Figure 11-23.—Revolution counter.
It magnetically opens or closes the reed switch as the
float passes over it. Figure 11-22 shows the construction
of the magnetically operated float switch. Magnetic
float switches may be constructed with more than one
float on a stem. Magnetic float switches can be installed
to detect multiple levels in the same tank; and this type
of switch can activate a high- and low-level alarm.
Figure 11-24.—stroboscope tachometer.
REVOLUTION COUNTERS AND
INDICATORS
Measurements of rotational speed are necessary for
the proper operation of pumps, forced-draft blowers,
main engines, and other components of the engineering
plants. Various types of instruments are used to measure
equipment revolutions per minute (rpm) and count the
number of revolutions a shaft makes.
PROPELLER INDICATORS
Propeller indicators are mounted on the throttle
board. They indicate the speed and direction of rotation
of the propulsion shaft or shafts. They also record the
number of revolutions the propulsion shaft has made.
The speed of rotation is important because it is related
to the ship’s speed. The total number of revolutions is
used to determine the total distance traveled by the ship.
A typical revolution counter is shown in figure 11-23.
OTHER REVOLUTION INDICATORS
Equipment speed is determined by either
permanently installed mechanical or electrical
tachometers or by portable tachometers. Portable
tachometers are hand-held, mechanical types. They
require access to the end of the rotating machinery shaft.
Another type of tachometer is the stroboscope
tachometer (fig. 11-24). This device allows rotating
machinery to be viewed intermittently, under flashing
light, so that the rotation appears to stop.
WARNING
If you use a stroboscopic tachometer,
NEVER reach into the rotating machinery.
Although the machinery appears to be stopped,
it is still rotating.
11-13
