A comparator is an instrument used to obtain precise optical measurements of small characteristics, dents and scratches on flat surfaces. The instrument is placed on top of the region to be measured, the appropriate scale is selected (e.g. linear, circular or radius) and the measurement is read off of the display screen. Calibration of this device must be carried out for each of the available scales. These require different calibration processes.

Calibration

Proper calibration of the linear scale setting should result in a range of between 0.0 and 0.8 inches and an accuracy of plus or minus 0.005 inches. Proper calibration of the circular scale setting should result in a range of between 0.0 and 0.25 inches and an accuracy of plus or minus 0.0025 inches. Finally, proper calibration of the radius scale setting should result in a range of between 0.0625 and 0.375 inches and a range of plus or minus 0.0025 inches.

Linear Scale

First, modify the worktable micrometer so that the first major division of the device’s analog scale coincides with the vertical line on the comparator’s screen. Next, compare the device’s analog reading with the digital readout on the comparator. Modify the worktable micrometer until the subsequent division coincides with the vertical line and the comparator readings.

Circular Scale

Modify the worktable of the comparator so that the left side of the inside diameter of the circle being measured coincides with the vertical line on the comparator screen. Using the micrometer head found on the worktable, alter the table so that the right hand side of the circle coincides with the vertical line on the screen. Compare the device's analog reading with the digital readout on the comparator. If the readings match, the circular scale calibration process is complete.

Radius Scale

Modify the comparator’s worktable so that the radius of the analog instrument coincides with the reference line on the digital readout of the comparator. Set the digital readout to zero. Modify the micrometer head so that the right side of the radius coincides with the reference line on the digital readout of the comparator. Compare the device’s analog readout with the digital readout on the comparator. If they match, the radius scale calibration process has been performed successfully.