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The U-tube manometer is a primary measuring device indicating pressure or vacuum by the difference in the height of two columns of fluid. ‪

Connect the manometer to the source of pressure, vacuum or differential pressure. When the pressure is imposed, add the number of inches one column of fluid travels up to the amount the other column travels down to obtain the pressure (or vacuum) reading. ‪

The height of a column of mercury is read differently from that of a column of water. Mercury does not wet the inside surface: therefore, the top of the column is convex (shape). Water wets the surface and therefore is concave. A mercury column is read by sighting horizontally between the top of the convex mercury surface and the scale, see Figure 20580 . ‪

Comparison of Column Height for Mercury and Water Manometers

A water manometer is read by sighting horizontally between the bottom of the concave water surface and the scale. ‪

Should one volume of fluid travel further than the other column, due to minor variations in the inside diameter of the tube or to the pressure imposed, the accuracy of the reading obtained is not impaired. ‪

Listed in Table are manometer readings which can be converted into other units of measure. ‪

Pressure Conversions

1 in. H2 O = 0.0735 in. Hg ‪

1 in. H2 O = 0.0361 in. lb/in.2

1 in. Hg = 13.61 in. H2 O ‪

1 in. Hg = 0.4912 lb/in.2

1 lb/in.2 = 27.700 in. H2 O ‪

1 lb/in.2 = 2.036 in. Hg ‪

1 lb/in.2 = 6.895 kPa ‪

1 kPa = 0.145 lb/in.2

Pressure Conversion Chart