# I Filling a Tractor Tire Up With Two Tanks

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1. Jun 13, 2016

### Hunter235711

My dad and I were going to fill up a tractor tire that was very flat. We had two air tanks, a small (~5 gallons) and a large air tank (~10 gallons) which were both filled to the same pressure (~100 PSI).

We wanted to pressurize the tractor tire as much as possible with the two tanks, and so my dad asked me "would it be better to fill up the tire as much as possible with the small tank and then attach the large tank, or fill it up as much as possible with the large tank first and then attach the small tank?"

What do you all think?

2. Jun 13, 2016

### phinds

I think you should tell us what YOU think and why and then ask for comments.

3. Jun 13, 2016

### Hunter235711

Ok! I was thinking that we should use the larger tank first. I was assuming the residual energy left in a tank is the product of its volume and pressure, so it would be more important to minimize the residual pressure in the larger tank.

4. Jun 13, 2016

### phinds

I don't have a good technical enlargement, just a practical one. I would use the smaller tank first for the simple reason that that then leaves you with (most likely) one empty tank and one partial tank rather than potentially two partial tanks. You can then toss the empty tank or get it refilled, whatever one does with such empty tanks.

5. Jun 13, 2016

### Hunter235711

Thanks! I guess my question is more about the theoretical basis of the problem. If you have two tanks (T1 and T2) with volume V1 and V2 where V2>V1 and both with pressure Ph, and the goal is to maximize the pressure in a third tank with volume V3 and pressure P0 where P0<Ph, should you connect tank T1 and T3 first, allow them to reach the same pressure, then connect T2 and T3 and allow them to reach the same pressure? Or should you first connect T2 and T3 and then T1 and T3?

Thanks!
-Hunter

6. Jun 13, 2016

### phinds

Isn't that going to depend on the volume of the tire and the pressure you want to get into the tire? I do have to say I didn't really think this through completely. I, for some reason (stupidity?), did not consider the fact that the pressure will try to equalize, not just move gas willy nilly from the tank to the tire regardless of the pressure of the tire.

Last edited: Jun 13, 2016