Temperature, altitude and espresso extraction - a question - Page 2

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lancealot

Postby lancealot » Feb 13, 2019, 6:24 pm

forbiddenbeat wrote:is 1.0 bar in a sealed system actually going to be a lower temp at altitude?


[s] INSIDE the closed vessel, physics says no. [/s]

Bret

Postby Bret » Feb 13, 2019, 6:58 pm

If boiler pressure is measured purely internally, via a piezo-electric sensor or something, then the boiler pressure will be altitude-independent. But if it a relative pressure measurement (inside the boiler vs outside the boiler), altitude will have an effect.

I have no idea how the pressure measurement is implemented, if it is a standard or if it can be different between types.

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homeburrero
Team HB

Postby homeburrero » Feb 13, 2019, 10:23 pm

forbiddenbeat wrote:Somewhat tangential question: is 1.0 bar in a sealed system actually going to be a lower temp at altitude?

Yes, Bret has it right. The gauges on espresso boilers measure gauge pressure -- the pressure inside the boiler relative to the pressure outside the boiler. So at sea level 1.00 bar gauge pressure is 1.00 bar plus the atmospheric pressure of 1.01 bar, so the absolute pressure inside the boiler will be 2.01 bar. In Denver, 1.00 bar gauge pressure is 1.00 bar plus the atmospheric pressure of 0.84 bar, so the absolute pressure inside the boiler will be 1.84 bar. Boiling point is related to absolute pressure.

When you see something like 1.00 barg, that means the author is making it explicit that they mean gauge pressure and not absolute pressure. But most people don't use that terminology and just use the term bar when talking about espresso machine pressures - it's understood that it must be gauge pressure.

Bret wrote: I have no idea how the pressure measurement is implemented, if it is a standard or if it can be different between types.

Most use simple mechanical Bourdon tube gauges.
Pat
nínádiishʼnahgo gohwééh náshdlį́į́h

jeffmattel

Postby jeffmattel » Feb 14, 2019, 1:17 am

Question:

Say your steam boiler has a recommended max temperature setting of 255F and that setting gives you 1.5bar which is also the max recommended on that particular machine. Say these numbers were calculated at sea level. Now say you are at 6,000' with the same machine and a temperature setting of 255F results in 1.8bar(instead of 1.5bar) which is past the maximum recommended. In order to stay at 1.5bar at 6,000' I have to set a temperature of 250F(5 degrees lower than max recommended)

Is this normal?
Can I set the temp to a max of 255F and even though it shows 1.8bar(.3 higher than recommended) be ok?

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homeburrero
Team HB

Postby homeburrero » Feb 14, 2019, 1:51 pm

jeffmattel wrote:Is this normal?

Yes.

jeffmattel wrote:Can I set the temp to a max of 255F and even though it shows 1.8bar(.3 higher than recommended) be ok?

You could probably get away with that, but I think it's better to set the temp a few degrees lower so that you come out within the recommended max gauge pressure for the boiler. Two reasons for that are: 1. The recommended max is a pressure that is safely below the pressure where the safety valve on the boiler opens. Safety valves, like gauges are relative pressure devices, so you want to go by the gauge pressure as the safe max, irrespective of altitude. 2. The steam velocity coming out of the wand is also related to the relative rather than the absolute pressure, So the steaming oomph you get is about the same at 1.5 bar gauge pressure irrespective of whether the machine is at altitude and at a lower temperature. Admittedly, there is a slight loss of steaming/heating capacity due to the lower temperature steam and lower heat capacity in the boiler water at altitude, but not enough to worry about.
Pat
nínádiishʼnahgo gohwééh náshdlį́į́h

jeffmattel

Postby jeffmattel » Feb 14, 2019, 3:54 pm

Thx Pat.