DC vs. Pulse Calipers
Resistive or "dc calipers" are still in use for open hole / mineral logging, and to a lesser extent for cased hole logging. The Dowell / Worth Well / Bell solenoid release caliper is a classic example of a resistive dc caliper. The Dowell calipers were made in 2-1/2 inch and 3-1/2 inch diameter body sizes, and offered interchangeable arms for maximum logging diameters of 12, 24, 36, 48, and 60 inches. The 1-1/4 inch Well Reconnaissance, Inc caliper was originally manufactured as a resistive caliper, with early pulse circuits for said tool being little more than an afterthought. Depending on how dc calipers are configured, they can be subject to some drift; the ground path should preferably not be the armor of the cable. Further, in deep or geothermal hot environments, the resistance change of the logging cable copper conductors can actually become an issue.
Pulse calipers began to appear in the 1950's. A pulse caliper produces a pulse train with a frequency proportional to arm extension. Early pulse calipers used several novel schemes to vary the frequency of vacuum tube based oscillators of the time, but most modern pulse calipers simply use a pot actuated by the movable arms to vary the voltage fed to a voltage-to-frequency circuit of some type. Pulse calipers typically have better resolution than dc calipers, but that is not necessarily true. Pulse calipers are immune to variations in line resistance, so may offer advantages in geothermal or deep logging, though it should be noted that Dowell dc calipers have been used to depths of 23,000 feet. All things considered, we prefer pulse calipers for cased hole type work.