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A pressure gauge is essential for safe operation of a boiler plant. However, the use of additional instruments such as flowmeters and draft gauges, increases safety and promotes efficiency. All of these instruments may be either indicating or recording.


A boilerman must be able to identify the different types of monitoring instruments and understand their operation and use. Meters used to measure quantities are divided into two general types:

1. Those indicating rate, such as flowmeters
2. Those indicating the total, such as scales

Many devices are designed to measure and indicate the pressure of steam flow. One of these devices is shown in figure 1-23. This meter uses a weighted inverted bell (called a Ledoux bell) sealed with mercury. The bell moves up and down as the rate of flow changes. The movement is transmitted to a pen that records the flow.

Figure 1-23.—Flowmeter.


A combustion air and steam flowmeter is shown in figure 1-24. This meter is used as a guide in controlling the relationship between air required and air actually supplied to burn the fuel. The rate of steam generation is used as a measure of air necessary to burn the required amount of fuel. The flow of gases through the boiler setting is used as a measure of air supplied.

Figure 1-24.—Airflow mechanism of a boiler air flowmeter.

The essential parts of this meter are two airflow bells supported from knife-edges on a beam, which is supported by other knife-edges, and a mercury displacer assembly supported by a knife-edge on the beam. The bottoms of the bells are sealed with oil, and the spaces under the bells are connected to two points of the boiler setting.


A draft gauge is a form of pressure gauge. In boiler practice, the term draft usually refers to the pressure difference producing the flow. Drafts are pressures below atmospheric pressure. They are measured in inches of water. A draft gauge is essential to boiler operation. Its use increases the safety of operation.

A simple type of draft gauge is the U-tube gauge. The source of draft is connected to one leg of the U and the other end is left open. The difference between the levels of the liquid in the two legs is a measure of the draft. Water is generally used in this type of gauge. Take a close look at figure 1-25 that shows a comparison of an inclined-draft gauge and a U-tube gauge.

Figure 1-25.—Comparison of inclined-draft gauge and U-tube gauge.

When one leg of the U tube is arranged on an incline, the distance moved by the liquid in the inclined portion is increased for a given draft change which makes more accurate reading possible.

Two or more draft gauges are required for economical boiler operation. The gauges inform the operator of the relative amount of air being supplied to burn the fuel and the condition of the gas passages. Draft gauges are made as indicators, recorders, or both. The measuring element uses a column of liquid, a diaphragm, 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).

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

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.


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).

Figure 1-27.—CO2 meter (analyzer).


A digital CO2 analyzer.
(Photo courtesy Electricals Electronics Enterprise, Chennai)

Q20. Meters are divided into what two general categories?

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

David L. Heiserman, Editor

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Revised: June 06, 2015