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I just used a simple car tire air pump with a gauge like the one in the picture. The device has a handle, a gauge and a button. Squeezing the handle fills the tire with air, and the button releases it.

I noticed, that when the handle is squeezed, the gauge reading momentarily drops to almost zero. When I let go, the reading jumps back up, this time with a higher pressure reading. How do these gauges work internally, and why does the reading momentarily drop when the tire is filling?

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The implementation may vary but, to my understanding, the basic idea is that the switch of the handle rotates a plunger (for lack of a better word), which connects the tire to either the pressure gauge or the air supply

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figure: a generic schematic - only for illustration purposes.

The handle trigger is spring loaded so it returns to its position.

So what happens when you connect the pressure gauge, is that the tire gets connected with the pressure gauge and you read the pressure.

When you press the button, the rotation of the plunger/valve results into the external air pressure feed to be connected to the tire. This results in increasing the pressure in the tire.

When you release the handle, the plunger returns to its default position and the pressure is properly read again.

When the handle trigger is activated, after the momentary drop at the bottom, you can see that the pressure gauge sometimes reads a lower value. This may be due to leakage from the port, or how the ports are connected (again this may vary).

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  • $\begingroup$ Thank you, good answer. So the reason is quite simple. I realized however, that some gauges do the opposite of what I described - the pressure reads quite higher when the handle is pressed. What could be the reason for this? What comes to mind is that this higher pressure is the pressure inside the air tank (I assume the air held in the tank is quite much higher than the tire pressure). Maybe the plunger moves somewhere between the tank and the gauge, thus putting the air from the tank in contact with the gauge? $\endgroup$
    – S. Rotos
    Feb 11, 2022 at 13:10
  • $\begingroup$ That is also probably depended upon the handle implementation. E.g . if there is a rotational valve involved then as the valve rotates then momentarily the air can be compressed and show that higher reading. $\endgroup$
    – NMech ♦
    Feb 11, 2022 at 14:10
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Because the handle operates a valve which cuts the supply to the pressure gauge while air is pumped into the tire.

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