Psi is the basic unit of pressure in the international system of units, referred to as Pa, named in memory of Pascal, a French physicist and mathematician. Pascal made a famous experiment in 1648. He used a closed bucket filled with water, inserted a slender tube into the lid of the bucket, and poured water into the slender tube from the balcony of the building.

As a result, it took only a few glasses of water to crack the bucket, and the water in the bucket flowed out of the crack. It turns out that the bottom area of the thin tube is small, and a few glasses of water are poured into it, and its depth h is very large, resulting in great pressure. The liquid in the container only exerts the weight of the liquid on the bottom of the container, but it can crack the barrel, which is incredible to many people.

Torricelli experiment;

Torricelli experiment measured the specific value of atmospheric pressure. The glass tube with one end closed is filled with mercury, and the nozzle is blocked. Then insert it backwards in the mercury tank. When you let go of your finger blocking the nozzle, the mercury level in the pipe will not drop again. At this time, the height difference between the inside and outside of the tube is 760mm, and the reason why there is a mercury column with a height of 760mm in the tube is precisely because of the existence of atmospheric pressure.

According to the characteristics of liquid pressure, the pressure on the liquid surface in the mercury tank should be equal to the pressure below 760 mm Hg in the glass tube. The pressure on the surface of the liquid in the mercury tank is atmospheric pressure. Because there is a vacuum above the mercury column in the glass tube, the pressure in the tube can only be generated by the mercury column with a height of 760 mm. ..

Reference to the above content: Baidu Encyclopedia -—psi