I've been trying to get a simple understanding of water temperature inside my Cremina's boiler. I assumed (perhaps incorrectly) that the pressure reading on the PSTAT will give me an indirect reading of the temperature. My PSTAT will range from about 0.8-0.9 bar, which I assumed corresponds to a temperature of approximately 201F-206F (approximate boiling temperature of water at 0.8 and 0.9 bars respectively). What confuses me is that people routinely claims that the temperature of water will go up to ~250F (Espresso Machines 101): Post #4 states "Most of these machines operate around .8 bar, or roughly 250F". I've seen several other instances of this. Am I missing something here? Assuming that I purge my boiler of air (and allow the boiler to equilibrate), shouldn't the water temperature correspond directly steam pressure?

It is a Saturated system. The pressure in the boiler AFTER venting off the non-condensible gases is the saturation temperature or pressure, you may have to correct the gauge pressure to absolute pressure depending on the copy of the steam table you are using. To look the temperature up you use a steam table. This table reads in PSIG which does not require correction to absolute pressure. Convert the BAR reading to PSIG and read off the temperature. Forgot to link table. http://www.copper.org/applications/plum ... tables.pdf

See this thread where the following chart comes from: How does high altitude affect brewing?

Patagent wrote:What confuses me is that people routinely claims that the temperature of water will go up to ~250F (Espresso Machines 101): Post #4 states "Most of these machines operate around .8 bar, or roughly 250F".

See Saturated Steam Table and note it expects absolute pressure, not gauge pressure. When we refer to steam boiler pressure in the forums, it's gauge pressure. You have to add 1.0 to get absolute pressure. So, for example, if your gauge reading is 0.8 bar, that's absolute pressure of 1.8 bar at sea level, or 242.44°F.

Simply put, when your gage is reading 0.0 Bar it is actually at 1 Bar because of atmospheric pressure. When you are reading on the gage .8 Bar it is actually 1.8 Bar (approx). There are a whole bunch of corrections but altitude is the biggest, which is why Yakster's graph has different altitude lines.

Edit: What Dan said.

HB wrote:See Saturated Steam Table and note it expects absolute pressure, not gauge pressure. When we refer to steam boiler pressure in the forums, it's gauge pressure. You have to add 1.0 to get absolute pressure. So, for example, if your gauge reading is 0.8 bar, that's absolute pressure of 1.8 bar at sea level, or 242.44°F.

flathead1 wrote:Simply put, when your gage is reading 0.0 Bar it is actually at 1 Bar because of atmospheric pressure. When you are reading on the gage .8 Bar it is actually 1.8 Bar (approx). There are a whole bunch of corrections but altitude is the biggest, which is why Yakster's graph has different altitude lines.

Why would atmospheric pressure come into play here? As far as I understand, my boiler is a closed (relatively speaking) system. The atmospheric pressure shouldn't affect steam production which determines the PSTAT reading.

. . . and note it expects absolute pressure, not gauge pressure.

That's a "for sure" on steam tables.

But, with an espresso machine boiler that is NOT PID controlled, it can get a little complicated. Here is but ONE example:



It has been a while since I did the calcs but the mean pressure is pretty close to 85% of the maximum pressure. Certainly the mean temperature and the mean pressure would be as I previously published. The relationship between the mean pressure and the maximum pressure would be different (Olympia Cremina vs Quickmill Anita) for various machines.

Patagent wrote:Why would atmospheric pressure come into play here? As far as I understand, my boiler is a closed (relatively speaking) system.

Think of an inflated balloon that rises into the sky. Its volume will increase. Submerge it in deep water and its volume will decrease. Now imagine the same situations with a fixed volume boiler. What would you expect to happen to the pressure reading inside the boiler?

Mike,

You do open the boiler to put in water? When you do, is it under vacuum, under pressure or does it just open without any pressure? If the latter your boiler is at atmospheric pressure when you start. I can envision possibly it being under vacuum if you have a very tight boiler but then your gauge would be tightly pegged at 0 trying to read a negative Bar pressure. Given enough time (over night for example), even with a tight system atmospheric pressure will be forced into the boiler. I'm betting your cold boiler 99.9% of the time is at 1 Bar (atmospheric) pressure and the gauge is reading 0.0 Bar (gauge).

At that point you close the system and begin heating, (ignoring False Pressure (it gets too complicated)). At Atmospheric pressure 0 Bar (gauge) and 1 Bar (absolute) water will boil at 212F. The pstat is also reading the same pressure the gauge is reading and will remain closed until steam pressure builds up to your set point of .9 Bar (gauge) 1.9 Bar (absolute) (water at 242F) at which time it opens and the water begins to cool. Temperature and pressure drop until .8 Bar (gauge) and the pstat closes and the heater kicks on. Without the pstat to turn off the heater at .9 Bar (gauge) the heater would keep heating the water until enough pressure built up to blow open the weakest point of the boiler, that is usually the safety valve, otherwise things can get extremely dicey.

PS. I noticed you have a 2014 Cremina. In the boiler fill cap is a vacuum breaker. It is open at room temperature and then closes as steam is being pushed past it as the boiler heats up. When the boiler cools it opens and the boiler goes to atmospheric pressure. The older Creminas didn't have this and as the boiler cooled the indications of a tight system were the lever rising up because of the vacuum in the boiler.

HB wrote:Think of an inflated balloon that rises into the sky. Its volume will increase. Submerge it in deep water and its volume will decrease. Now imagine the same situations with a fixed volume boiler. What would you expect to happen to the pressure reading inside the boiler?

I can see your point using a balloon as an example but a stainless steel boiler is fairly rigid. Wouldn't a submarine be a better example? As far as I know, a submarine can maintain 1 atm pressure at extended depths without decompression. Shouldn't atmospheric pressure have minimal impact on the inside of a closed rigid system?