Originally Posted by
dwmckee
It would be interesting to have someone try this out to see how accurate it is. The slider type gauges I have used like this in the past tended to jump around 5 - 7 PSI on each stroke. I really like the digital gauge on the pump mentioned above because it steadily rises in pressure as you pump with just very small fluctuations.
The reality is that there are pressure fluctuations at the pressure gauge as you pump, so the jumping around accurately portrays what is happening. As you pump, the pressure in the pump body (where the gauge sits) exceeds the pressure in the tire, so air flows to the tire. There is energy (observed as pressure differential) lost as air flows in the hose and especially through the valve, so it takes some time for the pressure on the pump and tire sides to equalize. As air flows from the pump, the pressure in the pump decreases (remember PV=nRT from general chemistry?). If you had a separate pressure gauge on a second tire valve, you would see pressure rise as you pumped, but it would never overshoot the pressure that you observe at rest between strokes.