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Propane: volumes, weights, tanks, gauges...oh my
Lots of questions about propane lately probably driven by the fact it is cold out. Since everyone at my house is still crashed from last night I thought I’d take a stab at propane in general based on my experience as a homebrewer and knowledge of my coach. Bear with me…I’m an engineer.
I will use my coach as a reference - it has a 40.9 pound propane capacity according to the Thor spec sheet.
Tanks: Propane tanks are designed for various volumes in gallons. Volumes are then converted to weights - more on this later. My propane tank has stamped on it a volume of 12.2 gallons. This is the amount of water that the tank can hold or water volume. For propane, we do not exceed 80% of the tank water volume for expansion and safety. So, my tank holds 80% of 12.2 gallons or 9.76 gallons of propane. When my propane tank gauge reads F there are 9.76 gallons of propane in my tank, The gauge says right on it F, full at 80%. Full is full at 9.76 gallons, forget about the 80%, it has been taken care of in the design of the tank.
Filling a tank: Propane tanks these days are different than days gone by in that they have an Overfill Protection Device (OPD) installed. This is to keep the tank from being filed past the 80% point. There is also a float that senses the level of the propane in the tank and connects to an external gauge that we read to see how full the tank is. When the tank is filled it is done by volume these days and not by weight. The propane place hooks up and fills the tank, the fill will shut off when the tank maximum volume is reached (9.76 gallons in my case), the gallons are recorded and you are charged accordingly.
Weight vs volume: Most everything we do with propane is by volume with a few exceptions. For us RVers we need to know weight to calculate our total weight. A gallon of propane weighs 4.23 pounds give or take due to temperature - 4.2 is commonly used. So, my tank of 9.76 gallons will carry a propane load of roughly 41 pounds - hey, my spec sheet says that. This is the only time I care about weight - except for homebrewing since I can’t measure the volume of my BBQ tank so I weigh it to determine how much propane I have left.
Propane tank gauge: The gauge on the propane tank is connected magnetically to a float inside the tank. The gauge accurately reads the float and we can look at the gauge and get a fairly good idea of the volume of propane left. E = 0 gallons, 1/4 = 2.4 gallons, 1/2 = 4.8 gallons, 3/4 = 7.3 gallons and F = 9.7 gallons in my case. Good enough for government work as we use to say. The gauge also has a sending unit attached to it so we can monitor the tank volume in the coach - monitor being the key word here. The sending unit is resistive from 0 to 90 ohms and the resistance changes as the float level changes.
Propane monitor in the coach: The sending unit on the propane gauge drives the coach monitor panel. This is were things get weird. First and foremost, the unit in the coach is a monitor and should be treated that way - it gives us a rough idea of tank volume whereas the gauge on the tank gives us close to actual volume. In the Navy we called them idiot lights, worth watching but don’t bet the farm on them. Second, the monitor lights are calibrated in thirds (1/3), really, who the heck measures in thirds…must be some obscure reason Thor picked this. Anyways, the key here is the monitor panel is not an actual reading but a “close to” reading depending on how the monitor is calibrated. E is “close to” empty, 1/3 is “close to” 3.2 gallons +/- 1 gallon or so, 2/3 is “close to” 6.4 gallons +/- 1 gallon of so, and F is “close to” 9.7 gallons. The problem with the monitor is we are always “somewhere between” two levels. My rule of thumb, when the monitor says 1/3, I fill up.
So I ask…if my tank gauge is reading 1/4 (actual) and my monitor is reading reading 1/3 (close to) is there a problem - probably not, 1/4 is 2.4 gallons, 1/3 is 3.2 +/- 1 - could be 2.2, could be 4.2. Bottom line, treat the monitor as that, a rough idea. You want to know for sure…check the tank gauge. Find your comfort level and fill your propane tank when you hit it.
Ok, got it. I know my tank volume when full, watch my coach monitor to get an idea the range of what I have left, look at the tank gauge to get a really good idea what I have left. Fill when the idiot light reads 1/3….maybe sooner f I’m going into cold weather like I did these past 2 weeks.
Next installment - why does this matter? It’s all about BTUs.
My favorite saying: somedays you’re the statue and somedays you’re the pigeon. We’ll see how this flies (no pun intended).
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That's an awful lot of Math for New Year's Day... My head hurts! 
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