| Vol.1 No.4 Date: Feb. 10, 1999 Subject: Flow Measurement News From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com ........ ******************************************* For more information in our complete line of flowmeters go to: http://www.mccrometer.com ******************************************* Ask The Experts... TYPES OF PRESSURE UNITS Pressure is a vital measurement in many flow systems. Not only does pressure have safety implications, a flow measurement's accuracy may also depend on it. This is true more for gasses than liquids. Gasses are a compressible fluid in which the density of the fluid will change as the pressure changes. However, liquids, in general, do not change density with pressure changes. The units in which we measure pressure are often misunderstood or misapplied. There are two types of basic pressure: gauge and absolute. An instrument must call out what type of pressure is being measured in order to be accurate. The difference between the types of pressure units is the reference point. Gauge pressure is referenced against the atmospheric pressure at the measurement point. The units of gauge pressure are followed by a "g", such as psig. A gauge pressure instrument should always read zero when exposed to atmospheric pressure. An important point to remember is that atmospheric pressure does not always equal "standard pressure". Standard pressure is a constant value used for calculation purposes. Atmospheric pressure can change with many influences, such as elevation and weather. Absolute pressure is referenced against absolute zero pressure, or a complete vacuum. The units of absolute pressure are followed by an "a", such as psia. If you hold an absolute pressure instrument in the open air, the reading should be well above zero, in the range of 14.7 to 12 psia. The relationship between absolute pressure and gauge is as follows: ABSOLUTE PRESSURE = GAUGE PRESSURE + ATMOSPHERIC PRESSURE Absolute pressure is used when calculating flow and fluid properties. This is important to understand because gauge pressure is normally converted to an absolute pressure value for calculating purposes. To ensure accuracy, the gauge pressure must be accompanied by the atmospheric pressure at the time of the reading. Does it really make much of a difference? Yes, but more so at lower pressures. Here's an example: lets say 1 psig is being measured with a gauge pressure instrument. This needs to be converted to absolute units. To save time, it will be assumed that the atmospheric pressure is 14.7 psia, so 1 + 14.7 = 15.7 psia. The actual atmospheric pressure though is 12.5 psia. The absolute pressure is actually 13.5 psia. If you were trying to calculate the density using the incorrect absolute pressure, the density calculation would be off by 14%. ******************************************* |