# CO2 tank pressure



## epicfish (Sep 11, 2006)

750 to ~800 (or a little higher) is right.


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## Chris333 (Jul 29, 2006)

Cool, guess no need to worry.
Thanks.


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## Hoppy (Dec 24, 2005)

CO2 is a liquid in those tanks. The liquid only fills part of the volume, so there is room for gas above the liquid. At room temperature that gas is at about 750 psi, but when you bring home a newly filled bottle, which I did this morning, the bottle is very cold, and the pressure can be as low as 500 psi, but it is still "full" - they go by weight. Your 20 pound bottle contains 20 pounds of liquid CO2 when full.

My Milwaukee regulator acts unusual when hooked up to a cold bottle of CO2. It can be adjusted to 30 psi, but as the bottle warms up the pressure drops back to near zero, so I had to keep adjusting the pressure setting all day. I recall having the same experience when I first filled the bottle, ten months ago.


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## xcooperx (Jun 21, 2006)

mee too i just refill my tank, then when i get home the psi reads 700psi, im suspecting now my refill station if they fully refill it, i dont feel any gain weight on my tank when it was refilled. so you mean if the cyclinder is 20lbs the co2 will be 20lb = 40lbs total weight


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## cwlodarczyk (Sep 13, 2005)

This is actually really easy to check...

CO2 tanks should have several markings stamped on them, one of which usually will be "TW" followed by a number. TW stands for "tare weight" and is the weight of the tank when empty. In the case of a 20# tank, the tank itself might weigh 15# or so and will hold another 20# of liquid CO2.


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## xcooperx (Jun 21, 2006)

My 5lbs Tank has a TW of 7.5,


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## cwlodarczyk (Sep 13, 2005)

xcooperx said:


> My 5lbs Tank has a TW of 7.5,


If that's the case your full tank should weigh in at 12.5 lbs.


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## xcooperx (Jun 21, 2006)

i knew it the refill guy didn't fully refill my tank, thats why im getting 700 PSI, i weight the tank and guess what it weight 7.5, thats why im wondering when the tank is refilled i didn't feel gain weight. I weight the tank now with the regulator and it weight 10.5


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## Chris333 (Jul 29, 2006)

I just re-checked mine now that it's warm. Up to 800# / 55#.

I'm hopping this 20# tank will last about 2 years for me. (using a solenoid) The place I bought it at said the tank price was the same no matter what size it was. I paid $75 for the tank and the CO2 in it. It had a brand new valve and they freshly painted the tank.


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## Rex Grigg (Dec 10, 2002)

xcooperx said:


> i knew it the refill guy didn't fully refill my tank, thats why im getting 700 PSI, i weight the tank and guess what it weight 7.5, thats why im wondering when the tank is refilled i didn't feel gain weight. I weight the tank now with the regulator and it weight 10.5


The amount of CO2 in the tank has little to do with the pressure. As long as there is liquid CO2 in the tank you will have normal pressure readings. As soon as the liquid CO2 is gone the pressure will drop below 700 psi very quickly.

Now I see a couple of things going one here. You have already been told that the scale you are using could be off. If your mother uses it to weigh herself chances are it could be at least 5 lbs off. Your regulator could be having problems. That's not uncommon. Take the damn cylinder down to the Post Office and put it on the Automated Postal Center (the blue box in the lobby that looks like a fancy ATM) and weigh it there. Next start reading what we are telling you.

You have a 5 lb cylinder. If it was only filled with 2.5 lbs of CO2 the high side pressure would read 750 psi, not 700, not 600, not 500. But 750 psi at 70 degrees. Note that if it's 85 out the tank will read very close to 1000 psi. There is a direct relationship between tank temperature and the pressure in the tank.


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## LS6 Tommy (May 13, 2006)

55# is high for the low side. You should be running around 10#-15# tops.


Tommy


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## Chris333 (Jul 29, 2006)

My Azoo regulator is not adjustable:
http://www.tropiacuariumbilbao.com/images/products/Manometro-azoogrande.jpg
It has an acorn nut where the adjustment knob would be. If there is a way to loosen the nut and adjust, please let me know. If not I will leave it alone.


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## Canoe2Can (Oct 31, 2004)

Chris333 said:


> My Azoo regulator is not adjustable:
> http://www.tropiacuariumbilbao.com/images/products/Manometro-azoogrande.jpg
> It has an acorn nut where the adjustment knob would be. If there is a way to loosen the nut and adjust, please let me know. If not I will leave it alone.


I have the same regulator, and mine runs around 40-45 PSI. I'm not worried about it, since it's always done this and never been a problem. It's just a pain to fine tune it. It takes me forever to get it tuned in to just the right BPS that I want. If there's a way to adjust the pressure, I'd also like to know.


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## Rex Grigg (Dec 10, 2002)

Canoe2Can,

Easy to fix that problem. A good in-line needle valve.


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## Canoe2Can (Oct 31, 2004)

Rex,

I was checking those out on your website the other day. Do they just attach directly to the CO2 tubing, or do they need to be hardwired to the regulator? I was thinking I might get one sometime.


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## Rex Grigg (Dec 10, 2002)

The in-line models can go right into your tubing. They have hose barbs on each end.


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## CmLaracy (Jan 7, 2007)

I just got my 5lbs filled and its reading about 950 on the large side... I guess they filled it up a lot


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## Naja002 (Oct 12, 2005)

CmLaracy said:


> I just got my 5lbs filled and its reading about 950 on the large side... I guess they filled it up a lot



Read this part:




Rex Grigg said:


> The amount of CO2 in the tank has little to do with the pressure. As long as there is liquid CO2 in the tank you will have normal pressure readings. As soon as the liquid CO2 is gone the pressure will drop below 700 psi very quickly.
> 
> Now I see a couple of things going one here. You have already been told that the scale you are using could be off. If your mother uses it to weigh herself chances are it could be at least 5 lbs off. Your regulator could be having problems. That's not uncommon. Take the damn cylinder down to the Post Office and put it on the Automated Postal Center (the blue box in the lobby that looks like a fancy ATM) and weigh it there. Next start reading what we are telling you.
> 
> You have a 5 lb cylinder. If it was only filled with 2.5 lbs of CO2 the high side pressure would read 750 psi, not 700, not 600, not 500. But 750 psi at 70 degrees. Note that if it's 85 out the tank will read very close to 1000 psi. There is a direct relationship between tank temperature and the pressure in the tank.


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## strizzi16 (Nov 13, 2007)

Ok so my questions is...what is the formula to see what your high side pressure gauge should read?


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## tazcrash69 (Sep 27, 2005)

from our own FAq section:
http://www.plantedtank.net/forums/planted-tank-faq/33365-co2-bottle-pressure-relation-room-temperatures.html


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## strizzi16 (Nov 13, 2007)

okay well based on that chart... my room temp is around 65-67 degrees... and my high pressure side gauge reads 900..
So what does that mean..its extra full?


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## nvrmor2paradise (Nov 7, 2007)

CO2, like any other liquid compressed gas (ie. nitrogen, propane, hydrogen, ect.), will always remain at a constant pressure, in constant temperature, no matter how full the tank is. When some pressure is relieved from the tank, the liquid in the tank boils creating more gas to bring the pressure back up. This will happen untill there is no more liquid in the tank. So, in other words, if you see a dramatic pressure drop on the high side of your pressure gauge, and the room's temperature is constant, there is no liquid left in the tank (your tank is empty). The "high side" of a compressed liquid gas gauge is therefore virtually useless. Ever see a pressure gauge on a LP or propane tank? That's why.

The only exception is under rapid release of gas. The liquid boils rapidly (evaporation) causing a dramatic temperature drop of the liquid and tank. This in turn causes the liquid to boil less rapidly untill the temperature drops to a point below the liquid's boiling point (thus less able to pressurize the tank). This is how you are able to "pour" liquid nitrogen from a bottle. The liquid nitrogen's temperature had dropped below it's boiling point and ceases to boil (but don't dare touch it as it will be -320 F).

To sum things up, the only way the pressure should drop in your tank is if your tank is empty or you decrease the temperature of the co2. The co2's pressure will steadily fall (asuming there is some release) until the temp drops below -109 F in which your gauge will read zero while still containing co2.

Science. Don't you just love it? There is so much of it in this hobby!

Below is a video demonstrating co2 under pressure (and not) in liquid and solid states. Note the use of "pressure".

http://youtube.com/watch?v=TA8zIl10-Z8


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## strizzi16 (Nov 13, 2007)

so is my reading of 900 psi for 67 degrees reasonable... it seems kind of high, in comparison to the chart


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## nvrmor2paradise (Nov 7, 2007)

No, it does seem high. But then again, what difference does it make? I work in the fire service and we have no pressure gauges at all on our co2. Everything is done by weight. Your gauge could be inaccurate, but it makes little difference. Your low side however, makes a big difference as this is your actual working pressure. It costs us $12 per gauge (nitrogen) to have them verified (or calibrated) and certified eash year per Florida law. Perhaps you could take the gauge to the "Weights and Measures" authority in your area and have it checked?


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## strizzi16 (Nov 13, 2007)

okay..but if everything is stable [working gauge (what should this be set as in terms of psi?) and high pressure gauge] then everything should be okay...?


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## Naja002 (Oct 12, 2005)

It was set and worked fine. All you had to do was hook it up to a full tank, get your diffusor setup and set the needle valve. But that would have been too easy. You should have done all of this research before-hand.

What do you need to adjust? Why do you need to adjust it?

The needle valve. That's it.....


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## strizzi16 (Nov 13, 2007)

I know... I just like cuadrupal checking things when i'm not sure of something. 
I have the bubble count set to 1 bps, so I should be fine..
Its just that most people say they have their working presure set to between 10-15 psi... and mine is at like 40, so I was wondering if i should do some adjusting or if it simply does not matter


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## tazcrash69 (Sep 27, 2005)

It shouldn't matter. I have mine set to 45 psi.


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## strizzi16 (Nov 13, 2007)

okay thats all I needed to know. Sorry if I got anyone worked up through this whole thing
Thanks!


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## nvrmor2paradise (Nov 7, 2007)

lol, oh heck, this has been great fun! I myself have to understand everything even if it doesn't matter. "I have no specialty, I'm just infinatly curious" Albert Instein


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