Gas gauge physics

Discussion in 'Propane Smokers' started by little_wolf, Oct 27, 2009.

  1. Alright, I know the best way to know how much gas left in a tank is to weigh it, and I know the best way to be prepared for gas exhaustion is a spare tank - no disagreement on either.

    However ... curious minds ....

    From basic physics, I remember that the pressure in a closed space is dependent on temperature, not fluid mass (assuming a gas/liquid mix). Raise the temperature and the partial pressure of the fluid goes up and more liquid turns to gas and the pressure of the tank goes up. Likewise, if you cool the tank far enough, the partial pressure will drop and gas will go back into liquid.

    So why does a pressure gauge on the tank side of the regulator work? Obviously if you draw the gas out at a high rate there will be some cooling and the pressure would go down. But during normal smoking the flow is very low and this shouldn't be a factor.

    Sorry to put this question on the smoking forum, but every time I use my smoker I am puzzled as to how the gauge is giving me a volume indication - my gut tells me it shouldn't work!

  2. mballi3011

    mballi3011 Smoking Guru OTBS Member SMF Premier Member

    In theory but I just have a bunch of tanks (5) so I don't worry about running out but some people here might not have that luxory. I don't know anyone with a gauge so I'm not sure how they work. It's just one of thoses thing you don't second guess.
  3. grampyskids

    grampyskids Meat Mopper SMF Premier Member

    Since it has been over 4 decades since college, I believe that the answer is: with more propane in the tank, there is a smaller volume of space in the tank. The mass of gas produces more pressure in a smaller space. This higher pressure on the diaphragm shows the level of gas in the tank. As the propane is burned, more space is created and the resultant pressure drop on the diaphragm will reflect on the guage.
  4. oldschoolbbq

    oldschoolbbq Smoking Guru OTBS Member

    I have an in-line guage on one of my tanks(I too have 5,but only 2 are fill at any one time-$$$$$ issues). I have no problem with mine and use it as a guide to when to start changing.Always a little left when it is cooler ,but in Summer it all gets used.I also use a magnetic guage,temp. change when in use showing the level of liquid.
  5. OK, after doing research and a simple experiment, I think I now understand how the gas gauge works. First of all, and this is what confused me, is that the pressure in the tank, and thus measured by a gauge upstream of the regulator, is only dependent on the temperature of the gas/liquid system, and not the amount of liquid. Look up "vapor pressure" on Wikipedia and you will see a chart showing propane pressure vs. temperature.

    When you start drawing gas from a tank, the liquid will boil at a rate to keep the pressure at the "right" temperature for the tank. But the boiling and evaporation of the liquid will cause it to cool down as it takes heat to produce the gas. In a full tank, there is more mass, and it will cool down less - a nearly empty tank will cool down more. As a result, the full tank will show a higher pressure, the empty tank tank a lower pressure. That also explains why if you draw your tank into the "yellow" zone one day, then come back the next day, you may find you are back in the green, but not for long once you start drawing gas.

    I let a mostly empty tank come up to room temperature overnight and when I connected the gauge, it showed mostly full. I then let the tank cool down to outside temps (about 50 degF today), then checked again and the level was lower. I didn't use any gas between readings.

    Propane boils at around -40 degF, so you will get some gas pressure as long as your temp is above that (boy, that'd be a tough smoke!). But of course, the colder you start at, the lower your starting point will be.

    Again, sorry for filling the smoking forum with this, but I think this explains how the gauge works for anyone who happens to care!
  6. bwsmith_2000

    bwsmith_2000 Master of the Pit OTBS Member

    Interesting .... thanks for the post.

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