What is a bar?

[Tex]

There's an art to doing just enough to get under someone's skin!  

Robert ... are you saying that Milo has the art well in hand?

 

Don't know about that, but the boy certainly has lots of potential!

[mp]

Don't know about that, but the boy certainly has lots of potential!

Ha ha.

 

[nimbus]

Alright guys, I'll try to set you straight...

a bar is approx. equal to the atmospheric pressure on the surface of the earth. It is one pascal (Pa)  , named after the famous physicist

Technically, 1 pascal is 100,000 N/m2 (newton per meter squared). That is in SI units (Systeme Internationale).

Realize, that when we measure pressure (and this is a critical point), that our pressure guage doesn't show the true pressure. It shows what we call "guage pressure." The pressure guage is calibrated to subtract off the atmospheric pressure, so that a flat tire shows "zero" pressure. But really, we have substantial atmospheric pressure all around us.

One can think of the atmospheric pressure merely as the weight of all of the gasses in the atmosphere, contained in a cylnder directly above you, pushing down on you. That is how I like to think of it. It is really what is going on. It is very cool conceptually, and most people don't know it, and even fewer "get it." It is simply gravity at work.

Cheers

-Nimbus, physics geek

p.s. For more reference, to think about what 100,000 N/m2 (meter squared) is. Think about a 1 kg block of cement. The weight is mg = 9.8 kg per meter squared. Spread that over one square meter, and you're at a pressure (force per area) of 9.8 N/m2. So 100,000 is pretty huge. The gasses in our atmosphere weigh surprisingly much. And it is all pushing on us. Very cool concepts. But it'd take (approx) 10,000 kg of gasses in our 1 square meter cylinder pushing down due to gravity to create that pressure. That is what is physically going on ....(1 kg = 2.2 pounds for you gringos)

[donn]

The pressure guage is calibrated to subtract off the atmospheric pressure, so that a flat tire shows "zero" pressure.

Really?  To me, that seems like an odd way to do it.  I mean, a gauge `calibrated to subtract off' a certain atmospheric pressure, say sea level, would accordingly be inaccurate in say Denver where the atmospheric pressure is lower.  Seems easier to just design something that just implicitly measures pressure difference.  A spring loaded chamber or something.  In this case, atmospheric pressure is immaterial - you're just measuring the difference, between the pressure inside and outside the chamber, as recorded by the spring.

That's what I would want to measure for a bicycle tire, for example, because the point of the pressure is to support the walls of the tire with a certain amount of force, as a function of pressure difference in just the same way as the spring loaded chamber.

If I were looking for some kind of effect of pressure on a chemical reaction, for example, I'm not so sure - that would be a function of absolute pressure, wouldn't it?  And I'm not sure where espresso fits in this.  My guess is that it really is about relative pressure, not absolute, so you do not need more (mechanical, and hence relative) pressure in Denver than in Seattle, but it's only a guess.

[nimbus]

The pressure guage is calibrated to subtract off the atmospheric pressure, so that a flat tire shows "zero" pressure.

Really?  To me, that seems like an odd way to do it.  I mean, a gauge `calibrated to subtract off' a certain atmospheric pressure, say sea level, would accordingly be inaccurate in say Denver where the atmospheric pressure is lower.  Seems easier to just design something that just implicitly measures pressure difference.  A spring loaded chamber or something.  In this case, atmospheric pressure is immaterial - you're just measuring the difference, between the pressure inside and outside the chamber, as recorded by the spring.

That's what I would want to measure for a bicycle tire, for example, because the point of the pressure is to support the walls of the tire with a certain amount of force, as a function of pressure difference in just the same way as the spring loaded chamber.

If I were looking for some kind of effect of pressure on a chemical reaction, for example, I'm not so sure - that would be a function of absolute pressure, wouldn't it?  And I'm not sure where espresso fits in this.  My guess is that it really is about relative pressure, not absolute, so you do not need more (mechanical, and hence relative) pressure in Denver than in Seattle, but it's only a guess.

Pretty darn sure I'm right on this one. The pressure is different when you're a mile up (Denver). It is slightly less. But if you buy the same walmart/target/kmart pressure guage, you're going to read zero for a flat tire, and the same reading for a full tire.

So while it isn't totally accurate (there do exist super accurate pressure meters - at high cost), I'm simply talking about what you and I use when we talk about our tires being at 30PSI.

(I was answering the What is a bar question)

Nimbus

Of course absolute pressure is what is critical for chemical reactions. But that simply isn't how we measure it. We subtract off some kind of average atmospheric pressure (which is slightly less than one bar)....

[donn]

Of course absolute pressure is what is critical for chemical reactions. But that simply isn't how we measure it. We subtract off some kind of average atmospheric pressure (which is slightly less than one bar)....

OK, so apparently it's only a semantic point.  I suppose a device that measures pressure difference, like a tire gauge, could be said to `subtract' one pressure from another, but at any rate we appear to agree that it won't need to be calibrated in any way to one atmospheric pressure or another.

[nimbus]

Of course absolute pressure is what is critical for chemical reactions. But that simply isn't how we measure it. We subtract off some kind of average atmospheric pressure (which is slightly less than one bar)....

OK, so apparently it's only a semantic point.  I suppose a device that measures pressure difference, like a tire gauge, could be said to `subtract' one pressure from another, but at any rate we appear to agree that it won't need to be calibrated in any way to one atmospheric pressure or another.

Not sure what you mean by the last sentence. Yeah, I was talking about common things like tire guages (I mentioned some big boxes where you can buy them). That is what 99.9% of us use.  To be really accurate, you WOULD need to calibrate it, which is what I thought was what you're getting at. From memory, I think atmospheric pressure in Denver is 20% lower, so more like 0.80 bar (rather than 0.98 at sea level). I guess I should check these numbers, but they're pretty close.

This is how we teach the stuff in intro physics at least.

So if you had a "tire guage" that one could calibrate, you'd want to zero it out when it isn't hooked up to anything. That is basically just subtracting off the atmospheric pressure of where ever you are (thus obtaining a "zero" ) pressure...

[YasBean]

Of course absolute pressure is what is critical for chemical reactions. But that simply isn't how we measure it. We subtract off some kind of average atmospheric pressure (which is slightly less than one bar)....

OK, so apparently it's only a semantic point.  I suppose a device that measures pressure difference, like a tire gauge, could be said to `subtract' one pressure from another, but at any rate we appear to agree that it won't need to be calibrated in any way to one atmospheric pressure or another.

Not sure what you mean by the last sentence. Yeah, I was talking about common things like tire guages (I mentioned some big boxes where you can buy them). That is what 99.9% of us use.  To be really accurate, you WOULD need to calibrate it, which is what I thought was what you're getting at. From memory, I think atmospheric pressure in Denver is 20% lower, so more like 0.80 bar (rather than 0.98 at sea level). I guess I should check these numbers, but they're pretty close.

This is how we teach the stuff in intro physics at least.

So if you had a "tire guage" that one could calibrate, you'd want to zero it out when it isn't hooked up to anything. That is basically just subtracting off the atmospheric pressure of where ever you are (thus obtaining a "zero" ) pressure...

As long as you are at standard temperature.

Anyhow, so I have a FancisFrancis that does not seem to have any pressure guages.  I assume it is simply a rough reference to the standard pressure that the pump can put out.  Not to expose my ignorance and inexperience too much, but are there household espresso machines on which the user can adjust the pressure?  All I know is that sometimes, the pressure is fine, and sometimes it cannot get the water through the plug, and there is never enough pressure for steaming milk for cappuccino art.

[SusanJoM]

Not to expose my ignorance and inexperience too much, but are there household espresso machines on which the user can adjust the pressure?  All I know is that sometimes, the pressure is fine, and sometimes it cannot get the water through the plug, and there is never enough pressure for steaming milk for cappuccino art.

Yes.
My only experience is with Gaggias, so I can tell you about them.   All Gaggias come with some sort of an Over Pressure Valve.  In the less expensive models (like the Espresso and most of the Coffees) the OPV is not adjustable.  On the older Baby and on the Classic, there is an adjustable OPV mounted on the boiler.

Testing of Gaggias has seemed to indicate that the pressure is usually too high;  espresso seems to be best when made at 8-9 bar;  most Gaggias seem to come off the line at around 11-13.

For many of us Gaggia owners, having an adjustable OPV has been important enough that we have modified our machines and added one to the boiler.  I have done this with my current daily Gaggia (a Dandy), and my first Gaggia (a Coffee). 

Susan