Boilers, Pstats, and Gauges

[Matthew Brinski]

I'm trying to comprehend the workings of the boiler on my QM Vetrano. The boiler pressure is obviously contained and independent of the outside atmospheric pressure. My question is does high elevation/low atmospheric pressure play a role with the accuracy of the boiler pressure gauge? Further, is the pstat affected by this possible variable? From my understanding, the pstat is basically a big spring and relay switch which activates as adjusted by the user to the corresponding readings on the boiler pressure gauge. So I believe the pstat does not pose a problem related to ambient pressures, that is to say if a problem even exists. The boiler pressure gauge, however, is referenced when adjusting the pstat. What does the boiler gauge use as a baseline and/or what is it calibrated against. Is it fixed and not influenced by outside pressures, or does it calibrate its baseline to ambient pressures which are normally at or close to sea level?

Matthew Brinski

[HB]

How does high altitude affect brewing? recently answered your question: No, the boiler pressure, boiler pressure gauge, boiler temperature, and pstat are not affected by outside pressure per se.

Imagine for a moment you drive your car from Death Valley to the top of the nearest mountain. Would your tires' air pressure change? Darn right! But let's assume you take a tire gauge and adjust the pressure to read 32 PSI at your new locale. You drive back to Death Valley and check / adjust your tire pressure again, setting it to 32 PSI. Now, what was the force on the interior walls of the tires in both cases? (answer: 32 PSI)

The same is true for your boiler pressure, which directly determines the boiling point of water, independent of the ambient pressure (i.e., 1.2 bar inside your espresso machine's boiler is 1.2 bar wherever it may be located). What happens to water as it exits the group near the local boiling point of water, well, that's another story.

[Matthew Brinski]

Right, I have always understood that boiler pressure is independent of ambient pressure, and also that the boiling point in the boiler is independent of the relative ambient boiling point. I was just wondering about the function of the gauge itself. Your analogy of the tire pressure gauge makes the answer REALLY simple and makes my question about the gauge sound REALLY STUPID, but (and I'm probably over-complicating it) what I'm questioning is whether or not the ZERO POINT on the pressure gauge is effected by being at high altitude, thus possibly indicating a false boiler pressure. I ask because when I came home from work this morning, I measured the temperature of brew water from the machine which has been idle for 12+ hours, and the initial flush was 194F. The second flush 2 minutes later was 194F. The boiler pressure was "1.0". I was initially disturbed by this, but then common sense kicked in and I contributed it to a cold pf and group (yes, I'm calling the idle E61 group cold and not hot). I raised the pstat to a boiler reading of ~1.35 and have been experimenting with various flush routines and their correlating brew temps throughout the day - this has been challenging with my one year old son "assisting" me :D . The data from those experiments are for a whole different discussion, but my point is that the machine has undergone heavy use today. I dropped the pstat setting to reflect a boiler reading of 1.0 again. I flushed three ounces of water after a five minute idle interval. I repeated this five times with the brew temp being between 201.9 and 203.1 for all five measurements as a whole. Isn't this low??? I know you probably see me as a PITA poster, but I am just questioning everything that I think is relevant. I really do have my sights on what's in the cup, not what's going to be "techie cool".

Matthew Brinski

[HB]

...I'm questioning is whether or not the ZERO POINT on the pressure gauge is effected by being at high altitude, thus possibly indicating a false boiler pressure.

Sorry, I don't know the precise mechanics of your gauge. Don't they work by pushing against a spring, or straining a hollow piece of metal formed in a spiral? If so, neither would be affected by the ambient pressure.

I repeated this five times with the brew temp being between 201.9 and 203.1 for all five measurements as a whole. Isn't this low???

For brew purposes, that sounds like a pretty good start. For your machine, the flush amount and the rebound time have much more to do with the brew temperature than the boiler pressure because the HX is fairly small (IIRC, around 110ml). In contrast, the Cimbali Junior's HX is much larger and fiddling with flush amounts / rebound times wasn't fruitful; adjusting the pressurestat was the right answer. This is covered in detail in How I Stopped Worrying and Learned to Love HXs and more recently in Monitoring Brew Temperature - E61 & Silvia.

[PaulTheRoaster]

I think the question does make sense.

Most pressure gauges show a difference in pressure.

The boiler pressure isn't pressing against just a spring or a spiral, it's pressing against a spring or a spiral plus the ambient atmospheric pressure. Right? I think that is the nature of the OP's question.

Moreover, even if the absolute pressure in the boiler is kept constant, the membrane in the p-stat will flex more or less depending upon the ambient pressure.

I haven't done the math to determine whether or not either of these are significant effects.

-Paul

[mtnwoman]

I measured the temperature of brew water from the machine which has been idle for 12+ hours, and the initial flush was 194F. The second flush 2 minutes later was 194F. The boiler pressure was "1.0".

I dropped the pstat setting to reflect a boiler reading of 1.0 again. I flushed three ounces of water after a five minute idle interval. I repeated this five times with the brew temp being between 201.9 and 203.1 for all five measurements as a whole

Hi Matt,

I gather you got your Scace! I'm confused by your findings so far. Are you getting these temps at the beginning of the flush? Are you saying that the brew group was cold because you had not been flushing water through it...or were you talking about the portafilter? How did you get higher temp results later?

Since we are in the same boat...altitudally speaking...I'm most interested in hearing your thoughts as to what setting/flush sequence is giving you the best temp results!

Thanks!

[Matthew Brinski]

Hi, "mtnwoman". Sorry for not posting sooner, but time has been limited the past few days.

I'm confused by your findings so far. Are you getting these temps at the beginning of the flush?!

Yes, the "194's" were a walk up set of readings after the machine was idle for some time.

Are you saying that the brew group was cold because you had not been flushing water through it...or were you talking about the portafilter?!

Yes and yes.

How did you get higher temp results later??!

It was after a long series of temp readings when everything was warm.

Since we are in the same boat...altitudally speaking...I'm most interested in hearing your thoughts as to what setting/flush sequence is giving you the best temp results!!

I'll post some stuff in the morning on the thread "...high altitude..." that you started.

Matthew Brinski

[KaffaFiend]

First of all, when referring to pressure in PSI or BAR it's usually considered to be referenced to atmospheric pressure. That is PSIG and BARG, the G is for Gauge pressure meaning it's referenced to atmosphere. If it's shown as PSIA or BARA the "A" is referenced to an "Absolute" vacuum. So atmospheric pressure at sea level is roughly 14.7 PSIA(1.014 BARA) but is 0 PSIG (0 BARG).

I love the analogy of the tire pressure used above but it doesn't hold water, using "My Cousin Vinny's" lingo.

If you put 32 pounds of pressure in a tire, and drive it between two different elevations say Death Valley and Kingman, AZ - you no longer will have the same internal pressure. The reason is that atmospheric pressure at sea level (14.696 PSIA) is higher than at say 6,000 feet above sea level (11.778 PSIA). This atmospheric pressure (not the tire itself) is what actually serves as the resistance or outside force to contain the 32 PSI. So at higher elevation the static pressure in the tire (referenced to atmosphere) will actually rise. The airlines will vouch for that.

The pressure gauge's actual sensing element is most likely a bourdon tube. This bourdon tube is essentially a coiled decreasing apex and decreasing diameter tube. The inside of the tube is full of the "process variable" (boiler pressure) and the outside of the tube is engulfed in whatever atmospheric pressure is prevalent. Similar to the tire in the atmosphere in the example above.

Why does food cook faster in a pressure cooker?... because at atmospheric pressure the water turns to steam at 212°F/100°C such that the "boiling/simmering" water never goes above that temperature. In a pressurized container (say 15 PSIG/1.03 BARG) the water turns to steam at a much higher temperature (249°F/121°C) so the boiling water is actually hotter. The opposite is true at higher elevations.

So, unless you have an absolute (vacuum based gauge), when you see 1.1 BAR; what you are actually seeing is 1.1 BARG (1.1 BAR above atmosphere) which depending on what elevation you are at puts you at a different point on the steam table (where water turns to steam) and a different maximum water temperature. So relatively speaking, as you change elevation the temperature of the boiler's water will change even though your gauge still reads the same pressure.

From an engineering perspective, using the same pressure gauge at a higher elevation, you would have to increase your boiler pressure (according to that pressure gauge) to get an equivalent temperature.

A quick check on saturated steam tables gives the following relationship:

   PSIG   PSIA   DegF    DegC  Bar(Gauge)  Elevation
      0   14.7  212.0   100.0           0  Sea level
  -2.95  11.75  200.9   93.83       -.203  6000 feet (approx)

There is a 12°F differential in steam temperature at 6000 feet of elevation. That makes you wonder how they keep the (so-called) coffee hot in an airplane eh?

[HB]

The boiler pressure isn't pressing against just a spring or a spiral, it's pressing against a spring or a spiral plus the ambient atmospheric pressure. Right? I think that is the nature of the OP's question.

I love the analogy of the tire pressure used above but it doesn't hold water, using "My Cousin Vinny's" lingo.

You're right! Please extend my apologies to Vinny. Your explanation coupled with Wikipedia's mechanical details of a Bourdon tube pressure gauge makes it clear (I went astray confusing absolute and gauge pressure ).

Bonus imponderable: Henry mentioned offline that he's still puzzling over the question When filling lever machine cylinder, where does the air go?