Instrument Calibration

 As previously discussed the principle of measurement is comparing a component against a known standard therefore any equipment used must be calibrated against one of these standards.  The standard used to calibrate any equipment should be at least four times more accurate than the equipment being calibrated.  The standards used are:-  Line (one point to another on a metre bar)  Light (distance travelled by Helium Neon beam in one second)  End (distance from one end to the other on a Gauge Block

 Gauge blocks are calibrated for length and parallelism by using the beam of light from a Helium-Neon light source (Interferometer)  The Gauge block is wrung onto a perfectly flat surface plate and the beam of light directed vertically on to the top surface of the block.  The reflection of the beam from the surface is measured by the Interferometer and the accurate size of the gauge block calculated from the results, the parallelism of the block is also checked at the same time.

 Gauge blocks are used to calibrate micrometers.  The anvil faces are cleaned and the micrometer is wound forward until the anvils touch check that it reads zero, if not use spanner to rotate barrel to line up datum line with zero.  Measure different size gauge blocks, to check micrometer readings at different intervals along its scale.  The recommended gauge blocks for a 0-25mm micrometer are shown opposite.

 The depth micrometer is also calibrated using gauge blocks.  The faces must be cleaned and it must be set to zero on face plate for 0-25mm micrometers or if longer rods are used set to zero, 25mm, 50mm etc, using correct size gauge blocks.  The micrometer is then tested in the same way as the external micrometer except it is seated on top of the gauge blocks as shown

 Gauge blocks are also used for calibrating the vernier calliper.  The jaws are cleaned and the gauge block is placed between the external jaws to check it is reading correctly.  It is important to place the gauge block in the correct position to ensure the reading is correct, and also to ensure it is square and parallel to the jaws.  The diagram opposite demonstrates the correct method and what can happen if the wrong method is used.

 The internal jaws are calibrated using gauge blocks which are held in a gauge block holder as shown, as with the micrometer it is tested across the range with different size blocks.  The calliper must be parallel to the gauge blocks to give accurate reading.  The internal jaws can also be checked/calibrated using a micrometer as shown, however the micrometer itself would need to be calibrated first to ensure accuracy