Secure the valve-stem of the safety-valve to the lever with wire or string, and attach a loop to the lever, into which pass the hook of an accurate spring-balance, arranging the loop so that it is directly over the centre of the valve-stem. Then take hold of the upper part of the spring-balance, and lift the valve slightly, noting the reading of the balance. Measure the lower diameter of the safety-valve, and find its area; divide the reading of the spring-balance by the area of the valve, and the result will be the pressure, in pounds per square inch, at which the steam will raise the safety-valve. Suppose, for instance, that the diameter of the safety-valve is 1 inch; its area will be about 7854/10000 of an inch. Now, if the tension of the spring-balance in raising the valve is 120 lbs., the pressure at which the valve will rise is the quotient arising from dividing 120 by 7854/10000 or 153 lbs. per square inch.

A table is appended, giving the areas of valves for the majority of cases that occur in practice:

Table Of Areas Of Valves Of Different Diameters

Diameter of valve in inches.

Area of valve in square inches

1/2 or 0.5 ........

...... 13-64 or 0.19635

5/8 or 0.625........

....... 5-16 or 0.30680

3/4 or 0.75

7-16 or 0.44179

7/8 or 0.875........

....... 19-32 or 0.60132

1 ........

....... 25-32 or 0.7854

1-1/4 or 1.25........

.......1 15-64 or 1.2272

1-1/2 or 1.5 ........

.......1 49-64 or 1.7671

1-3/4 or 1.75 ........

.......2 13-32 or 2.4053

2 ........

.......3 9-64 or 3.1416

2-1/2 or 2.5 ........

....... 4 29-32 or 4.9087

3 ........

....... 7 1-16 or 7.0686

3-1/2 or 3.5

....... 9 5 8 or 9.6211

4 .......

.......12 9-16 or 12.5664

4-1/2. or 4.5 ........

.......15 29-32 or 15.9043

Diameter of valve in inches.

Area of valve in square inches.

5 ........

.......19 41-64 or 19.635

5-1/2 or 5.5

.......23 49-64 or 23.7583

6 ........

.......28 9-32 or 28.2744