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Topic: effects of density altitude on roasts (2 msgs / 84 lines)
1) From: Kevin DuPre
It is a well known fact that the efficiency of a gas
burner changes with density-altitude, thus affecting
the roasting profile for batch-to-batch consistency.
Some roasteries who use gas burner drum roasters
compensate the duty cycle of the burner for changes in
density-altitude to achieve consistent results.
Does density-altitude have any affect on
batch-to-batch consistency with roasters that use an
electric heating element, such as the Fresh Roast??
I have been recording density-altitude (DA)in my
roasting logs, and the value is quite sensitive to
changes in both temperature and atmospheric pressure. 
While the latter is the more stable of the two during
a roasting session (unless a front is sweeping through
the area), there is a big swing in DA with even a few
degrees or fractions of degrees change in the
temperature.
Some on this list may already be familiar with DA, but
for those who are not, air changes density with
altitude, pressure, and ambient temperature.  Changes
in air density has an effect on things like what
temperature water boils at, how efficient heat sources
burn (at least gas ones - don't know about electric
ones).  The DA value represents the equivalent
altitude you WOULD BE roasting at given the current
conditions above.  I use a pilot's calculator on my
Palm to determine DA from [known] local altitude, and
measured temp and pressure.
I haven't yet been able to determine if there are
variations in roasts of the same bean from the same
lot, but should I decide to build a drum roaster for
the BBQ, this will have an effect since I'll be using
a gas burner.
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--
Kevin DuPre
obxwindsurfhttp://www.virtualfusion.com"The real voyage of discovery consists not in seeking new landscapes but in having new eyes -- Marcel Proust"
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2) From: Charlie Herlihy
Keven DuPre wrote>It is a well known fact that the
efficiency of a gas
burner changes with density-altitude, thus affecting
the roasting profile for batch-to-batch consistency.
Some roasteries who use gas burner drum roasters
compensate the duty cycle of the burner for changes in
density-altitude to achieve consistent results.
Keven, you seem like you might be the guy to figure
out for us the affect(if any) of ambiant humidity on
roasting profiles, once you get the DA thing licked.  
Meanwhile, listen for cracks, wait for the right
smells and say a prayer to the coffee goddess.
Saludos,
Charlie
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