Thermocouple meters

Thermocouple meters convert applied current (a-c or d-c) by thermoelectric effects into deflection currents which register on a PM moving-coil couple actuated by heat generated at the junction formed by two strips of dissimilar metal. One strip carries the heat-generating input currents, the other carries heat-produced currents to the meter movement.

Thermocouple meters are used to measure a wide range of either a-c or d-c currents and give an accurate picture of the effective, or heats producting, value of a current no matter what its nature. The meters are relatively expensive and the thermal delay in converting input electricity to heat makes them sluggish.

A-C Indicating Meters

The moving iron-vane type is the most common a-c meter. In it induced eddy currents are used to produce magnetic force on a structure bearing a pivoted pointer and a thin iron element called a vane. The vane has no coil. The stationary magnetic field is produced by a single current-carrying coil surrounding both the fixed metal element and the pointer movement. This coil is so arranged that its own field induces a field in the moving vane and in addition generates attractive or repulsive magnetic forces with respect to its own self-produced magnetic field. Deflection is basically proportional to the current through the main coil.

Moving iron-vane meters usually have relatively low impedance and are simple and inexpensive. They measure either voltage or current, but their use must be restricted to the frequency for which they are designed.

Rectifier type meters utilize PM d-c movements actuated by current developed from rectifying the applied a-c being measured. Rectifier elements mounted within the meter case may be copper oxide, selenium, germanium, or silicon. The developed d-c is proportional to the applied a-c while the rectifiers and associated circuitry are designed for operation over as wide a band of frequency as possible.

Basic Electrical Measuring Instruments

1. In the field of electricity, as well as in all other physical sciences, accurate quantitative measurements are essential. This involves two important items — numbers and units. Simple arithmetic is used in most cases, and the units are well defined and easily understood. The standard units are those of current, voltage and resistance.

2.  The operator commonly works with ammeters, voltmeters and electron-tube analyzers; but he may also have many occasions to use watt meters, watt-hour meters, power-factor meters, synchroscopes, frequency meters and capacitance-resistance-inductance bridges.

3. Electrical equipment is designed to operate at certain efficiency levels. A good understanding of the functional design and operation of electrical instruments is important. In electrical service work one or more of the following methods are commonly used to determine if the circuits of an equipment are operating properly.

a) Use an ammeter to measure the amount of current flowing in a circuit.
b) Use a voltmeter to determine the voltage existing between two points.
c) Use an ohmmeter or megger to measure circuit continuity and total or partial circuit resistance.

4. It may also be necessary to employ a wattmeter to determine the to tal power being consumed by certain equipment. If we wish to measure the energy consumed by certain equipment or certain circuits, a watt-hour or kilowatt-hour meter is used.

5. For measuring other quantities such as power factor and frequency it is necessary to employ the appropriate instruments. In each case the instrument indicates the value of the quantity measured, and in this information helps to understand the way the circuit is operating.

6. Occasionally the operator will need to determine the value of a capacitor or an inductor. Inductance or capacitance bridges may be employed for this purpose.

7. A thorough understanding of the construction, operation and limitati ons of the basic types of electrical measuring instruments, coupled with the theory of circuit operation is most essential in servicing and maintaining electrical equipment.