Most RVs use Propane to power various items. In our 2018 Grand Design Momentum 397TH, we have our fridge (LP/Electric), Stove (and Oven), Water Heater (LP/Electric), and Furnace. When boondocking, it's much more efficient to use our LP/Electric appliances on LP versus using batteries and solar.

With so many items needing LP, it's good to know how much you have at any given time. This is particularly true if you have a Propane generator or it's winter and you need to use the furnace.

The Physics of Liquid Propane (LP)

Like any liquid, LP expands and contracts with temperature. It does this seventeen times more than water, expanding or contracting 1.5% for every 10 degrees Fahrenheit.

This is why LP tanks are designed to only be filled to 80% of the tank's true capacity. Don't worry, your 30lb LP tank will still hold 30lbs. But, it may hold a little more or less depending on the temperature when filled since the tank is filled by volume and not weight.

A gallon of LP weighs 4.2lbs at 60F. Drop the temp to 20F and a gallon of LP will weigh ~4.5lbs (6% more). Keep this in mind when you get your LP tank filled and you're looking for a certain number of gallons. The temperature might affect the end result.

The rate of vaporization of Liquid Propane decreases with temperature, bottoming out at -44F when no vaporization at all occurs. But, even before that (above -44F), the rate of vaporization can become unusable for appliances that require a higher rate of flow. For instance, from the graph below, you can see that even at +20F, the rate of vaporization is not enough to keep up with a 35,000BTU furnace. To be usable at that level or colder, a tank heater needs to be used for sufficient vaporization. Also, keep in mind that as LP is used (evaporated), that evaporation cools the LP and the tank, further exacerbating the issue.

While we're on the topic of BTUs, each pound of LP contains 21,500 BTUs of energy. Knowing this, we can do some math to see how long 30lbs of LP can run our 35,000BTU Furnace. (21,500 * 30) / 35,000 = 18.4 hours of constant run time. Of course, furnaces don't usually run constantly, and even when they are running, they cycle the burner about 6 times an hour. The best bet to get a real measurement of how long your LP will last is to take two measurements one day apart at the same time of day, take the difference and use that for your calculations.

Measuring Your LP Tank Levels

There are a few ways you can measure your LP levels. The most accurate way is by weight. If you know the empty weight of your tank, you can simply weigh it, and subtract the empty tank weight. While it's certainly not the most convenient way, it is the most accurate. The mass of the LP doesn't change with temperature or pressure.

Another way is the hot water method: Boil a small pot of water and pour the water down the side of the tank. Carefully feel the side of the tank and you will notice a very distinct difference in temperature where the Liquid Propane stops and Propane vapor begins. The line where this difference is felt is the top of the Liquid Propane. The reason this works is that the Liquid Propane absorbs the heat much faster than the vapor in the tank. This is very clearly visible in the FLIR images in the video.

Pressure gauges can be another way to estimate the level of LP in the tank. I say “estimate” because they are not super accurate. But, they are a quick way to at least know if it's full or empty or somewhere in between.

Last, but not least, are the Mopeka ultrasonic sensors.

Mopeka Ultrasonic Tank Sensors

When we first started our Full-Time RV life back in 2017, we researched ways to monitor LP and found the Mopeka Tank Sensors. We used those (the standard version) for about 3 years till one stopped communicating. Heading to Amazon to buy a replacement, we noticed the newer “pro” sensors and have been using those ever since. I'm not sure how much better the pro version is but I did notice they accept firmware updates wirelessly and I never saw that on the older sensors. They are also easier to center on the tank, being round versus an odd shape.

These sensors work by sending an ultrasonic pulse through the liquid and measuring the depth. You connect to them via a dedicated (free) app over Bluetooth to get an actual % reading based on the size of the tank and the level of the LP. While susceptible to temperature fluctuations, the measurement is still very accurate according to our tests.

The Test!

The test plan was pretty simple. Take a full tank and weigh it. Bleed off the propane in 10% increments (according to the sensors) and record the weight all the way down. Weigh the completely empty tank and use that to adjust the actual weight measurements to just LP weight measurements.

You can see in the spreadsheet below that our empty tank weight was 24.1lbs. The measurements in the first column (B) are the raw weights and in the column next to that (C) are the weights adjusted to remove the tank empty weight. The next two columns are the sensor readings compared to the weight measured percentages and the final column is the difference between the two percentages.

LP Sensor Testing Measurements and Calculations

One issue we ran into is that bleeding off the LP as fast as possible cools the LP and tank a LOT! This, in turn, slowed the rate of evaporation but also condensed the liquid more than what we would see with normal use (much slower evaporation). This gave the sensor a shallower liquid and thus a lower % reading. This is why the delta column gets larger as the test goes on. You can also see where I stopped around 30% and let it warm back up and melt the ice (see video).

Knowing that the speed of testing skewed the numbers a bit, we feel very confident in these sensors providing very accurate readings under normal use.

We always had a decent level of confidence in the sensors but it's great to do an actual test and confirm it!

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