Telemetering is the indicating or recording of a quantity at a distant point. Telemetering is employed in power measurements to show at a central point the power loads at a number of distant stations and often to indicate total power on a single meter, but practically any electrical quantity which is measured can be transmitted, together with a large number of nonelectrical quantities such as levels, positions, and pressures.
Telemetering systems may be classified by type: current, voltage, frequency, position, and impulse.
1. In current systems, the movement of the primary measuring element calls for a current in the attached control member to balance the torque created by the quantity measured. This balancing current (usually dc) is sent over the transmitting circuit to be indicated and recorded.
Totalizing is possible by the addition of such currents from several sources in a common indicator. The receiver may be as much as 50 mi from the transmitter.
2. In voltage systems, a voltage balance may be produced through a control-member voltmeter, or a voltage may be generated by thermocouples heated by the quantity to be measured, or produced as an IR drop as a result of a current torque balance, or generated by a generator driven at a speed proportional to the measured quantity.
These voltages, however produced, are recorded at a distance by a potentiometer recorder. Here, also, the recorder may be 50 mi from the transmitter.
3. A variable frequency may be produced for telemetering by causing the primary element to move a capacitor plate in an rf oscillator or to change the speed of a small dc motor driving an alternator. High-frequency systems cannot be used for transmission over many miles.
4. In position systems, the movement of the primary element or of a pilot controlled by the primary element is duplicated at a distance. The pilot may be a bridge balancing resistance or reactance, a variable mutual inductance, or a selsyn motor where the position of a rotor relative to a 3-phase stator is reproduced at the receiver end.
Satisfactory operation is usually limited to a few miles.
5. The impulse type of transmission of measured quantities is represented by the largest number of devices. The number of impulses transmitted in a given time may represent the magnitude of the quantity being measured, and these may be integrated by a notching device or by a clutch, or the duration of the pulse may be governed by the primary element and interpreted at the receiver.
If the impulses are transmitted at high frequency, inductance and capacitance effects in the transmitting line limit the distance of satisfactory transmission; systems using dc impulses operate over 50 to 250 mi.