Dimensional indicators and comparators are instruments for comparative measurements where the linear movement of a precision spindle is amplified and displayed on a dial or digital display. Indicators utilize electronic, mechanical or pneumatic technology in the amplification process; e.g., dial indicators, digital indicators and electronic amplifiers or columns. These gaging amplifiers or instruments are available in three main types:

1) Comparators or high precision amplifiers (including columns or electronic amplifiers).
2) Indicators (higher precision compared to test indicators, used for inspection).
3) Test indicators (lowest precision, widely applied in production checking).

Different Kinds of Comparators

    
Mechanical comparators, electronic comparators, or appliers and pneumatic or air comparators are gaging devices for comparative measurements where the linear movement of a precision spindle is amplified and displayed on a dial / analog amplifier, column, or digital display. Mechanical comparators have sophisticated, low friction mechanism, better discrimination (~0.00001"), and lower range (~+/- 0.0005") compared to compared to indicators. Comparators have a higher level of precision and less span error compared to conventional dial or digital indicators. The level or precision is sufficient for measurement of high precision ground parts and for the calibration of other gages.

What are Indicators?

   
Indicators are gaging devices for comparative measurements where the linear movement of a spindle or plunger is amplified and displayed on a dial, column or digital display. Typically, indicators have a lower discrimination (~0.001" to 0.0001") and greater range (~+/- 1.000" to +/- 0.050" total) compared to comparators. The level or precision is sufficient for final part inspection.

Video Credit: Starrett / CC BY 3.0

Test Indicators

    
Test indicators have the lowest discrimination when compared with indicators and comparators. They are used mainly for set up and comparative production part checking. Test indicators often use a cantilevered stylus or level style probe that facilitates inspection of hard to reach part features, but results in high cosine errors. A cosine error of 0.0006" may result over a travel range of 0.010". Test indicators are not considered absolute measuring instruments, but comparative tools for checking components against a standard or zeroing-out setups.

Other devices that fall within the category of indicators and comparators include gage sets, gaging stations and gaging systems.

Related Standards

• A-A-2348—Dial indicators and test set accessories
• A-A-59006—Sight flow indicators for pipelines
• BS EN ISO 8503-1—Assessment of steel substrates using profile comparators

Image credits:

Inspec Inc. | KEYENCE