preset tare value>, which can be recalled or entered without weighing each individual empty container.<Non-automatic weighing instruments, Part 1>[T.2.7.5, printed p12; T.5.3.1–T.5.3.2, printed p16]{ref} For the same chosen load, using a preset of $0.100\,\mathrm{kg}$ gives calculated <net mass> $0.420-0.100=0.320\,\mathrm{kg}$: \b[the contents are overestimated by 20 g]. A persistently low preset produces a <systematic error> even with otherwise exact readings.
<Composite tare mass> includes several excluded components, such as all the jars and an outer crate. The <gross mass> example shows how to account for these components once each. The symbol $m_{\mathrm t}$ here denotes a <mass>; the terminology's use of the word weight does not change the quantity's unit from a <kilogram> to a unit of <force>.
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