You lose some air by measuring the pressure, especially if you slip a little bit. As John says, a large change in altitude can change measured gauge air pressure. It would take about a 45 degree Fahrenheit change in temperature to change the tire pressure 3 psi when in the 20 psig range. A combination of these could be cumulative.
So I went back and assembled my unicycle again, and pumped it to 22 psi.
If I understand you guys correctly, if I keep putting the tire gauge in, the pressure will decrease. If the weather changes, (where I live is in my opinion the town with “weather moodswings”) that could raise or lower the pressure.
What I’ll do is not measure it any more. It’s at 22 right now and I know it.
I’ve never personally experienced this, but back a month or so ago on a ride with Terry, he told me that temperature plays a pretty big role. He said that car tires inflated to 40 PSI on a cold morning might have 55 PSI in them on a hot afternoon. Now, if air were an ideal gas, this would be impossible, but it’s clearly not, and I don’t know enough more about it to know for sure anything So… could it be the temperature at which you pumped it up?
Was it really hot when you pumped it up, and cold when you just tested it?
I AM sure, though, that tires do lose air quite naturally. I usually pump my 36er back up to 60 from 40 or 45 once every week or two. That’s a cool 25 percent change. Even that, though, isn’t nearly as fast a leak as what you’ve described. Couldn’t tell ya from here.
With low pressures like you are talking about it is most likely losing air when you are checking the pressure, but it could be a membrane leak. I have seen old tubes lose considerable air through the rubber, and it can happen if you are running a tube that is thin walled, or too small for your tire.
Check the size of your tube and if the valve is OK!
I bought a 2.0 tube at Denmark and used it in there and it was always 18lbs. When I came back I made a hole while assembling the wheel and changed the tube for a national 1.75 tube… I ALWAYS have to pump it… Everyday…
Bad quality tubes are around and good ones are expensive… I’m riding flat I don’t care, but if you ride street always check… and with big drops you’ll probably loose more air!
Check that the valve itself isn’t leaking. Use a valve core tool to unscrew and remove the guts of the valve. Clean off the valve, lubricate the gasket part of the valve with spit and screw it back in. That process will usually solve a leaky valve. You can get various styles of valve core tools at a local auto parts store.
A common valve core tool looks like this:
The part that unscrews the valve core is the bit at the top of the image.
Temperature can certainly play a big roll in pressure variation. That said, the temperature change required for the pressure change you describe is about 135 degrees Fahrenheit. That’s a big change in temperature. That’s a REALLY cold morning followed by a REALLY hot afternoon.
Air is pretty much an ideal gas in the parameter region we’re discussing. Why what you describe would be impossible given an ideal gas is unclear to me.
Gas temperature/pressure interdependence is not complicated to approximate. Most folks make mistakes by using the wrong pressure or temperature scales. To do these problems one must merely know PV=nRT and that the pressure P is the absolute pressure and the temperature T is the absolute temperature. Then reconverting to gauge pressure and relative temperature gives correct approximate results.
As for JC, although he is the only main man and there can be no other, hungry4uni clearly stated that he submerged his tube in the bathtub. I’m going to assume the tube was inflated, the tub was full of water, and the valve stem was under water. A leaky valve stem would have been apparent. JC rarely plays fast and loose, however.
It could be that the valve only leaks when there is more pressure inside the tube. A tube pumped up and put in a tub of water isn’t going to have much pressure inside. Put the same tube inside a tire and inflate it to 20 psi and that might be enough to get the valve to leak.
I’ve had a few leaky valves. It happens so it’s something that is worth checking.
The other possibility is that hungry4uni is pedaling so rapidly that the reactive centrifugal force is strong enough to depress the valve and cause air to leak. I leave it as an exercise for the reader to calculate how fast hungry4uni would need to be pedaling.
I experienced this a little over a week ago. The valve didn’t leak in the tub because I it’s not easy to put 20 psi in the tube without it being assembled inside the tire, and the valve didn’t leak at lower pressures. Once I reassembled and pumped up the tire to 30+ psi it became apparent that the valve was leaking.
To check for a leaky valve, just spit on your finger and hold it over the valve. You’ll feel the spittle sputtering if there is air leaking.