CO2 pressure, consumption, and bubble counters

[mcubed45]

In the planted aquarium hobby, CO2 is often infused into the aquarium water using a compressed cylinder of CO2 which runs through a regulator, a needle valve, a bubble counter, and then into the tank. At the end of the gas line, a diffuser is used. The diffuser is porous ceramic material which permits tiny bubbles of CO2 to permeate into the water when sufficient pressure is applied.

My question is whether the working line pressure affects the density of the CO2 gas in the line. Bubble counters are used as a visual means to gauge how much gas is being delivered to the aquarium and gives an easy references for titrating up or down to meet the needs of the plants. Will the bubble counter still accurately reflect the quantity of gas being delivered when a different working pressure is used?

Some newer diffusers have come into the market that require a working pressure of at least 30 psi. Previous diffuers typically required only about 10-15 psi. The company claims that because only half as many bubbles per second (bps) are being used to achieve the same level of CO2 saturation, only half as much CO2 is being consumed. Is this accurate or does the change in working pressure need to be factored in? Does a single bubble in the bubble counter at 15psi contain the same quantity of CO2 at 30psi?

 

[Fish4Fun]

The short answer is "NO", there are more grams of CO2 in a fixed volume at 30psi than at 15psi; however, this does NOT imply that you would use more CO2 to carbonate the water at a higher CO2 pressure.

I am not familiar with aquarium based carbonation levels or needs, but the general process of carbonating water is well documented. As CO2 is fairly cheap, I would assume that the process being used is designed to minimize system cost/complexity rather than CO2 usage. That is, there are certainly plenty of ways to approach minimizing CO2 usage, but they are prolly not worth the trouble/expense.

Fish

 

[mcubed45]

The short answer is "NO", there are more grams of CO2 in a fixed volume at 30psi than at 15psi; however, this does NOT imply that you would use more CO2 to carbonate the water at a higher CO2 pressure.

I am not familiar with aquarium based carbonation levels or needs, but the general process of carbonating water is well documented. As CO2 is fairly cheap, I would assume that the process being used is designed to minimize system cost/complexity rather than CO2 usage. That is, there are certainly plenty of ways to approach minimizing CO2 usage, but they are prolly not worth the trouble/expense.

Fish

i'm mostly interested in whether a bubble counter, which is very simplistic volumetric flow meter, is affected by line pressure. my thinking, based on natural gas delivery and metering systems, is that line pressure needs to be accounted for when measuring gas consumption. the company states otherwise and claims that because their new diffusers achieve the same saturation levels at 1/2 the bps count, 1/2 as much CO2 is being consumed. the caveat is that their new diffusers require a siginificantly higher line pressure to operate (nearly double that of regular diffusers).

while the cost of CO2 is fairly inexpensive, for the planted tank hobby, it's more about the inconvenience of needing to refill your bottle and compesnate for the CO2 deficit your tank faces in the interim. CO2 cost is somewhat of an issue in this case because upgrading to these new diffusers require refitting your system with higher pressure connectors and gas lines. this cost is supposedly offset by the CO2 cost savings they are claiming. the new diffusers produce much finer gas bubbles which are more easily absorbed into the water, but the company is claiming twice the efficiency based strictly on half the number of bps in the bubble counter being used to achieve the same level of saturation. I have no doubt that their diffusers are more efficient and disolve gas more readily, I just don't think it's accurate to claim twice the efficiency based on a VOLUMETRIC flow meter.

also, in the planted tank hobby, we're interested not so much in carbonation, as in saturation. bubbles quickly outgas into the air. the goal is to maximize the CO2 saturation in the water so it can remain available for plant usage. CO2 saturation is typically measured using a drop checker, which is basically a pH indicator. the goal is to get your CO2 levels as high as possible without killing your fish.

 

[Fish4Fun]

I am confused. Are you asking if:

PV = nRT

Yes, it does. This implies that for a given volume of gas, the higher the pressure the more moles of gas you will consume. That is, if you are measuring the same volume reading @ twice the pressure then you are using more gas.

This does not confirm or refute the MFG's claims that their smaller bubbles achieve the same concentrations using less gas than their competitors unless the volume flow is equivalent AND the concentration level remains the same. If, for example, their product uses the same volume of gas at twice the pressure and quadruples the concentration level then their claim of using "half the gas is mathematically correct, and implies that using 1/4 the volume at twice the pressure might achieve the same concentration as your existing system.

In general, smaller bubbles increase the surface area of CO2 in contact with the water thus increasing absorption. Additionally smaller bubbles tend to rise slower than larger bubbles thus increasing absorption. So, if you currently have CO2 bubble reaching the surface, then it is likely a system that creates smaller bubbles would use less gas to achieve a similar concentration. There is absolutely no way of guessing how much less gas, there are simply too many variables.

Fish

 

[mcubed45]

I am confused. Are you asking if:

PV = nRT

Yes, it does. This implies that for a given volume of gas, the higher the pressure the more moles of gas you will consume. That is, if you are measuring the same volume reading @ twice the pressure then you are using more gas.

This does not confirm or refute the MFG's claims that their smaller bubbles achieve the same concentrations using less gas than their competitors unless the volume flow is equivalent AND the concentration level remains the same. If, for example, their product uses the same volume of gas at twice the pressure and quadruples the concentration level then their claim of using "half the gas is mathematically correct, and implies that using 1/4 the volume at twice the pressure might achieve the same concentration as your existing system.

In general, smaller bubbles increase the surface area of CO2 in contact with the water thus increasing absorption. Additionally smaller bubbles tend to rise slower than larger bubbles thus increasing absorption. So, if you currently have CO2 bubble reaching the surface, then it is likely a system that creates smaller bubbles would use less gas to achieve a similar concentration. There is absolutely no way of guessing how much less gas, there are simply too many variables.

Fish

Oh I don't question that the new diffusers are more efficient. Smaller bubbles leads to greater surface area and subsequent better absorption. The problem is the company has measured CO2 consumption strictly using BPS, which is a volumetric measurement and does not take into account line pressure. I've tried to explain why this is erroneous and misleading to the owner but he has gotten very defensive. I don't think he's deliberately misleading customers, but he doesn't have the physics background to understand what I've been trying to explain.

I understand the ideal gas law and have tried using it to explain my case, to no avail. Another poster questioned the application of the law in a dynamic situation. My thinking though, is that although the system is open, it is in steady state so the pressure/volume relationship should hold. The poster also pointed out that pressure in the line is not necessarily uniform and is significantly lower as you travel downstream. However, bubble counters are typically placed almost immediately after the pressure gauge coming off the regulator so I don't think the pressure drop should be that significant.

Is there something wrong in my reasoning?

 

[mcubed45]

Here's the post where another member raised some points:

http://www.plantedtank.net/forums/equipment/123561-green-leaf-aquariums-new-co2-diffuser-4.html#post1239003

 

[Borek]

If I understand your description correctly, bubble counter works at elevated pressure, that means each bubble holds more gas. Pressure inside the bubbler is fairly constant and time scale is such that you can safely use PV=nRT.