Application reports

Question: I have difficulties understanding units within vacuum technology – can you help me ?

Answer: Since 1960 there is an international official system of units called „SI system“. “SI” is the abbreviation for “Système International d'unités“. There are seven fundamental units. Examples are length in metres, time in seconds and mass in kg. The most important physical value in vacuum technology is pressure. The pressure unit can be derived from standard SI units. According to definition pressure equals force (unit Newton = m·kg/s2) per area (unit square metres). So the pressure unit is Newton per square metre or with a specific name “Pascal” which commemorates to the French “vacuum pioneer” Blaise Pascal.

Additional information: In vacuum technology and leak detection there are still many historically grown units in use. These are different according to their geographical origin. The SI unit Pascal (Pa) ist he unit of choice in Japan. In the USA the unit “Torr” still is frequently used and Central Europe still describes pressuers in bar or millibar (milli = 1/1000). The following table summarizes units and conversion factors for frequently used pressure units
 

Pumping speed of vacuum pumps is also given in different units. Pumping speed of displacement pumps in rough and fine vacuum often is given in m3/h (possibly cfm = cubic feet per minute in the USA). The pumping speed of high vacuum pumps is measured in l/s.

In addition the gas flow or throughput of a pump in a vacuum process is of interest. The same units apply for the leak rate which is a gas flow through a leak channel. Please find below the most important units for gas flow.

“NTP” stands for „Normal Temperature and Pressure“. Please be aware that this is not equal to „STP“! Standard Temperature and Pressure (STP) are following regulations with world wide validity. According to DIN 1343 “Reference conditions, normal conditions, normal volume; concepts and values” standard conditions are defined for an absolute temperature of 273.15 K (0°C) and a pressure of 1013.25 mbar (= 101325 Pa). However, NPT are not unambiguously identified. There are variations according to region and applications which are covering pressure, temperature, and possibly humidity.

If you discuss a leak rate specification with a customer or a supplier please make sure that both parties are talking about the same conditions for the test. However, the deviations often are very small. An example: if you need to convert a given leak rate to the unit “atm cm3/s” (which is frequently used in the USA) the conversion factors for time (1 h = 60 min = 3600 s) and volume (1 m3 = 1000 l = 1000000 cm3) are precisely defined. However, pressure in “atm” can be a “technical” atmosphere with a value of 1,02 x 105 Pascal or a “standard” atmosphere with a value of 0,987 x 105 Pascal.

Another example: in the table above the conversion of “Pa·m3/s” in “cm3 (NTP)/s” was calculated with a pressure of 1013.25 mbar. The conversion of sccm and slm was made with the even number of 1000 mbar. The error you are making in this conversion is around 1 %. Our recommendation: be precisely in unit conversions but keep an eye on practical considerations. The errors due to deviations of pressure gauges or mass flow meters or theoretical assumptions and simplifications in calculations are definitely bigger !

So far for the most important units. And if somebody specifies a flow rate in “LUSEC” you may know that this is 0,001 Torr·l/s but please ask your partner to use a more conventional unit.

Did we forget a unit which is important for your daily work ? Do you think that a unit conversion tool would be helpful on this web site ? If so, please contact us.