or a bellows. The liquids used are oil, water, or mercury. The gauge shown in figure 1-26 is an indicating type that operates on the same principle as the U tube (difference between the levels of the liquid in the two legs is a measure of the draft).

The bottom of the inverted bell is sealed with oil or mercury, depending on the magnitude of the draft or pressure to be measured. It is supported by knife-edges on the beam to reduce friction as much as possible. The weights counterbalance the weight of the bell. and the pointer is returned to zero. The source of draft is connected to the tube, which projects into the inverted bell, so an increase in draft causes the pointer to move down.

CO₂ METERS (ANALYZERS)

Figure 1-27 shows one type of carbon dioxide meter. The meters are also known as analyzers and are designed for determining, indicating, and recording the percentage of CO₂ (carbon dioxide) in the products of combustion. The principle of this instrument is based on the fact that the specific weight of flue gas varies in proportion to its CO₂ content (CO₂ being considerably heavier than the remaining parts of the flue gas).

Q20. Meters are divided into what two general categories?

Q21. In reference to boiler operations, what does the term "draft" mean?

BOILER WATER TREATMENT AND CLEANING

Learning Objective: Describe the methods and procedures for the testing of and treatment of boiler water. A Utilitiesman must understand the methods, tests, and safety precautions involved in boiler water treatment and the procedures for cleaning boiler firesides and watersides. To ensure a boiler operates at peak efficiency, you must treat and clean it. Water testing, treatment. and cleaning go hand-in-hand. The reason for this is because the effect of the impurities in the water on interior surfaces determines the method and frequency of boiler cleaning. In this section, we will discuss the relationships between water testing. treatment. and cleaning and the procedures for each.

WATER IMPURITIES

All natural waters contain acid materials and scale-forming compounds of calcium and magnesium that attack ferrous metals. Some water sources contain more scale-forming compounds than others; therefore, some waters are more corrosive than others. Subsurface or well waters are generally more scale-forming, while surface waters are usually more corrosive. To prevent scale formation on the internal water-contacted surfaces of a boiler and to prevent destruction of the boiler metal by corrosion, chemically treat feedwater and boiler

Figure 1-26. - Liquid-sealed draft gauge.