I am thinking about making an espresso maker and one of the design
considerations is the ability to tightly control the temperature of the
water as it is forced through the coffee. It appears that one of the week
points of the current designs is the lack of thermal stability of the brew
head.

As water flows through the head, there is a heat loss from the water to the
brew head so it is hard to maintain a stable water temp.

To address this I want to introduce water hotter than the desired
temperature into a holding chamber attached to the brew head and allow the
bleed off heat to soak into the brew head while the water is cooling to the
optimum brew temperature.

To further complicate matters, the coffee extraction process relies on a
stable pressure as the water is forced through the coffee, so I want to use
regulated air or gas pressure to be able to give me a easily adjustable and
steady pressure.

Now to my question. Since the optimal extraction temperature is just under
the boiling point, I suspect the heat loss will be such that starting with
boiling water will not be hot enough, so I want water hotter than boiling to
start with. This will require heating the water under pressure.

What I need to know is the relationship between pressure and temperature.
Lets say I wanted to get the water to 225F. How much pressure would there
need to be to raise the boiling point of water to 225F? I figured one of
the many steam engine buffs here would know.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
they come up with this striped stuff.

"Roger Shoaf" wrote in message
...
I am thinking about making an espresso maker and one of the design
considerations is the ability to tightly control the temperature of
the
water as it is forced through the coffee. It appears that one of
the week
points of the current designs is the lack of thermal stability of
the brew
head.

As water flows through the head, there is a heat loss from the water
to the
brew head so it is hard to maintain a stable water temp.

To address this I want to introduce water hotter than the desired
temperature into a holding chamber attached to the brew head and
allow the
bleed off heat to soak into the brew head while the water is cooling
to the
optimum brew temperature.

To further complicate matters, the coffee extraction process relies
on a
stable pressure as the water is forced through the coffee, so I want
to use
regulated air or gas pressure to be able to give me a easily
adjustable and
steady pressure.

Now to my question. Since the optimal extraction temperature is
just under
the boiling point, I suspect the heat loss will be such that
starting with
boiling water will not be hot enough, so I want water hotter than
boiling to
start with. This will require heating the water under pressure.

What I need to know is the relationship between pressure and
temperature.
Lets say I wanted to get the water to 225F. How much pressure would
there
need to be to raise the boiling point of water to 225F? I figured
one of
the many steam engine buffs here would know.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the
tube, then
they come up with this striped stuff.


From the steam tables, 20 psia is 227.96 degrees

220 deg is 17.186 psia
230 deg is 20.779 psia

Note these are absolute pressures...

Roger Shoaf wrote:
I am thinking about making an espresso maker and one of the design
considerations is the ability to tightly control the temperature of the
water as it is forced through the coffee. It appears that one of the week
points of the current designs is the lack of thermal stability of the brew
head.

For your interest, some of the best quality espresso machines (e.g.,
Faema) use an electric pump to pressurise cold water, which is then fed
through a heat exchanger before it reaches the brew head. The water in
the boiler acts as a heat reservoir but doesn't actually end up in the
coffee. If you search for a history of the Faema company online you
should find more information. There was also a Faema dealer who had many
detailed pictures of Faema machine parts online, but I'm afraid I can't
remember the address right now.

I've always hankered after a Faema E 61, but can't afford it. Maybe one
day I will pick up a broken one and restore it.

Chris

"Rick" wrote in message
hlink.net...


From the steam tables, 20 psia is 227.96 degrees

220 deg is 17.186 psia
230 deg is 20.779 psia

Note these are absolute pressures...



Does absolute pressure mean that gage pressure is in addition to atmospheric
pressure? In other words when 5 PSI was indicated on a the pressure vessel
gage then water boiling inside would be about 230F?

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
they come up with this striped stuff.

"Christopher Tidy" wrote in message
...

For your interest, some of the best quality espresso machines (e.g.,
Faema) use an electric pump to pressurise cold water, which is then fed
through a heat exchanger before it reaches the brew head. The water in
the boiler acts as a heat reservoir but doesn't actually end up in the
coffee. If you search for a history of the Faema company online you
should find more information. There was also a Faema dealer who had many
detailed pictures of Faema machine parts online, but I'm afraid I can't
remember the address right now.

I've always hankered after a Faema E 61, but can't afford it. Maybe one
day I will pick up a broken one and restore it.


What I am looking for is the ability to have a machine that is able to
squirt water through coffee at what ever temperature I want and what ever
pressure I want. Commercially available machines have several drawbacks,
the price is one and the "tweakability" is another.

I built a roaster I cobbled together out of a scrap gas BBQ grill and now I
want to build something that will give me a brew that tastes as good as my
beans smell.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
they come up with this striped stuff.

"Roger Shoaf" wrote in message
...

"Rick" wrote in message
hlink.net...


From the steam tables, 20 psia is 227.96 degrees

220 deg is 17.186 psia
230 deg is 20.779 psia

Note these are absolute pressures...



Does absolute pressure mean that gage pressure is in addition to
atmospheric
pressure?
^^^^^^^^^^^^^
Yes
^^^^^^^^^^^^
In other words when 5 PSI was indicated on a the pressure vessel gage then
water boiling inside would be about 230F?
^^^^^^^^^^^^^^^
No. 230 degrees occurs at 20.8 psia, which is 6.1 psig. (not 5 psig.)

Leo Lichtman wrote:
"Roger Shoaf" wrote in message
...

"Rick" wrote in message
rthlink.net...


From the steam tables, 20 psia is 227.96 degrees

220 deg is 17.186 psia
230 deg is 20.779 psia

Note these are absolute pressures...



Does absolute pressure mean that gage pressure is in addition to
atmospheric
pressure?

^^^^^^^^^^^^^
Yes
^^^^^^^^^^^^
In other words when 5 PSI was indicated on a the pressure vessel gage then
water boiling inside would be about 230F?
^^^^^^^^^^^^^^^
No. 230 degrees occurs at 20.8 psia, which is 6.1 psig. (not 5 psig.)


Yes BUT! It also depends on "where" you are. Frinstance at Albuquerque
we are at a little over 5000 ft so the "atmosphereic" pressure is lower
by quite a bit than say LA or DC etc. (sea level)
Just to keep all you "hair splitters" honest. :-)
...lew...

I dug out my 1966 Thermo textbook, and according to the steam tables in the
back of it,water boils at 225 degrees F. if it is at an absolute temp of
18.983 PSI. That is a gauge pressure of 4.287 PSI.
Good luck on maintaining that last decimal place.

Tom Miller



"Roger Shoaf" wrote in message
...
I am thinking about making an espresso maker and one of the design
considerations is the ability to tightly control the temperature of the
water as it is forced through the coffee. It appears that one of the week
points of the current designs is the lack of thermal stability of the brew
head.

As water flows through the head, there is a heat loss from the water to
the
brew head so it is hard to maintain a stable water temp.

To address this I want to introduce water hotter than the desired
temperature into a holding chamber attached to the brew head and allow the
bleed off heat to soak into the brew head while the water is cooling to
the
optimum brew temperature.

To further complicate matters, the coffee extraction process relies on a
stable pressure as the water is forced through the coffee, so I want to
use
regulated air or gas pressure to be able to give me a easily adjustable
and
steady pressure.

Now to my question. Since the optimal extraction temperature is just
under
the boiling point, I suspect the heat loss will be such that starting with
boiling water will not be hot enough, so I want water hotter than boiling
to
start with. This will require heating the water under pressure.

What I need to know is the relationship between pressure and temperature.
Lets say I wanted to get the water to 225F. How much pressure would there
need to be to raise the boiling point of water to 225F? I figured one of
the many steam engine buffs here would know.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube,
then
they come up with this striped stuff.