The MGA With An Attitude
Using a Portable Air Tank -- AR-104

At 12:37 PM 3/25/04 -0500, Richard Pence wrote:
".... Can someone there tell me or direct me to a chart representing the amounts of air used for different applications?"

Generally no. There are specifications for nearly all individual air tools telling the rate of air consumption in Cubic Feet per Minute (which is really Standard Cubic Feet per Minute). There are specifications for nearly all air compressors telling the rate of supply in Cubic Feet per Minute (which is really Standard Cubic Feet per Minute).

"In particular, with a 7 gallon tank filled with 100 psi, how many tires could you 'top-off' adding five psi per tire assuming standard 15 in tires?"

The question is too broad, not enough information about the size of the tire (need to know the internal volume) or the required peak pressure in the tire.

"Is there a formula or chart that you could direct me to?"

See formula for adiabatic expansion of air.
      V2/V1 = (P1/P2)^0.71
Volume is in Cubic Feet
Pressure is in PSIA   (Absolute pressure, not atmospheric or gauge pressure)

"With so many gas stations charging for air now, I'm considering just buying an air tank and keeping it filled with the remaining compressor in my area and just using it to keep my tires properly inflated."

That's not a bad idea, but I can tell you that the "portable" tank will need be larger than you may have in mind if it is to save you many trips to the gas station. One cubic foot is 7-1/2 gallons. A small car tire is close to 1 Cu Ft in volume. Tires on a lot of sport utility vehicles are fairly large, possibly more than 2 Cu Ft in volume.

When using the portable air tank the final pressure in the tank has to be higher than the final pressure in the tire. If you need 35 psi in the tire the tank pressure can never drop below 35 psi. The calculation for supply volume available from the tank has to be based on the variation in the working pressure range. If you can only get 100 psi from the gas station, then the tank working range will be from 100 to 35 psig (gauge pressure). Because atmospheric pressure is 14.7 psi (at sea level), the absolute pressure range will be 114.7-49.7 psia.

V2/V1 = (P1/P2)^0.71
V2 = (P1/P2)^0.71 x V1
V2 = (114.7/49.7)^.71 x 1cu.ft.
V2 = 1.81 cu ft.

Since 1 cu ft of air has to remain in the tank, only 0.81 cu ft of air (at the new pressure) will come out of the tank with this pressure change.

The relatively small tires on my MGA, 165-15, have about 1.2 cu ft internal volume (including the space in the rim). If I want to raise pressure here from 30 to 35 psig (44.7-49.7 psia), I need to calculate how much the air originally in the tire will be reduced in volume.

V2 = (P1/P2)^0.71 x V1
V2 = (44.7/49.7)^.71 x 1.2 cu.ft.
V2 = 1.113 cu ft.

The amount of air needing to be put into the tire at 35 psi is then
1.2 - 1.113 = 0.087 cu ft.

Your 1 cu ft tank can give out 0.81 cu ft at that pressure (going from 100 psi to 35 psi). So, ....
0.81 / 0.087 = 9.3
You could possibly do that 9 times (with this size tire) before you need to refill the tank. That's the optimistic view of the situation.

If the tire was twice the volume (like on a sport ute), then you could only do it 4 times. If the larger tire was half flat and needed to be raised from 20 psi to 35 psi, then the 1 cu ft air tank would be hard pressed to do it just once.

In short, a small portable air tank is good for making small adjustments in tire pressure, but not so good for filling flat or half flat tires. For the use you have in mind, I might recommend getting a larger portable tank, maybe 1.5 or 2.0 cubic feet in volume. At the gas station you should fill the car tires first, then put air in the tank to the maximum pressure and take it home for future use. You also need to be sure that the tank is absolutely sealed, as the smallest leak could deflate the pressure in a short time.

The tank can store much more air at higher pressure. Some service stations will have air supply pressure as high as 150 psi for use in large truck tires. Most portable air tanks are rated for maximum working pressure of 150 psi. The portable tanks also commonly have a pressure relief valve which may be preset or can be reset to 150 psi. You should set the pressure relief valve on the tank to no higher than the tank's maximum working pressure rating. Then fill the tank to a pressure slightly below that setting. Keep in mind that warming of the tank can increase pressure in the tank rather dramatically, so do NOT fill it right to the max and then leave it sitting in the sun.

When autocrossing I commonly carry the race tires in the trailer, leave them near the desired working pressure, and carry a small (1 cu ft) air tank to make minor pressure adjustments at the track. Occasionally I may lend the small portable tank to someone who needs to raise their tire pressure from 25 psi street pressure to 35 psi competition pressure. The trick there is to do the front tires first, which usually need higher pressure for competition, and the rear tires last, in case there's not enough air left in the tank to finish out at 35 psi. The 1 cu ft tank starting near 150 psi can usually service one car in this manner (if the tires are not too big), and that's about it. Lots of serious race enthusiasts carry larger tanks to the track, commonly 2 cu ft filled to 150 psi.

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