Really? That doesn't make sense from a physics standpoint. The gauge pressure is water_vapor_pressure + air_pressure - external_air_pressure. So if there is a significant amount of air in the boiler the steam tables will give the wrong number.
Admittedly, I know next to nothing about espresso machines. But isn't there something called a vacuum check valve that allows the air to vent and be replaced by water vapor in the boiler as it is initially heating?
Probe I would have used then Click Here (www.sweetmarias.com) not the fastest but fast enough. I used a Musa 6cup and a Moka Express 6cup, I was trying to see noticeable difference from SS and Aluminum. I also have the 10 cup SS and 1,3, and 6 in aluminum. The 3 cup is my favorite.
Not sure where you measured temps but you mention at the end of the brew, I am long off the heat before that, the aluminum pot response is the quickest when removed from the heat.
Thanks for that. I measured the whole cycle with two small thermistors coming in from below through the safety valve port (example: Click Here (coffeegeek.com)). I didn't remove the moka pot from the heat until it started to sputter, so perhaps that's the main difference -- you're stopping it sooner.
My last espresso machine I created a leak when I completely filled it with water for a descale. Where it went horrible wrong was when I heated a vessel that had no room for its contents to expand, my intention was not to heat to full temp 255f temp but heat the solution to 200f, I walked away and got distracted. 35 min later the tell tell his was coming from my machine and it wasn't the obviously not working safety pressure valve. All boilers have this safety valve, if they didn't it is a bomb, I am just lucky water found an escape in my case.
Oh, so is that what you mean when you say there's "air" in the boiler? Some room for gas, not all liquid? I'm not 100% sure but I think that the gas is nearly all water vapor in a pressurized espresso boiler.
germantownrob Senior Member Joined: 2 Dec 2007 Posts: 2,152 Location: Philadelphia Expertise: I love coffee
Espresso: Duetto 3, A Dead Oscar Grinder: Vario-W, Preciso w/Esatto,... Drip: Brazen Roaster: Diedrich IR-1, HT B
Posted Fri Feb 1, 2013, 2:21pm Subject: Re: Crazy idea for a big Moka pot
John I am no expert on any of this but I do like to figure things out, I have been wrong about many things I have been certain of in my life.
I am new to double boilers so I can say nothing about my brew boiler at this point in time but the pressure is delivered from a rotary pump or a vibe pump for brewing. A steam boiler is going to deliver steam so it has to have head room to deliver it, the pressure gauge is on the top of my boiler. On my heat exchange machine the brew path of water is all tubing being heated by the steam boiler, on this boiler that lays on its side ( the Duetto DB has them standing up) and at the top was a fill probe that is adjustable from about 1inch of head space or less depending on how it was adjusted. The vacuum breaker allows the boiler to heat with out dealing with false pressure by releasing pressure and water once the boiler heats above boiling so it can continue to heat and build pressure. The Oscar did not have a vacuum breaker and has to be manually released by opening the steam knob, if this is not done the machine will heat above the boiling point but not to full temp.
I think the key with a moka pot brew is to pull at the first start of flow, some exceptions are when I pack tight and the flow will be slow so I may leave it on heat a little extra, taste dictated this, other wise the brew is burnt tasting to me. From you measurements and mine I think the results say it is best to pull the moka pot soon after the first flow of coffee starts to flow. This make a lot of sense to me now, the first say half of the water has extracted most everything we want from the coffee but if the remaining water is getting hotter, especially over 205f it is now extracting undesirable elements from the beans. It sure is great when to people get to be correct!
The Brikka, worth getting if you like moka pots. I bad mouthed them for a few years with statements like " I don't need some freaking widget to get a good brew from a moka pot" but curiosity got the better of me and I sure am glad. It really opens a wide range of brews from a single bean with dose and grind but it opened up beans and light roasts I could never get to taste good in a moka.
That's true, it isn't quite the same. But you could rig things up to vent the air if you wanted. The point is that without the "false pressure" caused by the air you'd need to have the water well above 100°C to drive liquid through the grounds. Instead what happens is it starts percolating upward at something like 50°C.
Moka is odd in that respect. Assuming you start with cold water, a large fraction of the brewing occurs well below the optimal temperature range and (at least potentially) some fraction occurs above that range. You can't tell in the graph I linked because the volume data aren't shown, but only 15-20% of the coffee brewed at 90-95°C.
Not sure if that was what you meant by "porn" but I loved the Martha Stewart moka video.
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