CO2 TO AIR UP TIRES

[Gnarly4X]

I learned something today, and lately I've been learning how much I don't know.

I've been actively doing scientific research for about 52 years, mostly focused on health science.

My last 6 years of serious rock crawling here in AZ, I used a 20 lbs. aluminum CO2 tank to fill up my tires from 10 to 15 psi to 25 psi on 33" tires. When wheeling the tank was securely strapped to the corner of my bed next to the cab. The CO2 tank would last about 6 months, so about 6 to 8 air downs, and I could fill up all 4 tires from 15 to 25 psi in about 5 or 6 minutes. A 20 lbs. aluminum CO2 tank is about $200. To fill it is about $20 to $30.

Fast forward to today...

12 months ago, I had a new set of tires put on my 2013 Corolla with new TPMS sensors. One sensor failed and I didn't want to mess with waiting for the other 3 to fail. Over the past 4 months, I have noticed the air pressure has been dropping by 5 to 6 psi from 32 psi causing the TPMS light to come on in the dash. I have been refilling the tires with my old CO2 tank. In 40 days, the pressure in all 4 tires was dropping to 23 to 24 psi. THAT I've never experienced. My first guess was the valves, but I checked them several months ago. My second guess was the TPMS units were somehow leaking.  I doubted it was the beads on all 4 tires, but it's possible.

I took the car into Discount Tire today and they said they cannot find any leaks. I was more than unhappy.  They pumped the tires to 35 psi and suggested I bring the car back in when the pressure dropped again.  I came home and lowered the pressure to 32 psi, factory max cold pressure.

I then decided to do some research. Low and behold I discovered that CO2 molecules are smaller than compressed air molecules and can pass through rubber tires quicker than air molecules. ALSO... CO2 can do something called permeation. The CO2 molecules can actually enter, permeate, the rubber. This can cause mild degradation and damage to the rubber over time.

SO.... for all you CO2 air up guys and gals, I'm just passing along some information that was a big surprise to me.