CO2 for filling tires- truths and myths

***NOTE***
I'd like to treat this thread as an engineering analysis of CO2 and its uses. If you're a member of the "fill with nitrogen or nothing" church, it might sting a little. Disagreements are easily solved by simply backing up claims with data and/or calculations.

So I'm building myself a "home-built" CO2 system for filling tires, and it's amazing how many myths are floating around out there regarding the pros and cons of filling with CO2 vs. Nitrogen/Air. I thought I would try and do my part in confirming/debunking many of these myths in one spot. My goal is this thread will be linked to as a reference for future questions (in any forum) regarding CO2 and its advantages/disadvantages.

Hopefully, it becomes a useful resource. :wrenchin:

CLAIM #1: You can carry more CO2 in a tank of a given size than N2

Status: True!

Reason: CO2 condenses to a liquid at about 700psi (depending on temperature). This means two things:

• CO2 as a liquid is dense (lbm/ft^3), so in a tank of a given size you can have more of it in a tank at lower pressure
• Pressure is essentially constant in the tank as long as there is liquid in it. (Note that pressure will change as the tank is used and cools, but will stabilize once the temperature stabilizes after use.)

Thermophysical properties of many gases (including CO2 and N2) can be found for free here: http://webbook.nist.gov/chemistry/fluid/

Nitrogen is typically stored in a tank at 3000psi. Even at this pressure it's still a gas. This means that pressure in the tank goes down as its used, and you need really high pressures to store a lot of it. Typically N2 at 70F and 3000psi is about 14.12 lbm/ft^3. At 35 psi, N2 is 0.173 lbm/ft^3. This nets an expansion ratio of about 81.62.

Carbon Dioxide is typically stored in a tank at the saturation pressure of CO2, which is around 700-1000 psi depending on temperature. Typically a full tank is 3/4 fluid. CO2 at 70F and 1000psi is about 49.92 lbm/ft^3. At 35 psi, CO2 is 0.274 lbm/ft^3. This nets an expansion ratio of about 182.

So, for a fixed-volume tank, you can carry 2.23 times more in CO2 than N2

CLAIM #2: CO2 expands a lot more than Nitrogen when heated, screwing up your tire pressures.

Status: False because it's an exaggeration.

Reason: All gases expand when heated, and it's technically true that CO2 expands more than N2. But, it's possible to calculate HOW MUCH MORE using isentropic gas relationships (http://en.wikipedia.org/wiki/Isentro...r_an_ideal_gas)

Basically, it all comes down to the ratio of specific heats for the gas. Here's a nice list: http://www.engineeringtoolbox.com/sp...tio-d_608.html

The ratio of specific heats for CO2 is about 1.28
The ratio of specific heats for N2 is about 1.4 (same as air)

Take a tire that is at 70F and 35 psi, and that tire gets heated to 100F. If the tire's full of N2, it's pressure will rise to about 42 psi (calculation link). If the tire's full of CO2, it's pressure will rise to about 45 psi (calculation link).

Now take the same tire at 70F and 35 psi, and cool the tire to 30F. If the tire's full of N2 it's pressure will drop to 26.6 psi (calculation link). If the tire's full of CO2 it will drop to 24.5 psi (calculation link).

This seems to me to be a negligible difference considering all of the other factors that can play into tire pressure. If you're a racer and every pound counts (and your tires get really hot), it might matter. Off-road and daily driving, no problem.

CLAIM #3: CO2 "carries a lot more moisture" than nitrogen, adding water to the tire when you fill it.

Status: False, both gases are in pressurized cylinders and don't have water in them. AFAIK, CO2 doesn't "cling to water" any more than N2... EDIT: I've read a couple of places that a "dry gas" expands less with temperature than a "wet gas," so this myth basically goes back to misinterpretation of terminology. "Wet" or "dry" refers to the gas's thermal expansion properties, not water content. See Claim #2 for explanation of gas expansion.

CLAIM #4: CO2 "is more corrosive to tires" than nitrogen, breaking down your tires faster.

Status: False, there is no chemical reaction between the CO2 and tire rubber that I'm aware of. Someone can feel free to present the chemical process if they think it's true, but as far as I know the biggest causes of tire breakdown are oxidation (oxygen), heat, and UV light.

CLAIM #5: CO2 tanks cannot be used on their side, Nitrogen can.

Status: True, but False :tongueout:

Reason: It's true that CO2 cannot be used on its side, because the regulator is designed to work with gaseous CO2 at the top of the tank. If it sucks up liquid CO2, it can break the regulator and make you have a generally bad day.

BUT, N2 shouldn't be used/stored on its side because storing and/or using a high pressure cylinder (especially N2 at 3000 psi) is an accident waiting to happen. If you are involved in a crash or rollover and the regulator breaks off the bottle, you've got a projectile that's going somewhere you don't want. OSHA recommends all high pressure cylinders be stored and used in an upright position:

29 CFR 1926.350(a)

(9) "Compressed gas cylinders shall be secured in an upright position at all times except, if necessary for short periods of time while cylinders are actually being hoisted or carried".

http://www.osha.gov/pls/oshaweb/owad...ONS&p_id=20650