I know this message thread is a little old, but I was looking for information about filling level in Wavian Jerry cans and this was one few I found. I've been using these cans for a few years now and have really been happy with them, but had a really bad experience with them the other day. I use these cans to to hold gasoline that I have de-ethananolized (if that's a real word). Having a bit more gasoline produced than capacity to hold it, I really filled these cans as high as I could. I had no issue with this during the winter and spring. When I opened a can, it would just have slighly negative or neutral pressure in it. However, I opened a couple of these the other day during a long period of high temperatures. The gasoline violently sprayed out them. Straight up into the air, like a fountain. Not just a little spray- quite a bit of fuel. I tried searching for information about this on the internet. I found surprisingly little. The "official" instructions for using these states you should hold a rag over them when opening. There were also a couple of warnings not to over fill them, and a couple of references about only filling them 95%. But what does that mean? Does it mean fill it to 95% of what you can possibly get into it? Or does it mean only put 19 Liters into it (which is 95% of the labeled 20 Liter capacity)? I am going to perform some experiments while we have hot weather, to determine exactly how much should be put into these. However, knowing how well the German's originally engineered these, and how specific the current specifications for these are, I suspect that the answer is going to be exactly 20L. And it may also turn out that this is about 95% of the possible total fill. Looking carefully at these cans, and considering their design, I believe I understand exactly why they can blast gasoline out of them so strongly. If you look at them from the side, you can see that the cans have a high "hump" behind the handles. This hump is considerably higher than the opening. So when you fill gas all the way to the top, you have a pocket of air in it. That's great- it allows for fuel expansion. In warmer temperature it will be under pressure. If the can is not overfilled, that pressure will just push itself out accross top of the gasoline to the spout and blow out as vapor. However, if you have filled the can to the max, there is no open channel between that pressured pocket and the can opening. In fact, the most direct path for that pressure will be to travel through the air vent channel built into the top of the can. Because that channel is filled with gasoline (in a fully filled can), it will get forcibly pushed through that and ejected straight up from opening. Here are a couple photos of a dissected Wavian can where you can see the air vent channel from both top and bottom. I will report back further findings, but for now I would just say that while these cans are capable of holding more, you only want to put 20 Liters of fuel into them and you will see considerable unused volume. But you REALLY need that unused volume when you have warmer conditions. I'm just surprised that there is so little information about this on the internet.
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