Steam boiler pressure vs. temperature - no absolute number?

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#1: Post by BaristaMcBob »

I measured the brew water temp by holding a styrofoam cup under the group. Exactly like this, except I used a faster responding digital kitchen thermometer:

To get 202F, my pressurestat is set to 1.4 bar. Some people tell me "oh, that's too high," because they set their machine to 1.2 bar.

But isn't the formula something like PV = nRT? Thus, a higher volume boiler (i.e. more non-liquid volume) would require less pressure to achieve the same water temperature in an HX machine.

Is my understanding correct there?

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#2: Post by HB »

The saturated steam temperature or "steam boiler temperature" is based on the boiler pressure; the volume isn't relevant. See Saturated Steam Table, noting that the "absolute" pressure refers the actual pressure, not the gauge pressure you're reading (i.e., 1.2 bar gauge pressure = 2.2 bar absolute pressure).
Dan Kehn
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#3: Post by guijan12 »

I'm not sure if I fully get your point here; this is the ideal gas law (or Boyle en Gay-Lussac's law).

In your boiler with a pressure of 1,4 bar, the water temperature is 109C (228F) ( ... d_457.html)
Unless you have and Hx machine, that is the temperature of the water heading to your grouphead and being cooled down by the mass of it.

If that results in 94C (202F), also B2B shots, you're doing fine imo. :D


BaristaMcBob (original poster)

#4: Post by BaristaMcBob (original poster) »

Thank you all for the replies. As it always turns out, the actual physics is more complicated than it first appears. :D
Just to clarify my post:

Question 1: Does the "bar" reading on the gauge correspond to the same boiler water temperature across different machines. Sounds like the answer is yes. That is, 1.4 bar on my machine corresponds to 228F tank water and the same would be true on your machine too. Whether or not that results in the right brew temp on your particular machine is a different story.

Question 2: Is measuring the temperature with a foam cup a reasonable way to do it?

...and yes, my machine is an HX type.


#5: Post by JohanR »

As Dan said you have to keep the difference between absolute and gauge (or relative) pressure in mind. So the saturated steam table tells you that at 2.4 bar absolute pressure, that is about 1.4 bar gauge pressure, water boils at 126.09 degC (or 259.0 degF). The difference between gauge and absolute pressure will depend on the pressure at your location which in turn depends on the altitude and weather conditions, but typically 1 bar is used.

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#6: Post by Jeff »

I'm not sure that knowing the "exact" brew temperature is very helpful. Even if you can match what a roaster is pulling on their commercial machine, with their water, grinder, baskets, barista, ... , there are enough things different that it won't be "201° on a Scace" at your home.

What is very valuable is to be able to repeat the same temperature every time, and to be able to adjust quickly from there up or down "a notch" and come back. "A notch" might be 1-2°C or 2-4°F. You'll find your own "normal" and then can try cooler or warmer based on either experience or what you taste in the cup.

I found an EricS' group-head thermometer invaluable in becoming repeatable with an HX machine. /hx-love.html is a good read to understand some of the workings of an HX machine, and methods to manage its brew temperature.

BaristaMcBob (original poster)

#7: Post by BaristaMcBob (original poster) »

Just to clarify, when I said "absolute," I was not referring to pressure at sea level vs. what's actually inside the boiler. I meant 'absolute' with reference to other espresso machines. That is, a reading of, say, 1.3 bar means the same thing (i.e. same water temp) on any machine irrespective of the boiler size. If it were "relative", then boiler size would matter. That's how I meant it. Sorry for any confusion.

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#8: Post by Jeff »

Within what is probably something like a 5-10% tolerance for the gauge, before the needle spins on its shaft against the stop, perhaps due to a slight vacuum being drawn one day when the vacuum breaker doesn't release.


#9: Post by randytsuch »

I was wondering how much difference there is in pressure/temp between when a pressurestat turns on and when it turns off?

There has to be some difference, so it won't be exactly 1.4 bar for example.
If the heat is off, pressure will drift down until it kicks in and turns the heat on, at which point pressure rises.

Because of this, both pressure and temperature will vary some, you never know the exact pressure or temp unless you have a pressure gauge in the boiler.

BTW, I also have a EricS thermocouple that I use for flushing my HX, so this is just academic for me.



#10: Post by Bluenoser »

Yes, if the gauges are all perfect, 1.4bar means the same steam temp in all machines.. but the thermosiphon in each is so different (different restrictor diameters, thermosiphon volumes) that different machines produce different brew water temps because the circulation inside the TS is at different rates.. That is why the group thermometer is essential, IMO. There is a difference between the group thermometer and actual screen exit water and I used a SCACE to calibrate my difference for the "flush n wait" workflow that I use. Once done, the group thermometer allows you to calculate a pretty accurate brew water temp.

I used the foam cup method quite a bit before I borrowed a scace and I found it worked pretty well.. I used a faster thermocouple, though and put a small hole in bottom to let a bit of water exit.. I got within a degree or two of the scace. The beauty of the group thermometer is in letting you know when your group(or thermosiphon) has rebounded and you can pull the next shot. In some machines, if you pull too quickly, your thermosiphon may not have hot enough water and you may find your brew water into the 180sF (if you have a restrictor)
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