Despite their many "faces," dial indicators and test indicator dials tend to look pretty similar in that they all have graduations and numbers. This similarity is especially true of dial indicators that have balanced dials. Such a dial might have "0" at the top and a number on the bottom—maybe a "10." On both the right and left could be the number "5." These dial indicators are generally used for comparison measurements, and when the indicator hand moves clockwise or counterclockwise, it's counting 0-5-10. Both the test and dial indicators are virtually the same.
Most dial indicators are used in an upright position (usually in a height stand), and as the contact point is pushed in, the part is getting larger. Therefore, the dial indicator will have a plus sign (+) on the right, and a minus (-) sign on the left.
Dial indicators can also be used to measure long travel, such as on a machine table or mechanical slide. When these indicators are used for displaying position on a machine table, they tend to be placed in front and on the right side of the table. This is because the indicator is being used to measure the X and Y coordinates of the table, and the indicators have to follow those coordinates. When the table gets closer to the indicator, the clockwise movement caused by the inward movement of the indicator displays a plus value.
However, this is not to say that the movement of an indicator can't be changed to go in the opposite direction or that the minus sign might not be placed on the right. Exceptions like these are generally made for gages where an outward movement indicates a plus value, as in a mechanical fixed plug gage. In these indicators, as the spindle is pushed in, the part is actually getting smaller.
Test indicators are often without this important indication. This is because of the versatility they offer and the way they are used. Test indicators have long probes (contacts) that extend away from the indicator and allow it to get into grooves and measure on the tops and bottoms of lands. Because of this versatility, and depending on the way the test indicator is positioned, upward movement of the probe may sometimes mean plus, or it may mean minus. To reduce the potential for misreading these indicators, the plus and minus signs are left off. However, this really puts the weight of dial interpretation on the user.
To complicate matters even more, on some test indicators the dial is not lined up with the contact point movement, and the operator has to take care to properly interpret these values. These indicator dials may be mounted on the top or even the side. The type of measurement to be made and the ability to see the indicator face determine the choice of indicator style.
All of these test indicator choices share one common difference from a dial indicator face. While they all may have a balanced dial, they are all missing the plus and minus sign, and both direction and size interpretation must be determined by the user.
So the next time you pick up a test indicator and notice that the dial looks different from a dial indicator, take it as a reminder that you need to understand the way the test indicator will be used, and add the results up for yourself.