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 MEASURING DEVICES

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.

THERMOMETERS (MECHANICAL)

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:

0 Liquid-in-glass thermometers

Bimetallic expansion thermometers Filled-system thermometers

Liquid-in-Glass 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.

Figure 11-11.-Effect of unequal expansion of a bimetallic strip.

has a bulb and a very fine-bore capillary tube. The tube contains alcohol or some other liquid that uniformly expands or contracts as the temperature rises or falls. The selection of liquid is based on the temperature range for which the thermometer is to be used.

Almost all liquid-in-glass thermometers are sealed so atmospheric pressure does not affect the reading. The space above the liquid in this type of thermometer may be a vacuum, or this space maybe filled with an inert gas, such as nitrogen, argon, or carbon dioxide.

The capillary bore may be round or elliptical. In either case, it is very small; therefore, a relatively small expansion or contraction of the liquid causes a relatively large change in the position of the liquid in the capillary tube. Although the capillary bore has a very small diameter, the walls of the capillary tube are quite thick. Most liquid-in-glass thermometers have an expansion chamber at the top of the bore to provide a margin of safety for the instrument if it should accidentally overheat.

Liquid-in-glass thermometers may have graduations etched directly on the glass stem or placed on a separate strip of material located behind the stem. Many thermometers used in shipboard engineering plants have the graduations marked on a separate strip because this type is generally easier to read.

You will find liquid-in-glass thermometers in use in the oil and water test lab for analytical tests on fuel, oil, and water.

Bimetallic Expansion Thermometers

Bimetallic expansion thermometers make use of different metals having different coefficients of linear expansion. The essential element in a bimetallic expansion thermometer is a bimetallic strip consisting of two layers of different metals fused together. When such a strip is subjected to temperature changes, one layer expands or contracts more than the other, thus tending to change the curvature of the strip.

Figure 11-11 shows the basic principle of a bimetallic expansion thermometer. One end of a straight bimetallic strip is fixed in place. As the strip is heated, the other end tends to curve away from the side that has the greater coefficient of linear expansion.

When used in thermometers, the bimetallic strip is normally wound into a flat spiral (fig. 11-12), a single helix, or a multiple helix. The end of the strip that is not fixed in position is fastened to the end of a pointer that moves over a circular scale. Bimetallic thermometers are easily adapted for use as recording thermometers; a pen is attached to the pointer and positioned so that it marks on a revolving chart.

Filled-System Thermometers

Generally, filled-system thermometers are used in locations where the indicating part of the instrument must be placed some distance away from the point where the temperature is to be measured. For this reason, they are often called distant-reading thermometers. However, this is not true for filled-system thermometers. In some designs, the capillary tubing is very short or nonexistent. Generally, however, filled-system thermometers are distant-reading thermometers. Some distant-reading thermometers have capillaries as long as 125 feet.

Figure 11-12.-Bimetallic thermometer (flat, spiral strip).

Figure 11-13.-Distant-reading, Bourdon-tube thermometer.

There are two basic types of filled-system thermometers. One type has a Bourdon tube that responds primarily to changes in the volume of the filling fluid. The other type has a Bourdon tube that responds primarily to changes in the pressure of the filling fluid.

A distant-reading thermometer fig. 11-13 consists of a hollow metal sensing bulb at one end of a small-bore capillary tube. The tube is connected to a Bourdon tube or other device that responds to volume changes or pressure changes. The system is partially or completely filled with a fluid that expands when heated and contracts when cooled. The fluid may be a gas, an organic liquid, or a combination of liquid and vapor.

PYROMETERS

Pyrometers are used to measure temperature through a wide range, generally between 300F and 3,000F. Aboard ship, pyrometers are used to measure temperatures in heat treatment furnaces, the exhaust temperatures of diesel engines, and other similar purposes.

The pyrometer consists of a thermocouple and a meter (fig 11-14). The thermocouple is made of two dissimilar metals joined together at one end. It produces an electric current when heat is applied at its joined end. The meter, calibrated in degrees, indicates the temperature at the thermocouple.