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By: Ikedi Ani-okoye.

Caliper explained

A caliper (British spelling also calliper) is a device used to measure the distance between two symmetrically opposing sides. A caliper can be as simple as a compass with inward or outward-facing points. The tips of the caliper are adjusted to fit across the points to be measured, the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as a ruler.

Why own a caliper

Why should you own one, you ask? Well, even though you might not be planning to rebuild an engine or transmission (where the caliper is an indispensable tool) you are surely going to run into situations wherein you need to make two parts fit tightly, measure between two holes, manufacture a part, etc. Calipers are amazingly useful.

When choosing one, try finding a model that uses a thumb-roll device to open and close the jaws. Also, models that have adjustable "zeroing" are preferable to those that do not. No matter what you have you can be assured that the caliper's accuracy is at least to the nearest .001 inch, which is quite sufficient for routine use.

Caliper with a dial

Calipers with a dial are much easier to read than the old dial-less slide vernier calipers. They eliminate the eye strain.  Always double check a caliper reading!  Vernier calipers with a dial can jump a setting.  Even better are digital calipers below.  

Dial vernier calipers should be stored in a dry, humid free location, and 
checked often for correct calibration. They can be wiped off with oil, but any 
build-up of oil will collect dirt.  Also the tips should be checked for wear.
Digital calipers are becoming more popular, probably because they have less tendency to skip gears.  Digital vernier calipers are very accurate and the batteries last a long time on most brand units.

CONCLUSION

The three-arm caliper provides a single continuous log of borehole diameter as determined by three mechanically coupled arms in contact with the borehole walls. Both standard (38mm) and long-arm models (60mm) are available to suit a wide range of well diameters. The standard caliper range can be increased using extension arms supplied with the probe. The caliper is a useful first log to determine the borehole condition before running more costly probes or those containing radioactive sources.