The pressure of a gas can be measured by a simple device called a Utube manometer. For low pressures, a relatively light fluid such as water can be used in the manometer. For higher pressures, a heavier liquid such as mercury may be used. Mercury is more than 13 times heavier than water, and it takes a correspondingly higher pressure to push mercury up the tube on one side of the manometer. It is quite simple to convert pressure measured in inches of water to pressure measured in inches of mercury.
Things You'll Need
 Calculator

Write down the gas pressure as measured in inches of water. The ratio of the density of a substance to the density of water is called the specific gravity (SG). The SG of mercury is 13.6, meaning that mercury is 13.6 times heavier than water.

Take a calculator and divide the pressure measured in inches of water by 13.6. The answer is the pressure measured in inches of mercury. For example, if the gas pressure is measured at 6.8 inches of water, dividing by 13.6 equals 0.5 inches of mercury.

Note that gas pressure measured in mercury is more usually given in millimeters. An inch is 25.4 millimeters (mm). So a pressure of 0.5 inches of mercury is equal to 12.7 mm.
References
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