Jun
15
A refinement now popular is the replacement of the analog dial with an electronic digital display on which the reading is displayed as a single value. A good digital caliper can be switched
between centimeters or millimeters, and inches. All provide for zeroing the display at any point along the slide, allowing the same sort of differential measurements as with the dial caliper. Digital calipers may contain some sort of “reading hold” feature, allowing the reading of dimensions even in awkward locations where the display cannot be seen. Ordinary 6-in/150-mm digital calipers are made of stainless steel, have a rated accuracy of .001″ (.02mm) and resolution of .0005″ (.01mm). The same technology is used to make longer 8-in and 12-in calipers; the accuracy for bigger measurements declines to .001″ (.03mm) for 100-200mm and .0015″ (.04mm) for 200-300mm.
Many Chinese-made digital calipers are available very cheaply, and perform reasonably well. One point worth noting is battery current and switching. Many calipers do not stop drawing power when the switch is in the off position; they shut down the display but continue drawing nearly as much current. The current may be as much as 20 microamperes, much higher than for many established brands. Sometimes calipers may not work properly when the battery voltage has dropped relatively little; silver cells, preferably selected from a datasheet to have a constant voltage for most of their life, may give a much longer usable life than alkaline button cells (e.g., SR44 instead of LR44).
Increasingly, digital calipers offer a serial data output to allow them to be interfaced with a dedicated recorder or a personal computer. The digital interface significantly decreases the time to make and record a series of measurements, and it also improves the reliability of the records. A suitable device to convert the serial data output to common computer interfaces such as RS-232, Universal Serial Bus, or wireless can be built or purchased. With such a converter, measurements can be directly entered into a spreadsheet, a Statistical Process Control program, or similar software.