Summary of commonly used vacuum measurement units for helium mass spectrometer leak detector
Helium mass spectrometry leak detection often uses the concept of vacuum, vacuum detection sensitivity, accurate positioning, but also can realize the leak detection of larger containers or more complex structure products. We have discussed about vacuum leak detection before, and today we are talking about the unit of measurement of vacuum. Here we begin:
Is a vacuum really empty?
Most people may think that a vacuum is “really empty”, that is, “nothing exists”. In fact, even if the highest vacuum degree can be obtained at present (~10-11Pa), there are still about 3000 gas molecules per cubic centimeter (calculation conditions: temperature is assumed to be 25 ° C, vacuum vacation is set to 1×10-11Pa), which is obviously contrary to the previous statement. The meaning of vacuum refers to a gas state below a standard atmospheric pressure in a given space, which is a physical phenomenon, not a void space where nothing exists.
At the 7th International Metrology Conference in 1927, standard atmospheric pressure was defined. The definition conditions are as follows: gravitational acceleration is 980.665cm/s2; Mercury temperature is 0℃; Mercury density 13.5951g/cm3. Under these conditions, the pressure generated by the 760mm high mercury column is called standard atmospheric pressure. Its value:
1 ATM = 1013250.144354 dyn/cm2
Standard millimetre of mercury (mmHg) values are thus defined:
1mmHg=(1/760)atm
It can be seen that this standard atmospheric pressure depends on the measurement accuracy of mercury density and cannot give the final value. Therefore, the tenth International Metrology Conference in 1954 redefined the standard atmosphere. Its value:
1atm=1013250dyn/cm2=101325N/m2
Although the standard atmospheric pressure has been redefined, mmHg is still defined in 1927.
Mm of mercury is not an easy pressure unit to write. Therefore, in order to commemorate the discovery of vacuum Torricelli (Torricelli), Germany named the millimeter mercury “Torr”. Later, the International Organization for Standardization used “Torr” as a unit of pressure. The pallet value is defined as 1/760 of standard atmospheric pressure (as defined in 1954). To wit:
1Torr=(1/760) atm
Due to the difference between the standard atmospheric pressure values defined in 1927 and 1954, 1mmHg is 1.9×10-4 Dyn /cm2 larger than 1Torr. Due to the small difference between the two, it is generally considered that 1mmHg is approximately equal to 1Torr.
In 1958, at the first International vacuum Conference, it was formally stipulated that “Tuo” was used as the unit of measurement of vacuum degree. Subsequently, the unit was also widely used in our country.
The pressure unit in the International System of Units and China’s legal system of units of measurement does not use mmHg, nor does it use Torr, but uses N/m2.
In order to commemorate the 17th century world famous French mathematician, physicist Pascal, proposed by France, and passed by the International Metrology Commission, to N/m2 a special term, called “PASCAL”, referred to as “Pa” (Pa).
In 1971, the tenth International Conference on Measurement officially adopted the PASCAL as a unit of pressure, and its value is:
1Pa=1N/m2 =7.5006 ×10-3Torr
Because this unit is easy to read and write, and can be unified with other units of measurement, it is gradually used by various countries.
In the International System of Units, the pressure unit also uses bar (bar) and standard atmospheric pressure (atm), the relationship between the two and Pa is as follows:
1bar=105Pa
perhaps
1 ATM = 1.01325 x 105 pa
In engineering applications, the value of 1atm is often approximated as 1×105Pa.
In the manufacture of vacuum equipment, when the use pressure is higher than 100Pa, sometimes the percentage vacuum degree is used to indicate the pressure, that is
δ=[(105-p) /105]×100%
Shizhong
Delta – percent of vacuum;
p — Gas pressure in vacuum equipment, Pa.
The pressure units stipulated in China’s legal system of measurement units are Pa (Pa), gigpa (1GPa=109Pa), mpa (1MPa=106Pa), kpa (1kPa=103Pa), millipa (1mPa=10-3Pa), micropa (1μPa=10-6Pa). In the field of vacuum technology, the unit of measurement of vacuum degree is Pa.
At present, the unit of measurement of vacuum is different from country to country, and the commonly used units are Pa (Pa), bar (bar), millibar (mbar), Torr (Torr), millimeter mercury (mmHg).
After reading the above content, I believe you have a certain understanding of the vacuum measurement units, these units are used to measure the vacuum degree of a given space, and in order to achieve a specific vacuum degree, you need rotary vane pump, Roots pump, molecular pump, ion pump and other vacuum pumps to obtain equipment, then how to choose a vacuum pump?
Select vacuum pump need to pay attention to the following points:
The working pressure of the vacuum pump should meet the limit vacuum and working pressure requirements of the vacuum equipment. The vacuum degree in the selection is half to an order of magnitude higher than the vacuum degree of the vacuum equipment.
Considering the total discharge time requirement of the gas volume generated in the process of vacuum equipment, the pumping rate of vacuum pump with load is measured.
Fully consider the composition and characteristics of the pumped medium, select the type of vacuum pump, and also consider the combination of a variety of pumps.
Considering the vibration of the vacuum pump itself and the influence of the exhaust gas on the whole process and the surrounding working environment.