# [Wet Thumb Forum]-CO2 Tank Pressure



## Garmoni (Dec 24, 2003)

When I originally set up my CO2 system the tank pressure was reading 900psi. It’s now reading 650. How soon do I need to think about getting it refilled?


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## Garmoni (Dec 24, 2003)

When I originally set up my CO2 system the tank pressure was reading 900psi. It’s now reading 650. How soon do I need to think about getting it refilled?


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## gsmollin (Feb 3, 2003)

Hard to say. What was the original temperature, and what is it now? The CO2 is stored as a liquid, and you measure only a vapor pressure in the gauge. That pressure will remain constant at constant temperature until there is about 10% of the CO2 left in the tank. Then it becomes all gas, and the pressure drops quickly. So the answer is this: If thetemperature has been constant, you have about 7% of your gas left. If the temperature has dropped significantly since it read 900 psi, then it may just be a vapor pressure change. The way to be sure is to weigh the tank. The empty weight is stamped on the side.


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## BobAlston (Jan 23, 2004)

When my co2 tank pressure starts dropping like that i get it refilled. Why wait.

Since the pressure is dropping I would think ALL of the liquid co2 has been converted to gas. So there really isn't much left.

Of course, you could measure the pressure now and again in a week and calculate how long it will last. Of course, if you do this you probably should have a needle valve to reduce any potential end of tank dump problems.

Bob


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## Garmoni (Dec 24, 2003)

Thanks. I went ahead and got the tank refilled yesterday because I had the time and wanted to be safe. There was still some gas left in the tank but it only cost $10 to refill it and now I don’t have to worry about it taking care of it at an inconvenient time.


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## gsmollin (Feb 3, 2003)

> quote:
> 
> Originally posted by BobAlston:
> When my co2 tank pressure starts dropping like that i get it refilled. Why wait.


Because I'm really cheap.


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## Verminaard (Dec 18, 2003)

> quote:
> 
> Originally posted by Garmoni:
> Thanks. I went ahead and got the tank refilled yesterday because I had the time and wanted to be safe. There was still some gas left in the tank but it only cost $10 to refill it and now I don't have to worry about it taking care of it at an inconvenient time.


Where did you get yours filled? I am in the area and havent had to fill mine yet.......


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## bebop (Jul 8, 2003)

I'm about to post this question until i find this tread.... I have a pressurized co2 for like 5-8 months and I dont know if I should get it refilled I don't know how to read the gauges.

1 gauge says its on 1000 and the 2 gauge is on 3. Do I wait till it hit the zero to have it refilled.


And what is this end of tank dump thing is it bad for the fish?
Appreciate the help.


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## Desolas (Apr 10, 2004)

> quote:
> 
> 1 gauge says its on 1000 and the 2 gauge is on 3. Do I wait till it hit the zero to have it refilled.


The gauge that shows more pressure is the tank's pressure, the other gauge is your working pressure - or the pressure coming out of the regulator. Myself, I like to have my regulator turned up to 12-15 psi to get a even bubble count out of my needle valve.

With a typical 5lb steel canister you will often see the tank pressure drop to around 500 when there is only a pound or less of CO2 in the bottle, but that doesn't mean it is empty, it will show pressure until the last bit of CO2 is gone. The best way to know is to measure the tank with the scale. A full steel tank weighs 15lbs, and 10lbs when empty. I went to the tool store and bought a tank scale for $10, you hook it around the neck of the bottle and just lift it up off the ground. You can probably use a bathroom scale too, but it's more work.

You can wait until it runs out before getting it refilled but depending on your pH you may not want to as it can quickly jump up without CO2. I keep an extra CO2 bottle on hand to swap in when I run out just to avoid a pH fluctuation, but after watching the tank weight for a few months I have it down to the day when I switch bottles.

Some bottles may differ in weight, you can call the shop you get yours filled at and they will know for sure.


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## BobAlston (Jan 23, 2004)

The only question you asked that has not been answered - either before or after your post - is about end of tank dump. From my research, that seems to be a horror that has happened relatively infrequently at best. Also, it is commonly believed that use of a needle valve will avoid or at least minimize the potential adverse impact.

I believe avoiding the potential is the best insurance. As noted earlier in this thread, the tank pressure will stay at 850 lbs (or the 1000 you quoted - depending on the temperature) as long as there is liquid co2 remaining in the tank. Once the liquid is depleted, the tank is working off the gas only and the pressure will therefore begin to drop. Slowly. I simply get my tank refilled once I notice it is beginning to drop. By experience, I know my tank will last about 8 months for my 55 gallon, so I start expecting the pressure drop and start watching at the right time.

For more info on pressurized co2, take a look at the many articles listed on my web site.

Bob


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## gsmollin (Feb 3, 2003)

Yes indeed, and beebop will suffer a most agregious tank dump, if the regulator is really set at 3 psi. Even a very good single stage regulator will increase pressure by 2-3 psi as the tank pressure drops from nominally 850 psi to zero. The regulator pressure must have at least 12 psi dropped across the needle valve. In addition, the back pressure from the check valve and water depth must be added to get the proper regulator outlet pressure.


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