Pocket Comparator Calibration


The purpose of this procedure is to provide general instructions for the calibration of pocket comparators.


This general procedure shall be used in the absence of any specific calibration procedure for pocket comparators


The pocket comparator is a small precision optical measuring instrument used for measuring small characteristics and/or dents, scratches, etc. on flat surfaces. The instrument is equipped with an etched linear scale, angular scale, and various hole sizes. The instrument is placed over the characteristic to be measured, the appropriate scale selected and the size is read directly from the etched scale. The instrument employs a magnification factor for ease of reading the various scales.

Preliminary Instructions and Notes

  1. Read this entire procedure before beginning the calibration.
  2. Calibration shall be performed in an environment that conforms to Manufacturer Specifications.
  3. The pocket comparator will hereafter be referred to as the Instrument Under Test (IUT).
  4. Verify that the IUT is clean.
  5. Visually examine the IUT for any condition that could cause errors in the calibration.
  6. Thoroughly clean the lens of the IUT with a soft lens tissue.
  7. If any of the requirements cannot be met, refer to the applicable manufacturer manual.
  8. If a malfunction occurs or a defect is observed while calibration is in progress, the calibration shall be discontinued and necessary corrective action taken; if corrective action affects a measurement function previously calibrated, the function shall be recalibrated before the remainder of the procedure implemented.

Reference Material

Applicable Manufacturers Manual or Brochures.


The specifications of the IUT are determined by the applicable manufacturer's documentation. If the manufacturer's documentation is not available, then the specifications identified in this procedure are used.


Scale Range Accuracy
Linear 0.000 to 0.800 inch +/- 0.005 inch
Circular 0.000 to 0.250 inch +/- 0.0025 inch
Angular 0 to 90 degrees Reference Only
Radius 0.0625 to 0.3750 inch +/- 0.0025 inch

Equipment Required

The Standards listed below should be selected on the basis of their higher accuracy level when compared to the IUT. Equivalent Standards must be equal to or better than the Minimum-Use-Specification.

Minimum-Use-Specifications for Standards listed are 1/4 the accuracy required by the IUT.

Required Equipment

Standards Used Minimum-Use-Specifications
Optical Comparator Accuracy: +/- 0.0001 Inch

Detailed Procedure

  1. Place the Radius Chart on the Optical Comparator. This chart will be used for reference only.
  2. Remove the engraved scale from the body of the IUT.
  3. Mount the scale in a V-Block then mount the V-Block on the working table of the Optical Comparator.
  4. Set Comparator magnification switch to 20 and set the horizontal switch to the horizontal position. Adjust light intensity and focus control for best viewing.
  5. Rotate the IUT as required until the scale of the IUT coincides with the vertical line on the Comparator screen. Zero the Comparator electronic readout.
Linear Scale Calibration
  1. Adjust the worktable micrometer until the first major division of the IUT scale coincides with the vertical line on the Comparator screen.
  2. Compare the IUT reading with digital readout of the Comparator.
  3. Adjust worktable micrometer until next division coincides with the vertical line and compare readings.
  4. Repeat step 8 for the remaining major divisions on the IUT.
Circular Scale Calibration
  1. Adjust the worktable of the Comparator until the left hand side of the inside diameter of the circle to be measured is in coincidence with the vertical line of the Comparator screen.
  2. Utilizing the micrometer head on the worktable, move the table until the right hand side of the circle is in coincidence with the vertical line of the screen. Compare IUT reading with digital readout of the Comparator.
  3. Repeat steps 10 and 11 for the remaining circles on the IUT.
Radius Scale Calibration
  1. Adjust the Comparator worktable until the radius of the IUT coincides with the reference line on the Comparator. Zero the digital readout.
  2. Adjust the micrometer head until the right side of the radius coincides with the reference line on the Comparator. Compare IUT readout with the digital readout on the Comparator.
  3. Repeat steps 13 and 14 for the remaining radii on the IUT.

Reason for Reissue

Revision A - First Release