How to measure pressure TSI（如何衡量压力 TSI）.pdf

HOW TO MEASURE PRESSURE HOW TO MEASURE FLOW by G.J.Matthews ©Airflow Developments Limited HOW TO MEASURE PRESSURE. FUNDAMENTAL PRINCIPLES. Due to the depth of the earth’s atmosphere the air around us exerts a pressure, known as atmospheric pressure. This can be measured with a barometer and varies depending upon the weather conditions. It also varies according to altitude above or below sea level at which it is made. Pressure measurements in heating systems are usually made relative to atmospheric pressure, and may be corrected to ‘Standard Conditions’ to make possible a comparison between readings. The internationally agreed ‘Standard Conditions’ are for an air density of 1.2 kg/m³. ( Atmospheric air at a temperature of 16°C, a pressure of 100,000 Pa. and a relative humidity of 65% has a density of 1.2 kg/m³, but these conditions are not part of the definition.) The only pressure acting at any point in a stationary fluid is Hydrostatic pressure which acts equally in all directions. If the fluid is set in motion the pressure still persists, but may change in magnitude . If we consider a very thin disc placed in the fluid, and moving with it, the static pressure as it is called will act on the two faces of the disc. INFINITELY THIN DISC. PS PS 2 If the disc is stationary the pressure acting on the two surfaces will be the same providing the presence of the disc does not disturb the flow. PS PS NO DISTURBANCE However if the stationary disc is turned through 90° its front face will be subjected to not only s