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.

All measuring instruments with Least count

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

Vernier caliper types and Least count

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 calculation of vernier caliper

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 Types and  Least count

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.

Least count calculation for Micrometer

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. 

Vernier caliper least count calculation

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.

Bevel Protector Least count calculation

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.

Dial gauge with  least count calculation

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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