# When to refill CO2 tank?



## mattcham (Mar 7, 2014)

I am new to pressurized CO2. My new full tank has 2 gauges. High pressure gauge says 900 psi and low pressure gauge says 35 psi (I set it to 35 psi per protocol). How low should the high pressure gauge become before I need to refill the tank? 500 psi? 300 psi? 100 psi? (Lowest tick mark on my high pressure gauge is 100 psi)


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## Undertow (Dec 21, 2012)

I wait till mine gets right above the lowest point.


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## Higher Thinking (Mar 16, 2011)

mattcham said:


> I am new to pressurized CO2. My new full tank has 2 gauges. High pressure gauge says 900 psi and low pressure gauge says 35 psi (I set it to 35 psi per protocol). How low should the high pressure gauge become before I need to refill the tank? 500 psi? 300 psi? 100 psi? (Lowest tick mark on my high pressure gauge is 100 psi)



The gauge will stay at roughly the same spot until the tank is almost out. Once the pressure gauge starts dropping, it means you have exhausted your liquid CO2 and are currently using the remainder of the gas in the tank. Just how much is left depends on how much gas you are pumping out. 

With my 20lb tank, the bubble counter goes almost too fast to count for about 10 hours a day. When the pressure gauge starts dropping I can easily go a week or more. Just know that as the pressure starts to drop, the bubble rate will also decrease. So unless you want to fiddle with increasing the needle valve, just swap out your tank within the first week of the pressure dropping.


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## Daximus (Oct 19, 2011)

I run off paintball tanks, but my experiences are similar. Once the high pressure side starts to drop your nearing the end. I'd plan on getting it filled back up at your earliest convenience. How long you have will depend on your tank size and how fast your dumping it into the aquarium. That is one of the reasons I went with paintball tanks for storage...I just keep a spare. The downside is I run out out more often then someone with a proper sized tank.


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## aclaar877 (Feb 19, 2014)

I run a 10-lb cylinder at about 10-12 bps for 8 hrs a day. I have about a week left when I notice the pressure drop significantly combined with a bubble rate drop. At that point I have to open the needle valve each day to keep the bubble rate up.


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## mattcham (Mar 7, 2014)

Thanks for the info. Really helpful. Never knew that the pressure drop begins 1 week prior to becoming empty. I better get a spare tank because my local air gas does not routinely stock CO2 and could take them 2 weeks to get me a replacement tank.


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## mattinmd (Aug 16, 2014)

Agreed, with a 2-week lead-time having 2 cylinders might be helpful. Either that or start weighing your tank and see if the air gas can do an order-ahead for swapout when you are getting low.

Determining how much CO2 is in your tank:

If you want to really know how much CO2 is in your tank, use a scale and weigh it.

The cylinder should have its tare weight stamped on it, which is the weight of the tank when empty. Subtract this from the current weight and you have the weight of CO2 in the cylinder. 

The "pounds" rating of a CO2 tank is how many pounds of CO2 they hold. A 5 lb cylinder might have 7.5lb tare weight, and would weigh 7.5lbs when empty, and 12.5lbs when completely full.

If your cylinder lacks a tare weight stamp (very unusual) you can weigh them when you get them, and work backwards ie: if you get the cylinder at 12.8lbs, it is going to be empty when it hits 7.8, roughly speaking. There's always a chance of slight over/under charge, so it might not have exactly 5lbs of CO2 in it, but it should be close enough for estimation.


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