"The 521 series Industrial Electronic Counters count periodic and random events and display the total in a digital readout. When coupled to proper tranducers these counters can also measure speed, rpm, weight, pressure, temperature, acceleration, or any other quantity converted into appropriate electrical impulses. In addition the counters can measure the time between the closing and opening of either a front-panel switch or a pair of external contacts." (hp, Manual 521C, 1959-1964)

BRIEF SPECS

The 521C is one of the series of compact counters hp launched for industrial use. They direct compete against the according Beckman/Berkeley devices (like the 5210 or 5230). It has a direct phototube input for rpm or similar measurements. An additional connector supplies some B+ and heater voltage for external sensor circuitry. It took me long time to find a tube counter. As counters with semiconductors were extreme smaller and used less power, the tube counters were at the first trashed tube instruments in many labs.