Relations of Vacuum Degree, Positive Pressure and Negative Pressure and Corresponding Relations of Vacuum Unit Conversion

- Jul 14, 2019-

First, to put it simply, these three concepts correspond to the rarefied, normal and dense states of gases respectively.

Atmospheric pressure: refers to the atmospheric pressure, that is, the gas pressure produced by the atmosphere in which we normally live. A standard atmospheric pressure is 101325 Pa. 100,000 Pa = 100KPa, so "a standard atmospheric pressure" is also commonly expressed as 100KPa or 101KPa. The actual atmospheric pressure is not the same as the standard atmospheric pressure because of different geographical location, altitude, temperature and so on. But for simplification purposes, atmospheric pressure can sometimes be approximated as a standard atmospheric pressure, i.e. 100KPa.

Negative pressure: refers to the state of a gas that is lower than the atmospheric pressure, which is what we often call "vacuum". For example, when you drink a drink through a pipe, the inside of the pipe is negative pressure, and the inside of the suction cup used to hang things is negative pressure.

Positive pressure: refers to the state of a gas that is higher than the atmospheric pressure. For example, when inflating a bicycle or car tyre, positive pressure is generated at the exhaust end of the inflator or pump.

2. In many applications, such as scientific research, bioengineering, automatic control, environmental protection, water treatment and so on, gas sampling, gas circulation and object adsorption are often needed. At this time, vacuum pumps are needed. Its main parameters are vacuum degree, flow rate and so on.

(1) "Vacuum degree" generally refers to the limit pressure that can be reached when the pump is working, that is, the degree of rarefaction of the remaining gas after it is pumped out of the sealed container.

In industry, there are two kinds of limit pressure indicators, one is "absolute pressure". That is to say, "absolute vacuum" (absolute vacuum that can be achieved theoretically, nothing) is zero, and the values are positive. The smaller the number, the closer the absolute vacuum, that is, the higher the vacuum degree. For example, we have a "high vacuum" miniature vacuum. Its limit pressure is 10KPa (0.01MPa). In the miniature vacuum pump, the vacuum degree is very high.

The other is "relative pressure", that is, the atmospheric pressure as zero, lower than the atmospheric pressure with negative values, so called "negative pressure". The greater the absolute value of this negative value, the higher the vacuum degree.

In the international vacuum industry, "absolute pressure" is the most common and scientific mark, but because the method of measuring relative pressure is simple and the measuring instruments are universal (such as general vacuum gauges are relative pressure gauges), it is customary to use "relative pressure" to mark in China.

The relationship between them is: relative pressure = absolute pressure - local atmospheric pressure.

For example, the absolute pressure of VCH1028 is 10 Kpa, and its relative pressure is 10-100=-90 Kpa (-0.09MPa).

(2) In the fields of scientific research, laboratory, medical treatment and other fields, there are often applications of gas pressurization, such as: pumping air into a container with positive pressure itself, or the resistance of the system is relatively high, so it is necessary to overcome the resistance of the pump. At this time, it is necessary for the pump to output a positive pressure higher than the atmospheric pressure, usually expressed as "relative pressure". Our high-pressure micro-air pump and micro-vacuum pump can output a maximum positive pressure of more than 100 Kpa (0.1 MPa). They belong to dry vacuum pump. They do not need vacuum pump oil and lubricating oil, do not pollute working media, and can operate continuously for 24 hours. The exhaust end can be blocked, so they are especially suitable for these occasions.

Comprehensive examples: (not particularly rigorous, just to illustrate the relationship between the three)

Assuming that the gas pressure in the closed vessel is atmospheric, that is to say, there are 100 gas molecules in the vessel. VCH1028 with negative pressure of - 90Kpa can finally remove 90, and the remaining 10, then the negative pressure in the vessel is - 90Kpa; PH2506B can only remove 75, 25, and the corresponding negative pressure in the vessel is - 75Kpa.

If PCF5015N is used to blow air into the vessel, there will be 200 gas molecules in the vessel. The absolute pressure is 200 Kpa, and the relative pressure (positive pressure) is 100 Kpa.

The universal "vacuum degree" in the international vacuum industry refers to "extreme vacuum, absolute vacuum degree and absolute pressure", but "relative vacuum degree" (relative pressure, vacuum gauge pressure, negative pressure) is more widely used because of its simple measurement method and the widespread use of measuring instruments.

Conversion formula: relative vacuum = standard atmospheric pressure - absolute vacuum

For example, if the absolute vacuum is 80KPa, the relative vacuum is about 100-80=20KPa, which should be displayed as -0.02MPa on the relative vacuum meter.

Conversion of pressure unit: 1 Pa = 1.02 *10-5Kgf/cm2 = 1 *10-5bar = 0.01mbar = 9.87 *10-6atm = 7.5 *10-3torr = 4.01 *10-3inH20 = 7.5 *10-3mmHg = 1.45 *10-4PSI