High pressure gauge

Discussion in 'General Brewing Discussions' started by EvanAltman36, Jul 10, 2013.

  1. EvanAltman36

    EvanAltman36 New Member

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    I'm still pretty new to kegging, but the transition from bottling has been everything I had hoped for and then some. It's so nice to pull a fresh draft beer at home and not have to deal with sediment and the subsequent rinsing of the bottle. In any case, my high pressure gauge reads that the CO2 tank is completely empty, which is clearly not the case. I initially though that it was because I have a dual-gauge regulator and that the valve nearest the gauge was closed. However, the CO2 should be running to that gauge regardless. Then, after checking out some threads online, I have come to the conclusion that the high-pressure gauge is pretty much like a radar detector: it really only works when it's already too late. Is that pretty much the deal? It appears that the needle in the gauge is not quite square, so I was thinking that I might be able to monkey with it. But if the end result is that it'll still read empty the whole time, I won't mess with it.
     
  2. LarryBrewer

    LarryBrewer Active Member

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    I don't have one of those.

    What I do have is a second tank on hand in case the main one runs out. For me, at 9-10 psi, it takes 10+ kegs to run out of CO2. I recently switched the tank to a full one. This time around I will keep better records on exactly how many kegs I can get out of it.
     
  3. EvanAltman36

    EvanAltman36 New Member

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    So you just have the low-pressure gauge to see what your beer is at? The good thing is that it's my first keg, so I can tell exactly how much I use.
     
  4. W1IA

    W1IA New Member

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    The gauges are useless as CO2 maintains a constant till the tank empties. I have two and don't bother watching the high pressure gauge. When one empties, switch it out and I head to my gas supplier to get 'em refilled.
     
  5. bilhelm96

    bilhelm96 Member

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    The high pressure gauges aren't useless, they suffer from not being accurate at the low end. Any gauge is most accurate in the middle of the range and not at all accurate at either side. My HP gauge goes from 0 - 2000 PSI with marks for each 100 PSI and a red refil indicator at 300 PSI. Since I have the LP side set at 30 (I have a secondary regulator in my kegerator), the cylinder still has plenty of gas at 300 PSI. What I do is use the HP gauge to get an estimate of how much CO2 is left, but once it bottoms out, I watch the LP gauge like a hawk and as soon as it starts dropping below 30, I switch tanks and get that one filled.
     
  6. W1IA

    W1IA New Member

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    Fair enough...I should have said I find them useless as I run my tanks dry and don't bother paying attention to them. When its empty I switch it out. The following is a quote from another thread, very good info.


    "The pressure inside a closed cylinder containing a volatile non-polar liquid like CO2 is determined solely by the characteristic VAPOR PRESSURE of the substance, not by how many pounds of liquid are put into it. Vapor pressure is defined as the pressure of the vapor of a liquid in equilibrium with its liquid state. (Solids have vapor pressure, too, even though that seems odd at first. Just think of dry ice, which is solid CO2 but turns to vapor very quickly due to its high vapor pressure at room temp.)

    As long as a CO2 cylinder is at a reasonable, constant temperature, as gas is (slowly) used out of the cylinder liquid CO2 "boils" off as gaseous CO2, at the vapor pressure of CO2 at that temperature. This is about 860 psi at normal room temperature, or about 72 degrees Fahrenheit. This process of evaporation continues until all the liquid CO2 is gone. That is why CO2 tank pressure is constant (at a given temperature) until it's almost empty (i.e., no liquid is left), and then falls off rapidly as the remaining residual gas is quickly used up.

    Rapidly releasing gas will lower the tank temperature dramatically (as you observed in your frozen kitchen) due to expansion cooling as the CO2 soaks up its "heat content of evaporation" from the surrounding environment, but we can ignore that tiny effect for normal slow CO2 usage rates. (This is also why aerosol cans sometimes get very cold to the touch if you spray them too quickly for too long.).

    So, as strange as it seems, how much liquid CO2 (in pounds) was put into your cylinder had NOTHING to do with its internal tank pressure. Just as long as there was SOME liquid present inside the cylinder, the internal pressure was identically fixed at the vapor pressure of CO2.

    There are four basic things that can influence vapor pressure - the "polar" or "non-polar" nature of the molecules of the substance, how much surface area of the liquid is exposed to the gas in the cylinder (i.e., cylinder diameter in this case), the concentration of the vapor molecules above the liquid, and lastly TEMPERATURE. In your case, on the inside of a closed CO2 cylinder at equilibrium everything else is fixed (held constant), so TEMPERATURE is all that controls the CO2 vapor pressure, and thus temperature is also all that controls the pressure inside the tank. As other people in this thread have noted, CO2 tank pressure varies with the temperature, and this is why.

    "Polar" substances have di-polar molecules, i.e., they have positive and negative ends to the molecules. The inter-molecular forces acting on polar liquids are much stronger and the liquid molecules are kept in the liquid state easier. Polar substances tend to have lower vapor pressures at a given temperature.
    On the other hand, if the molecules of a liquid are non-polar like those of CO2, then there will be weaker intermolecular forces keeping them in the liquid state. Non-polar substances like CO2 have weaker forces between them and therefore their vapor pressures are characteristically higher than those of polar substances. The rate of evaporation for non-polar molecules is generally far greater than polar molecules in comparison. This gives CO2 its comparatively high vapor pressure.

    Any increase in the temperature of the CO2 tank increases the average kinetic energy present in the molecules of the liquid CO2. The more energy that the surface liquid molecules absorb (heat), the faster they will be able to overcome the intermolecular forces acting upon them, and the sooner they will become vapor.

    Here's the really nasty part that apparently bit you with your CO2 cylinder. Increasing the temperature of a liquid increases it's vapor pressure not linearly, but LOGARITHMICALLY. Doubling absolute temperature increases pressure by a factor of ten times.

    This is defined by the Clausius-Clapyron Equation, that you probably remember having seen in science class:

    Log P2 / P1 = Delta H vaporization [ 1 / T1 - 1/T2] / 2.303 ( R)

    where:

    R = universal gas law constant = 8.31 J/mol-K = 8.31 X 10-3 Kj / mol-K

    P1 and P2 = vapor pressure at T1 and T2

    T1 and T2 = Kelvin Temperature at the initial state and final state

    This tells us that at about 40 degrees Fahrenheit, CO2 has a vapor pressure of 600 psi. At 72 degrees F, it's up to about 860. At about 80 degrees F, the CO2 vapor pressure rises to about 970 psi. So, if your pressure gauge before your uncontrolled CO2 discharge was off-the-dial at over 1200 psi, the internal temp must have climbed to 90 degrees F or more due to where the tank was located, or due to being in the car.

    Does this seem reasonable to you given conditions in your house/car, Jeff? (Feeling the outside of the metal tank won't tell you much, because the thermal conductivity of metal is so high it will likely feel cool to the touch of your 98.6 degree hand, even if it is relatively hot inside.) If so, the CO2 was well into the highly dangerous and unpredictable so-called "critical zone."

    The critical zone is temperatures and resulting pressures above the "critical point," a unique combination of temperature and pressure for a given substance at and above which gas and liquid begin to behave the same, and normal fluid dynamics laws no longer hold. For CO2, this critical point is 88.88 degrees F, at about 1080 psi vapor pressure. It is unclear to science what state of matter the CO2 would be said to be in at and above this point - gas or liquid. At or above these temps, all bets are off as far as guessing the fluid behavior and/or the gas pressure of the CO2 inside the cylinder.

    (I'm pretty sure I did the calculation here correctly, but math is not really my thing and I did do these calculations on the fly, so corrections are welcome from you more math-savy reef keepers out there.)

    Bottom line - be very careful with heat and bottled CO2. If you leave it in a car, house, or closet with temps above 90 degrees or so, you are asking for big, big trouble. The only reason more people don't blow themselves off the face of the earth messing around with these compressed CO2 cylinders is because industry standard tanks are so tremendously over-engineered, to protect us from ourselves for both safety and liability reasons.

    The total pressure acting on the total square inches of the walls of a typical size cylinder at 1000 psi or more is mind-blowing (pun intended). If you really did badly overheat your CO2 cylinder, you were fortunate that a small valve failed before the tank walls ruptured. As mentioned before in this thread, tanks contain rupture disks to prevent this.

    So, keep those CO2 tanks cool, people. ALWAYS below 80 degrees, for sure. (Your pressure gauge should never read more than 1000 psi. If it does but your room temp is below 80, suspect that the gauge needs recalibrating.)

    Forewarned is forearmed"
     
  7. EvanAltman36

    EvanAltman36 New Member

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    Wow, I should have paid more attention in Chemistry. So, more or less, it appears that the efficacy of my gauge itself is less than or equal to my own estimation and measurement of the tank's fullness. I messed with the gauge itself, as the needle appeared to be perched on the little brass peg that serves to limit the needle at the low end of the gauge. While it slightly improved the aesthetics of the gauge, it still appears that the short part of the needle is actually in contact with the surface of the actual measurement circle. Further, the tank is in the fridge and it's my understanding that that will impact things. What it really comes down to is that this is really like having a very small scratch in your favorite pair of sunglasses. No one else notices and you yourself only see it when you're looking right through it, which is rare. But then when you do, it just irks you. But hey, as long as the tank keeps filling my kegs and beer with little bubbles, I'm good.
     
  8. W1IA

    W1IA New Member

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    In the end Evan, I just don't worry about it...run it dry and replace with another. Bottom line is buy a spare tank. I keep a 50lb bottle for charge and purging and a small 10 lb for portability.
     

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