Am I getting a full CO2 refill?

CO2 Regulator Gauges

There are a few ways to get CO2 for your aquarium. Some shops let you swap your empty cylinder for a filled cylinder, some refill your cylinder on the spot, and others help you send your cylinder to be refilled and let you collect it a few days later. Some hobbyists go to the gas suppliers directly for an on-the-spot or collect-later refill. Many of those who get their cylinder refilled wonder if they are getting back a full cylinder? This article will teach you how to tell.

If you doing a one-for-one swap, this doesn’t apply as the empty weight of each cylinder may vary. For swaps, you have to trust that the shop sent it to a professional gas supplier which should do a proper refill.

Capacity of the cylinder

First, you need to know the capacity of the cylinder. Some parts of the world use volume to indicate the capacity of CO2 cylinders. The rest use weight to indicate the capacity of CO2 cylinders. For example, in the USA, they use pounds (lbs or #). In Singapore, we use volume in litres (L).

Commonly found cylinder capacities in Singapore are: 1L, 2L, 3L and 5L

A 1L cylinder just means that it can hold 1L of compressed CO2.

There are bigger cylinders, but for most hobbyists, 5L or smaller are the most practical sizes for space and transportation reasons. The bigger cylinders available to hobbyists are 10L cylinders. Sometimes you might be able to find 13.5L (20lb) cylinders. These sizes are mainly used by hobbyists who have much larger aquariums, like 600L (6x2x2 feet) ones.

Weight of CO2

In a fully filled CO2 cylinder, the compressed CO2 should weigh 0.66kg/L. That means a litre of compressed CO2 should weigh about 0.66kg.

Theoretically, if your 1L cylinder weighs 1kg when empty, then when it is full, it should weigh 1.66kg.

However, in Singapore, gas suppliers will only fill 0.6kg of CO2 per litre of capacity. This is an industry-wide safety practice; they are not attempting to cheat you. So if your cylinder capacity is 1L and weighs 1kg when empty, they will fill your cylinder until it weighs 1.6kg. That is, they will only fill 0.6kg of CO2 .

Basically, take the capacity of your cylinder in litres and multiply it by 0.6kg.

Cylinder
Capacity (L)
Approximate Weight
of CO2 (kg)
10.6
21.2
31.8
53
Cylinder Capacity and Weight of CO2

Am I getting a full cylinder back?

To find out if you are getting a full cylinder back, just find out the weight of your empty cylinder once. Thereafter, weigh it every time you get it back from refilling to check if it is full.

Before weighing the empty cylinder, you should make sure it is completely empty. Very often we will detach the cylinder as soon as the gauge shows a very low or zero psi. However, it doesn’t mean it is completely empty. The gauges are not accurate and there could still be some pressurised CO2 inside. If you weigh it now, it may not be the empty weight yet. Depending on the size of your cylinder, what’s left inside may be negligible or significant enough to make a difference in the weight. For example, from my 13.5L cylinder with a dual-stage regulator, I can get at least 1 day’s worth of CO2 after the gauge reads zero.

If you would really like to know the weight of a really empty cylinder, you can detach the CO2 tubing from your bubble counter, and then let the CO2 flow for a few hours or even overnight. You are basically waiting for your bubble counter to stop bubbling.

You should make sure it is a well ventilated area. Although they shouldn’t be much CO2 left in the cylinder, still better to be safe.

Warning: You might be tempted to detach your regulator and then open the cylinder main valve to let the gas escape. I strongly discourage that. The hissing is sudden, deafeningly loud and could scare your household members and neighbours. Might get you an earful from your family members too, if you can still hear after that…

Once you have emptied the cylinder, weigh it and record the weight somewhere. Now, whenever you send your cylinder for a refill, weigh it when it comes back to check if it increased by the correct weight. Then you will know if you got a full refill. Do be aware it may vary slightly from the expected weight, but it should not be too far away.

So, if you send a 5L cylinder for refilling, your cylinder should be about 3kg heavier when it comes back.

Why can’t I just depend on the gauges?

The simple answer: the gauges measure pressure, not volume. And pressure cannot be converted directly to volume or weight.

A more detailed explanation: CO2 gas converts to its liquid state at about 1000psi. When the cylinder is being refilled, the pressure inside builds up as we pump more gas into it. At approximately 1000psi, the CO2 gas begins to convert to its liquid state. At this point, you would have a little bit of liquid CO2 at the bottom of the cylinder, and the rest of the cylinder is gaseous CO2. As we pump more CO2 in, more of the gas converts to the liquid state, until almost the entire cylinder is full of liquid CO2 . The pressure will remain at 1000psi this entire time.

So, when your cylinder is at 1000psi, it could be full of liquid CO2, there might be just a little bit at the bottom of the cylinder, or any amount in between. This is why the gauges can’t tell you how much CO2 is really in your cylinder.

Clarification: No, this is NOT “liquid CO2” like Seachem Excel. This is actual CO2 in its liquid state. Excel is made of a completely different chemical.

Fun Fact: 1L of liquid CO2 is approximately equal to 372L of gaseous CO2!

Nevertheless, the gauge readings can tell you something about the contents and state of your cylinder.

Pressure Reading (psi)What it Means
1000
(sometimes 900+ due to inaccurate gauges)
There is some liquid CO2 in your cylinder. Could be full, could be partially full. Should last quite a while yet.
Between 1000 to 0No more liquid CO2 in the cylinder; just pressurised CO2 gas. The cylinder will be empty soon. Depending on the capacity and the usage rate, it could be days or weeks. For the same usage rate, larger cylinders last longer at this stage. Obviously, a higher usage rate will mean the cylinder will be empty sooner. At this stage, you should be checking the gauges and injection rate (or CO2 levels) daily. Single-stage regulators will begin to slow down and may cause a dangerous End-of-Tank-Dump (Google it) in some situations,
0No more CO2 will be coming out of your cylinder or at least too slow for your tank requirements. If you are using a dual-stage regulator, you might get a bit more CO2 quite close to your required injection rate. But essentially time to swap in a full cylinder.
Translating the Gauge Readings

Bonus: Industrial refilling process

The following describes the typical refilling process if you bring your cylinder to a gas supplier. These are the steps they will take to refill your cylinder, just to give you an idea of how it is done, when done properly. Some of the fish shops that do on-the-spot refills may not follow the steps described below exactly, but the important thing is that they give you back a cylinder at the correct weight or close to it.

All the steps below are performed by the gas supplier employee. Please do not attempt this yourself.

  1. Open the main valve on your cylinder and let any remaining compressed CO2 escape.
  2. Attach the refilling hose. The other end of the hose is attached to a much bigger cylinder of CO2. The hose has a closed valve and CO2 will not flow yet.
  3. Place your cylinder on a weighing scale and tare the weighing scale. This means they set the weight reading of the scale to zero even though your cylinder is sitting on it. This allows them to weigh how much gas is being filled into your cylinder.
  4. Open the valve on the refilling hose. CO2 gas will start to flow from the big cylinder into your cylinder. Usually, they will also turn on a gas pump which makes the process faster and also helps to fill your cylinder to the proper pressure/capacity.
  5. They will watch the weighing scale and stop the refill once it reaches the correct weight. For example, if your cylinder has a capacity of 5L, they will stop the refilling when the weight reaches 3kg. Typically it will overshoot by a few hundred grams by the time they close the valve. So sometimes you might get 3.3kg of CO2 which is pretty much the “full” weight.
  6. The last steps are to switch off the pump, close the valves, detach the hose, etc.

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