A caliper is a widely used tool to measure the distance between two symmetrical points using two adjustable steel legs or jaws. After measuring with the steel jaws, the distance can be read using a ruler or whatever measuring device is attached to the caliper. There are many types of calipers.
- Venier caliper which is an upgrade of the traditional calipers (inside caliper, outside caliper, divider caliper and odd-leg caliper); uses a Venier scale
- Dial caliper which has a dial indicator for direct reading
- Digital caliper which has an electronic digital display and can switch between metric and inch units
- Micrometer caliper used for precision measurements
Calipers have many applications. They are used in engineering, woodworking and metalworking. As such, they come in different sizes like 4", 6", 8", and 12" up to 80". There is also what is known as a brake caliper where the brake pads, rotors and the pistons are mounted between steel jaws.
A digital caliper, like an electronic weighing scale, may lose its accuracy with use. Once this happens, the caliper needs to be calibrated. You can either try to calibrate the device yourself or send it for professional calibration. This can be done by the manufacturer. Caliper makers Starrett, Mitutoyo, Brown&Sharpe all offer calibration services.
Close the jaws as tight as possible. Set the caliper to zero. Open the jaws as far as you can then close them again tightly. The display should read zero. At the very least, the difference between the two readings should be a single unit of the smallest measurement. For example, a ".001" difference is acceptable for calipers that can distinguish this amount. If the reading is greater, try to calibrate the caliper.
- Using a soft lint-free cloth, wipe the caliper clean. For the device to function properly the beams should be dry. Remove all the visible dirt and oil from the caliper.
- If the caliper is new, it sometimes comes with an instruction to clean the caliper with an oil-soaked rag before use. This is because the caliper has been coated with grease to protect it while it is being stored and shipped. Simply spray some WD40 on a soft cloth and wipe the jaws and the beam. Next, using a dry cloth, wipe off the WD40.
- Calibrate the outside jaws using certified gauge blocks. These are metal blocks that have a specific measurement or size. Gauge blocks are routinely used for checking the accuracy of the caliper. Insert the gauge blocks between the outside jaws then record the caliper readings. Do this several times per block. If the measurement returned by the digital caliper is different from the actual size of the block, the caliper should be calibrated. The accuracy may depend on the model of the caliper so check the manufacturer's specification. Use several gauge blocks.
- Next, calibrate the inside jaws by using a set of certified ring gauges. You can also use a micrometer for this purpose. Take several measurements as well.
- Test for repeatability. This simply means that the caliper should return the same results each time you measure the same gauge block or ring. If not, it's time to have the caliper serviced by the manufacturer.
If you use the caliper frequently, make sure to check it for accuracy as often as possible. Compared to other tools, digital calipers are more prone to damage. When in doubt, it is best to seek professional calibration than to get inaccurate results.