The statutory units used in vacuum technology
Introduction
Two federal German laws and the related implementing provisions stipulate which units must be used for measurements in business and official documents and communications. The provisions resulted in a number of fundamental changes that also have to be taken into account in vacuum technology. Many of the units commonly used in the past, such as torr, gauss, standard cubic meter, atmosphere, poise, kilocalorie, kilogram-force, etc., are no longer permissible. Instead, other units are to be used, some of which are new while others were previously used in other fields. The alphabetical list below contains the major variables relevant for vacuum technology along with their symbols and the units now to be used, including the SI units (see below) and legally permissible units derived from them. The list is followed by a number of remarks. The purpose of the remarks is, on the one hand, to establish a connection with previous practice wherever this is necessary and, on the other hand, to provide explanations on practical use of the content of the alphabetical list. The statutory units for measurements are based on the seven basic SI units of the Système International (SI). Statutory units are:
a) the basic SI units (Table 10.4.1)
b) units derived from the basic SI units, in some cases with special names and unit symbols (Tables 10.4.2 and 10.4.4)
c) units used in atomic physics (Table 10.4.3)
d) decimal multiples and decimal parts of units, some with special names
Examples: 105 N (m-2 = 1 bar)
1 dm3 = 1 l (liter)
103 kg = 1 t (ton)
Detailed descriptions are provided in publications by W. Haeder and E. Gärtner (DIN), by IUPAP 1987 and by S. German, P. Draht (PTB). These should always be referred to if the present summary tailored to vacuum technology leaves any questions open.
10.4.1 Basic SI units
Table 10.4.2 Derived coherent1 SI units with special names and symbols (alphabetical)
Table 10.4.3 Atomic units
Table 10.4.4 Derived noncoherent SI units with special names and symbols
Alphabetical list of variables, symbols and units frequently used in vacuum technology and its applications
Table 10.2 Alphabetical list of variables, symbols and units frequently used in vacuum technology and its applications
Table V Important values
Remarks on the alphabetical list
3/1: Activity
The unit previously used was curie (Ci).
3/2: (°C) Celsius temperature
The term degrees Celsius is a special name for the SI unit kelvin (K) [see no. 122] for indicating Celsius temperatures. The term degrees Celsius is legally approved.
3/3: Pressure
The revised version of DIN 1314 must be complied with. The specifications of this standard primarily apply to fluids (liquids, gases, vapors). In DIN 1314, bar (1 bar = 0.1 MPA = 105 Pa) is stated in addition to the (derived) SI unit, 1 Pa = 1 N · m-2, as a special name for one tenth of a megapascal (Mpa). This is in accordance with ISO/1000 (11/92), p. 7. Accordingly, the millibar (mbar), a very useful unit for vacuum technology, is also permissible: 1 mbar = 102 Pa = 0.75 torr. The unit “torr” is no longer permissible.
Special note
Exclusively absolute pressures are measured and used for calculations in vacuum technology.
In applications involving high pressures, frequently pressures are used that are based on the respective atmospheric pressure (ambient pressure) pamb. According to DIN 1314, the difference between a pressure p and the respective atmospheric pressure (ambient pressure) pamb is designated as overpressure pe: pe = p – pamb. The overpressure can have positive or negative values.
Conversions
1 kg · cm-2 = 980.665 mbar = 981 mbar
1 at (technical atmosphere) = 980.665 mbar = 981 mbar
1 atm (physical atmosphere) = 1013.25 mbar = 1013 mbar
1 atmosphere above atmospheric pressure (atmospheric overpressure) =
2026.50 mbar = 2 bar
1 meter head of water = 9806.65 Pa = 98 mbar
1 mm Hg = 133.332 Pa = 1.333 mbar = 4/3 mbar
The pressure as mechanical stress (strength) is generally given in pascal
(Pa) and in N · nm–2.
Conversions:
1 Pa = 1 N · m–2 = 10–6 N · mm–2
1 kg · cm–2 = 98,100 Pa = 0.981 N · mm–2 = 0,1 N mm–2
1 kg · mm–2 = 9,810,000 Pa = 9.81 N · mm–2 = 10 N · mm–2
3/5: Dynamic viscosity
The unit previously used was poise (P).
3/5a: Energy dose
Rad (rd) is no longer permissible.
3/6: Weight
DIN 1305 is to be complied with in this context. Because of its previous ambivalence, the word weight should only be used to designate a variable of the nature of a mass as a weighing result for indicating quantities of goods.
The designations “specific weight” and “specific gravity” should no longer be used. Instead, one should say density.
3/7: Weight force
See DIN 1305. The previous units pond (p) and kilopond, i.e. kilogramforce, (kp) as well as other decimal multiples of p are no longer used.
1 kp = 9.81 N
3/8: Ion dose
The previously used unit was the Röntgen (R).
3/9: Kinematic viscosity
The previously used unit was stokes (St).
3/10: Force
The dyne, the CGS unit for force, is no longer used.
3/11: Length/wavelength
The unit Ångström (Å) (e.g. for wavelength) will no longer be used in future.
3/12: Leak rate
In DIN 40.046 sheet 102 (draft of August 1973 issue), the unit mbar · dm3 · s-1 (= mbar · l · s-1) is used for the leak rate. Note that the leak rate corresponding to the unit 1 mbar · l · s-1 at 20 °C is practically the same as the leak rate 1 cm3 · s-1 (NTP). (See also 3/17)
3/13: Magnetic field strength
The previously used unit was the oersted (Oe).
3/14: Magnetic flux density
The previously used unit was the gauss (G).
3/15: Magnetic flux
The previously used unit was the maxwell (M).
3/16: Standard volume
DIN 1343 must be complied with.
The designation m3 (NTP) or m3 (pn, Tn) is proposed, though the expression in parentheses does not belong to the unit symbol m3 but points out that it refers to the volume of a gas in its normal state
3/17: Partial pressure
The index “i” indicates that it is the partial pressure of the “i-th” gas that is contained in a gas mixture.
3/18: Gas permeability
The permeation coefficient is defined as the gas flow m3 · s-1 (volumetric flow pV) that goes through a fixed test unit of a given area (m2) and thickness (m) at a given pressure difference (bar).
According to DIN 53.380 and DIN 7740, Sheet 1, supplement, the gas permeability (see no. 40) is defined as “the volume of a gas, converted to 0 °C and 760 torr, which goes through 1 m2 of the product to be tested at a certain temperature and a certain pressure differential during a day (= 24 hours)”.
3/19: pV throughput/pV value
DIN 28.400, Sheet 1 is to be taken into account here. No. 86 and no. 87 have a quantitative physical significance only if the temperature is indicated in each case.
3/20: Relative atomic mass
Misleadingly called “atomic weight” in the past!
3/21: Relative molecular mass
Misleadingly called “molecular weight” in the past!
3/22: Specific gas constant
As mass-related gas constant of the substance “i”. Ri = Rm (Mi-1; Mi molar mass (no. 74) of the substance “i”. See also DIN 1345.
3/23: Specific heat capacity
Also called specific heat:
Specific heat (capacity) at constant pressure: cp.
Specific heat (capacity) at constant volume: cV.
3/24: Temperature difference
Temperature differences are given in K, but can also be expressed in °C. The designation degrees (deg) is no longer permissible.
3/25: Quantity of heat
The units calorie (cal) and kilocalorie (kcal) are no longer be used.
3/26: Angle
1 radian (rad) is equal to the plane angle which, as the central angle of a circle, cuts out an arc having a length of 1 m from the circle. See also DIN 1315 (8/82).
