# 500psi - Time to refill Co2 tank??



## Mass-Diver (Jan 13, 2008)

I've read in several places that after a CO2 tank's pressure falls to 500psi that the reg becomes unstable and can at any time "dump" the rest of the gas into the aquarium . Is this true? 

What pressure do you guys run your cylinders down to?

Thanks!


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## imeridian (Jan 19, 2007)

I refill mine as soon as possible whenever there is a noted drop in pressure, as at that point all of the liquid CO2 is gone. As the cylinder pressure lowers, the pressure on your needle valve increases, increasing the bubble rate. You can still gas the fish to death even if you don't experience the EOTD. Wasting the bit of gas left is far less expensive than risking the population of my aquarium.


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## moogoo (Dec 7, 2007)

It is not good to have an end of tank dump (EOTD) as it allows for the chance for moisture to enter the tank. Once that happens, the tank has to be retested and certified. As a diver you should know this! 500psi is definitely time to refill. As stated, the liquid CO2 is gone and only the CO2 gas is left, which will deplete much quicker than the liquid CO2.


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## fishscale (May 29, 2007)

There are ways to prevent an EOTD, such as a really good needle valve, or a low pressure regulator. But, it's probably best to go and get your tank refilled.

It is not very common for moisture to get into the tank if you have check valves installed correctly.


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## Mass-Diver (Jan 13, 2008)

moogoo said:


> As a diver you should know this! 500psi is definitely time to refill. .


Thanks guys. I think it's actually the diver part that had added to my confusion. In SCUBA diving, we (obviously) don't dive with liquid gas. SCUBA cylinders can be dropped down to 200-300psi without any need to re-visual the tank, not to mention that an Al SCUBA cylinder can be filled to 3200psi. With a tank that is not filled with liquid, the drop in PSI remains that same until the bitter end. 

Again, thanks for the help, I'll be sure to refill as soon as I see a pressure drop!


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## imeridian (Jan 19, 2007)

I'm glad you've never had a problem, I've read of so very many horror stories of people killing all of their fish due to not refilling right after the pressure drop, they experience the EOTD or otherwise have rapidly rising CO2 levels. I suspect many of them had the lower quality regulators and needle valves commonly available, of which I'm guilty of too. The worst case is if someone actually follows Milwaukee's directions and sets the pressure low with the needle valve wide open... geez. 

I take the extra precaution because I have that cheapie regulator, someday I'll upgrade it, but it really does work fine otherwise.


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## eds (Feb 12, 2003)

Last refill, I let my cylinder empty. My brass check valve failed, and I got water in the cylinder. 

Doesn't appear to have been any permanent damage to the regulator or cylinder, but it was a bit of a hassle when I went to refill the cylinder, and instead of coming home with a refill, I had to make another trip as they dried out the cylinder.

In my mind, it all comes down to your personal level of risk tolerance. I was glad to rely on the check valve to protect me. Probably 999 times out of 1000 it will. But it kinda sucks if that 1/1000th happens to you. From now on, I'll just pay more attention to the pressure, and replace it when it starts to drop. IME, it probably takes a week to zero out once the pressure starts dropping, so it isn't as tho you have to rush out as soon as you notice any drop. But, IMO, I'll figure out when is the earliest time that I can fit it into my schedule (the shop I use has no weekend or evening hours, so I need to either go on a day off work, or adjust my work hours to do it.)


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## Mass-Diver (Jan 13, 2008)

Thanks again for all the replies. I've put a lot of effort into my aquarium and I have a low tolerance for risk. I've tired not to cut corners with this tank. I've had plently of jury-rigged SW systems and while most of them did well, I just don't want to deal with the stress and constant tinkering anymore. I think I'm going to buy a second cylinder since my shop has a 3 or 4 day turnaround on fills.


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## cleekdafish (Jun 13, 2007)

I change mine when it is almost empty and i never had any problems thus far


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## frostby (Sep 22, 2006)

How are you getting above 500PSI? I only hit 450 or so on a full tank?! Or is it this cold wisconsin weather that's keeping it down?


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## cleekdafish (Jun 13, 2007)

lol U probably have a half empty tank


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## Rex Grigg (Dec 10, 2002)

Oh man. There is a sticky here that shows pressure and temperature for a CO2 cylinder. 

The cylinder will hold a constant pressure at a constant temperature till all the liquid CO2 is gone. Then it will drop quickly.

A cylinder can show 450 psi and be full. The same cylinder at 90° would show over 1100 psi.


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## tazcrash69 (Sep 27, 2005)

Rex Grigg said:


> Oh man. There is a sticky here that shows pressure and temperature for a CO2 cylinder.


It's in the FAQ section 
http://www.plantedtank.net/forums/planted-tank-faq/33365-co2-bottle-pressure-relation-room-temperatures.html



frostby said:


> How are you getting above 500PSI? I only hit 450 or so on a full tank?! Or is it this cold wisconsin weather that's keeping it down?


Wow, how cold is your house? I guess the name fits. :biggrin:


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## frostby (Sep 22, 2006)

House temp? hummm, maybe mid to low 60's in the basement.

Thanks for clarifying rex, i know this is simple stuff for you but, you are helping some of us grow (smooch smooch)

I guess the 500psi indicating a near empty tank is simply a 'busted myth' - in most cases!


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## EdTheEdge (Jan 25, 2007)

> I guess the 500psi indicating a near empty tank is simply a 'busted myth' - in most cases!


I guess the trick is just keep a close eye on the regulator until you see a drop in pressure.

Am I correct in assuming that as the pressure starts dropping no matter how much or little the high pressure gauge shows it is time for a refill?

Just want to be sure I am understanding this thread.

Thanks for sharing.


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## mott (Nov 23, 2006)

fishscale said:


> There are ways to prevent an EOTD, such as a really good needle valve, or a low pressure regulator. But, it's probably best to go and get your tank refilled.
> 
> It is not very common for moisture to get into the tank if you have check valves installed correctly.


In case of the all in one reg from Milwaukee where the check valve is installed before the bubble counter, is this check valve good enough?


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## imeridian (Jan 19, 2007)

frostby said:


> I guess the 500psi indicating a near empty tank is simply a 'busted myth' - in most cases!


Absolutely Not! 

If your nominal tank pressure is something like 1000 PSI and the guage is now showing 500 PSI, you have already used every drop of liquid CO2 and are "running on vapor." 

If you get a standard sized refilled tank and it shows only 500 PSI once it has acclimated to room temperature, you better weigh the thing to see if you're really getting what you've paid for.



EdTheEdge said:


> Am I correct in assuming that as the pressure starts dropping no matter how much or little the high pressure gauge shows it is time for a refill?


Yes, that is correct. The pressure gauge will read the nominal pressure at a given temperature until the liquid CO2 has been exhausted, once CO2 is solely in vapor phase, the pressure will begin to drop. It is then time to refill the cylinder.


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