Somewhere between our first caravan trip and our most recent one, I became convinced that I had developed a reliable intuition for how much gas we had left. I could just tell, I thought. A kind of sixth sense. The weight of the bottle when we loaded up. The confidence of the flame. The vague memory of when we last filled it.

I was wrong, and I was wrong in a very specific way: I was consistently and significantly more optimistic than reality. This is, I've since learned, essentially universal among caravan and camper owners. We all think we have more gas than we do.

The reason for this is simple. Checking a gas bottle properly is genuinely awkward. Our 4kg bottles live in a cupboard, strapped in snugly, connected to the gas line. To actually assess the level, you'd need to unhook the bottle from the regulator, unstrap it, haul it out, and then — the scientific climax of the whole operation — give it a shake. You're essentially trying to detect a slosh. A slight asymmetry in the way a metal cylinder swings. It's the caravan equivalent of a wet finger in the air.

“It’s the caravan equivalent of a wet finger in the air. A quarter-full bottle and a half-full bottle both shake with the kind of rock-solid reliability that says ‘probably fine.’”

A quarter-full bottle and a half-full bottle both shake with roughly the same sloshiness. The shake test, performed correctly, tells you essentially nothing useful — but we do it anyway, nod sagely, say "yeah, should be right," and put the bottle back. We are not learning anything. We are performing a ritual.

Our particular situation adds a layer of complexity that many caravanners will recognise. We run 4kg bottles rather than the standard 9kg. The advantage is that they're lighter, easier to manoeuvre and (just) fit into an external cupboard. The disadvantage is that you cannot simply pull into any hardware store or petrol station and swap an empty for a full one — that's the 9kg world, and it is much better served. For 4kg bottles, you need an actual gas filling station. You need to find one, go there during opening hours, and have the bottle refilled. The number of places that do this has been quietly shrinking for years.

What this means in practice is that you can't afford to run out and then figure it out. You need to fill up when you're passing a station that does it, even if the bottle isn't empty — because the next opportunity might be a significant detour. The question, then, is always: is it worth stopping now, or do we have enough to wait? And answering that question, pre-sensor, required the shake test, the ritual, and a guess.

And we all know what running out of gas actually means, because it's easy to shrug it off as a minor inconvenience. It is not always a minor inconvenience. It is, depending on timing and temperature, a fairly comprehensive unravelling of the comfortable caravan life.

None of this is catastrophic. But it is avoidable. And it tends to happen, as these things do, at the least convenient moment — a cold night at a remote campsite, a Sunday when nothing is open, a stretch of coast where the next town is further than you thought.

The Mopeka sensor clips to the bottom of the gas bottle and stays there. It reads the level using ultrasound — actual measurement, not estimation — and sends that reading to an app on your phone. You can see, in real numbers, how much gas you have and roughly how long it should last at your current rate of use.

The part that made the biggest difference for us was this: you can check it without leaving the car. We're driving along, spot a gas filling station coming up, glance at the phone, and know instantly whether it's worth stopping. No pulling over. No unpacking the cupboard. No shake. Just a number, accurate, on a screen, in three seconds.

“You can check it without leaving the car. Spot a filling station, glance at the phone, know instantly. No unpacking. No shake. Just a number.”

Whether you're running 4kg bottles like us or the more common 9kg, the principle is the same: you want to swap or refill before you need to, not after. An empty bottle on a remote stretch of coast is an empty bottle regardless of its size. The ability to make an informed stop — not a precautionary one, not a panicked one, but an informed one — changes how you plan a trip. We now fill up when we're around 25–30% remaining, which is comfortably before it matters, and we've had exactly zero cold showers since fitting the sensor.

My sixth sense for gas levels, it turned out, was nonsense. It was hope with a shake and a guess. The Mopeka, by contrast, actually knows. I find this a significant improvement.