Check the air tightness of the pressure gauge, and press the rubber film of the metal box with your fingers. For example, the liquid level on both sides of the U-shaped tube has not changed.

The pressure gauge senses pressure through the rubber membrane. When the rubber film is under pressure, the gas pressure in the hose becomes larger. If it is greater than atmospheric pressure, there will be a height difference in the liquid level in the U-shaped pipe. The greater the height difference, the greater the pressure on the rubber film. If the hose connected with the rubber membrane leaks, whether the rubber membrane is compressed or not, the pressure of the gas in the hose is equal to atmospheric pressure, and the liquid level in the manometer will not be high or low.