1、z n (o 0 o - .- E L 3 C c 0 3 i 3 L U c m z DEUTSCHE NORMEN February 1977 UDC 532.11 : 001.4: 531.787.081.1 Pressure Basic Definitions, Units DIN - 1314 Druck; Grundbegriffe, Einheiten 1 Scope The stipulations in this Standard apply to the pressure in liquids, gases and vapours. 2 Basic definitions
2、2.1 The physical magnitude p re s s u re p is the quotient of the normal force FN acting on a surface divided by said surface A: FN p = - A 2.2 In the field of engineering, various pressure magnitudes are commonly used; in the majority of cases, these represent the difference between two pressures,
3、but in engineering parlance they are again referred to as “pressure“. In order to avoid any mis- understandings, it is therefore recommended to adopt the denominations according to Sections 2.2.1 to 2.2.3 below. 2.2.1 Absolute pressure The a b s o I u te p r e s s u r e Pabs is the pressure vis-vis
4、the pressure zero in a vacuum. 2.2.2 Pressure difference, differential pressure The difference between two pressurespi andp2 is called pressure difference Ap=p1 -p2, or, if it happens to be the measured variable, it is called d iff er entia I pressure 1.2. 2.2.3 Atmospheric pressure difference, over
5、pressure The difference between an absolute pressure pa it is called o v e r p r e s s u r e : The overpressurep, is positive if the absolute pressure is higher than the atmospheric pressure; it is negative if the absolute pressure is lower than the atmospheric pressure. N o te 1 : Previously the te
6、rm “overpressure“ was only used in cases where the absolute pressure was higher than the atmospheric pressure; in cases where it was lower, the magnitude defined by the difference Pmb - pabs was called “underpressure“ or partial vacuum “. As from now, the zone of underpressure will be charac- terize
7、d by negative values of overpressure. The word “underpressure“ or the expression “partial vacuum“ shall in future no longer be used to denominate a magnitude, but only in connection with the qualitative designation of a state or condition, for example: “Low pressure (underpressure) chamber“; “A part
8、ial vacuum reigns in the suction pipe“. In the case of composite words which incorporate the word “overpressure“, the “over“ part of the word may be omitted in cases where the associated magnitude is clearly defined as an overpressure. Examples: Bursting pressure, blood pressure, sonic pressure, tyr
9、e pressure. N o t e 2 : The pressure zone below atmospheric pressure is also referred to as vacuum zone (see DIN 28400 Part 1). In the vacuum technology, pressures are always given in the form of absolute pressures. Pe =pabs -Pamb. N o te 3 : The graphic representation below illustrates the relation
10、ship between the various pressure magnitudes. f + 1 I Continued on page 2 Explanations on pages 3 and 4 Alleinverkauf der Normen durch Beuth Verlag GrnbH. Berlin 30 und Kln 1 05.79 DIN 13 14 eng/. Preisgr. 4 Vertr.-Nr. 0104 Page4 DIN 1314 in Section 2.2.1 of this Standerd. On the other hand, mainly
11、in the engineering jargon, the word “pressure“ is used as a blanket definition which encompasses all pressure magnitudes, irrespective of their individual definition. The reason for this difference in approach lies in the fact that pressure to the physicist is a state variable which determines many
12、of the properties of matter, whereas to the engineer and technician it represents a control magnitude which has to be moni- tored; it hardly needs saying that these two viewpoints represent opposite extremes. Because of these differences in approach, no attempt has been made to give headings to Sect
13、ions 2.1 and 2.2. The physicist can thus continue to refer to “pressure“ where he means absolute pressure, whilst on the other hand the measured variables in the industrial field are more clearly differentiated than heretofore. Industry is mainly concerned with pressure differences; even the absolut
14、e pressure can be conceived as the difference between two pressures, one of which is equal to zero. This representation is in keeping with measuring technique; the fact that absolute pressure was not defined in this way in the Standard is due to the introduction of the concept of pressure difference
15、 later on. In this conception, the denomination “abso- lute“ might appear to the physicist at least to be inappro- priate, in view of the meaning of the word. However the Standard stipulates quite clearly what is meant. A pressure difference is of course a pressure once again. It is however necessar
16、y to make a distinction between pressure and pressure difference in order to pinpoint quite clearly the datum point of the latter magnitude and thus the point of origin of the pressure graduation scale used. A concession was made to certain desires expressed on this subject, and the concept of diffe
17、rential pressure was accordingly incorporated in the Standard. In this connection, it should be borne in mind that the differential pressure is obtained as a direct measurement reading on differential pressure measuring instruments incorporating a venturi or an orifice plate. The wording of Section
18、2.2.2 makes it perfectly clear that the denom- ination “pressure difference“ is the preferred one, The fact that the “over“ portion may be omitted in composite words which include the word “overpressure“ is the result of a quest for a simplification in terms commonly used. Although the last paragrap
19、h of Section 2.2.3 may not read quite as clearly as had been hoped, it should be adequate to dispel any misunder- standings. The zone of overpressures with negative values (minus sign) coincides (in the manner in which it has been defined) with the vacuum zone as laid down in DIN Standard 28400 Part
20、 1: “A vacuum within the context of vacuum technology is the state of a gas, the pressure of which is lower than atmospheric pressure“. In usual engineering parlance, this is hardly ever the case, as no one, or hardly any one is likely to refer to an absolute pressure of say 0.7 bar as a “vacuum“, t
21、he latter term being reserved to describe very low and extremely low pressures. The qualification “within the context of vacuum technology“ which was appended should however be adequate to differentiate the appli- cation ranges of the two words. The graphic representation of overpressure, absolute p
22、ressure and pressure difference or differential pressure was incorporated in the Standard at the express desire of a large number of objectors, despite the fact that the relationships of the various pressure magnitudes to one another are so simple that a graph hardly seemed necessary. The pressure u
23、nits in force today and the conversion calculations for units which are no longer admissible for use have been incorporated in the present Standard as a result of a majority vote, despite the fact they already appear in DIN 1301 “Units; names and symbols“ The reason for adopting the “bar“ has been included in the present Standard, because it seemed desirable to do so in view of the discussions concerning its future use which took place at the ISO, and which has raised doubts in the minds of users of the present Standard. There should be no objections to the further use of the “bar“.
