Vernier Caliper, Micrometer and other length measuring devices

 1 (d) Describe how to measure a variety of lengths with appropriate accuracy using tapes, rules, micrometers and calipers using a vernier as necessary.



In this part we are going to focus on how to measure different lengths, using the appropriate equipment. First, here are the most commonly used units for length

1km = 1000m

1m = 100cm 

1cm = 10mm

Back to the topic, the instrument's we need to know about are the following .....

 1) Vernier Caliper

Vernier caliper is used to measure short lengths i.e. 0 to 15 cm. It is accurate to 0.01 cm. A pair of vernier calipers consists of a Main Scale and a Vernier Scale as shown in the diagram below. The outside jaws, are used to measure the outer dimensions of an object and the inner jaws for the inner ones. The stem is used to measure the depth.

 

To measure , gently grip the object with the straight edges of outside or inside jaws.

  How to take reading

First read the main scale, and note down the reading before the 0 on the vernier scale, as shown in the diagram below. The reading on it is 2.8 cm, as the .8 after the 2 on the main scale is before the 0 on the vernier scale.

For the second place of decimal, look at the vernier scale. Find  a marking on the vernier scale that coincides exactly with the a reading on the main scale. In the diagram below, the 6 on the vernier scale coincides exactly with a line on the main scale (it does not matter with which line on the main scale this line coincides). So the second place of the decimal would 6, ie .06.

To get the total reading, add the two readings i-e 2.8+.06. The final reading is 2.86cm or 286mm.

 

How to avoid zero error

Before using vernier caliper you should close it to find whether a zero error exists or not

IF THE SCALES ARE LIKE THIS

Then there is no zero error

Else if:

• The zero on the vernier scale is on the right side of the zero (on main scale ) then a positive zero error exists . It should be subtracted from the final reading at the end i.e. final reading - positive zero error = accurate reading e.g. if the reading is 2.15cm and the error is of 0.02cm (positive zero error). Then the reading will be 2.15-0.02=2.13cm.

• The zero on the vernier scale is on the left side of the zero on main scale then a negative zero error exists to get an accurate reading then we will add the error in the final reading i.e final reading + negative zero error = accurate reading e.g. from above values 2.15+0.02=2.17.

2) Micrometer

Micrometer is used to measure very short readings i.e. 0 to 2.5cm. it has an accuracy of 0.001cm or .01 mm. Micrometer has two scales, main scale (on the sleeve) and the circular scale (on the thimble ). Each division on the main scale represents 1 mm.  According to o level's pattern each division on the thimble represents a distance of 0.01mm.

(Diagram above shows some main parts of the micrometer)

How to take reading

• Turn the thimble until the object is gripped gently.
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• Read the main scale on the sleeve. This reading would be in millimeters. In the diagram below, the reading is 5.5mm
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• Then read the line on the circular scale that coincides with the line on the main scale. In the diagram below, the 28th line on the circular scale coincides with the line. So, the reading would be 0.28mm.
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• Then add 5.5 with 0.28 and you will obtain your answer in millimeters.

How to avoid error

When the zero on the vernier scale i.e. circular scale coincides exactly the horizontal line than no error exist.

Else if the zero is above the horizontal line than a negative zero error exists and can be avoided by measuring the error and adding the error into the final reading.

While if the zero is below the horizontal line than a positive zero error exists and can be avoided by subtracting the error from the final reading.


 Note: The micrometer should never be tightened to much as it can damage it. it may also introduce errors so aviod over tightening the ratchet. a 'tic' sound is heard when micrometer has been tightened.

General formulas for errors (for vernier caliper and micrometer)

Accurate reading = Final reading plus Negative zero error

Accurate reading = Final reading minus Positive zero error.


3&4)  Meter rule and Measuring Tape

• A meter rule is used to measure medium lengths i.e. 0 to 1m. It has an accuracy of 0.1cm.
• A measuring tape is used to measure long lengths i.e.  0 to 100m . It has an accuracy of 0.1 cm

Reading both tape and rule is very simple and easy. Parallax error is the main type of error asked about in the exams. Parallax error is the error caused due to noting the reading from the side of the Tape or the Rule. The reading should always be taken by keeping your eye perpendicular and directly above the reading.