Perplexed about PSI Change when Airing Down/Up

[OzX]

A couple of weeks ago I was on a short run and as usual I put my Stauns on the valve stems at the trailhead to air down.

I usually pay more attention to them so that they "stop" at the selected PSI (14psi) by pulling on them, but I got distracted and just let them stop by themselves.

When the Stauns 'finished' all tires were at 10 psi (I always check afterwards with a tire pressure gauge). Even though it was lower than I usually run them at I figured what the heck and let it go.

Air temp was about 70 degrees and the elevation I'm guessing at the trailhead is about 4500'.

NOW 3 hours later after wheeling moderately on the egress, back at the trailhead the PSI at each tire was between 18-20 PSI. YEs the ambeint air temp was up to about 80-82 degrees and I imagine the action of wheeling will warm the inside air of the tires and increase the PSI.

WHAT I want to know is it possible to figure the TRUE PSI of what your tires are doing, no matter what the external temps and/or elevation is.

Visually the tires felt fine and had a nice bulge when aired down.

ANYONE else ever notice this discrepancy between PSI after airing down at beginning and end of a run. Is there a good rule of thumb here?

THANKS

 

[Herbie]

Well, the PSI you measured at the end of the run was still the "true" pressure.

Pressure of a gas is defined as a function of temperature, volume, and the number of "moles" of gas present in the container.

If you want to try to calculate a "before" and "after" pressure based on temperature, you can derive it from the ideal gas law:

pV=nRT where:
p=pressure
V=volume
n=number of moles of gas
T=temperature
R=a constant, you can lookup the value according to the units you'll use.

The volume will not change significantly (slightly due to ballooning of a rubber tire, but neglect this), the number of moles is fixed, and R is a real constant, so you can re-juggle the formula:

p/T = nR/V

this way, everything on the right side is "constant" (or at least constant-ish), so therefore we can say this:

p1/T1 = p2/T2
where:
p1 is the initial pressure
p2 is the later pressure
T1 is the initial temperature
T2 is the later pressure

We re-juggle again:

p2 = p1 * T2 / T1

So, your "new" pressure is the old pressure times the ratio of the new and old temperatures. If you can accurately measure (or predict) the temperature before and after a run, you should be able to predict the resulting change in pressure.

Now above I mentioned that this was the "ideal" gas law. This means that it covers the behavior of an "ideal" gas, which doesn't really exist. But for basic calculations, this should be accurate "enough", much as we can safely ignore a small change in tire volume.

Bottom line: Heat in your tires changes the pressure. More than most people think. Moreover, your tires change temperature more than most people think. Tire temperature can shoot up 50-100 degrees over ambient on a normal tire, and much more on a tire doing serious work like a race car tire (or possibly even an offroad tire)

If you want to try to minimize this effect, you could fill your tires with 100% nitrogen like race cars, etc. do, since nitrogen changes pressure against temperature less than "air", but since air is like 78% nitrogen anyway, you're really chasing a small delta with that strategy.

 

[njjeepthing]

Yep, I aired down to 14psi last time out, when I aired up at the end of the day they read higher than the 14psi they were in the morning, only by a couple psi. Than when I aired up to 30 psi and checked when I got home they were again higher by a few psi. I didn't really give a lot of thought to it, just figured I was in a hurry and the gauge got a inaccurate reading. But I would like to hear some "technical" reason this happens!

I had a set of Staun's they never worked right, I went back to the old fashion way of doing things!

 

[jcbrandon]

...the ambeint air temp was up to about 80-82 degrees and I imagine the action of wheeling will warm the inside air of the tires and increase the PSI...

I think tire temperature explains the increase. At 10 psi, you probably had a lot more friction and subsequent heat than you were used to. Heat expands the air inside the tire. The air takes up more space and exerts more pressure, as shown on your tire gage.

 

[Rando]

Your tires heating up wouldn't explain an increase form 10 to 20 psi. As herbie explained your tires will follow the ideal gas law to within the uncertainty of your pressure gauge at least. However you need to realize that the ideal gas law is based on absolute temperatures and pressures. So if your tires were at 10 psi ( = 24 PSI absolute) at 70F (295 K), they would have to heat up to 418K or 290F! to reach 20 psi (34 psi absolute), which they clearly didn't. Changes in tire pressure due to temperature are on the order of 10%.

I suspect there is something up with your pressure gauge.

 

[Hill Bill E.]

A jump from 10 to 20 psi does seem extreme.

The pressure will go up after driving, that's why they reccomend checking the pressure when the tire is 'cool'.

My Stuans don't work well either, so I set them around 15, then use my guage to get them all equal at the psi I want.

I use a 0-20 guage made for ATV's, for a more accurate reading. Using a tire guage that's made for 10-60+ pounds isn't really accurate at low psi.


Last time out, I set mine at 18psi (usually go down to 10-12, but it was a 20 mile drive to where we could air up)

3 of the tires were at 22 psi, and one at 24, when we aired up.

Not sure the exact change, as I was using my higher pressure guage, becuase the 0-20 went off the chart when I went to double check.

(I was double checking the pressure, becuase 2 of the tires looked lower than when I started)

 

[rezdiver]

one reason i can see for this issue is that when you rapidly air down, you are cooling the air in the tire to below normal temp as a result of air expansion. so once you think you are at 10 psi, that is the pressure at the cooler temp inside the tire.

once it reaches ambient temp then the pressure will rise, and once you start driving and it heats up it rises even more.

 

[Herbie]

one reason i can see for this issue is that when you rapidly air down, you are cooling the air in the tire to below normal temp as a result of air expansion. so once you think you are at 10 psi, that is the pressure at the cooler temp inside the tire.

once it reaches ambient temp then the pressure will rise, and once you start driving and it heats up it rises even more.

This is a good point. The process is called adiabatic cooling.

A good parallel is that when filling CO2 canisters, they are usually chilled first to get a good fill. The easiest way to chill them is to add a small amount of liquid CO2, then vent it rapidly. I have had paintball tanks vent so fast that the temperature dropped far enough to form small chunks of dry ice inside the tank (which rapidly sublimated off).

 

[OzX]

one reason i can see for this issue is that when you rapidly air down, you are cooling the air in the tire to below normal temp as a result of air expansion. so once you think you are at 10 psi, that is the pressure at the cooler temp inside the tire.

once it reaches ambient temp then the pressure will rise, and once you start driving and it heats up it rises even more.

Excellent Point!! Makes sense for the big jump.

Thanks for everyone's contributions

 

[T.Low]

I thought you said you were in cell phones!

Herbie, where do you come up with this stuff?

 

[Hilldweller]

FWIW, I also carry an infrared thermometer with me and make sure my tires don't get too hot. 160 to 180 degrees is about as high as I like it; if they're hotter, I add a little air.

 

[I Leak Oil]

Herbie, where do you come up with this stuff?

Basic thermodynamics. I still have my text book from Koledge if you want some (cough) good ready by the camp fire!

 

[Lynn]

I got a BS Computer Engineering, and had to take Thermo and Statics and Dynamics.

Matter of fact, they kept dropping Thermo and Solid State Physics due to low enrollment, so to graduate I had to take them both as self-study my last quarter.

Sucked.

Big time.

My Statics and Dynamics instructor told us that the only reason it was in the curriculum was because he had to take a Circuits course when he was in school for Civil Engineering, so he insisted EEs and CEs take Statics and Dynamics as his 'get even.'