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

a close look at figure 1-25 that shows a comparison of an inclined-draft gauge and a 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,

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