Micrometer Screw Gauge

Among the tools that are used for measurement of length, an  instrument that is more precise than an ordinary Vernier Caliper is Micrometer Screw Gauge. It can measure upto a precision of 0.01 mm or in other words 0.001 cm. However its range is restricted to a few centimeters only, which is why it can only be used for fine measurements of very small objects. Due to its design only external diameters be measured, which is another disadvantage compared to a vernier caliper.   

 Parts with purpose:

How to take a reading through an error free Screw Gauge:

Step 1: Note the Main Scale Reading (MSR):

Take that reading on the Main Scale (the sleeve). It is noted in mm. Its value is noted as a whole number as a long line or on top of what is known as the 'datum line'; while as 0.5 mm smaller or below the 'datum line'.

Vernier Caliper - Zero Errors and how to overcome them

Zero Error:

 When zero of the vernier scale does not co-inside with zero of the main scale.

 
No Zero Error:

Positive Zero Error:

When zero of the vernier scale is ahead of zero of the main scale.

        

As the 3rd vernier division is co-insiding with main scale, hence zero error is 

 ZERO ERROR = VERNIER SCALE DIVISION x LEAST COUNT

Z.E = 3 x 0.01 cm

      = 0.03 cm 

Negative Zero Error:

When zero of the vernier scale is behind the zero of the main scale.

 Although the 4th vernier division is co-insiding with main scale, zero error is counted from the backward direction i.e -6

  ZERO ERROR = VERNIER SCALE DIVISION x LEAST COUNT

Z.E = -6 x 0.01 cm

      = -0.06 cm  

 

Overcoming a zero error 

Apply:

 Actual (Corrected) Reading = Total (Observed) Reading  - Zero Error