Tips What is the relationship between relative pressure, absolute pressure and vacuum

Due to the working relationship, I often deal with fluid/gas pressure. Sometimes I chat with my friends on the spot and find that many of my friends are not very clear about the concept of relative pressure and absolute pressure. Today, we will talk about relative pressure. , The relationship between absolute pressure and vacuum.

Physical pressure belongs to the category of force, and the unit is "Newton (N)"; in engineering, what we call pressure

Force refers to the pressure per unit area, that is, pressure, and its international unit is "Pascal (Pa)". The "relative pressure" and "absolute pressure" discussed in this article belong to the category of pressure, but we are accustomed to the term pressure, such as "pressure sensor". Few people say "pressure sensor", so here we still use pressure To be called.

The environment in which we live is covered by the atmosphere, and the pressure generated by the atmosphere is called atmospheric pressure. How big is the atmospheric pressure? The earliest recorded atmospheric pressure measurement was by the Italian scientist Toricelli. As early as 1643, Toricelli filled a 1-meter-long thin glass tube with mercury and placed it upside down in a water tank containing mercury. He found that the height of mercury in the glass tube would drop after being inverted, but it would no longer drop after it dropped to about 760 mm. No air enters the space above the 760 mm scale, which is a vacuum state.

When the mercury rod is tilted, the height of the mercury in the vertical direction does not change, which is still 760 millimeters.

Meter. Based on this, Torricelli infers that the pressure of the atmosphere is equal to the pressure produced by the mercury column. This is the famous Torricelli experiment.

Scientists also found that the atmospheric pressure is not fixed, it changes with temperature and altitude. In order to calculate the pressure in science and engineering, a standard atmospheric pressure value is needed. Therefore, a resolution was made at the 10th International Conference on Metrology in 1954, stipulating that the standard atmospheric pressure is101.325kPa.

The commonly used units for calculating pressure in engineering are "bar (bar)" and "megapascal (mPa)". Their conversion relationship is as follows: 1 Bar (bar) = 1 standard atmosphere = 100 000 Pa (Pa) =100

kPa=0.1MPa (megapascal)=760mm mercury column. In engineering, "kg" is often used to express pressure

Strong, it means the pressure generated by an object weighing 1 kilogram acting on an area of 1 square centimeter, and the pressure of 1 kilogram = the pressure of bar.

Let's talk about relative pressure (pressure) and absolute pressure (pressure). The so-called relative pressure is the pressure calculated using the value of 1 standard atmospheric pressure as the reference point (zero point); the absolute pressure is the pressure calculated using absolute vacuum as the reference point (zero point). Engineering pressure refers to relative pressure (relative to standard atmospheric pressure) unless otherwise specified. For example, the project requires a pressure of 5 bar (bar), which means that an additional 5 bar (bar) is added to 1 standard atmosphere, and its absolute pressure is 6 bar (bar).

In engineering, the pressure above 1 standard atmosphere is called positive pressure, and the pressure below 1 standard atmosphere is called negative pressure. The magnitude of the negative pressure is measured by the vacuum value. When a closed container is pumped with a vacuum pump for a long enough time, there is no gas in the container, which is an ideal vacuum state, and the absolute pressure in the container is zero bar at this time. The vacuum value is the value of the pressure of the gas in the container relative to the absolute pressure of zero, and its unit is millibar (mbar). The lower the vacuum value and the closer to absolute pressure zero, the better the vacuum.

So remember one sentence: pressure is relative (relative to standard atmospheric pressure), vacuum is absolute (absolute pressure)