What Is Vernier Caliper? | How to Read Vernier Caliper?

What is Vernier Caliper?

An 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).

Understanding Vernier Caliper

Vernier calipers are precision instruments used for measuring internal and external dimensions accurately. Their measurements are presented in centimeters (cm) and are precise to two decimal places, for example, 1.23 cm.

This guide will cover the essential aspects of reading measurements from a vernier caliper, applicable to models that use inches as well.

Main Scale:

• The main scale is a fixed scale on the caliper and is typically graduated in millimeters or inches, depending on the unit of measurement.
• Each division on the main scale represents a specific measurement unit, and it provides the whole number part of the measurement.

Vernier Scale:

• The Vernier scale is a secondary scale that slides parallel to the main scale. It has a series of divisions that are slightly smaller than those on the main scale.
• The zero (0) point on the Vernier scale corresponds to a specific point on the main scale, ensuring alignment during measurements.

Alignment:

• To read the Vernier scale, align the zero (0) point of the Vernier scale with the closest point on the main scale.
• Look for the first line on the Vernier scale that aligns perfectly with a line on the main scale. This indicates the fraction or decimal part of the measurement.

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 acuity

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

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 the 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.

How to Read a Vernier Caliper

Accurately reading a vernier caliper requires a combination of understanding its parts and using a systematic approach. Here’s how you can obtain precise measurements:

#1. Zero Calibration.

Before measuring, ensure that the caliper is correctly calibrated. Close the jaws fully, and check if the zero marks on the main scale and vernier scale align. If not, you may need to recalibrate or adjust for zero error.

#2. Taking the Measurement.

For external dimensions, place the object (such as a screw) between the fixed and sliding jaws. For internal dimensions, use the internal measuring jaws.

If measuring depth, extend the depth rod into the hole or recess. Gently close the jaws or extend the rod until it snugly fits the dimension you are measuring.

#3. Reading the Main Scale.

Note the last visible mark on the main scale that lines up with the vernier scale’s zero. This is your main scale reading. In this example, 3 as the zero point of the vernier scale has not yet passed the 4th indicator.

#4. Reading the Vernier Scale.

Next, look for the line on the vernier scale that vertically aligns perfectly with a line on the main scale. There should only be one. This is your vernier scale reading. The divisions on the vernier scale will give you your measurement past the decimal point.

A simple scenario would be if the division indicator marked as 9 on the vernier scale lines up perfectly, that would be 0.9mm.

In our scenario, the line that aligns perfectly is the 2nd after 9. Because each division denotes 0.02mm as indicated by the caliper, this would be 0.94mm.

#5. Add Your Main & Vernier Scale Together.

Now that you’ve got your main and vernier scale reading, all that’s left to do is add them together to get your final reading.

So, (Main Scale) 3mm + (Vernier Scale) 0.94mm leaves us with a reading of 3.94mm.

FAQs.