How does a co2 tank regulator read a higher pressure then ~850psi.

[GeorgeOrr]

This is a very old thread, but I came across it when researching what pressure a CO2 bottle would reach when warmed. It is so full of inaccuracy, with minimal challenge, I felt compelled to add a reply in the hope that others might not be mislead or confused.

A charged bottle of CO2 that is below 88°F contains part liquid and part gas (vapor). The CO2 is said to be in a saturated state, or in vapor–liquid equilibrium. The pressure of any saturated vapor correlates to a unique temperature. That is, if the bottle contains both liquid and gas, then for any given temperature, there is only one possible pressure. The pressure in a bottle of pure CO2 at 72°F is 875 psi. (That is absolute pressure. For gauge pressure, subtract atmospheric pressure.) Things change at the “critical point”, which is approximately 88°F & 1070 psi. A fluid above its critical temperature and pressure takes on characteristics of both a liquid and a gas, and evenly fills the available volume. This is called a supercritical fluid. Temperature and pressure are still directly related, but not uniquely correlated. The density of the supercritical fluid is now a third variable.

@shinybrain

What I don’t understand is how the regulator can read a higher pressure if the maximum pressure of a gas form of co2 is about 850 psi at room temp.

It can’t. Either the temperature of the bottle is substantially higher than you think or the gauge is wrong.

@minor7flat5

…CO2 becomes a supercritical fluid above 73 ATM.

Not exactly right.

@snailboy

It also has to be above 31.1 degrees celsius though, which isn’t very hot but above room temperature. And since liquids aren’t compressible, I don’t see how you could have that pressure below 31.1 degrees in a sealed container like that.

EXACTLY right. Not unless the temperature is higher then “room temperature”.

@minor7flat5

No it doesn’t. The phase chart shows that liquid and gas differ only up to 73 ATM and 31.1 C. Exceed either one and you end up with a supercritical fluid. That’s why the line dividing liquid from gas stops at that point instead of continuing straight up.

To be clear, it is very possible to have a room-temperature tank of CO2 in supercritical fluid state as long as the pressure is above 73 ATM.

Unequivocally wrong. Both temperature and pressure will be above the critical point when the CO2 becomes a supercritical fluid.

@WarmNPrickly

But how would the pressure ever get that high when it becomes a liquid at 850 psi?

Mostly right. The pressure at which CO2 “becomes a liquid” (reaches equilibrium) depends upon its temperature. At 81°F the pressure is 982 psi (absolute).

@Uncertain

…you have supercritical CO2 at any pressure on a somewhat hot day. You could walk around with a balloon full of it in summertime.

Not true. Everyplace you are going to walk around will be near standard atmospheric pressure. CO2 is a gas at standard pressure on a summer day. I only qualify it as a summer day because in the dead of winter in the middle of Antarctica CO2 might be solid.

@minor7flat5

Sure. I used to fill balloons with supercritical CO2 all the time when I was a child, during the hot summer days.

No, you didn’t. The CO2 you exhale is simply a gas. There is nothing supercritical about it.

@lazybratsche

Any chance the tank was overfilled? My understanding is that a given tank will only be partly full of liquid when “filled” to its rated capacity. If it was actually topped off, and cold from the filling, it might be conceivable that the pressure could get out of control when it warms to room temperature.

Professionals are careful to fill CO2 tanks to no more than 90% of capacity. Its practically impossible to fill a tank entirely with liquid. (Ever tried to completely eliminate the bubble when refilling a clear cigarette lighter?) Snailboy hits this on the head. If somehow you did pack the bottle with liquid, the pressure in the tank would skyrocket as it warmed and the tank would safety vent, or explode. With any amount of gaseous CO2 in the tank, the temperature and pressure would still be at equilibrium.

@shinybrain

…today, on a day that I thought it was cooler, it was actually about 90f by the tank. On a warmer day I could easily imagine it being about 100 by the tank… Perhaps that could simply be the reason…that temperature would mean it is in a super fluid state.

Yes, a supercritical fluid. At 100°F the pressure would reach around 1200 psi.