Least Count of measuring instruments

 

Least Count of measuring instruments

In this article, we are going to learn about least count of various instruments and calculation of least count for measuring instruments. Following content is covered in this post.

Content

What is least count?

It is smallest measurable value of any measurement instrument. For example steel scale can measure 1mm, so least count is 1.0 Least count of an instrument has direct relation with precision and accuracy of that measuring instrument.

Precision–
It talks about how close is the measurement of same item to each other.

Accuracy–
It refers how close measurement of an item to its true value is.

Least count vs Precision 

Least count is inversely proportional to precision.
Mean smaller least count; more will be its precision. 

Least count Vs Accuracy

Least count is inversely proportional to accuracy.
Lower the least count more will be accuracy. Accuracy means measurement close to true value. For example- 4.0 mm is true value; I am using Digital vernier caliper to measure this dimension. My results are 4.002 but not 4.000. Here I am 0.002 mm far away from true value (Which is instrument accuracy level) and least count is 0.01 mm. So accuracy will be always less than least count of any instrument.

Let’s understand with an example. We have mechanical vernier caliper and Digital vernier caliper. Least count of mechanical vernier caliper is more than digital vernier caliper. If I measure any dimension with mechanical vernier caliper, then result of that dimension will not come same each time, there will be variation each time.

But if I check same dimension with digital vernier caliper, then we will get same value of that dimension on each measurement. That is precision.

So digital vernier caliper is more precise as compared to mechanical vernier caliper. It is because digital V.C has low least count.

Least count of various instruments

Instruments used in industries for measurement are- Vernier caliper, Micrometer, Profile Projector, Dial gauge, Bevel protector, CMM, VMM, Hardness Tester, Roughness Tester, contour machine, Rapid Eye, Height Gauge, etc. Each instruments has its least count and there is calculation for least  count of various instruments, which is explained below.

Least count of Vernier Caliper

There 2 types of vernier caliper. Type 1 Mechanical vernier caliper (Also called Manual vernier caliper) and Type 2 is digital micrometer. Least of both type of vernier caliper is different, which is explained below.

Least count of Manual Vernier caliper= 0.02 mm

Least count of Digital Vernier Caliper= 0.01 mm

Manual vernier least count calculation

In mechanical vernier caliper, there are 2 scales (Main scale and another one vernier scale) so we have to calculate least count of both scale.

First calculate least count of main scale– Smallest reading on main scale/No of division on main scale. Here 1cm/10 Division=0.1 cm

Now calculate least count of vernier scale- Least count of Main scale/No of division in vernier scale. Here 0.1/50=0.002 cm, if we convert to mm, it will be 0.02mm.

Refer below picture for more understanding.

Least count of Micrometer

There are different types of micrometers like- outside micrometer (or screw gauge), digital micrometer, Vernier micrometer, etc. Least count of these micrometers is different, which is explained below.

Least count of Outside Micrometer= 0.01 mm (10 micron)

Least count of Vernier Micrometer= 0.001 mm (1 micron)

Least count of Digital Micrometer= 0.001 mm (1 micron)

Micrometer least count calculation

Before going direct on calculation of least count of micrometer, first understand it important parts like- Main scale division, main sub division and thimble scale.

Now for least count of any instrument we have to found smallest measurable unit of that instrument, then divide that unit by its numbers of divisions.

Here in micrometer, we have main scale which is divided into 2 parts- Main scale (Upside) and Main Sub division (Downside). On main
scale we can measure 1 mm and on rotation of thimble division by one round, 0.5mm measurement is completed on main sub division (It means I have to rotate thimble division by 2 round to complete 1 mm). Here 0.5 is least count of main scale and total divisions on thimble scale are 50. Then least count will be 0.5/50= 0.01 mm.

Refer below picture for more understanding.

Vernier micrometer least count calculation

To write measurement result of vernier caliper, we have to write in 3 decimal places. Because least count of vernier micrometer is 1 micron (0.001 mm).

In micrometer, the reading is accomplished by adding the whole millimeter, the half millimeter, and hundredth of millimeter just as we do in outsite micrometer (Simple micrometer). But in vernier micrometer, we
have to add the number of 1 thousandth of millimeter. See below picture for better understanding. 

Steel Scale /Measuring tape least count

Steel scale can measure 1 mm minimum. So least count of
steel scale is 1mm. Same least count is applicable for measuring tape.

Bevel protector Least count

Bevel protector least count is 5 minute (5’). Bevel
protector is used for angular measurement. Range of bevel protector is 360˚.

Bevel protector least count calculation

Formula for least count calculation of vernier bevel protector is division of least count of main scale division by total number of division on vernier scale). In bevel protector least count for main scale is 1˚ or 1 minute and total divisions on vernier scale are 12 (Left side & Right side). On converting calculation (1/12×60) in minute we get 5 minute.

For more understanding about least count of bevel protector refer below picture.

Least count of Dial Gauge

Least count of dial gauge is 0.01 mm. Because 1 revolution of large hand indicator is equal to 1 mm movement. It is used to measure small distance from range of 5mm to 10 mm approx. Dial gauge or Dial indicator is device used to measure angle, concentricity, flatness, straightness of any object. Dial gauge come with different types depends its application.

Dial gauge least count calculation

Least count calculation for dial gauge is simple. Here in dial gauge we see two scales. One is large scale contain 100 division. Small scale containing around 10 divisions. 1 division in small scale is equal to 1 mm. So one revolution of large scale is equal to 1 division movement of small scale. Formula for least count will be smallest measurement of small scale divide by total number of division on large scale. Here 1 mm divided by 100 comes 0.01mm. This is least count. Refer below picture for more understanding.

Least count of CMM & VMM

CMM stand for coordinate measuring machine. It work on pressure and its accuracy level is very high. As we know accuracy is inversely proportional to least count. Since accuracy high then least count will be very low. CMM can measure upto 1 micron. So least count is 0.001 mm.

Least count of profile projector

Profile projector least count is 0.001 mm ( 1 micron). This is same as CMM.

Least count of contour machine

As we know least count is lowest possible measurement of an instrument. There are various type of roughness tester which have different capacity and range of measurement. But minimum measurement of roughness is 0.5 micrometer by contour machine. So we can say least count of contour machine is 0.5µm. But 0.1 micrometer is also possible with high precision profile tester.

Difference between Vernier caliper and Micrometer

• Vernier caliper is used to measure large dimension measurement(Length, Height) range upto 150 ~300 mm,
  while Micrometer is used to measure small distance measurement (Dia, Thickness).
• Least count is different for both instruments.
• Micrometer is more precise than vernier caliper.
• Micrometer is costly compare to Vernier caliper

This was all about least count of measuring instruments and their calculation method and Formula. Hope you found this post informative and useful. If you like follow me on below social media platform and also share with friends. 

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