It is February, the furnace has been cycling since dark, and at some point in the evening you put a coat on, walk round to the propane locker and lift the tank an inch off its base. Feels like maybe half. That is the fuel gauge most of us are actually running on, and there is no shame in it, because the alternatives are not obvious.
It works better than it has any right to, right up until the night it does not, and you find that out at four in the morning with the fan pushing cold air round a rig losing a degree every twenty minutes. Why a tank is so awkward to read from outside is also why the cheapest gadget on the shelf will not help.
Pressure is not quantity
Propane sits in the tank as a liquid with vapour above it, and the pressure you can measure at the valve comes from the vapour. That pressure is a function of temperature, not of how much liquid is underneath it. On a warm afternoon, a tank at three quarters and a tank at an eighth read close enough to the same that you cannot tell them apart, and the number only falls away right at the end, by which point it is not information, it is a warning you can no longer act on.
That is the whole story with the little green-yellow-red discs that screw on between the tank and the regulator. They are not lying, they are answering a different question, and their answer stays nearly constant across the whole range you care about. What you want is the level of the liquid, which means measuring the liquid rather than the gas sitting on top of it.
Three ways to get a real number
Weigh it
The low-tech answer, and more accurate than most of the gadgets. Every tank is stamped with its tare weight, the empty weight, usually marked TW somewhere on the collar. Weigh the tank, subtract the tare, and the remainder is propane. A 20 lb tank holds 20 lb of propane when full, so the arithmetic will not trouble anybody. A luggage scale covers portable tanks, and cradle scales live under the tank permanently if you would rather not lift it.
The limitation is that you have to be standing next to it. A scale tells you nothing at two in the morning with the furnace running and you asleep, nothing at all when the rig is in storage and you are two hundred miles away, and on a mounted horizontal tank it is not really an option, because nobody is lifting that. Camp a few weekends a year and mostly want to know before you pull off the driveway? Buy the scale and skip the rest of this.
Ultrasound, through the steel
The Mopeka Pro Check is the product that turned propane monitoring into a category. It magnets onto the underside of the tank and fires a sound pulse up through the steel. The pulse reflects off the surface of the liquid and comes back, and the time it took is the depth. Nothing is plumbed, nothing is drilled, and nothing sits in the gas path anywhere.
Five things come off that broadcast and we read all five: liquid level, temperature, battery voltage, battery level and signal quality. The compatibility database carries the same list with the caveats attached, and two of those caveats are worth reading before you order anything.
Signal quality is the one people skim past, and it is unusually honest engineering. The sensor reports how much it trusts its own reading. Stand a tank on a painted base or on uneven ground and the pulse has a worse path to travel, so rather than quietly handing back a plausible wrong number, the sensor says it is struggling. When a level looks off, it has usually told you so already.
Two caveats we publish rather than bury. The level is broadcast in millimetres of liquid, so turning that into a percentage needs your tank height, which is configured per tank in the hub rather than read off the sensor. And the Pro H2O water version uses the same frame, but water needs its own sound-speed curve; until that ships, a water tank reports a raw level rather than a calibrated depth.
Set against that, you take on a battery to change eventually and a magnet that wants clean bare steel. The thing to think through before ordering is range: a sensor in a front propane locker on a long fifth wheel sits a fair distance from wherever the hub lives, and a broadcast nobody is listening to is not a reading.
When propane is not really the complaint
Sometimes propane is just the gauge that annoyed you most recently. If the factory tank readings have all lost your trust, the sensible move may be to replace the lot rather than bolt one sensor onto one tank.
The Garnet SeeLevel II 709-BTP7 reads fresh, grey and black tanks, propane, and system voltage from a single panel, and broadcasts the whole set over Bluetooth. We read all five. The 709-BTP3 is the same thing minus the propane channel, so the model number is worth checking twice before you hand anybody money.
People rarely fit these because of propane. They fit them because factory holding tank sensors are probes mounted inside the tank, probes inside a black tank foul, and a black tank that insists forever it is two thirds full is one of the most hated failures in the hobby. SeeLevel senders read from outside the tank wall, so there is nothing in there to get dirty.
It costs money and an afternoon, and both are real: senders bonded to the outside of every tank, wire run, a panel mounted somewhere you will look at it. If the whole gauge cluster has stopped being believable, that is a good afternoon. If propane is your only complaint, it is a lot of installing to answer one question.
Choosing between them
- Weekend camping, portable tanks. A scale. Genuinely.
- You run the furnace overnight in the cold. A Mopeka. Waking up cold because the tank ran dry at four in the morning is the exact thing it prevents.
- You leave the rig heated while you are away. A Mopeka, with an alert. This is our own main use for it.
- Your holding tank gauges are lying to you anyway. A SeeLevel, and get propane included by choosing the BTP7.
Where to get one
The Mopeka if you are adding propane monitoring to tanks you already have. The SeeLevel if you are replacing a panel that is already wrong.
Mopeka Pro Check
The sensor that made propane monitoring a category. It magnets to the underside of the tank and measures the liquid level ultrasonically, with no plumbing and no drilling.
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Garnet SeeLevel II 709-BTP3
The earlier SeeLevel Bluetooth board. It cycles one sender per broadcast rather than sending them together, so readings arrive staggered.
We may earn a commission if you buy through this link. It does not change your price.
Set the alert higher than feels necessary
Whatever you fit, the threshold is the part people get wrong. An alert at ten percent is a notification that you have a problem right now. Something nearer a quarter is a reminder to throw a tank in the truck next time you are already driving into town anyway, and the gap between those two numbers is the entire difference between an errand and a bad night.
Cold weather leans on this from both ends. The furnace runs far more hours, and a nearly empty tank at low temperatures cannot vaporise quickly enough to keep up with demand, so the last of your propane does not fade out gently. It steps off a ledge. Set the number generously and the walk out to the locker with a coat on becomes something you do because you felt like checking, not because you are standing in the dark guessing.
