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All About Measuring Pressure All About Measuring Pressure 33 Preface Alongside temperature, pressure is one of the most important and most measured variables for applications in research and engineering. A large number of measuring tasks have become indispensable for a wide range of applications in the foodstuffs industry, in heating, sanitation and air-conditioning, in power generation and technology, in process engineering and in many other areas. Whatever the application, when it comes to measuring pressure there are some important parameters which have to be observed if reliable results are to be achieved. In addition to knowledge of the various units, familiarity with the different types of pressure, and their definition, is fundamental to measurement. The Technical manual deals first and foremost with the context surrounding the electronic pressure measuring process. The aim of this practical guide is to both enable newcomers to gain an overview of the relevant parameters, and to serve as a reference work for professionals in the industry. We are grateful for any ideas you may have and will be happy to incorporate these into the next edition. The Board of Directors Burkart Knospe Wolfgang Hessler Martin Schulz Parameter Pressure Page Content Page Content 6 I. The definition of pressure 17 IX. Accuracy 1. The right meter for the particular application 6 II. Units 2. Linearity 1. SI units 3. Temperature coefficient 2. About the units 4. Hysteresis 7 III. Conversion table 20 X. The measuring process 8 IV. Types of pressure and their definition/basis 20 XI. Overload versus static pressure 1. Absolute pressure = p abs 2. Positive pressure 22 XII. Measuring in liquids 3. Negative pressure 1. Static admission pressure 4. Differential pressure = ³ p 2. Measuring pressure in liquids 5. Atmospheric air pressure = p atm 6. Pressure meters and their areas of 25 XIII. Which gases can be measured? application 6 XIV. Pressure shocks 10 V. Pressure measuring processes 1. Various liquid pressure meters 27 XV. Measuring flows with a pitot tube – U-tube manometers 1. Determining air density – inclined-tube manometers 2. Sources of error in pitot tube measurement – float manometers 3. Measuring flows above 100 m/s 2. Elastic element pressure meters 4. Advantages of measuring flows using a differential 3. Electric pressure measuring processes pressure meter and pitot tube 13 VI. Advantages of electric pressure meters 34 XVI. Calibration 13 VII. Description of the Testo measuring principle 36 XVII. Some applications 1. Measuring pressure according to the piezoresistive principle 2. Inductive pressure measurement 15 VIII. Pressure and temperature 4 5 Parameter Pressure I. The definition of pressure The hPa (= mbar) is used primarily in meteorology, but also to some Force F Pressure p = Pressure is defined as a force (F) acting on an area (A). extent in contracting and industry as well. Area A 750 micron = 1 hPa The units bar, KPa and MPa are the standard units in industrial II. Units pressure measuring technology. SI units are the names given to the basic units of the international The mmH2O is now almost never used. system of units. The name comes from the French “Système International d’ Unités.” The SI was established by the General The mmHg is used most frequently in medical engineering. Blood Conference on Weights and Measures (founded by the Metric pressure, for instance, is measured in mmHg. Agreement of 20 May 1875). The system is now administered and revised by the Bureau International des Poids et Mesures in Sèvres The micron is the smallest unit (750 micron = 1 hPa) and is used (France). Both the International Standardization Organization (ISO) primarily in vacuuming, e.g. of refrigeration installations. and the International Union of Pure and Applied Physics (IUPAP) compile international recommendations for the use of the SI which Imperial units: are then laid down as binding at national level. In Germany, the Law - psi (pounds per square inch) on Units in Metrology is the legal foundation for the indication of - inH2O (inches of water) physical quantities in statutory units. It is mandatory for use in - in Hg (inches of mercury) official and commercial transactions. The “guardian” of the units in Germany is the PTB. The older units of Torr, atü, ata, atu, atm and kp/cm2 are no longer The Implementing Order to the Law on Units in Metrology (Units common and under the Units Law may no longer be used in official Order) refers to the DIN 1301 standard. or commercial transactions. The statutory units are listed in alphabetical order in the annex to the Units Order. III. Conversion table Pa hPa/mbar kPa MPa bar psi mmH O inH O mmHg inHg The pascal. 2 2 This can be derived from the SI units of the metre and the Newton. Pa 1 100 1,000 1,000,000 100,000 6,895 9.807 249.1 133.3 3,386 2 1 Pa = 1 N/m . In meteorology pressure is usually indicated in hPa. hPa/mbar 0.01 1 10 10.000 1,000 68.948 0.09807 2.491 1.333 33.864 This unit has replaced the mbar that was previously common. The 0.001 0.1 1 1.000 100 6.895 0.009807 0.2491 0.1333 3.386 pascal was named after Blaise Pascal (1623 - 1662), a French kPa mathematician and natural scientist. In industrial applications the MPa 0.000001 0.0001 0.001 1 0.1 0.006895 0.000009807 0.0002491 0.0001333 0.003386 unit bar, Kpa or MPa is frequently used. bar 0.00001 0.001 0.01 10 1 0.0689 0.00009807 0.002491 0.001333 0.0339 0.0001451 0.0145 0.14505 145.05 14.505 1 0.001422 0.0361 0.0193 0.4912 Since the pascal is a very small unit of pressure, it is primarily used psi 0.102 10.2 102 102,000 10,200 704.3 1 25.4 13.62 345.9 for measuring pressure in clean rooms. But the Pa is also used as a mmH2O unit of measurement when measuring flows in conjunction with a 0.004016 0.4016 4.016 4,016 401.6 27.73 0.0394 1 0.5362 13.62 inH2O pitot tube (see also point 14). mmHg 0.007501 0.7501 7.501 7,501 750.1 51.71 0.0734 1.865 1 25.4 1 N m 1 Pa = where 1 N = 1 kg m2 s2 inHg 0.0002953 0.0295 2953 295.3 29.35 2.036 0.002891 0.0734 0.0394 1 The table must be read from top to bottom, e.g. 1 Pa = 0.01 hPa/mbar 6 7 Parameter Pressure IV. Types of pressure Patm Pressure measurement compares a current pressure with a This is the most important pressure for life on earth. Atmospheric reference pressure. Pressure measuring technology distinguishes pressure arises through the weight of the atmosphere that between the following types of pressure, enabling a statement to be surrounds the earth. The atmosphere extends up to an altitude of made about the relationship between the measuring pressure and about 500 km. Air pressure decreases constantly up to this altitude the reference pressure. (absolute pressure p abs = zero). Atmospheric air pressure is also affected by weather-related fluctuations. On average, p atm at sea Absolute pressure relates to the vacuous space of the universe level is 1013.25 hPa. However, it can fluctuate by up to ±5 % (zero pressure). according to highs or lows in the weather. Absolute pressure: – measured pressure above absolute zero Pressure meters and their areas of application – reference, ideal vacuum What are known as differential pressure meters can measure – measuring pressure is always greater than reference pressure positive pressure and negative pressure as well as differential Positive pressure: pressure. The important thing here is to connect the right pressure to – measured pressure above the barometric daily air pressure the right connection, i.e. a positive pressure to the + and a negative – reference ambient pressure pressure to the – connection. With the right connection, a differential – measuring pressure is always greater than reference pressure pressure meter can cover the entire measuring range in both the positive and the negative direction. Negative pressure: If a meter has a measuring range of 0...200 hPa, for instance, a – measured pressure below the barometric daily air pressure positive pressure, a negative pressure and a differential pressure – reference ambient pressure within the 200 hPa range can be measured. – measuring pressure is always less than reference pressure Differential pressure: Why always the correct pressure at the right connection? – measured pressure above or below any desired reference pressure If a meter is used to measure a negative pressure and this is – measuring pressure is always greater than reference pressure connected to the + connection, some meters will indicate a part of their measuring range with a – sign, but will stop after a certain value (both to protect the sensor and because all pressure sensors Differential pressure are only calibrated in the positive range). If the user does not take this into account, incorrect measuring results will be obtained. Positive pressure Some other meters indicate “out of range” on the display after a certain measured value. Negative pressure Barometric pressure can be measured with absolute pressure Absolute pressure meters. There are two types of barometric atmospheric pressure. One is the pressure related to the particular altitude, the other the Vacuum Pressure Pressure types absolute pressure converted to sea level. The converted absolute pressure is primarily used in meteorology in order to ensure 8 9 Parameter Pressure comparability. It is known that the average value related to sea level Parallax error is 1013.25 hPa.
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