# single vs dual stage regulators



## AndreyT (Apr 28, 2011)

Single-stage pressure regulator works perfectly well as long as the CO2 bottle has liquid CO2 in it, i.e. as long as the input (bottle-side) pressure is high. Under such circumstances there's no difference between single-stage and dual-stage regulators. Both provide identically stable output pressure (AKA "working pressure")

The only situation when the difference between single-stage and dual-stage comes into play is when the CO2 bottle is about to become empty, i.e. when all liquid CO2 is gone and all that's left is pressurized gaseous CO2. As the leftover CO2 gas gets spent, the bottle pressure gradually drops. At some point the pressure will drop to the point when a single-pressure regulator will "fail" and allow all remaining gas to come through very quickly (so called "end-of-bottle dump"). Dual-stage regulators are designed to prevent this from happening.

The period between the moment all liquid CO2 is gone and the moment the dump occurs is typically measured in weeks. The bottle-side pressure gauge provides an advanced warning for that: once all liquid CO2 is gone, the input pressure begins to drop slowly but steadily. In other words, single-stage regulator will give you a warning well in advance when it is time to refill the bottle. For this reason, there's no definitive point in using a dual-stage regulator in aquarium CO2 setups. (And again, when the bottle is not yet empty of liquid CO2, the operation of single-stage and dual-stage regulators is indistinguishable.)

Basically, the only situation when a dual-stage regulator might be necessary is when the Co2 system is left unattended for months in a row.


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## Slippryrock (Mar 15, 2011)

so basically it's only when the last little drizzle is about to run out. what are the tell tale signs to watch for ?


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## AndreyT (Apr 28, 2011)

Slippryrock said:


> so basically it's only when the last little drizzle is about to run out. what are the tell tale signs to watch for ?


As I said above, when the input pressure gauge begins to drop, it is time to refill the bottle.

As long as you have even any liquid CO2 in your bottle, the input pressure gauge will show the same constant pressure (around 1000 psi). It will stay at 1000 psi constantly, even as the level of liquid CO2 in your bottle drops lower and lower.

Once all liquid CO2 is gone, the input pressure will begin to drop gradually. This is the sign you are supposed to be watching for. Once the input pressure begins to drop steadily, refill the bottle. You will have plenty of time to do that, i.e. several weeks typically from the moment when all liquid CO2 was spent.

P.S. The steady input pressure value might vary around 1000 psi depending on the ambient temperature in your room. But in any case, it will remain fairly constant. So, again, you have to ignore temperature-induced variations and watch for the moment when the pressure begins to drop steadily. A single-stage regulator will allow it to drop way down before it "dumps", so you can rest assured that you will never mistake a temporary temperature-induced variation with the steady drop that indicates the empty bottle.


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## KH2PO4 (Jul 18, 2009)

From what people describe as "dump". It's like a jet of CO2 being dumped quickly. 

My Azoo doesn't behave like that. When the tank pressure gradually drops, 
the working pressure rises, the bubble rate slowly increases. During that time 
I have to adjust the needle valve once every a few days until the tank is 
completely emptied.

You can avoid this by refilling/changing the CO2 tank when the pressure starts to go down.


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## Slippryrock (Mar 15, 2011)

those were the exact answers i was looking for thx. i think i have enough knowledge now to decide. just need to find a high quality single stage. i can upgrade from a chevette to cadillac down the road.


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## AndreyT (Apr 28, 2011)

KH2PO4 said:


> From what people describe as "dump". It's like a jet of CO2 being dumped quickly.
> 
> My Azoo doesn't behave like that. When the tank pressure gradually drops,
> the working pressure rises, the bubble rate slowly increases. During that time
> ...


I believe such difference in behavior can be caused by differences in regulator design. More precisely, adjustable single-stage regulators are usually implemented differently from non-adjustable single-stage ones.

An adjustable single-stage regulator is typically implemented around a flexible membrane (AKA diaphragm), as shown in this picture (see the sticky in this forum)










Meanwhile, non-adjustable regulators often don't have any membrane at all. They are built around a "piston" instead, as can be seen in this picture










This is actually my non-adjustable AquaticLife regulator. I believe non-adjustable Azoo is actually the same thing inside.

The difference in internal mechanics of these regulators can easily lead to different end-of-tank behavior. 

Although I have to admit I don't remember seeing that gradual ramp-up behavior from my AquaticLife. But I don't let it run for too long once the input pressure begins to drop either...


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## kevmo911 (Sep 24, 2010)

This has been discussed in detail in many threads. It's not an especially fast "jet" of CO2. In single stage regulators, as has been said, when all the liquid CO2 has changed to gas (probably the last 3-5% of the life of the cylinder fill) there's an increase in working pressure, which will therefore increase the rate of CO2 injected into the tank.

In cheaper, lower-quality single stage regulators (Azoo, Milwaukee, AquaticLife, Aquatek, and any of their clones), this will be a significant increase. In higher quality single stage regulators, this rise in working pressure will be less signifcant, and in many regulators not even noticeable. If you run your CO2 at or below 30ppm or so, the increase will likely do no harm to any fauna in the tank. It's only when the hobbyist regularly runs CO2 at a higher range that they have difficulty (i.e., fish deaths) due to a pressure rise.

In dual stage regulators, there won't be any noticeable change in working pressure.


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## KH2PO4 (Jul 18, 2009)

AndreyT said:


> This is actually my non-adjustable AquaticLife regulator. I believe non-adjustable Azoo is actually the same thing inside.


I agree. They look really alike. But my Azoo is the older/original model. 
It's adjustable. Just uncap it and use an Allen key.










Looks like it's reborn as Up Aqua's UA-165.
http://upaqua.net/products/co2-systems/regulators/ua-165-co2-regulator-adjustable/


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