A typical counter drive train is shown in Figure 500-24. The counter drive train consists of a gear train, a rotary shaft seal or magnetic coupling, and a calibrator.
The gear ratio of the gear train is chosen to convert the fixed volume per revolution of the measuring element to some nominally convenient volume per revolution of the counter input shaft.
The rotary shaft seal is required where the counter drive train penetrates the meter internal housing or measuring chamber. It is normally designed as a module or gland for easy access. Alternately, a magnetic drive coupling can be used instead of a packing gland to eliminate the need for frequent servicing of a shaft seal.
A meter calibrator is a device containing gears for adjusting in fine (e.g., 0.05%) increments the output speed (RPM) from a meter counter drive train over a relatively narrow (e.g., 1.0%) total adjustment range. Use of a calibrator may produce error; therefore, it should be used only when necessary.
A calibrator is necessary whenever the mechanical counter on the meter must register actual volume throughput. If a meter factor (ratio of actual/registered volume) is to be applied to the registered volume (a common practice with pipeline and bulk marine custody transfer meters), nominal 100% meter gearing and no calibrator (or a “dummy” calibrator) would typically be used.
One or more pulse transmitters can be connected to the output shaft. The pulse transmitters are used to generate high and low resolution pulses to a remote receiver or computer that performs calculations and operates the metering system.