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Caliper jaw tilt error is a change in the indicated dimension caused by rotation of a caliper's moving jaw relative to its reference geometry. If a contact lies distance $h$ from the displacement reference, a small rotation $\theta$ gives a signed offset contributionThe sign depends on the direction of rotation and contact geometry. The approximation requires small $|\theta |$; it describes this angular contribution rather than every possible measurement error.

Mitutoyo gives a concrete geometry: a jaw slope of $0.01\mathrm{mm}$ over $50\mathrm{mm}$ and a contact depth of $40\mathrm{mm}$ produce a tip error of $0.008\mathrm{mm}$. Excessive contact force and wear of the guide can cause the moving-jaw rotation.^[ref]

An original diagnostic repeats a gauge block comparison at contact depths $h_1=10\mathrm{mm}$ and $h_2=40\mathrm{mm}$. With stipulated constant $\theta =2\times 10^{-4}\mathrm{rad}$ and other effects unchanged, the angular contributions are approximately $2\mu \mathrm{m}$ and $8\mu \mathrm{m}$. Their $6\mu \mathrm{m}$ difference isolates a depth-dependent effect that a single closed-jaw zero reading could miss. Real contacts may also change force or deformation, so this comparison does not uniquely identify rotation without controls.

A Vernier zero correction removes an estimated constant offset; it cannot generally remove an error proportional to contact depth or varying force. Use consistent light contact, a clean reference and a stable temperature, and compare several separations and contact positions. A fine Vernier scale improves reading increments without guaranteeing that these mechanical contributions are small.

This lever-arm mechanism is distinct from merely projecting a fixed length onto a tilted measurement axis: the latter involves $\cos \theta$ and has a quadratic small-angle change, whereas the offset term above is linear in $\theta$. Keeping the geometry in the measurement model prevents applying the wrong angular correction.

A gauge block is a physical length standard with carefully finished, approximately flat and parallel opposing measuring faces. Its calibrated length transfers a reference dimension to a comparator, a caliper or another measuring system. The nominal inscription identifies its intended size; the certificate supplies the measured departure and measurement uncertainty.^[ref]

The reference definition uses a block face wrung to an auxiliary plane and a specified point on the opposite face. Gauge block wringing also permits blocks to form a stack; the defined length convention includes a wringing layer. Clean contacts, the calibrated departures and the thermal conditions matter.^[ref]

An original stack uses nominal lengths $10\mathrm{mm}$, $2\mathrm{mm}$ and $1.5\mathrm{mm}$, giving nominal total $13.5\mathrm{mm}$. If their certified departures under compatible reference conditions are $+0.10$, $-0.04$ and $+0.02\mu \mathrm{m}$, the sum is $13.50008\mathrm{mm}$. The nominal sizes alone would omit the $0.08\mu \mathrm{m}$ correction. Combining uncertainties also requires their covariance, not just adding the certificate values.

For a separate illustrative thermal expansion calculation, a $100\mathrm{mm}$ block with stipulated linear expansion coefficient $11.5\times 10^{-6}{\mathrm{K}}^{-1}$ lengthens by $2.3\mu \mathrm{m}$ when heated by $2\mathrm{K}$. These are chosen model parameters, not a certificate for a particular block. NIST's handbook treats correction to the reference temperature $20{}^{\circ }\mathrm{C}$ and temperature differences in comparisons.^[ref]

NIST describes two complementary routes: optical interferometry establishes block lengths, while a differential length comparator transfers calibrated differences. The apparatus and thermal control determine which uncertainty contributions matter. Handling a block warms it; waiting for thermal equilibration is part of the measurement rather than idle time.^[ref]

A Vernier scale is an auxiliary graduated scale whose spacing differs slightly from the main scale. A fine displacement is read by finding which pair of graduation lines coincides. The device converts a difficult visual estimate of a small gap into a more conspicuous coincidence index. A caliper can attach the auxiliary scale to its sliding jaw.^[ref]

For an original ideal direct vernier, let one main-scale division be $d$ and let $N$ auxiliary divisions span $(N-1)d$. Each auxiliary division isIf the auxiliary zero is displaced by $x$ beyond main mark $m$, its graduation $j$ lies at $md+x+jv$. Coincidence with main mark $m+j$ gives $x=j(d-v)=j\Delta$. Thus the reading is $md+j\Delta$, for the stated geometry.

With $d=1\mathrm{mm}$ and $N=20$, the auxiliary divisions are $0.95\mathrm{mm}$ apart and the reading increment is $0.05\mathrm{mm}$. If the auxiliary zero is just beyond 12 mm and graduation 7 coincides, the reading is $12+7(0.05)=12.35\mathrm{mm}$.

This is a constructed example. The manufacturer guide gives the same 0.05 mm spacing-difference principle and distinguishes a longer vernier, whose auxiliary division is 0.05 mm shorter than two main divisions. That longer scale can improve readability while retaining the same increment; the reading rule must match the actual graduation geometry.^[ref]

A Vernier zero correction is needed when the closed or reference setting does not indicate the intended zero. For example, if the instrument reads $+0.10\mathrm{mm}$ at a true zero setting, subtracting that offset changes a later raw reading of $12.35$ to $12.25\mathrm{mm}$. The sign of the correction follows the observed error, not the direction of the object's length.

Fine graduations do not establish measurement accuracy. Jaw tilt, measuring force, thermal expansion, wear and viewing geometry can introduce errors larger than one increment. The primary guide identifies those physical sources explicitly.^[ref] Check reference closure and suitable calibrated artefacts, control contact force and record the relevant measurement uncertainty rather than treating the last readable digit as the whole error budget.

A Vernier zero correction removes an indicated offset observed at a known zero setting of a Vernier scale. If the raw indication is $r$ and the indication at true zero is $z$, the additive model givesThe zero error is $z$; the correction applied to a later reading is its negative. This model addresses an offset, not every possible scale or contact error.

For an original signed example, a raw reading $r=12.35\mathrm{mm}$ with closed-jaw indication $z=+0.10\mathrm{mm}$ gives $L=12.25\mathrm{mm}$. If the zero indication is instead $-0.10\mathrm{mm}$, the corrected reading is $12.45\mathrm{mm}$. A negative zero indication therefore requires adding its magnitude. It does not imply a negative object length.

The reference setting must represent the intended zero: clean, properly contacting outside faces on a caliper are one example. Record the actual signed indication and the scale convention, rather than relying on a memorized rule about which engraved zero lies to the left. The manufacturer guide explains the coincidence reading and identifies jaw tilt, force and viewing geometry as separate error sources.^[ref]

For standard uncertainties, the additive model givesWith stipulated independent $u(r)=0.03\mathrm{mm}$ and $u(z)=0.02\mathrm{mm}$, the combined value is approximately $0.0361\mathrm{mm}$. Subtracting a measured offset does not make that offset known exactly. Correlation can matter when both observations share an uncertain reference or drift contribution.^[ref]

A second reference checks the limitation. Suppose true zero reads $0.10\mathrm{mm}$ but a known $50.00\mathrm{mm}$ separation reads $50.60\mathrm{mm}$. Removing the zero offset leaves $50.50\mathrm{mm}$, still $0.50\mathrm{mm}$ high. A model $r=aL+z$ would have $a=1.01$ here. Measurement calibration across the working interval is needed to distinguish scale error from a zero correction, with the relevant measurement uncertainty retained.