Which of the following is NOT a common type of portable tachometer?

The correct response identifies a type of tachometer that does not fall into the category of commonly used portable tachometers. Portable tachometers are typically designed for ease of use and quick measurement of rotational speed.

Centrifugal tachometers rely on centrifugal force to measure speed, and are based on mechanical principles, making them a familiar choice in many applications. Chronometric tachometers use a time-based measurement, employing springs and gears to track speed over time, which is easily portable and practical for various uses.

Optical tachometers measure the speed of a rotating object by analyzing the light reflected from it, providing quick and accurate readings, which reinforces their acceptance as a portable option.

In contrast, electronic tachometers, while widely used and highly accurate, are not typically classified as "portable" in the same context as the other types listed. They often require more extensive setups, including power supplies and can be less convenient for quick, on-the-spot measurements. This distinction leads to the conclusion that electronic tachometers are not recognized exclusively as portable devices compared to the other types mentioned.