CHAPTER 11
INSTRUMENTS
As a watch stander, you observe or monitor
operating equipment and take the necessary steps to
detect malfunctions and prevent damage to the
equipment. The word monitor means to observe, record,
or detect an operation or condition using instruments.
Measurement, in a very real sense, is the language of
engineers. The shipboard engineering plant has many
instruments that indicate existing conditions within a
piece of machinery or a system. By reading and
interpreting the instruments, you can determine whether
the machinery or the system is operating within the
prescribed range.
Recorded instrument readings are used to make sure
the plant is operating properly. They are also used to
determine the operating efficiency of the plant. The
instruments provide information for hourly, daily, and
weekly entries for station operating records and reports.
The data entered in the records and reports must be
accurate since they are used to determine the condition
of the plant over a period of time. Remember, for
accurate data to be entered on the records and reports of
an engineering plant, you must read the instruments
carefully.
In this chapter, we describe various types of
indicating instruments that you, as a Fireman, come in
contact with while working and standing watch on an
engineering plant. Upon completion of this chapter, you
should be able to describe the various types of
temperature and pressure measuring instruments,
indicators, alarms, and the functions for which they are
used.
Engineering measuring instruments are typically
classified into the following groups:
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Pressure gauges
Temperature detectors
Temperature measuring devices
Electrical indicating instruments
Liquid-level indicators
Revolution counters and indicators
. Salinity indicators
. Torque wrenches
We will discuss each of these categories in the
following sections.
PRESSURE GAUGES
The types of pressure gauges used in an engineering
plant include Bourdon-tube gauges, bellows, diaphragm
gauges, and manometers. Bourdon-tube gauges are
generally used for measuring pressures above and below
atmospheric pressure. Bellows and diaphragm gauges
and manometers are generally used to measure
pressures below 15 pounds-per-square-inch gauge
(psig). They are also used for low vacuum pressure. Low
vacuum pressure is slightly less than 14.7
pounds-per-square-inch absolute (psia). Often, pressure
measuring instruments have scales calibrated in inches
of water (in. H2O) to allow greater accuracy.
NOTE: On dial pressure gauges, set the adjustable
red hand (if installed) at or slightly above the maximum
normal operating pressure, or at or slightly below the
minimum normal operating pressure, (Refer to Naval
Ships’ Technical Manual, chapter 504, for specific
instructions.)
BOURDON-TUBE GAUGES
The device usually used to indicate temperature
changes by its response to volume changes or to pressure
changes is called a Bourdon tube. A Bourdon tube is a
C-shaped, curved or twisted tube that is open at one end
and sealed at the other (fig. 11-1). The open end of the
tube is fixed in position, and the scaled end is free to
move. The tube is more or less elliptical in cross section;
it does not form a true circle. The tube becomes more
circular when there is an increase in the volume or the
internal pressure of the contained fluid. The spring
action of the tube metal opposes this action and tends to
coil the tube. Since the open end of the Bourdon tube is
rigidly fastened, the sealed end moves as the pressure of
the contained fluid changes.
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