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Date: 24 Sep 2006 20:13:23
From: Alan
Subject: can someone explain "false pressure" ?
Every once in a while, I hear references made to opening the steam valve a
bit to get rid of "false pressure" before pulling the shot.
This question may betray my ignorance, but what is meant by "false"
pressure? I mean, pressure is pressure, isn't it? I'd love to know, first
of all, what it is, and secondly, why it's desirable to get rid of it. Thank
you for your answers.






 
Date: 25 Sep 2006 18:27:07
From: Paul Pratt
Subject: Re: can someone explain "false pressure" ?

Danny wrote:
> I->Ian wrote:
>
> >
> > An old wives tale:
> >
> > Probably a corruption of:
> >
> > The HX cooling flush.
> >
> -snip-
>
> Absolutely untrue.
>
> False pressure is very real - it turns the pressurestat off, and
> boiler pressure appears to be at the setpoint, until the steam wand is
> opened, when all pressure dissapears until replaced with steam
> pressure (on machines with no FPV).

Yup. All my machines pre- late 1980's do not have vacuum breakers and
as such I do exactly as you do. This morning i'm using a 2 group from
the 70's. After switching it on I'll return about 5 mins later and
bleed off the steam boiler and then leave it for half an hour.

Some modern fully auto machines bleed off the pressure without a VB by
opening either the hot water or steam solenoid. About 10 mins after
being turned on you will then hear a blip every 30 secs. This will
happen about half a dozen times and then the steam will build up. I
think they don't use VB's on these machines to stop steam going into
the machine as it is so compact in there and there is a risk of
shorting components.

Paul



 
Date: 25 Sep 2006 18:07:29
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Sun, 24 Sep 2006 20:13:23 GMT, "Alan" <in_flagrante@hotmail.com >
wrote:

>Every once in a while, I hear references made to opening the steam valve a
>bit to get rid of "false pressure" before pulling the shot.
>This question may betray my ignorance, but what is meant by "false"
>pressure? I mean, pressure is pressure, isn't it? I'd love to know, first
>of all, what it is, and secondly, why it's desirable to get rid of it. Thank
>you for your answers.
>

An old wives tale:

Probably a corruption of:

The HX cooling flush.

OR
Bleeding steam pressure after steaming from single boiler machines,
like Silvia.

OR
Temp surfing on machines with notoriously poor temperature regulation,
like Silvia.

OR
Black magic to wake up a sticky pStat.

Gas pressure is related to temperature, not the composition of the
gas, at least at the temperatures and pressures in question, if I
remember my physics.

There will be some striation of temperature in the 'air' as the boiler
heats, but that will rapidly disappear as thermal equilibrium is
attained. The water molecules escaping the liquid water will transfer
some energy to the existing air and water 'blanket mixture', heating
it. The higher energy water molecules will rise to the top of the
space, forcing the cooler 'air' to the bottom and into contact with
the hot water, where the 'air' will be heated and rise to the top to
replace the now cooler water molecule that has transferred some energy
to the boiler wall. As the 'air' heats it will mix with the steam and
the mixture will very soon be homogenous.

As far as 'bleeding' the pStat pipe, assuming the vac breaker is
functioning: There will be a mix of air and water vapor in the pipe.
As boiler pressure builds, this mixture will be forced up the pipe and
intermixed with the air and steam from the boiler. Assuming a movable
blockage in the pStat pipe preventing the mixing of the steam and
'air', at 1bar boiler pressure [2bar absolute], the blockage would be
1/2 way up the pipe. If you bleed the boiler to .5bar [1.5bar
absolute], the blockage would be stil be 25% of the way up the pipe.
To evacuate the pipe, the pressure would have to drop to 0bar
absolute. With a failed vacuum breaker and without a pump to evacuate
the boiler, there will still be significant gas pressure at room
temperature inside the boiler due to the temperature of the water.



  
Date: 25 Sep 2006 20:04:09
From: Danny
Subject: Re: can someone explain "false pressure" ?
I- >Ian wrote:

>
> An old wives tale:
>
> Probably a corruption of:
>
> The HX cooling flush.
>
-snip-

Absolutely untrue.

False pressure is very real - it turns the pressurestat off, and
boiler pressure appears to be at the setpoint, until the steam wand is
opened, when all pressure dissapears until replaced with steam
pressure (on machines with no FPV).

I have two identical machines, one with a FPV and one without. The
equipped machine reaches correct boiler pressure, with full steaming
ability, whereas the non-eqipped machine requires a couple of openings
of the steam wand before true steam pressure is present.


--
Regards, Danny

http://www.gaggia-espresso.com (a purely hobby site)
http://www.malabargold.co.uk (UK/EU ordering for Malabar Gold blend)



  
Date: 25 Sep 2006 14:45:14
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
No its not a wive's tale and it has nothing to do with the cooling flush or
single boiler machines or temp surfing. You remember your physics wrong. A
boiler with a mixture of atmospheric pressure air and steam in it behaves
differently than a boiler with pure steam. It is a real phenomena that
exists in machines that don't have a (functioning) vacuum breaker, such as
my Oscar (and my Elli before that). Until you "burp" the air out the steam
wand the air pressure in the boiler confused the pstat and the machine does
not heat up fully.

Try this though experiment to understand why: Imagine that you have a vacuum
pump hooked up to your espresso machine and the pstat is set at 1.2 bar
(gauge). You evacuate all the air from the boiler and capture it in a
balloon. Now you heat the boiler until .2 bar (gauge pressure ) of steam
pressure is achieved. Now you switch the vacuum pump into reverse and you
pump all the air from the balloon back into the boiler. What happens to the
pressure inside the boiler? (The boiler has the same amount of air in it
that it had to begin with).


"I- >Ian" <someone@nowhere.com> wrote in message
news:9d2gh2liejtjcj5ka8o9t7hs5nnfkdjnst@4ax.com...
> On Sun, 24 Sep 2006 20:13:23 GMT, "Alan" <in_flagrante@hotmail.com>
> wrote:
>
>>Every once in a while, I hear references made to opening the steam valve a
>>bit to get rid of "false pressure" before pulling the shot.
>>This question may betray my ignorance, but what is meant by "false"
>>pressure? I mean, pressure is pressure, isn't it? I'd love to know, first
>>of all, what it is, and secondly, why it's desirable to get rid of it.
>>Thank
>>you for your answers.
>>
>
> An old wives tale:
>
> Probably a corruption of:
>
> The HX cooling flush.
>
> OR
> Bleeding steam pressure after steaming from single boiler machines,
> like Silvia.
>
> OR
> Temp surfing on machines with notoriously poor temperature regulation,
> like Silvia.
>
> OR
> Black magic to wake up a sticky pStat.
>
> Gas pressure is related to temperature, not the composition of the
> gas, at least at the temperatures and pressures in question, if I
> remember my physics.
>
> There will be some striation of temperature in the 'air' as the boiler
> heats, but that will rapidly disappear as thermal equilibrium is
> attained. The water molecules escaping the liquid water will transfer
> some energy to the existing air and water 'blanket mixture', heating
> it. The higher energy water molecules will rise to the top of the
> space, forcing the cooler 'air' to the bottom and into contact with
> the hot water, where the 'air' will be heated and rise to the top to
> replace the now cooler water molecule that has transferred some energy
> to the boiler wall. As the 'air' heats it will mix with the steam and
> the mixture will very soon be homogenous.
>
> As far as 'bleeding' the pStat pipe, assuming the vac breaker is
> functioning: There will be a mix of air and water vapor in the pipe.
> As boiler pressure builds, this mixture will be forced up the pipe and
> intermixed with the air and steam from the boiler. Assuming a movable
> blockage in the pStat pipe preventing the mixing of the steam and
> 'air', at 1bar boiler pressure [2bar absolute], the blockage would be
> 1/2 way up the pipe. If you bleed the boiler to .5bar [1.5bar
> absolute], the blockage would be stil be 25% of the way up the pipe.
> To evacuate the pipe, the pressure would have to drop to 0bar
> absolute. With a failed vacuum breaker and without a pump to evacuate
> the boiler, there will still be significant gas pressure at room
> temperature inside the boiler due to the temperature of the water.
>




   
Date: 25 Sep 2006 21:20:37
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Mon, 25 Sep 2006 14:45:14 -0400, "Jack Denver"
<nunuvyer@netscape.net > wrote:

> Until you "burp" the air out the steam
> wand the air pressure in the boiler confused the pstat and the machine does
> not heat up fully.

Presumably, you only have to do this once when the machine is new?

Assuming a sealed HX system, once the air is 'removed' from the boiler
and no new water is introduced, without a VacBrkValve, the system will
remain purged and only temperature change will effect a pressure
difference in the boiler.

With a single boiler machine, each time fresh water is loaded, some
disolved air will escape, polluting the steam. After how many shots
must one repurge the system to purify the steam?

Purging steam is lowering the pressure, causing the pStat to turn on
and add heat to the boiler a little more quickly. Assuming a good
pStat and sufficient time, it would arrive there on its own.

Burping steam pressure just before pulling a shot is upsetting the
equilibrium and forcing the pStat to turn the element on. This may
have advantages on specific machines.


>Try this though experiment to understand why: Imagine that you have a vacuum
>pump hooked up to your espresso machine and the pstat is set at 1.2 bar
>(gauge). You evacuate all the air from the boiler and capture it in a
>balloon. Now you heat the boiler until .2 bar (gauge pressure ) of steam
>pressure is achieved. Now you switch the vacuum pump into reverse and you
>pump all the air from the balloon back into the boiler. What happens to the
>pressure inside the boiler? (The boiler has the same amount of air in it
>that it had to begin with).

[g = gauge, a = absolute pressure in bar]
Cold 0g 1a
Evac boiler -1g 0a
Heat boiler 0.2g 1.2a
Add air back 1.2g 2.2a ignoring pressure difference to the
temperature differences in the air.

When you evac the boiler, you remove a mass of air with a density
directly related to the temperature. It contains a certain amount of
energy.

If you add enough air @ 240F back to raise the pressure to 1.2g there
will be some air left in the balloon. But you have supplied energy to
the system to raise the temperature of the air extracted at room temp.

If you add all the air back, the pressure will be higher, the pStat
will shut off, the water will cool, some of the water vapor gas will
go back to liquid, the pressure will drop, the pStat will turn on and
raise the water temperature until equilibrium is restored.

While there is certainly a difference in the specific heat of dry air
and pure steam, the likelihood of flushing all air from the system and
the ability of a typical pStat to react leaves me unconvinced that any
of the pressure is "false".


    
Date: 26 Sep 2006 06:27:28
From: Danny
Subject: Re: can someone explain "false pressure" ?
I- >Ian wrote:
> On Mon, 25 Sep 2006 14:45:14 -0400, "Jack Denver"
> <nunuvyer@netscape.net> wrote:
>
>
>>Until you "burp" the air out the steam
>>wand the air pressure in the boiler confused the pstat and the machine does
>>not heat up fully.
>
>
> Presumably, you only have to do this once when the machine is new?
>
> Assuming a sealed HX system, once the air is 'removed' from the boiler
> and no new water is introduced, without a VacBrkValve, the system will
> remain purged and only temperature change will effect a pressure
> difference in the boiler.
>
> With a single boiler machine, each time fresh water is loaded, some
> disolved air will escape, polluting the steam. After how many shots
> must one repurge the system to purify the steam?
>
> Purging steam is lowering the pressure, causing the pStat to turn on
> and add heat to the boiler a little more quickly. Assuming a good
> pStat and sufficient time, it would arrive there on its own.
>
> Burping steam pressure just before pulling a shot is upsetting the
> equilibrium and forcing the pStat to turn the element on. This may
> have advantages on specific machines.

I think you misunderstand. False pressure only builds when a machine
is switched on from cold. Once up to temperature it won't occur.


--
Regards, Danny

http://www.gaggia-espresso.com (a purely hobby site)
http://www.malabargold.co.uk (UK/EU ordering for Malabar Gold blend)



    
Date: 25 Sep 2006 17:53:33
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
If you ever worked with a machine without a vacuum breaker you'd be
convinced. See Danny's post. The phenomenon is real and quite obvious - the
false pressure shuts the pstat off prematurely before the machine is at full
temp (there's 1.2 bar of total pressure in the boiler but the temperature
only corresponds to .2 bar gauge of steam pressure). When you purge the
false pressure the machine omits a little burp of mixed steam/air , the
"false" pressure in the machine drops instantly to near 0 gauge because the
water is barely above boiling, the pstat kicks back on and then it takes a
while to heat back up to "true" pressure. It's very obvious when you see it
happening.



"I- >Ian" <someone@nowhere.com> wrote in message
news:atbgh21tp3kgmmr4rlm08maln4fb195chg@4ax.com...
>
> While there is certainly a difference in the specific heat of dry air
> and pure steam, the likelihood of flushing all air from the system and
> the ability of a typical pStat to react leaves me unconvinced that any
> of the pressure is "false".




     
Date: 28 Sep 2006 19:11:44
From: jggall01
Subject: Re: can someone explain "false pressure" ?
I- >Ian wrote:

> Since we're stating the obvious, a cylindrical object is stronger in
> compression than in tension. ;-)
>

Not usually true. Metal shell structures whose walls are "thin"
relative to the diameter will fail in compression, initiated by
buckling, at a much smaller membrane force than that required to fail
them in tension. Unless the walls of the cylinder are quite thick,
instability (buckling) will cause failure to occur at a stress that is
well below the strength of the metal.

If the boiler were constructed of unreinforced concrete, your statement
could be true ;-)

JGG



      
Date: 29 Sep 2006 21:57:08
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On 28 Sep 2006 19:11:44 -0700, "jggall01" <jggall01@yahoo.com > wrote:

>I->Ian wrote:
>
>> Since we're stating the obvious, a cylindrical object is stronger in
>> compression than in tension. ;-)
>>
>
>Not usually true. Metal shell structures whose walls are "thin"
>relative to the diameter will fail in compression, initiated by
>buckling, at a much smaller membrane force than that required to fail
>them in tension. Unless the walls of the cylinder are quite thick,
>instability (buckling) will cause failure to occur at a stress that is
>well below the strength of the metal.
>
>If the boiler were constructed of unreinforced concrete, your statement
>could be true ;-)
>
>JGG

I was thinking subines or was I thinking?

Removing the other shoe....


       
Date: 29 Sep 2006 23:49:33
From: Steve Ackman
Subject: Re: can someone explain "false pressure" ?
In <ij5rh295a0aauq1vacv4u8n5vfgl00ubfo@4ax.com >, on Fri, 29 Sep 2006
21:57:08 GMT, I- >Ian wrote:
> On 28 Sep 2006 19:11:44 -0700, "jggall01" <jggall01@yahoo.com> wrote:
>
>>I->Ian wrote:
>>
>>> Since we're stating the obvious, a cylindrical object is stronger in
>>> compression than in tension. ;-)
>>>
>>
>>Not usually true. Metal shell structures whose walls are "thin"
>>relative to the diameter will fail in compression, initiated by
>>buckling, at a much smaller membrane force than that required to fail
>>them in tension. Unless the walls of the cylinder are quite thick,
>>instability (buckling) will cause failure to occur at a stress that is
>>well below the strength of the metal.
>>
>>If the boiler were constructed of unreinforced concrete, your statement
>>could be true ;-)
>>
>>JGG
>
> I was thinking subines or was I thinking?
>
> Removing the other shoe....

Boilers don't have bulkheads and frames.


      
Date: 28 Sep 2006 23:21:16
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
Like this:

http://onewest.net/~garywl/Pic1Jun00.jpg


"The tank car in the attached photos is ....a general purpose LPG car that
was being steam cleaned to prepare it for some maintenance work. The job was
still in progress at the end of the shift so the employee decided to block
it in. Problem was there was no vacuum relief."

I imagine an LPG car is built to withstand positive pressure greater than 1
bar, at least I hope so.


"jggall01" <jggall01@yahoo.com > wrote in message
news:1159495904.652132.312710@m7g2000cwm.googlegroups.com...
> I->Ian wrote:
>
>> Since we're stating the obvious, a cylindrical object is stronger in
>> compression than in tension. ;-)
>>
>
> Not usually true. Metal shell structures whose walls are "thin"
> relative to the diameter will fail in compression, initiated by
> buckling, at a much smaller membrane force than that required to fail
> them in tension. Unless the walls of the cylinder are quite thick,
> instability (buckling) will cause failure to occur at a stress that is
> well below the strength of the metal.
>
> If the boiler were constructed of unreinforced concrete, your statement
> could be true ;-)
>
> JGG
>




     
Date: 26 Sep 2006 01:18:23
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
>
>(there's 1.2 bar of total pressure in the boiler but the temperature
>only corresponds to .2 bar gauge of steam pressure)

Are you saying the gauge reads 1.2bar [in which case the "total
pressure" is 2.2bar] and the water temperature is at 0.2bar? That's a
~35F temperature differential !!


>When you purge the
>false pressure the machine omits a little burp of mixed steam/air , the
>"false" pressure in the machine drops instantly to near 0 gauge because the
>water is barely above boiling, the pstat kicks back on and then it takes a
>while to heat back up to "true" pressure. It's very obvious when you see it
>happening.

I could see this happening if the machine got considerably colder than
normal ambient temperature or was exposed to higher than normal
atmospheric pressure. The seals meant to keep the steam in could allow
higher external pressure to leak into the boiler.

In this case, the boiler would be pre pressurized and the pStat would
shut off early as described. Releasing the pressure would allow the
overpressure to escape and the pressure would drop to that supported
by the water temperature.

I could also see the heater boiling up a head of steam due to
localized heating and until the whole unit is up to temp, the water
temp could be lower. Burping this pressure would force the pStat back
on.

On a machine with a VacBrk, no pressure at all is built until the unit
is quite hot. The boiler, plumbing and group are much closer to
operating temperature, so there is less heat load on the boiler once
pressure starts to build. The boiler water is more agitated due to
more steam formed and bubbled off and therefore much closer throughout
to normal operating temperature.

A higher temperature may also have some effect on the pStat mechanics
due to lubrication stiction and mechanical friction due to thermal
expansion.

In response to the OP, can we agree on :
The pressure is not "false", it's just the rest of machine has not
caught up to the boiler pressure. Burping the pressure is a method to
accelerate the ramp up to operating temperature, a temperature it
would achieve eventually without the burp.

Whether or not it is of any benefit depends hugely on the machine
type.


      
Date: 26 Sep 2006 03:37:18
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?
On Tue, 26 Sep 2006 01:18:23 GMT, "I- >Ian" <someone@nowhere.com>
wrote:

>In response to the OP, can we agree on :
>The pressure is not "false", it's just the rest of machine has not
>caught up to the boiler pressure. Burping the pressure is a method to
>accelerate the ramp up to operating temperature, a temperature it
>would achieve eventually without the burp.

No. This is not correct. An unburped machine will sit at the
setpoint pressure, but at an undesireably low temperature, until such
time as the air is vented from the boiler.

Dalton's law of partial pressures: For a mixture of gases in any
container, the total pressure exerted is the sum of the pressures that
each gas would exert if it were alone. The problem arises because the
partial pressure of air increases as the machine warms so the
combination of air pressure and water vapor pressure cause the pstat
to cycle at a significantly lower temperature than if the boiler were
full of water vapor alone.



       
Date: 26 Sep 2006 17:15:49
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Tue, 26 Sep 2006 03:37:18 GMT, Barry Jarrett
<barry@rileys-coffee.com > wrote:


>Dalton's law of partial pressures: For a mixture of gases in any
>container, the total pressure exerted is the sum of the pressures that
>each gas would exert if it were alone. The problem arises because the
>partial pressure of air increases as the machine warms so the
>combination of air pressure and water vapor pressure cause the pstat
>to cycle at a significantly lower temperature than if the boiler were
>full of water vapor alone.

I'm fully cognizant of Dalton's Law, it's just been too long for me to
remember whether the partial pressure is due to molecular mass or
specific heat of the gas at temperature.

The molecular weights are higher for N, O than HO, so I suspect its
due to specific heat. The specific heat for HO is almost double that
of dry air at 400K

The 'air' is going to be fairly saturated to start with as it is an
almost closed system over a water bath.

The sticky bit in the explaination is the idea that one can completely
purge the 'air' from the boiler with a 'burp' and end up with a 100%
steam cushion and that the unit will immediately start it's ramp to
normal operating temperature.

Of course if one is repeatedly steaming, which no one has yet
mentioned, the gas mixture would eventually approach 100% steam and
the water temperature would rise incrementally with each steam bleed.

Thanks for the education. ;-)

"The difference between theory and practise is... ...that in theory
there is no difference."


        
Date: 27 Sep 2006 03:07:16
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?

i happen to have a temp probe in the steam boiler of my home zocco.
i've just turned the machine off, and when it has cooled, i'll clamp
the vac breaker closed and datalog the boiler temp as it heats up.



         
Date: 27 Sep 2006 08:34:13
From: GeeDubb
Subject: Re: can someone explain "false pressure" ?

"Barry Jarrett" <barry@rileys-coffee.com > wrote in message
news:qjqjh2pi71haecc1araple23d55k1816u7@4ax.com...
>
> i happen to have a temp probe in the steam boiler of my home zocco.
> i've just turned the machine off, and when it has cooled, i'll clamp
> the vac breaker closed and datalog the boiler temp as it heats up.
>

What will clamping the vb do? Isn't the vb there to open when the boiler is
cooling to prevent boiler collapse? The vb on my Unic closes at some low
pressure when the boiler is heating which requires later burping to speed up
the heating process. If I were to not burp it would just take longer to
come to operating temperature as the p-stat would still cycle. I suppose
clamping the vb would maximize the false pressure phenomenon so maybe I just
answered my own question..........

Maybe a good comparison would be to heat up the zocco several ways,
clamped vb-no burp, clamp vb-burp, unclamped vb-no burp, unclamped vb-burp,
of course all this in your spare time, to see how long it takes to reach
operating temperature.

Gary



          
Date: 28 Sep 2006 04:34:29
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?
On Wed, 27 Sep 2006 08:34:13 -0700, "GeeDubb" <geedubb@qwest.net >
wrote:

>What will clamping the vb do? Isn't the vb there to open when the boiler is
>cooling to prevent boiler collapse? The vb on my Unic closes at some low
>pressure when the boiler is heating which requires later burping to speed up
>the heating process. If I were to not burp it would just take longer to
>come to operating temperature as the p-stat would still cycle. I suppose
>clamping the vb would maximize the false pressure phenomenon so maybe I just
>answered my own question..........
>

i let the machine cool down with the vac breaker "normal". then i
removed the vac breaker cap, and replaced it with a modified cap which
has had the hole on top silver soldered closed.

the machine has been through a couple of pstat cycles already. i just
vented out the steam wand, down to 1 bar, and it's reheating now.

data later.

--barry "floors & paint this weekend"


          
Date: 27 Sep 2006 18:08:31
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Wed, 27 Sep 2006 08:34:13 -0700, "GeeDubb" <geedubb@qwest.net >
wrote:

> Isn't the vb there to open when the boiler is
>cooling to prevent boiler collapse?

If the boiler were to collapse with +1bar on the outside, it would
probably explode with +1bar on the inside


           
Date: 28 Sep 2006 05:11:26
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?

graph is on a.b.c.

or, here's the raw data, one second intervals, deg. F:

75.4
75.4
75.4
76.2
77.9
79.5
80.5
81.8
82.9
84.3
85.8
88.1
89.2
91.4
92.9
94.5
96
97.8
99.7
101.9
104.5
106.5
108.8
110.7
111.9
114.3
117.5
118.8
122
124.6
126.4
129
132
134.4
137.8
140.2
143.1
145.7
148.5
150.3
152.7
155.6
158.5
160.8
163
165.7
167.6
170.5
173
174.4
177.4
179.4
181.9
183.9
186.3
189.5
191
193.8
195.8
198
201.2
202.5
204.7
206.3
208.6
210
212.8
214.9
217.7
218.4
220.8
222.5
224.2
226.1
227.5
229.8
232.6
234.3
234.6
234.5
230.5
229.5
229.7
229.5
229.1
230.1
229.9
229.2
229.7
230.1
229.2
228.6
228.9
229.4
228.4
228
227.3
229.1
230.8
232.6
236
237.7
238.9
240.5
241.7
243.3
244.8
245.4
245
244.1
242.2
241.8
241.3
241.1
239.5
239.7
239.2
240.7
240.2
240.2
240
240.1
239.2
239.1
238.8
238.5
238.6
237.2
240.2
241.8
243.4
244.6
245.9
247.1
247.9
249
249.8
250.8
251.3
251.6
248.2
245
245.5
244.2
245.4
247.1
248.9
250.5
252.2
253.7
255.4
256.9
258.5
259.4
258.7
257.9
257
256.3
256.1
255.5
255.2
255
254.8
254.8
254.6
254.3
256.5
257.9
257.4
256.6
255.7
255.1
255.4
257.9
257.4
256.7
255.5
255.4
255.4
256.9
256.9
256.2
256.2
257.8
257.2
256.5
255.6
255.3
255.7
257.5
256.8
256
257.1
257.2
256.5
255.6
255.3
256.4
258.1
257.4
256.8
255.9
255.4
255.2



            
Date: 28 Sep 2006 01:54:35
From: Johnny
Subject: Re: can someone explain "false pressure" ?

"Barry Jarrett" <barry@rileys-coffee.com > wrote in message
news:hammh2tgbqrg4qkc9ogpks3qptbnu3op64@4ax.com...
>
> graph is on a.b.c.
>
> or, here's the raw data, one second intervals, deg. F:
>
> 75.4
<snip/ >

thanks for the data Barry.
is that the pstat kicking out initially at 234.5F/1.25 mins and then back in
at 227/~1.6mins ?





             
Date: 28 Sep 2006 14:42:59
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?
On Thu, 28 Sep 2006 01:54:35 -0700, "Johnny"
<removethis.huuanito@hotmail.com > wrote:

>
>thanks for the data Barry.
>is that the pstat kicking out initially at 234.5F/1.25 mins and then back in
>at 227/~1.6mins ?
>
>


crap, that didn't even sink into my brain last night. the datalogger
must have been recording at 10 second intervals.... no way that
thing heated up in a minute and a half; i stood down there way longer
than that!!


--barry "new datalogger + no spare time = boo boos"


           
Date: 28 Sep 2006 04:28:28
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?
On Wed, 27 Sep 2006 18:08:31 GMT, "I- >Ian" <someone@nowhere.com>
wrote:

>On Wed, 27 Sep 2006 08:34:13 -0700, "GeeDubb" <geedubb@qwest.net>
>wrote:
>
>> Isn't the vb there to open when the boiler is
>>cooling to prevent boiler collapse?
>
>If the boiler were to collapse with +1bar on the outside, it would
>probably explode with +1bar on the inside

uh, no.

--barry "birthday balloons litter the house"


            
Date: 28 Sep 2006 17:20:24
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
dblValue = dblValue / dblRefCurrent
On Thu, 28 Sep 2006 04:28:28 GMT, Barry Jarrett
<barry@rileys-coffee.com > wrote:

>On Wed, 27 Sep 2006 18:08:31 GMT, "I->Ian" <someone@nowhere.com>
>wrote:
>
> >On Wed, 27 Sep 2006 08:34:13 -0700, "GeeDubb" <geedubb@qwest.net>
> >wrote:
> >
> >> Isn't the vb there to open when the boiler is
> >>cooling to prevent boiler collapse?
> >
> >If the boiler were to collapse with +1bar on the outside, it would
> >probably explode with +1bar on the inside
>
>uh, no.

There is identical force on the inside or the outside if there is a
1bar delta across the metal.

A boiler flimsy enough to collapse with +1bar on the outside would not
give me a warm and fuzzy feeling.


>
>--barry "birthday balloons litter the house"


             
Date: 28 Sep 2006 17:37:55
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
If you were talking about a flat membrane dividing a box in half, it
wouldn't make any difference whether 1 side of the membrane was at 1 bar and
the other in vacuum, or if 1 side was at atmos. and the other was at 2 bar.
But, you are talking about a boiler with a specific shape (generally
cylindrical) - The boiler walls have different stresses when under pressure
(generally tension) and under vacuum (mostly compression) and may be
stronger in one that the other.



"I- >Ian" <someone@nowhere.com> wrote in message
news:gi0oh21btvb1ve03ncjk9onpimh80ikga4@4ax.com...
> dblValue = dblValue / dblRefCurrent
> On Thu, 28 Sep 2006 04:28:28 GMT, Barry Jarrett
> <barry@rileys-coffee.com> wrote:
>
>>On Wed, 27 Sep 2006 18:08:31 GMT, "I->Ian" <someone@nowhere.com>
>>wrote:
>>
>> >On Wed, 27 Sep 2006 08:34:13 -0700, "GeeDubb" <geedubb@qwest.net>
>> >wrote:
>> >
>> >> Isn't the vb there to open when the boiler is
>> >>cooling to prevent boiler collapse?
>> >
>> >If the boiler were to collapse with +1bar on the outside, it would
>> >probably explode with +1bar on the inside
>>
>>uh, no.
>
> There is identical force on the inside or the outside if there is a
> 1bar delta across the metal.
>
> A boiler flimsy enough to collapse with +1bar on the outside would not
> give me a warm and fuzzy feeling.
>
>
>>
>>--barry "birthday balloons litter the house"




              
Date: 28 Sep 2006 22:41:58
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Thu, 28 Sep 2006 17:37:55 -0400, "Jack Denver"
<nunuvyer@netscape.net > wrote:

>But, you are talking about a boiler with a specific shape (generally
>cylindrical) - The boiler walls have different stresses when under pressure
>(generally tension) and under vacuum (mostly compression) and may be
>stronger in one that the other.

Since we're stating the obvious, a cylindrical object is stronger in
compression than in tension. ;-)

Any boiler unable to withstand a severe over or underpressure without
mechanical distortion is too flimsy and probably a safety hazard.


               
Date: 29 Sep 2006 08:05:46
From: Sheldon T. Hall - DO NOT MAIL
Subject: Re: can someone explain "false pressure" ?
On Thu, 28 Sep 2006 22:41:58 GMT, "I- >Ian" <someone@nowhere.com>
wrote:

>On Thu, 28 Sep 2006 17:37:55 -0400, "Jack Denver"
><nunuvyer@netscape.net> wrote:
>
>>But, you are talking about a boiler with a specific shape (generally
>>cylindrical) - The boiler walls have different stresses when under pressure
>>(generally tension) and under vacuum (mostly compression) and may be
>>stronger in one that the other.
>
>Since we're stating the obvious, a cylindrical object is stronger in
>compression than in tension. ;-)
>
>Any boiler unable to withstand a severe over or underpressure without
>mechanical distortion is too flimsy and probably a safety hazard.

Nah.

Grab an empty 2-liter soda bottle. Blow into it, and see if you can
change its shape.

Now suck the air out of it and see if you can change its shape.

-Shel



                
Date: 29 Sep 2006 15:41:30
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?
On Fri, 29 Sep 2006 08:05:46 -0700, Sheldon T. Hall - DO NOT MAIL
<aquaman@tandem.artell.net > wrote:

>Now suck the air out of it and see if you can change its shape.

coffee related: at the shop, we usually blew steam into empty milk
jugs before capping them and tossing them in the trash. it was a good
ongoing physics lesson for the staff, and it saved space in the trash
bins.



                 
Date: 29 Sep 2006 18:15:24
From: Danny
Subject: Re: can someone explain "false pressure" ?
Barry Jarrett wrote:
> On Fri, 29 Sep 2006 08:05:46 -0700, Sheldon T. Hall - DO NOT MAIL
> <aquaman@tandem.artell.net> wrote:
>
> >Now suck the air out of it and see if you can change its shape.
>
> coffee related: at the shop, we usually blew steam into empty milk
> jugs before capping them and tossing them in the trash. it was a good
> ongoing physics lesson for the staff, and it saved space in the trash
> bins.
>

Love it - going to try that tomorrow!!!

--
Regards, Danny

http://www.gaggia-espresso.com (a purely hobby site)
http://www.malabargold.co.uk (UK/EU ordering for Malabar Gold blend)



               
Date: 28 Sep 2006 19:16:32
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
Along which axis?

"I- >Ian" <someone@nowhere.com> wrote in message
news:frioh2tn81fema16m9cbfs4n1ofvs2hm1q@4ax.com...
> On Thu, 28 Sep 2006 17:37:55 -0400, "Jack Denver"
> <nunuvyer@netscape.net> wrote:
>
>>But, you are talking about a boiler with a specific shape (generally
>>cylindrical) - The boiler walls have different stresses when under
>>pressure
>>(generally tension) and under vacuum (mostly compression) and may be
>>stronger in one that the other.
>
> Since we're stating the obvious, a cylindrical object is stronger in
> compression than in tension. ;-)
>
> Any boiler unable to withstand a severe over or underpressure without
> mechanical distortion is too flimsy and probably a safety hazard.




           
Date: 27 Sep 2006 12:24:31
From: Johnny
Subject: Re: can someone explain "false pressure" ?

"I- >Ian" <someone@nowhere.com> wrote in message
news:veflh29r2p9obgeaeh9l2puj7nlmg9t7he@4ax.com...
> On Wed, 27 Sep 2006 08:34:13 -0700, "GeeDubb" <geedubb@qwest.net>
> wrote:
>
> > Isn't the vb there to open when the boiler is
> >cooling to prevent boiler collapse?
>
> If the boiler were to collapse with +1bar on the outside, it would
> probably explode with +1bar on the inside

I don't think it's that simple. A lot depends on geometry. It's easier to
dent a ring by crushing than it is to distend it by expansion.
For instance i'm thinking a soda can take way more than 1 bar inside but
they have a problem with the1 bar outside:
http://scifun.chem.wisc.edu/HOMEEXPTS/COLLAPSE.html

it'd be fun to test out what a soda can would take, might have to try that
some time with the compressor :-)




          
Date: 27 Sep 2006 13:53:19
From: Neal Reid
Subject: Re: can someone explain "false pressure" ?
In article <451a99e5$0$23852$882e0bbb@news.ThunderNews.com >,
"GeeDubb" <geedubb@qwest.net > wrote:

> What will clamping the vb do? Isn't the vb there to open when the boiler is
> cooling to prevent boiler collapse? The vb on my Unic closes at some low
> pressure when the boiler is heating which requires later burping to speed up
> the heating process.

My machine has been through a number of forms of V.B. Clearly, it
performs as per your 1st point - but it also allows air out on
heating to avoid the need for burping. I mention both 'avoid' and
'a number of forms' because iteration has lead me to a V.B. that
leaves no need to burp.

--
M for N in address to mail reply


         
Date: 26 Sep 2006 22:42:13
From: Johnny
Subject: Re: can someone explain "false pressure" ?

"Barry Jarrett" <barry@rileys-coffee.com > wrote in message
news:qjqjh2pi71haecc1araple23d55k1816u7@4ax.com...
>
> i happen to have a temp probe in the steam boiler of my home zocco.
> i've just turned the machine off, and when it has cooled, i'll clamp
> the vac breaker closed and datalog the boiler temp as it heats up.
>

One thing no-one has mentioned is another contributor to this effect: when
you heat up the water in the boiler the liquid itself expands so the space
for the gas is reduced increasing the pressure further. For a 0.75 full
boiler this can expand to to 0.79 at full temp.
For a 1.2bar gauge tstat setting I'd expect the temp to only reach about
237F instead of about 254F on start up without burping.





        
Date: 26 Sep 2006 15:45:12
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
Nobody said that a single "burp" would purge 100% of the air. You don't need
to - if you purge 80 or 90% of it, that will get the machine close to full
operating temperature and then the next time you "burp" it you'll blow out
80 or 90% of what remains, etc. As Paul mentioned, the superautos are
programmed blow the wand out several times over an interval - this is what
human operators do also.

When you say incrementally, I suppose you are right but as a practical
matter after 2 or 3 cycles the percentage of air would be so low that it
would no longer have a detectable effect on temperature.


"I- >Ian" <someone@nowhere.com> wrote in message
news:q6lih2dv6botp857g741dmotc4bi8ho9bn@4ax.com...
> On Tue, 26 Sep 2006 03:37:18 GMT, Barry Jarrett
> <barry@rileys-coffee.com> wrote:
>
>>
> The sticky bit in the explaination is the idea that one can completely
> purge the 'air' from the boiler with a 'burp' and end up with a 100%
> steam cushion and that the unit will immediately start it's ramp to
> normal operating temperature.
>
> Of course if one is repeatedly steaming, which no one has yet
> mentioned, the gas mixture would eventually approach 100% steam and
> the water temperature would rise incrementally with each steam bleed.
>
> Thanks for the education. ;-)
>
> "The difference between theory and practise is... ...that in theory
> there is no difference."




      
Date: 25 Sep 2006 21:51:12
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
You're making this all much more complicated than it really is. False
pressure is due to a cushion of atmospheric pressure air in the boiler,
period. It doesn't have to be cold or at above normal pressure or anything.
I suggest that you take up Brent's invitation and see this for yourself and
it will be obvious. All these cockamamie theories can only come from someone
who has never seen the real thing in action.



"I- >Ian" <someone@nowhere.com> wrote in message
news:6jqgh2pt1r22ceamoistvcokmabqe9om64@4ax.com...
> >
>>(there's 1.2 bar of total pressure in the boiler but the temperature
>>only corresponds to .2 bar gauge of steam pressure)
>
> Are you saying the gauge reads 1.2bar [in which case the "total
> pressure" is 2.2bar] and the water temperature is at 0.2bar? That's a
> ~35F temperature differential !!
>
>
>>When you purge the
>>false pressure the machine omits a little burp of mixed steam/air , the
>>"false" pressure in the machine drops instantly to near 0 gauge because
>>the
>>water is barely above boiling, the pstat kicks back on and then it takes a
>>while to heat back up to "true" pressure. It's very obvious when you see
>>it
>>happening.
>
> I could see this happening if the machine got considerably colder than
> normal ambient temperature or was exposed to higher than normal
> atmospheric pressure. The seals meant to keep the steam in could allow
> higher external pressure to leak into the boiler.
>
> In this case, the boiler would be pre pressurized and the pStat would
> shut off early as described. Releasing the pressure would allow the
> overpressure to escape and the pressure would drop to that supported
> by the water temperature.
>
> I could also see the heater boiling up a head of steam due to
> localized heating and until the whole unit is up to temp, the water
> temp could be lower. Burping this pressure would force the pStat back
> on.
>
> On a machine with a VacBrk, no pressure at all is built until the unit
> is quite hot. The boiler, plumbing and group are much closer to
> operating temperature, so there is less heat load on the boiler once
> pressure starts to build. The boiler water is more agitated due to
> more steam formed and bubbled off and therefore much closer throughout
> to normal operating temperature.
>
> A higher temperature may also have some effect on the pStat mechanics
> due to lubrication stiction and mechanical friction due to thermal
> expansion.
>
> In response to the OP, can we agree on :
> The pressure is not "false", it's just the rest of machine has not
> caught up to the boiler pressure. Burping the pressure is a method to
> accelerate the ramp up to operating temperature, a temperature it
> would achieve eventually without the burp.
>
> Whether or not it is of any benefit depends hugely on the machine
> type.




    
Date: 26 Sep 2006 10:07:02
From: Brent
Subject: Re: can someone explain "false pressure" ?
>
>> Until you "burp" the air out the steam
>> wand the air pressure in the boiler confused the pstat and the machine
>> does
>> not heat up fully.
>
> Presumably, you only have to do this once when the machine is new?
>
> Assuming a sealed HX system, once the air is 'removed' from the boiler
> and no new water is introduced, without a VacBrkValve, the system will
> remain purged and only temperature change will effect a pressure
> difference in the boiler.
>

Nope,

I get it on a single group HX, as well as my two group.

From off, you get a build up (as Barry described) of pressure in the boiler,
so the guage reads "correct", open the steam wand (especially on the two
group) and the wand almost spits at you, and 90% of the pressure goes. Leave
it a few minutes and the pressure builds up properly, then when you open the
wand the pressure is there, constant, and useable - ie I can open the steam
wand and leave it open sending steam everywhere :)

I have no idea on the temperature, but any time I have to turn my two group
off to do repairs (pump, brainbox whatever) I know when I start it up, I
have to go and bleed the false pressure, and reasonably soon after turning
it on, so the pressure builds up correctly, often depending on my
impatience, I will have to do a minor second bleed before it is usable.

The single group is the same, it just doesn't take as long to heat up.

As for the dedicated boilers like zocco's - pretty sure it happens
everytime I do work on a friends one, except the one time when the valve
stuck open - that time it just hissed until I reset it :)

Brent




    
Date: 25 Sep 2006 17:45:26
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
No, in practice you have to do this every time the machine is allowed to
cool. Theoretically a machine without a VBV should hold a vacuum when the
boiler cools and the steam condenses as long as you keep the steam valve
closed but in practice the seals are not that tight (especially when
everything contracts upon cooling) and air finds its way in every time -
"nature abhors a vacuum".

As long as the machine remains on 24/7 there's no further need to purge.




"I- >Ian" <someone@nowhere.com> wrote in message
news:atbgh21tp3kgmmr4rlm08maln4fb195chg@4ax.com...
> On Mon, 25 Sep 2006 14:45:14 -0400, "Jack Denver"
> <nunuvyer@netscape.net> wrote:
>
>> Until you "burp" the air out the steam
>> wand the air pressure in the boiler confused the pstat and the machine
>> does
>> not heat up fully.
>
> Presumably, you only have to do this once when the machine is new?
>
> Assuming a sealed HX system, once the air is 'removed' from the boiler
> and no new water is introduced, without a VacBrkValve, the system will
> remain purged and only temperature change will effect a pressure
> difference in the boiler.
>




     
Date: 25 Sep 2006 22:05:20
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Mon, 25 Sep 2006 17:45:26 -0400, "Jack Denver"
<nunuvyer@netscape.net > wrote:

>As long as the machine remains on 24/7 there's no further need to purge.

I would be most curious to look at a log of boiler water temperature
with and without burping to determine the point at which they merge.

OR

If they never do, then pStats are much better than I ever imagined.


      
Date: 26 Sep 2006 12:39:55
From: Brent
Subject: Re: can someone explain "false pressure" ?
>
>>As long as the machine remains on 24/7 there's no further need to purge.
>
> I would be most curious to look at a log of boiler water temperature
> with and without burping to determine the point at which they merge.
>
> OR
>
> If they never do, then pStats are much better than I ever imagined.

Bring a thermometer and play on mine :)




       
Date: 26 Sep 2006 01:01:27
From: I->Ian
Subject: Re: can someone explain "false pressure" ?
On Tue, 26 Sep 2006 12:39:55 +1200, "Brent" <me@privacy.net > wrote:

>>
>>>As long as the machine remains on 24/7 there's no further need to purge.
>>
>> I would be most curious to look at a log of boiler water temperature
>> with and without burping to determine the point at which they merge.
>>
>> OR
>>
>> If they never do, then pStats are much better than I ever imagined.
>
>Bring a thermometer and play on mine :)
>

Are you in SoCal?


        
Date: 26 Sep 2006 14:29:09
From: Brent
Subject: Re: can someone explain "false pressure" ?
no New Zealand - just down the road a bit from SoCal :)

>>> I would be most curious to look at a log of boiler water temperature
>>> with and without burping to determine the point at which they merge.
>>>
>>> OR
>>>
>>> If they never do, then pStats are much better than I ever imagined.
>>
>>Bring a thermometer and play on mine :)
>>
>
> Are you in SoCal?




    
Date: 25 Sep 2006 14:38:04
From: Paul Sack
Subject: Re: can someone explain "false pressure" ?
"I- >Ian" <someone@nowhere.com> writes:

> When you evac the boiler, you remove a mass of air with a density
> directly related to the temperature. It contains a certain amount of
> energy.
>
> If you add enough air @ 240F back to raise the pressure to 1.2g there
> will be some air left in the balloon. But you have supplied energy to
> the system to raise the temperature of the air extracted at room temp.

What's missing in this thread is mention of Dalton's Law of Partial
Pressure. The gist though is that you add the pressure you get from
steam and air. The p-stat reacts to total pressure, not partial
pressure.


 
Date: 24 Sep 2006 20:26:40
From: Mldhab
Subject: Re: can someone explain "false pressure" ?

Alan wrote:
> Every once in a while, I hear references made to opening the steam valve a
> bit to get rid of "false pressure" before pulling the shot.

If the anti-syphon (also called anti-vacuum or breaker valve) is
functioning correctly you will probably not have a problem with 'false
pressure'. This valve allows air to escape the boiler as the water
heats up, (you hear hissing from inside the machine while it is
initially heating up) and then closies when the boiler water produces
enough steam flow past the valve to raise the pressure a bit and close
it.

If the anti-syphon valve isn't working properly, it will either be
leaking a lot of steam and you will not be able to miss the problem, or
it will be stuck closed. If stuck closed, the previously mentioned air
will not be able to escape the boiler without opening the steam valve a
bit.

If the boiler has a substantial air blanket that hasn't been relieved
thru the steam valve, it will do a poor job of transferring heat to the
HX tube, and frequently dis-allows the formation of the required
temperature gradient necessary to generate the convection flow in the
HX / Group Head circuit.

Mike



 
Date: 24 Sep 2006 16:26:57
From: Jack Denver
Subject: Re: can someone explain "false pressure" ?
We live surrounded by a ocean of pressure. The air all around us (including
the air in your boiler) is at around 1 bar of pressure. An "empty" boiler
is not empty at all - its filled with a big cushion of air at atmospheric
pressure. Until you let the air out of the boiler it confuses the
pressurestat because the total pressure that the pressurest "sees" is the
sum of the air pressure inside the boiler plus the steam pressure. So if you
have your pstat set at say 1.2 bar, as soon as there is .2 bar of steam
pressure, the pstat will kick off even though there is very little "head" of
steam built up. Once you let the air out of the boiler, the "false"
pressure blows out the wand and the pstat can sense the "true" steam
pressure only. Actually the pressure is not "false" it is just not the kind
of pressure that you want to sense (steam pressure). If you operated your
espresso machine in the vacuum of outer space, there would be no "false"
pressure.




"Alan" <in_flagrante@hotmail.com > wrote in message
news:DFBRg.5902$7I1.4215@newssvr27.news.prodigy.net...
> Every once in a while, I hear references made to opening the steam valve a
> bit to get rid of "false pressure" before pulling the shot.
> This question may betray my ignorance, but what is meant by "false"
> pressure? I mean, pressure is pressure, isn't it? I'd love to know, first
> of all, what it is, and secondly, why it's desirable to get rid of it.
> Thank you for your answers.
>




  
Date: 25 Sep 2006 08:18:49
From: Bill Barner
Subject: Re: can someone explain "false pressure" ?

"Jack Denver" <nunuvyer@netscape.net > wrote in message
news:j9ednTXaaZOPd4vYnZ2dnUVZ_oqdnZ2d@comcast.com...
[...]
> If you operated your
> espresso machine in the vacuum of outer space, there would be no "false" >
pressure.
[...]

I wonder whether they have an espresso machine on the space shuttle. I'll
bet they get their lattes from a tube. :)




   
Date: 25 Sep 2006 18:15:27
From: Paul Monaghan
Subject: Re: can someone explain "false pressure" ?
On Mon, 25 Sep 2006 08:18:49 -0400, "Bill Barner" <bbarner@cox.net >
wrote:


>I wonder whether they have an espresso machine on the space shuttle.

If they do I hope they don't try the naked portafilter...



  
Date: 24 Sep 2006 21:15:23
From: Barry Jarrett
Subject: Re: can someone explain "false pressure" ?
On Sun, 24 Sep 2006 16:26:57 -0400, "Jack Denver"
<nunuvyer@netscape.net > wrote:

>sum of the air pressure inside the boiler plus the steam pressure. So if you
>have your pstat set at say 1.2 bar, as soon as there is .2 bar of steam
>pressure, the pstat will kick off even though there is very little "head" of
>steam built up.

you're doing what i usually do, forgetting that psig and psia are
different.

;)

the explanation, though, is correct, iirc. the partial pressure of
the thermally expanded air will cause the pstat to kick off at the
correct boiler pressure, but at an undesired lower temperature than if
the pressure in the boiler was due entirely to steam.




   
Date: 25 Sep 2006 01:43:54
From: Alan
Subject: Re: can someone explain "false pressure" ?

"Barry Jarrett" <barry@rileys-coffee.com > wrote in message
news:6lrdh25a2061qs3jles83qmtv7gnhqquik@4ax.com...
> On Sun, 24 Sep 2006 16:26:57 -0400, "Jack Denver"
> <nunuvyer@netscape.net> wrote:
>
> >sum of the air pressure inside the boiler plus the steam pressure. So if
> >you
> >have your pstat set at say 1.2 bar, as soon as there is .2 bar of steam
> >pressure, the pstat will kick off even though there is very little "head"
> >of
> >steam built up.
>
> you're doing what i usually do, forgetting that psig and psia are
> different.
>
> ;)
>
> the explanation, though, is correct, iirc. the partial pressure of
> the thermally expanded air will cause the pstat to kick off at the
> correct boiler pressure, but at an undesired lower temperature than if
> the pressure in the boiler was due entirely to steam.

Thank you both (Jack Denver and Barry Jarrett) for your explanation --- now
it makes sense to me .. .