Higher psi in a rated 150psi tank

[smokenyall]

Okay I have this air tank

http://bagriders.com/modlab/products/ANODIZED-ALUMINUM-2-GALLON-TANK.html

It's rated at 150psi , right now I have a 120/150 cutoff switch. Would it be safe to run 175psi or 200psi in the tank with a proper pressure switch. Reason I'm asking is because every time I go from layed out to ride height my compressors turn on.

[Oh Damn, it's Sam]

I wouldn't personally do it, but I'm kind of a safety Nazi at times. Even at 150 psi, there is an enormous amount of stored energy in that tank.

Another issue is whether 200 psi would solve your problem, which I suspect it wouldn't. At 200 psi, your pressure switch is going to be something like 175 on, 200 off, and you're still extracting the same amount of air from the tank every time you air up. It sounds to me like the compressor is still going to kick on every time you go to ride height.

I think the real issue here is tank capacity, not pressure.

[smokenyall]

There's no room for a larger tank, my theory was that if there's more dense air in the tank it would make the pressure drop less drastic.

[Oh Damn, it's Sam]

I don't think that's the case, but we can easily do a little math and find out.

Give that at STP, one mole of garden-variety air occupies a volume of 24.5L, we know that your 2-gallon tank holds 0.3 moles of air (2 gallons is approximately 7.57L, 7.57/24.5 = 0.31, rounded for significant figures to 0.3 moles)

Now, we need to figure out how many moles of air are in the tank at any given pressure. To do this, we convert PSI to ATM, and multiply by our 0.3 mol figure we found above. Since we know your compressor is kicking on when you air up, we know you're dropping to at least 120 psi, so we'll use that as our reference:

PSI:  120   150   200
ATM:  8.2  10.2  13.6
Mol:  2.4   3.1   4.1

If we subtract the number of moles of air at 150 from 120, we see that your are using a minimum of 0.7 mol of air to fill your bags. Thus, we can subtract 0.7 mol from our 200 psi figure to see how many moles of air will be in the tank when you air up:

4.1 - 0.7 = 3.4 mol

Now, we need to know what the pressure inside the tank will be at when there are 3.4 moles of air inside. Using our ideal gas law, we know that if we divide the number of moles we have at our unknown pressure by the number of moles at STP, we will get the pressure in the tank in atmospheres:

3.4 mol / 0.3 mol = 11.3 atm

Now we convert atm to psi:

11.3 atm / 0.068046 atm per psi = 166.6 psi.

Thus we know that with a standard 175 on / 200 off pressure switch, the compressor will kick on.

[smokenyall]

Thank you for the in depth answer, guess ill just be keeping my 150psi switch.

[Alex@BagRiders]

You will be completely fine running 200psi in a tank that is stamped for 150psi. Tank manufacturers pressure test tanks up to 3 times the amount stamped on them.

An example of this would be the AccuAir 3 and 5 gallon tanks. These tanks are rated for 150psi but AccuAir's systems will allow you to run 150psi, 175psi, or 200psi. As the manufacturer, they would not allow this to be possible if it was not safe.

I have an AccuAir 3 gallon tank with their management running at 200psi right now and it's been like that for a few years. No issues there!