Inch of water


Inches of water, inches of water gauge , inches water column , inAq, Aq, or inHO is a non-SI unit for pressure. The units are conventionally used for measurement of certain pressure differentials such as small pressure differences across an orifice, or in a pipeline or shaft.
It is defined as the pressure exerted by a column of water of 1 inch in height at defined conditions. At a temperature of 4 °C pure water has its highest density. At that temperature and assuming the standard acceleration of gravity, 1 inAq is approximately 249.082 pascals.
Alternative standard conditions in uncommon usage are 60 °F, or 68 °F, and depends on industry standards rather than on international standards.
In North America, air and other industrial gasses are often measured in inches of water when at low pressure. This is in contrast to inches of mercury or pounds per square inch for larger pressures. One usage is in the measurement of air that supplies a pipe organ and is referred simply as inches. It is also used in natural gas distribution for measuring utilization pressure which is typically between 6 and 7 inches WC or about 0.25 lbf/in.
1 inAq ≈ 0.036 lbf/in, or 27.7 inAq ≈ 1 lbf/in.