yakster wrote:I bet some of the posters in this thread could explain to me why a full balloon weighs more than an empty one. I know from experiments that it does because the air inside is not weighless, but since the column of air is pressing down on the balloon either way it doesn't seem to make sense.

Air has weight, and if you can weigh in a vacuum you'll see the weight of the air involved - pulling a vacuum on a balloon is a little difficult though. You can use displacement in water to compute density and show evidence of air weight there too. (I don't remember the steps though.)

Air inside the balloon is compressed by the tension of the balloon skin, causing a higher pressure inside the balloon than outside. Since gasses compress well you'll have more air particles inside the balloon per cc resulting in more density. Just slightly more density, so the balloon is often near buoyancy.

Think of the balloon skin as a PSTAT if you will. They both increase pressure inside the vessel over and above the atmospheric pressure present.

That makes sense, thanks. The middle school science experiment I read didn't explain that part of it.

It's kind of tangential to this discussion, but here's an article I saw on Sprudge discussing a slightly unusual protocol for high altitude brewing:

http://sprudge.com/in-colorado-deliciou ... oxcar.html

Yes, theoretically even percolators become usable at high altitudes.

During the next weekend I intend to be camping at fairly high altitude. I have made "stove top espresso" on other occasions with these. They are far far better at high altitude than low. I like the stainless steel ones best because the aluminium ones conduct heat upward too much.

The 1974-2000 group La Pavoni really is far easier to manage at high altitude. The espresso is excellent even when it emerges from the spout so it is boiling. I suspect certain other machines would have a problem getting the group hot enough.

another_jim wrote:So, couldn't the overheating problem of the group could be solved at sea level with a pstat adjustment or weakening the spring of the old relief valves?

Absolutely, positively, no doubt about it.

One could use these curves to see where the pstat setting at sea level would equate to the temperatures experienced at whatever altitude.



A larger version of this pic is in the downloads section ( /downloads/ ) and the MS Excel file used to create it is available for the asking.

OldNuc wrote:Boiler pressure/temperature will not be constant as the p-stat is responding to the differential pressure across the p-stat diaphragm which is boiler pressure on 1 side and atmospheric pressure on the other. The atmospheric pressure decreases with increasing altitude. The actuation point is determined by the absolute value of this differential pressure and the opposing pressure of the set point adjusting spring. The spring pressure is not affected by atmospheric pressure, it is strictly mechanical in this case. The sum of the atmospheric pressure force and set point spring force on 1 side of the diaphragm is opposing the boiler pressure force on the other side. Reducing the atmospheric pressure force will result in the boiler pressure force moving the diaphragm to the mechanical set point at a lower actual boiler pressure/temperature at a higher altitude than it will at sea level.

You are comparing atmospheric pressure (an absolute pressure) and a spring force to a gauge pressure. I believe this to be incorrect.

Just as an FYI:

If you add the 14.7psi to both sides of the equation to correct to absolute pressure then it may be factored out and ignored as it is just adding an unnecessary step.

-- The La Pavoni is a thermodynamic saturated system and when the boiler contains non-condensible gases the final temperature at P-stat operation to shut off the heating element will be below the saturation temperature.

-- Boiler pressure may be stated as absolute pressure referenced back to sea level or stated as absolute pressure at the actual boiler elevation, these will not be the same if there is an altitude difference.

-- When addressing the spring relief valve operation point the actual lift point is a function of the differential pressure across the valve disk so the adjustment for absolute pressure factors out.

-- Google Saturated Steam Table for more info. This one is referencing boiler pressure to temperature at sea level with pressure in psig units. 1BAR = 14.7psi http://www.engineeringtoolbox.com/satur ... d_273.html

Edit: I think I got all this wrong, so ignore it. I'll leave it around so the rest of the thread might make more sense.

If I run a machine at 1 BAR of pressure with a PSTAT, and I'm at sea level the total pressure inside the boiler when compared to a vacuum will be 2 BAR or 2x 14.7PSI = 29.4PSI. In this case nature provides the first BAR and the second bar is by a PSTAT adjusted to 14.7PSI.

Take that same machine to 4500ft and now you have a machine running with 1 BAR supplied by nature equaling 12.5PSI and another 14.7PSI from the PSTAT as previously adjusted. Total pressure is 27.2PSI for 2.176BAR of pressure.

So if we leave OPV and PSTAT set the same as we go up in altitude we'll increase the BAR pressure of the boiler, because the value of a BAR drops while the added pressure of the regulator remains the same. I wonder how that effects Robert's La Pavoni operation at altitude.

I think that if you read these posts very carefully, Robert, Eric, Rich, and Jim are all fundamentally in agreement. The explanations are easily misinterpreted though, and I think that's the cause of most of the disagreement. I think all would agree that if Robert sets his pStat to 1 barg in Kansas City, it will still read 1 barg up in Pagosa Springs, but will be running at a lower temp (and lower absolute pressure.)

But I think these statements confuse the issue a bit:

DanoM wrote:Take that same machine to 4500ft and now you have a machine running with 1 BAR supplied by nature equaling 12.5PSI

DanoM wrote:because the value of a BAR drops

Here Dan is using 'BAR', I suspect, to mean whatever pressure the atmosphere is providing. But a bar (as used in espresso machine gauges) is not that - it is a precise measure equal to 100,000 pascals (100 kPa.) It is close but not exactly the value of atmospheric pressure at sea level. It can be a measure of absolute pressure, or of gauge pressure (which is the difference between the atmospheric pressure and the pressure behind a gauge.) When gauge pressure is meant you often see it spelled 'barg'.

OldNuc wrote:Boiler pressure may be stated as absolute pressure referenced back to sea level or ....

Can't say I've ever seen that, and is a good thing. If you had a boiler at 1 barg in Denver and simply added the weather report's (sea-level corrected) millibar reading to state that it was at 2.0 bar sea-level corrected absolute, I would be very confused, and still need to know the altitude to know the real pressure. If I know the gauge pressure, the altitude, and the current barometric pressure, then I have everything I need to calculate the absolute pressure. For practical coffee boiler purposes I don't really need the barometric pressure.

rpavlis wrote:During the next weekend I intend to be camping at fairly high altitude.

Be careful about concluding how things taste when camping at altitude. My experience is that everything tastes way better on a camping and hiking trip.

I wonder how that effects Robert's La Pavoni operation at altitude.

That was contained in his initial post. What was unintentionally omitted from his post was some data as regards the reading on his pressure gage (maximum reading would be preferred). The difference between atmospheric pressure at sea level and an elevation of 7500 feet is ~ 0.25 bar neglecting any weather changes for that quick jaunt. .

Google Saturated Steam Table for more info. This one is referencing boiler pressure to temperature at sea level with pressure in psig units. 1BAR = 14.7psi http://www.engineeringtoolbox.com/satur...d_273.html

The curves were derived from Thermodynamic Properties of Steam by Keenan & Keys ~ 1936 and 1.0 bar = 14.50377 psi (14.5 amongst friends). The Engineering Tool Box you mention is good but, then again, not so good if they think 1.0 bar = 14.7 psi.