Apparatus for precision calibration of skinfold calipers

Despite the fact that skinfold calipers are widely used to measure subcutaneous adipose tissue, the current methods of calibration are quite crude. Methods such as hanging masses from the caliper jaws until they remain open, lack validity and reliability because the caliper jaws are stationary instead of dynamic, and the opening jaws give an upscale reading of the jaw gap not a downscale reading that occurs when the calipers are being used to measure skinfolds. This report describes how to build an apparatus capable of measuring static and dynamic, upscale and downscale jaw pressures of a variety of caliper types and also provides guidelines specifically for calibration and servicing of Harpenden calipers. The key areas to maintain are the caliper springs (which should have spring coefficients ranging 1.104-1.153 N.mm^-1 ), the pivot joint (which should operate smoothly), the indicator mechanism (which should require only 0.78-0.88 N for full movement), and the jaw alignment which should be square to ensure that the full effective jaw surface area of 90 mm² is applied to the skinfold. While there are insufficient data at this time to prescribe rigid calibration criteria, assessment of approximately 100 new and used Harpenden calipers indicates that, after servicing, the important dynamic downscale jaw pressure will range 7.7-8.4 g.mm^-2 at 5 mm of jaw gap and 7.3-8.0 g.mm^-2 at 40 mm of jaw gap. Dynamic upscale jaw pressure should be within 1.0-1.5 g.mm^-2 of the corresponding dynamic downscale jaw pressure. Am. J. Hum. Biol. 10:689-697, 1998. © 1998 Wiley-Liss, Inc.