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:

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. H20) 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.

Figure 11-1.-C-shaped Bourdon tube.

There are many types of Bourdon-tube gauges used in the Navy. The most common ones are the simplex, duplex, vacuum, compound, and differential pressure gauges. They operate on the principle that pressure in a curved tube has a tendency to straighten out the tube. This curved tube is made of bronze for pressure under 200 psi and of steel for pressures over 200 psi.

Simplex Bourdon-tube Gauge

Figure 11-2 shows a simplex Bourdon tube installed in a gauge case. Notice that the Bourdon tube is in the shape of the letter C and is welded or silver-brazed to the stationary base. The free end of the tube is connected to the indicating mechanism by a linkage assembly. The threaded socket, welded to the stationary base, is the pressure connection. When pressure enters the Bourdon tube, the tube tends to straighten out. The tube movement through linkage causes the pointer to move proptionally to the pressure applied to the tube. The simplex gauge is used for measuring the pressure of steam, air, water, oil, and similar fluids or gases.

Duplex Bourdon-tube Gauge

The duplex Bourdon-tube gauge (fig. 11-3) has two tubes and two separate gear mechanisms within the same case. As shown in view B, a pointer is connected to the gear mechanism of each tube. Each pointer operates independently. Duplex gauges are normally used to show pressure drops between the inlet and outlet sides of lube oil strainers. If the pressure reading for the inlet side of a strainer is much greater than the pressure reading for the outlet side, you may assume that the strainer is likely to be dirty and is restricting the flow of lube oil through the strainer.

Bourdon-tube Vacuum Gauge, Compound Gauge, and Differential Pressure Gauge

Bourdon-tube vacuum gauges are marked off in inches of mercury (fig 11-4). When a gauge is designed to measure both vacuum and pressure, it is called a compound gauge. Compound gauges are marked off both in inches of mercury (in.Hg) and in psig (fig 115).

Figure 11-2.-Simplex Bourdon-tube pressure gauge.

Figure 11-3.-Duplex Bourdon-tube pressure gauge.

Differential pressure may also be measured with Bourdon-tube gauges. One kind of Bourdon-tube differential pressure gauge is shown in figure 11-6.  This gauge has two Bourdon tubes, but only one pointer. The Bourdon tubes are connected in such away that they are

Figure 11-4.-Bourdon-tube vacuum gauge.

Figure 11-5.-Compound Bourdon-tube gauge.

Figure 11-6.-Bourdon-tube differential pressure gauge.

Figure 11-7.-Bellows gauge.

the pressure difference, rather than either of the two actual pressures indicated by the pointer.