Calipers range in price from a few dollars to $50 or more, depending on the material, accuracy, and readout type you want. On the cheap end, Lee Valley sells a ½ dozen plastic vernier style for $8. However, they also sell a top end, machined steel digital unit for $50. In this case, you do get what you pay for. The plastic ones are great to have around the shop for checking nuts and bolts, dowel sizes, or doing rough measures. The more precise instruments are more suited to our purposes as modellers.

There are three styles of caliper, named for the way they give a reading. Vernier calipers rely on a scale etched into the body, with good eyesight required to determine the actual measurement from an increasingly smaller set of lines. Dial calipers have a stopwatch-like face that indicates the measurement using a pointer. Digital calipers have the advantage of giving a direct reading in numeric form; some convert between millimetres, and inches (decimal and/or fractional) at the push of a button.

Most calipers are accurate to 1/1000^th (0.001) of an inch, or 1/128^th when using fractional measurements, and 0.01mm (0.00001m or 10 microns) in metric. Note that micrometers are 10 times more accurate, but this is rarely needed unless you are machining your own live steam engine parts.

There are four basic measurements that can be taken with a caliper: inside, outside, depth, and off-set or step. The geometry of the caliper also makes it possible to transfer measurements, because on most calipers, these four lengths are always equal. See references and attached Lee Valley information for all four measures and parts of the caliper.

Measurement technique

Obtaining accurate, consistent readings depends as on proper technique even more than the quality of the measuring tool. Here are some pointers:

   Make sure the measuring jaws are clean and free from dust or grit.
   Check the zero setting periodically, especially if you are working to close tolerances
   Use the thin tips of the jaws for most measurements. The wider portion of the jaws is useful for measuring discontinuous surfaces such as drill bits or screw diameters.
   Use the thumbwheel (if present) or light pressure to gently advance the jaws until they close on the work. Use only enough pressure to ensure a snug reading.
   Use the locking screw to keep the reading from changing as the jaws are withdrawn from the work (when transferring measurements). Lift the jaws straight off of the work being careful to keep the caliper perpendicular to the workpiece. Twisting the caliper slightly to either side will change the reading.
   When measuring inside diameters, rotate the caliper slowly around a small arc while applying gentle pressure to the thumbscrew.  This will enable the jaws to settle in to the true diameter of the workpiece. Watch the readout and stop when it reaches a maximum reading.

Using the caliper without turning it on…

One of the biggest sources of error in building occurs when a measurement is taken and then recreated at a later time. Locking the head of the caliper at a given measure, and transferring it to another site, rather than re-measuring is the simplest way to avoid this discrepancy. Remember that all four measurements (above) are always equal, allowing transfer between types of measurements as well. E.g. Take the diameter of a dowel using the outside measure, then verify that the receiving hole is big enough using the inside measure.

Care & Feeding

The more expensive calipers are precision instrument
s. It is important to keep them clean & dry, not subject them to shock, and not use their straight edges for any cutting, tempting as it may be in the moment! Keep them in their cases, and keep the case in a place where other tools and projects will not be piled on top or there is a chance of knocking it to the floor.

Additional information

http://www.mini-lathe.com/Mini_lathe/Reviews/HF_digital_caliper/caliper.htm

Using a Caliper – Lee Valley Tools

Go to http://www.leevalley.com/en/home/Articles.aspx?p=32  and use the link to "Measuring".  This section also has a number of additional items of use to modellers.