In terms of long term accuracy, linearity, and ease of calibration, Models 120 tachometers have no peers. Their interference-free dc system makes shielded cable unnecessary so long to withstand difficult environments (chemicals, water, etc.) runs are possible at low cost with conventional wiring techniques. While magnetos operate reliably at armature shaft speeds up to 5,000 RPM, it should be noted that their performance at low speeds far exceeds that of photocells, magnetic pickups and other pulse devices.

The magnetos, digital & analog (dial) indicators, and accessories cover most industrial requirements. However, many modifications are regularly engineered for specific applications. The Type M-1200 B Magneto is the most popular. The choice of indicators depends largely on personal preference as to type of readout, physical size, scale length or readability, and possibly the need for illumination and explosion-proof or water-tight cases.

All M-1200 Magnetos have the same internal construction and electrical characteristics. They differ only in mounting arrangements, speed requirements, and methods of coupling to the speed source (via spur gearing, couplings, friction wheels, etc.). Each magneto generates a dc voltage exactly proportional to speed. It electrically connects to from one to ten analog or digital indicators calibrated in any desired speed function such as RPM, feet per second, units per hour, etc. Magnetos also readily adapt to process control systems.

Wherever possible, the magneto should be geared so that its armature speed is at least 200 RPM for full scale reading on standard indicators; at least 20 RPM for full scale with highly damped indicators. Models with integral gearboxes allow for input shaft speeds for full scale reading that are as high as 30,000 RPM, or as low as ½ RPM. Although the required drive is always based on the speed the armature turns for full scale on the indicator, the indicator will be accurate over the entire range from zero to full scale. Thus with a ½ RPM input shaft speed for full scale, an indicator marked 0 to 50 Inches Per Minute will read properly at 1 IPM even though the input shaft only turns at .01 RPM for this reading.

For longest life under continuous running conditions, armature speed should be kept under 2000 RPM although the magnetos give excellent service at speeds up to 5000 RPM.

M-1200 magnetos are individually run-in and aged to insure long term stability, neutralized to generate the same voltage for either rotation, and adjusted with a magnetic shunt to generate a standard voltage for easy interchangeability.