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I recently had a second 80 gallon propane tank installed so that I would have extended run time for my 9Kw whole house generator.

The second tank is connected to the first tank and to my generator, and the first tank has a line that runs into my home for a gas fireplace.

The first tank's gauge currently reads 70 gallons, but the second tank is sitting at around 10 gallons. I realize that propane is a liquid in the tank, but as a load is placed on the tank, the liquid turns into a gas and it's this gas at the top of the tank that exits the tank and is used by whatever is generating the load.

So my questions are:

  • Should I ever expect the tanks to equalize on their gauges? I expect the answer is no, as only the gas will readily transfer between the tanks.

  • Can I infer the overall quantity of propane left in both tanks by reading the gauge on just one tank? I ask this, because the new tank has a gauge compatible with automatic tank level reading devices, and I'd love to add this to my setup.

  • counter intuitively, the less-full (lower pressure) one should empty first. if that's the one with the sensor, it won't help you much on the bottom half of your capacity. – dandavis Oct 17 '18 at 15:57
  • The typical setup for a "two portable tanks" arrangement (as seen with 200 lb tanks on our house growing up - they were replaced, not refilled in place, by the gas company) is a valve that sits between and empties one before drawing on the other (and raises a red indicator when that happens) - sounds like you have a more basic setup. When the new tank was installed, the manual valve was switched to keep drawing down the older tank first. – Ecnerwal Oct 17 '18 at 16:12
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Sounds like the two tanks are connected at the top (above the liquid line) since you say 'only the gas will readily transfer between the tanks'.

In this case, nothing equalizes the liquid level. Imagine if you run the two tanks down pretty far, and then only refill one of them; you can't see how much total propane you have available by only looking at one gauge.

Here's a random idea: you could put a valve between the two tanks, and fill them both, then close that valve. Treat one tank as the primary (probably the one that allows automatic monitoring), and the other tank is your 'reserve' (you have to monitor that level manually, but it will change less often). It's not great to have to manually engage backup propane, but it lets you get the most usage out of that automatic monitoring.

Update: What happens if a cylinder filled with propane gas is connected to an empty one of the same size

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Equalizing the tanks is not what you want to do. Instead you should allow one tank to be fully consumed before switching over to the second. This gives you the consumption time of the second tank to get a replacement or refill of the first tank. If you equalized and drew down both tanks to empty then you are out of available fuel until you get both tanks filled or replaced. Also if you were in the habit of keeping spare full tanks available on site the first approach only requires one spare tank while the latter requires two spare tanks.

The method used to switch between the two tanks is something you can choose based upon what tradeoff of price versus convenience works best for you. Obviously the simplest is probably a manual switch over valve.

  • This is the way they install tanks to houses here in NZ. There is a valve that switches over automatically. Conveniently, an arrow points to the near empty tank. – Matt Oct 17 '18 at 23:02

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