The weight of water required to make 1 cu. ft. of steam at any pressure is the same as the weight of 1 cu. ft. of steam.

Therefore, the weight of water is obtained by multiplying the number of cubic feet of steam required by the weight of one cubic foot.

Example. - How much water will it take to make 300 cu. ft. of steam at 100 lbs. absolute pressure?

One cubic foot of steam at 100 lbs. pressure is given as weighing .2307 lbs. Therefore, 300 cu. ft. will weigh 300 X .2307, or 69.21 lbs. of water.

One cubic foot of water may, for any practical purpose, be reckoned to weigh 62 1/2 lbs., and the weight of one gallon of water may be taken as 8- 3/10- lbs. Therefore, 69.21 lbs. divided by 62.5 gives 1.1 cu. ft., or 69.21 divided by 8.3 gives 8.34 gals.

At atmospheric pressure one cubic foot of steam has nearly the weight of one cubic inch of water, and the weight increases very nearly as the pressure; therefore, the rule: Multiply the number of cubic feet of steam by the absolute pressure in atmosphere and the product is the number of cubic inches of water required to give the steam.

In all such calculations, for practical purposes, a liberal allowance must be made for loss and leakage.