I was surprised at the workings revealed, when the end fell
off the tire gage I was using. Then I was surprised that
I was surprised. The method is so intuitive.
In a cylinder, place a rubber piston facing the air valve.
Its movement is resisted by a helical spring along the length
of the cylinder. The piston's final position is indicated by
a plastic calibrated rule sticking out at the end.
The piston is reset by pushing the rule back in.