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CO2 Cylinder Capacity and EOTD


  • CO2 Cylinder Capacity and EOTD

    I cant think of a good title, but hey, mods feel free to change:

    Hopefully this helps a few people out.

    Firstly, one cannot simply determine how much CO2 remains in the tank based on your high side pressure gauge reading. The reason is due to the various phases in which CO2 is present in a pressurised environment at certain temperatures, and can easily be seen with the following temperature-pressure phase diagram:

    Here are some useful conversions first to help you out:

    Fahrenheit to Centigrade: Centigrade = (F – 32)/1.8
    1 bar = 100 kPa
    1 psi = 6.895 kPa (so kPa = PSI x 6.895)

    So lets say your room temperature is 80 degrees F = 26.67 degrees C, and your bottle was freshly filled from the shop. Your regulator pressure gauge would show 970 PSI all the way till you had 35% of your gas remaining, after which you can see on the graph it drops almost linearly down to 0 PSI as you run out of gas. The constant pressure is due to the presence of CO2 in a liquid phase (which is incompressible). After 35% you only have gas left and so pressure drops.

    IMPORTANT: note that at 120% rated fill at the same room temperature you would have a pressure of about 1500 PSI, and if the temperature went up by just 10 degrees F to 90 F (32.2 degrees C) such as in a hot car or hot day, you would be close to blowing your cylinder’s pressure relief valve. You can see why it is important for your shop not to overfill, I had to bleed one of my bottles once when I weighed it, since it was a ticking bomb!

    So to figure out how much gas you have left:

    1) Measure temperature of air around where bottle is
    2) Convert reading to Fahrenheit
    3) Look at corresponding curve on the graph
    4) Read your high side pressure gauge and read off what your remaining fill % is.
    5) Understand you will not be able to determine remaining fill over a portion of the curve, so remain diligent in checking your pressure regularly.
    6) Understand there will be pressure variation as temperature changes, and that this does not necessarily mean you are running out of gas.
    7) Determine your biggest temperature variation, and whatever your lowest temp is, read that curve and look at where pressure starts to drop, and THAT is a reliable pressure to start reading your gas remaining.
    8) Weigh your gas bottle all the time – a bit of a debacle for most people, but the most accurate way to know exactly how much gas you have left.

    Now this brings me to another point. The rated fill 100% of CO2 is equal to 0.66 x the cylinder water capacity. The water capacity will be marked on the bottle (eg a 540g bottle has a 0.82L capacity, and 540/820 = 0.66). The reason why this is done is to ensure no liquid CO2 is sucked into the regulator (which would very quickly destroy it).

    Next lets look at EOTD (End of Tank Dump). This is in fact a consequence of the operation of a regulator and what is called the Supply Pressure Effect (SPE), whereby the output pressure rises when the input pressure (bottle pressure) falls. This occurs with every regulator, whether it be single or dual stage, but a single stage has a much greater SPE than a dual stage. A dual stage regulator is essentially two regulators stacked together, so you are essentially fractioning the amount of SPE experienced.

    When the high side pressure approaches your outlet set pressure, it is common belief that the SPE becomes so pronounced that your set pressure goes so high that you end up flowing a massive amount of CO2 into your tank.

    1) Understand that physics dictate your outlet pressure can never exceed your cylinder pressure.
    2) Understand that most regulators can handle an inlet pressure = outlet pressure situation with no problem (at low flows which we have)
    3) Understand that you have a needle valve which will mean your flow rate will not increase that much due to SPE (but it will increase as your cylinder drops pressure since you are running out of gas). You will not somehow instantly shove massive CO2 bubbles into your tank, it is an event which will occur over a period of time.

    Now you are wondering “how do I know if EOTD will be an issue?”. This is how you test it:

    1) Unplug your CO2 line from your reactor/diffuser/needle wheel/whatever.
    2) Turn your solenoid on. If you don’t have one, turn whatever you have on.
    3) Check your bubble rate and note it, preferably averaging over a time period. Since you have disconnected the end, it may flow faster than expected due to no back pressure. Don’t worry, unless it is so fast that you cant any longer count the bubbles (shouldn’t be).
    4) Note your high side and low side pressures.
    5) Turn the tap on your CO2 bottle off – if you don’t have one, well you should.

    What you have done here is now pressurised a tiny amount of gas in your regulator to what the cylinder was, which will now quite quickly deplete, and thus your high side pressure gauge now starts to drop.

    6) Watch your high side gauge carefully, it will go down slowly but be patient.
    7) Keep recording your bubble rate
    8) As pressure drops, note low side pressure gauge readings.
    9) At the point where your bubble rate exceeds what your are comfortable with, note your high side and low side pressures. This is your EOTD limit. Never let your cylinder pressure get near this, typically this will be at very low remaining capacity (check picture posted above).

    99% of you will find you never reach step 9, because you have no EOTD, because the SPE isn’t what you think it is or as bad as you think it is. Your output pressure will then slowly drop with high side pressure and your bubble count goes to zero. You have run out of gas, and you can sleep easy knowing you have no EOTD issues to worry about.

    • cybernaut
      cybernaut commented
      Editing a comment
      I promoted this to article because it will benefit a lots of people. It is magistrally discussed by kkara4
    Posting comments is disabled.



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