What is Vernier Caliper?
A vernier scale, named after Pierre Vernier, is a visual aid to take an accurate measurement reading between two graduation markings on a linear scale by using mechanical interpolation, thereby increasing resolution and reducing measurement uncertainty by using vernier acuity to reduce human estimation error.
The vernier is a subsidiary scale replacing a single measured-value pointer, and has for instance ten divisions equal in distance to nine divisions on the main scale. The interpolated reading is obtained by observing which of the vernier scale graduations is coincident with a graduation on the main scale, which is easier to perceive than visual estimation between two points.
Such an arrangement can go to a higher resolution by using a higher scale ratio, known as the vernier constant. A vernier may be used on circular or straight scales where a simple linear mechanism is adequate.
Examples are calipers and micrometers to measure to fine tolerances, on sextants for navigation, on theodolites in surveying, and generally on scientific instruments. The Vernier principle of interpolation is also used for electronic displacement sensors such as absolute encoders to measure linear or rotational movement, as part of an electronic measuring system.
The vernier scales may include metric measurements on the lower part of the scale and inch measurements on the upper, or vice versa, in countries that use inches. Vernier calipers commonly used in industry provide a precision to 0.01 mm (10 micrometers), or one thousandth of an inch. They are available in sizes that can measure up to 1828 mm (72 in).
Measurement Reading Technique for Vernier Caliper
In order to read the measurement readings from vernier caliper properly, you need to remember two things before we start. For example, if a vernier caliper output a measurement reading of 13.42 mm, this means that:
- The main scale contributes the main number(s) and one decimal place to the reading (E.g., 13 mm, whereby 1 is the main number and 0.3 is the one decimal place number)
- The vernier scale contributes the second decimal place to the reading (E.g., 21 divisions)
We will just use a two steps method to get the measurement reading from this:
To obtain the main scale reading: Look at the image above, 13mm is to the immediate left of the zero on the vernier scale. Hence, the main scale reading is 13mm
To obtain the vernier scale reading: Look at the image above and look closely for an alignment of the scale lines of the main scale and vernier scale. In the image above, the aligned line corresponds to 21. Hence, the vernier scale reading is 21*0.02=0.42mm. (least count is 0.02)
In order to obtain the final measurement reading, we will add the main scale reading and vernier scale reading together. This will give 13mm + 0.42mm = 13.42mm.
Use the following formula: Obtained reading = Main scale reading + Vernier scale reading
Least count or vernier constant
The difference between the value of one main scale division and the value of one vernier scale division is known as the least count of the vernier, also known as the vernier constant.
Let the measure of the smallest main-scale reading, that is the distance between two consecutive graduations (also called its pitch) be S, and the distance between two consecutive vernier scale graduations be V, such that the length of (n − 1) main-scale divisions is equal to n vernier-scale divisions.
Then the length of (n − 1) main-scale divisions = the length of n vernier-scale division, or
(n − 1) S = n V, or
nS − S = nV.
Vernier scales work so well because most people are especially good at detecting which of the lines is aligned and misaligned, and that ability gets better with practice, in fact far exceeding the optical capability of the eye.
This ability to detect alignment is called vernier acuity. Historically, none of the alternative technologies exploited this or any other hyperacuity, giving the vernier scale an advantage over its competitors.
Zero error is defined as the condition where a measuring instrument registers a reading when there should not be any reading. In case of vernier calipers it occurs when a zero on main scale does not coincide with a zero on vernier scale.
The zero error may be of two types: when the scale is towards numbers greater than zero, it is positive; otherwise, it is negative. The method to use a vernier scale or caliper with zero error is to use the formula
Actual reading = main scale + vernier scale − (zero error).
Zero error may arise due to knocks or other damage which causes the 0.00 mm marks to be misaligned when the jaws are perfectly closed or just touching each other.