jim_schulman Senior Member Joined: 19 Dec 2001 Posts: 3,772 Location: Chicago Expertise: I live coffee
Posted Fri Sep 13, 2002, 3:12pm Subject: Boiler Pressure in HX Machines: Mini FAQ
Ok, you can adjust the boiler pressure on your new HX machine to control brewing temperature. But where to adjust it to?
First off, a few facts.
The guage shows "guage pressure" that is the pressure above the air's. At sea level, a guage reading of 1 bar is 2 bar boiler pressure.
The innards of the boiler are at "saturated steam" temperature for that pressure, that is, the boiling point for that pressure level. At 1 bar, this is 100C, the normal boiling point of water. The table below gives values for espresso boiler pressures.
The Heat exchanger draws water from the mains or tank and runs it through the boiler where it heats up. If the pump isn't running, it heats up to whatever the boiler is at. If the pump is running at espresso making flow rates, it heats up to a lower temperature, hopefully the right one for espresso. There is a sort of design standard out there for espresso machine heat exchangers that most manufacturers follow. The table gives the brewing temperatures implied by that standard.
Here's the table:
Boiler Pressure Boiler Temp HX & Espresso Temp 0.84 bar 117.5C 244F 88C-90C 190F-194F 1.00 bar 120.0C 248F 90C-92C 194F-198F 1.17 bar 122.5C 253F 92C-94C 198F-201F 1.33 bar 125.0C 257F 94C-96C 201F-205F
Remember, even if your machine is designed to this standard, there may be variations. Afaik, this table is roughly right for the E61 tribe, and the Livia. NS plays by it's own rules, so I don't know about the Oscar.
The tradition is to use higher temperatures in this range for lighter roasted bright blends, and lower settings for dark roasted mellow blends. In any case, for a given blend, higher brew temperatures = less brightness, lower temperatures = more brightness. But temperatures outside this range will mess up the extraction, so avoid them.
The boiler temperature versus pressure comes from steam tables, my entries are approximate. The equivalent HX temperature comes from a variety of sources, mostly alt.coffee, but also from manufacturers sites that carry machine set-up instruction pages.
But oops! I stripped the link and metadata from the pages I copied, so I can't give you the web sources. I'll edit the original post adding sources, and make a note of it, as soon as I've tracked them down.
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