Kitchen Myths

Facts and fiction about food and cooking, by Peter Aitken

A pressure gauge on your propane tank is useful

Many folks have paid for a pressure gauge on their propane tanks in the mistaken belief that it tells you how much propane is left. Me too, I got one before I figured out that it is useless. How come? High school physics class to the rescue!

In the tank, the propane is mostly liquid. At the top of the tank, above the liquid propane, is gaseous propane. When you open the valve, some of the propane gas is released, and some of the liquid vaporizes to maintain what physicists call the vapor pressure – the pressure at which liquid and gaseous propane are in equilibrium. As long as there is some liquid propane in the tank, this pressure remains the same (although it does vary with temperature, being higher when the temperature is warmer). Whether 95% full or 5% full, the pressure is the same. Only when there is no more liquid propane in the tank will the pressure shown on your gauge drop. You can tell how much propane is left by weighing the tank, but that’s a bother. You can also tell by feeling the side of the tank when it is in use (when your grill is on). At some place the tank will be colder below, warmer above. That’s the level of the liquid propane. Bottom line? Keep an extra tank on hand.

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3 responses to “A pressure gauge on your propane tank is useful

  1. Walter J Freeman May 19, 2014 at 2:38 pm

    Overall, I would place a pressure gauge as the next to last (penultimate) method of measuring a propane tank level of usable gas.

    The best way is to weigh the tank. There are inexpensive hand-held spring weight gauges available, for example on Amazon [search for Original Grill Gauge ~$14]. This is the same method used to fill tanks in the first place as the empty tank is placed on a scale and the propane is pumped in until the weight is proper for the tank. (If using a larger tank or one of other than standard steel construction for the common 20 lb tank one would have to make some corrections to use a precalibrated commercial scale.)

    The second method, which is not as good, is to measure the temperature of the tank using a liquid crystal stick on gauge which shows the tank level as a color change. Problems with this method are that the tank needs to have been in active use for the liquid level within the tank to have cooled enough (from heat of vaporization) to show the temperature difference. A second issue is just where do you place the strip on the tank — at the top, middle or bottom — to cover the total volume? For accurate and full coverage, you would likely need three of these gauges to be able to see the whole side at a glance. Incidentally, there are now becoming available some fiberglass tanks which if one purchased a magnetic LCD gauge would not work.

    The third method is the use of the pressure gauge which has been addressed, but which is little better than the last method, which is:

    Try to light a fire on your grill. Chances are the empty tank will not work.

    Solution to all of this? Have a spare tank, and remember to get the empty refilled!

    For this last part, I suppose one could tie a string around your finger to remind you to get another tank.

    • Terry January 4, 2017 at 1:05 pm

      I suggest buying a hand held infrared with lazer pointer and check temperatures to find the liquid level. These are not expensive and have many other uses as well. Primary use is checking any air conditioning unit for problems. Vent temp, return line temp, liquid line temp and condenser temperature. Works on autos or home a/c units.

  2. Amber Jewelry June 27, 2014 at 6:07 pm

    Organic refers to origin from a living thing, not the pesticide-free
    growing process.

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