If the ambient temperature rises, the resistance of RT-1 decreases. This shunts more current around the bridge network and allows RT-2 to cool. The resistance of RT-3 decreases, maintaining meter sensitivity independent of temperature changes. Cavity Z-1 in view B of figure 2-4 is an ABSORPTION-TYPE FREQUENCY METER. This type of meter will be discussed later.

FREQUENCY MEASUREMENTS

Frequency measurements are an essential part of preventive and corrective maintenance for electric and electronic equipment. Some examples of the various frequency measurements follow:

• Rotation frequencies of some electro-mechanical devices, such as electric motors, must be determined.
• The output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines.
• Equipment that operates in the af range must be adjusted to operate at the correct frequencies.
• Radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interference with radio circuits operating on other frequencies.
• Radar sets must be properly tuned to obtain satisfactory performance.

As you can see from the above examples, frequency measurement does indeed play a valuable role in maintenance. These measurements can be divided into two broad categories: MECHANICAL-ROTATION FREQUENCY measurement and ELECTRICAL-OUTPUT FREQUENCY measurement. Depending upon your job and/or the type of command to which you are assigned, you may be tasked with performing one or both of these types of measurements.

MECHANICAL-ROTATION FREQUENCY MEASUREMENT

The rotating frequency (speed in revolutions per minute) of armatures in electric motors and engine-driven generators, as well as the blade speed in turbines, is measured with devices called TACHOMETERS, STROBOSCOPES, and STROBOTACS.

Tachometer

A tachometer is an instrument that measures the rate at which a shaft is turning. Although tachometers are installed on machinery, such as generators and engines, you may need to determine the speed of a rotating machine that is not equipped with a tachometer. In these instances, you will be required to use a PORTABLE TACHOMETER. Portable hand-held tachometers measure speed by direct contact with the shaft of the measured unit. Portable tachometers are for use only during testing and should not be used continuously. The common types of portable tachometers are the CENTRIFUGAL and the CHRONOMETRIC.

CENTRIFUGAL TACHOMETER. - A centrifugal-type tachometer is illustrated in figure 2-5, view A. View B shows the internal arrangement of the centrifugal tachometer; refer to view B in this discussion. In the centrifugal tachometer, centrifugal force acts upon fly weights that are connected by links to upper and lower collars. The upper collar is affixed to a drive shaft; the lower collar is free to move up and down the shaft. A spring, which fits over the shaft, connects the upper and lower collars.

Figure 2-5. - Centrifugal tachometer.

Each portable centrifugal tachometer has a small rubber-covered wheel and a number of hard rubber tips. You fit the appropriate tip or wheel on the end of the tachometer drive shaft, and hold it against the shaft to measure speed of rotation. As the drive shaft begins to rotate, the fly weights rotate with it. Centrifugal force tends to pull the fly weights away from the center, causing the lower collar to rise and compress the spring. The lower collar is attached to a pointer, and its upward motion, restricted by the spring tension, causes an increase in the indication on the dial face.

When properly used, a centrifugal tachometer will indicate correct shaft speed as long as it is in contact with the machine shaft under test. A portable centrifugal tachometer has three ranges: low (50 to 500 rpm), medium (500 to 5,000 rpm), and high (5,000 to 50,000 rpm).

CHRONOMETRIC TACHOMETER. - The chronometric tachometer (figure 2-6) is a combination watch and revolution counter. It measures the average number of revolutions of a shaft per minute. The chronometric tachometer also comes with hard rubber tips, which must be inserted over the drive shaft.

Figure 2-6. - Chronometric tachometer.

When applied to a rotating shaft, the outer drive shaft of this tachometer runs free until a starting button is depressed to start the timing element. In figure 2-6, note the starting button beneath the index finger. The chronometric tachometer retains readings on its dial after its drive shaft has been disengaged from a rotating shaft and until the pointers are returned to 0 by the reset button (usually the starting button). The range of a chronometric tachometer is usually from 0 to 10,000 rpm and from 0 to 3,000 feet per minute (fpm).

Stroboscope

The rotation frequencies of recording devices and teletypewriter motors can be measured by the use of a STROBOSCOPE. The stroboscope is an instrument that allows you to view rotating or reciprocating objects intermittently and produces the optical effect of a slowing down or stopping motion. For example, electric fan blades revolving at 1,800 rpm will appear stationary if you look at them under a light that flashes uniformly 1,800 times per minute. At 1,799 flashes per minute, the blades will appear to rotate forward at 1 rpm; at 1,801 flashes per minute, they will appear to rotate backward at 1 rpm.

When the flashing rate of the light is adjustable, you can calibrate the control in flashes (or revolutions) per minute. The stationary image you see when the rate of the lamp and the rotational rate of a shaft are equal lets you record a very precise speed measurement.