Figure 11-9.A. Standard U-tube manometer. B. Single-tube
pressure is lowered, the diaphragm moves the pointer
back toward the zero point.
A manometer is perhaps the most accurate, least
expensive, and simplest instrument for measuring low
pressure or low-pressure differentials. In its simplest
form, a manometer consists of either a straight or
U-shaped glass tube of uniform diameter, filled with a
liquid. The most common liquids used are water and oil.
One end of the U-tube is open to the atmosphere, and
the other end is connected to the pressure to be measured
(fig. 11-9). The liquid reacts to the amount of pressure
exerted on it and moves up or down within the tube. The
amount of pressure is determined by matching the liquid
level against a scale within the manometer.
Temperature is one of the basic engineering
variables. Therefore, temperature measurement is
essential to the proper operation of a shipboard
engineering plant. As a watch stander, you will use both
mechanical and electrical instruments to monitor
temperature levels. You will frequently be called on to
measure the temperature of steam, water, fuel,
lubricating oil, and other vital fluids. In many cases, you
will enter the results of measurements in engineering
logs and records.
Mechanical devices used to measure temperature
are classified in various ways. In this section, we will
discuss only the expansion thermometer types.
Expansion thermometers operate on the principle that
the expansion of solids, liquids, and gases has a known
relationship to temperature change. The following types
of expansion thermometers are discussed in this section:
. Liquid-in-glass thermometers
s Bimetallic expansion thermometers
. Filled-system thermometers
Liquid-in-glass thermometers are the oldest,
simplest, and most widely used devices for measuring
temperature. A liquid-in-glass thermometer (fig. 11-10)
Figure 11-10.Liquid-in-glass thermometer.