Is the beer gas ratio measured by volume or by mass?

[lautzki]

I'm looking to fill my 5-liter nitrogen cylinder with a blended gas consisting of 75% N2 and 25% CO2, and I have reasons to believe that my welder filled the cylinder with too much CO2 as my brews were getting overcarbonated whilst on beer gas. And I want to get it right this time around.

Maybe this is a stupid question, but I was thinking whether that ratio should be by volume or by mass, since CO2 is heavier than nitrogen. If we go strictly by volume, wouldn't the mass of CO2 be much higher than 25% in the mixture?

Based on my calculations, you would need to fill around 4.13 liters of N2 and 0.87 liters of CO2 in order to get to a 75/25 ratio by mass, instead of 3.75/1.25 liters if you go strictly by volume.

So which way is it? I would assume everything is measured by volume.

 

[VikeMan]

I believe the ratio is specified by molar gas fraction. In practice, I would imagine that the actual blending process is by mass, but the ratio of masses needed to achieve the specified molar gas fraction ratio is a different ratio (because the molar masses of the two gases are different).

But I'm going to use a lifeline... @doug293cz

 

[doug293cz]

I believe the ratio is specified by molar gas fraction. In practice, I would imagine that the actual blending process is by mass, but the ratio of masses needed to achieve the specified molar gas fraction ratio is a different ratio (because the molar masses of the two gases are different).

But I'm going to use a lifeline... @doug293cz

Yes it is by molar gas fraction. I believe you are correct about gas mixers using mass flow controllers (How Do Gas Blenders Work?)

Gas volume is a meaningless piece of information by itself. You need to know the volume, partial pressure, and temperature to know how much of a particular gas you have (either moles or mass.)

If you want to fill your own cylinders, and don't have a gas mixer, you fill by pressure. For example if you have a regular gas cylinder (not a CO2 cylinder) you could fill it to 500 psi with CO2, and then top it up to 2000 psi with nitrogen. In this case the CO2 partial pressure would be 500 psi, and the N2 partial pressure would be 1500 psi, for a 25/75 ratio. With this method you do have to worry about the flow rate of the second gas being added to the cylinder. It must be higher than the diffusion rate of the gas already in the cylinder to prevent back-flow of the first gas into the source of the second gas, which would contaminate the second gas source.

You don't want to try to fill a CO2 cylinder up to 2000 psi, as they have a burst disk (emergency, non-resettable pressure relief) set to a nominal 2200 psi. So to give you a safety margin (e.g. for if the tank gets hot) you might want to limit the total pressure to 1600 psi (400 CO2, 1200 N2.)

Brew on

 

[lautzki]

If you want to fill your own cylinders, and don't have a gas mixer, you fill by pressure. For example if you have a regular gas cylinder (not a CO2 cylinder) you could fill it to 500 psi with CO2, and then top it up to 2000 psi with nitrogen. In this case the CO2 partial pressure would be 500 psi, and the N2 partial pressure would be 1500 psi, for a 25/75 ratio. With this method you do have to worry about the flow rate of the second gas being added to the cylinder. It must be higher than the diffusion rate of the gas already in the cylinder to prevent back-flow of the first gas into the source of the second gas, which would contaminate the second gas source.

Thank you Doug! The approach you described actually crossed my mind, but the welder stated that it wouldn't work, as the CO2 is filled in liquid form, and the pressure of the cylinder wouldn't exceed 500 psi even if it was full of CO2.

I wonder how I should go on about this.

 

[doug293cz]

Thank you Doug! The approach you described actually crossed my mind, but the welder stated that it wouldn't work, as the CO2 is filled in liquid form, and the pressure of the cylinder wouldn't exceed 500 psi even if it was full of CO2.

I wonder how I should go on about this.

In a CO2 cylinder, that isn't almost empty, you have both liquid and gas. The partial pressure of the gas, if there is any liquid present, is a function only of the temperature. The chart below shows how CO2 behaves when compressed:

If the pressure is lower than the value in the table, at a particular temperature, then there is no more liquid CO2 in the cylinder. With liquid present, the pressure is only 500 psi at about 32°F. If the tank is warmer, the pressure will be higher. When making a CO2/N2 mix, you don't want the CO2 partial pressure to be high enough to force some of the CO2 to condense to a liquid, as that will change the ratio of the gases. So, limiting CO2 partial pressure to 500 psi max is good practice, unless you are going to leave the cylinder below freezing temps.

Brew on

 

[lautzki]

Got it, thanks. However, the welder keeps insisting that the correct way is to convert the figures to gas cubic meters per kg, should I just assume that they know what they're doing?

 

[doug293cz]

Got it, thanks. However, the welder keeps insisting that the correct way is to convert the figures to gas cubic meters per kg, should I just assume that they know what they're doing?

Without seeing all of the welder's math, it's not possible to make an assessment of their methodology.

You can go to the trouble of calculating the weight of each gas to add to the tank (weigh while filling), but that's a lot of extra work vs. just using a pressure ratio. In the case of a 330 cu ft cylinder (common 5 ft tall compressed gas cylinder) which weighs about 90 kg, you have to put in ~4 kg of CO2 & ~8 kg of N2. You need a pretty accurate scale to be able to maintain an accurate gas ratio.

It's so much simpler to just ratio the fill pressures of the gas.

Brew on

 

[lautzki]

Without seeing all of the welder's math, it's not possible to make an assessment of their methodology.

You can go to the trouble of calculating the weight of each gas to add to the tank (weigh while filling), but that's a lot of extra work vs. just using a pressure ratio. In the case of a 330 cu ft cylinder (common 5 ft tall compressed gas cylinder) which weighs about 90 kg, you have to put in ~4 kg of CO2 & ~8 kg of N2. You need a pretty accurate scale to be able to maintain an accurate gas ratio.

It's so much simpler to just ratio the fill pressures of the gas.

Brew on

Below you can find his reply when I inquired about the calculation:

"We convert carbon dioxide into cubic meters at standard pressure. 200bar * 5L = 1000bar/litre, then we divide the gases by percentage.

If you try to fill 50 bar with carbon dioxide, the figure will be inaccurate as the pressure of the gas changes with temperature. At normal temperature you can fill a cylinder with 3.7 kg of CO2, which is the maximum amount allowed, and then the pressure is about 50 bar at 20°C (then about 80% of the CO2 is in liquid form in the cylinder). So how many cubic meters of each gas to put into a cylinder is probably the correct way.

The other way around putting nitrogen first will not work because the carbon pump will trip at 110-150bar."

I really have no idea what to make out of this - do you catch the drift? @doug293cz