Why do machines have pressure gauges or PID's but not both?

Postby thelanz on Sun Jun 26, 2005 12:00 am

I have seen some espresso machines that have pressure gauges (factory installed as part of the original design) or machines that have PID's (temperature gauges, only user installed as far as I can tell) but not both.

Why is that? Aren't both useful to the making of great espresso?

I am trying to understand how to make the best espresso I can so I am curious why I don't see machines with both gauges installed.
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Postby shadowfax on Sun Jun 26, 2005 11:34 pm

PV = nRT

pressure x volume = moles of whatever x gas constant x temp

The ideal gas law.

Measuring the temperature and the pressure is redundant. they are directly proportional.

What machines are you looking at?

PIDs are useful on single or double boiler machines where the boilers only sustain one temperature of water at a time. you can precisely control water temperature with a PID.

Pressure controlled boilers are usually HX (heat exchange machines). Here, a PID is totally useless in a direct sense. You can measure it, but it will give you something like 250F or 280F... I have no clue. I'd be kinda interested to find that out, actually.

The point, though, is that it doesn't matter on an HX what the temperature in the boiler is. The brew temp could be anything for any different machine at a given boiler temperature, depending on the length of the heat exchanger. The thing with these machines is that the pressurestat is accurate enough. It doesn't make a huge, huge difference where it is, as long as it's in range (.9-1.2 bars). You control brew temperature on an HX primarily by a cooling flush... I guess it'd be nice for real geeks to have a little thermistor in the brew path with a highly accurate monitor on it, but I'd much rather play it by taste, personally, and save the extra money it would cost to have that added to the machine. Honestly, you can just let someone else do the dirty work of testing the flush lengths and measuring the avg. water temp at each volume, and go off that. It's what most people do, that or follow the "water dance" guidelines.

I think that making coffee is a lot more complicated than just reading a bunch of dials. Having more dials that give you quantitative feedback, from what I understand, don't do much more than give you more feedback.

Your best bet is to get either one (the only PID machine I know of is the Expobar Brewtus, which is not that great of a PID, I have heard, and there is another one similar to it that is coming out from Chris Coffee by Christmas, I think. Check the boards) and learn how to use it. No matter how many dials you get, you are probably going to make swill for the first while unless you're already a coffee king (like Chris Tacy, heh).

Get the best equipment you can afford, of course. The better your equipment, the easier it is to blame yourself for all your crappy coffee. Then you'll get a feel for your machine, and make beautiful coffee together with it, whatever it is.

Sorry if I am wacky/all over the place. just some disjointed advice. ;)
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Postby Abe Carmeli on Mon Jun 27, 2005 9:41 am

shadowfax wrote:the only PID machine I know of is the Expobar Brewtus


Just a fact correction here. The Expobar Brewtus is not PID'd. It uses a narrow band thermostat. The temperature swing in its boiler can reach 3c.
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Postby shadowfax on Mon Jun 27, 2005 4:24 pm

wow, I knew it was pretty bad, but didn't realize it wasn't even a real one.

are you talking ± 3C, or ± 1.5C swings? the former seems bad enough to make me think you'd be better off with an HX machine, where you can get dead on within 1 degree if you know how to flush well (and aren't 7000 feet above sea level so you can see the water dance...)
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Postby Abe Carmeli on Mon Jun 27, 2005 7:37 pm

shadowfax wrote:are you talking ± 3C, or ± 1.5C swings? the former seems bad enough to make me think you'd be better off with an HX machine, where you can get dead on within 1 degree if you know how to flush well (and aren't 7000 feet above sea level so you can see the water dance...)


It is ~ +-1.5-2c. However, I believe you are putting too much stock on that variance. Boiler swings are only a part of the story, and the real question is what ends up in the portafilter. That is, how do those swings affect brew temperature. By the time the water gets to the P/F, those swings are reduce to a +- 0.5c. When you pull your shot, your average brew temperature from the 10 second on will be within 0.5c from your dialed in target temperature. This is better than most commercial machines. Flushing requirements are minimal 0-3 oz, and temperature variance within a shot is 0.5 c for a double ristretto and 1C for a 2 oz double.

As to a comparison with a prosumer H/X machine, they do not offer the same range of temp control. Flushing a H/X machine to control brew temperature can do it only to a point. Sooner or later you will hit the pre-imposed restriction of the boiler pressure stat, which is really what controls temperature on a H/X machine. More about it on my upcoming article.

EDIT 6/28/05: On a 2nd look at the data the swing in the brew boiler in +- 5-8 F. Only to show us that there is much more to temp stability than boiler temp swings.
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