Main problem is evaporation of the water that is on the inside wall. I tried measuring and calculating it, was about 1 g within something like 10 s (rate decreases and evaporation eventually stops when no more water, depends on conditions like room temperature, humidity etc) - can't mount the cylinder with tongs to PF and start pouring water much faster than that. It means temperature drop of that amount of steel around 10°C (temperature probe fitted between cylinder and silicone confirmed the calculation quite well). Tried covering the top while doing that, but didn't help much

Outside wall is less of the problem, silicone is a kind of insulator (though not especially good), exposed top of the cylinder doesn't get into direct contact with water (insulating it anyway didn't make a difference), so toweling it wouldn't help (you also need additional 2-3 seconds to do it). With steam preheat, it is dry anyway (assuming suitable vessel used)

When water is poured in, after some time temperature of the cylinder and water equalize, according to approximate formula 0.33 * Temp of cylinder + 0.67 * Temp of water (with assumption 60 g water 4.2 kJ/kgK, 250 g steel 0.5 kJ/kgK), minus of course other temperature losses. So if original preheat method used (which heats the cylinder to something like 70°C at best minus decrease due to water evaporation - luckily less than with boiling preheat because cylinder temperature isn't very high and outside is also dry), you have to be really quick to do most of extraction before water cools down below usual espresso brew range

Cylinder filled with water cools slower (water surface is much smaller than inner surface of the cylinder, water temperature now significantly lower than boiling point) and when plugged with piston, cooling is even more slow - only through convection from the outside surface.

Yes, I meant drying off the exposed metal surfaces, not the silicone jacket. Is it impossible to do in a few seconds?

I'm not sure what would be a practical way to dry the inner surface in a few seconds if using boiling method.

vit wrote:..., wait 20-30 s checking the water temperature in the cylinder with thermometer until it drops to around 94°C,

Vit, given the declining temperature during the extraction (unless you are using the ss piston), wouldn 't it be better to start from a somewhat higher temp?

I have a plastic piston

I tried a bit higher or lower with each coffee. With this workflow, for this Illy blend temperature around 94°C before mounting the piston proved to be somehow up to my taste. With lighter roasts 95-96°C, with darker as low as 91°C (so far, because I started using steam preheat method somewhere at beginning of april).

These days I just run boiling water to the cylinder while holding it carefully in its silicone sleeve. I run a slow pour of boiling water from a gooseneck kettle on the top outer surface (since it's what's accessible when the sleeve is on) and the inner surface of the cylinder for a couple of seconds (10 seconds?) and then assemble it to the pf as quickly as I can (the pf is always ready with screen on before I do the pre-heating) then mount the assembled brew chamber to Flair and add that water from the same source (must be at least less than boiling given the time of pouring to pre-heat and assemble the cylinder). Works fine for me although I don't measure my temps.

I've continued experimenting with preheat methods and still haven't settled on one.

I like the direct heat method the best, but does anybody have a recommendation for an instant and accurate way to measure temp on this (quite reflective) stainless steel? My IR thermometer doesn't seem accurate on it. My thermometer pen is very slow.

I'm also thinking of finding a cezve like Vit's as a backup idea in order to use a steaming method.

I'd also like to explore using an immersion boiler to boil water directly inside the cylinder with a partially inserted piston. Does anyone do this and have any recommendations on an immersion boiler?

Some people are using electric tea kettle for boiling preheat. I didn't go that way because I was trying to find a workflow usable for traveling, where I don't want additional relatively big item

However, if ordinary pot used, at least 4-5 dl is needed to submerge the cylinder completely (depending on the pot diameter), which takes time to boil. Also, it doesn't work well on induction (gas probably similar), because induction doesn't heat the center of the pot, so water starts boiling outside the cylinder relatively fast, while inside it takes much more time so the time for preheat it enough was considerably longer. To improve things, I put a piece of metal at the bottom of the pot and cylinder on that piece, to enable water to circulate around the cylinder which helped. Also, I didn't like using tongs, being afraid to drop the cylinder on the glass-ceramic stovetop by accident and crack it. And, tap water in my town is relatively hard and there was a lot of calcination on the cylinder after each preheat. So I didn't find this method especially practical

My kettle isn't ideal for boiling preheat either. The max fill line is only just enough water. It's pretty snug to get tongs into it, and the tongs don't hold the cylinder that well. For a few other reasons, it's just not convenient for me, and I'd like something I can use when I travel as well. I'm feeling like the cezve or immersion boiler will work best there. I want to continue exploring direct heat for at home, just need a better thermometer.

A travel kettle is about the perfect size for pre-heating boiling and - ironically - reasonably sized for travel!