HomeRoast Digest


Topic: roasting temp . . . some thermometer tests (2 msgs / 72 lines)
1) From: dewardh
Jack:
<Snip>
the color temperature of the roasted bean, not the sensible temperature.
For the purposes of this discussion, anyway, the two are the same . . . the 
extent to which they differ *is* the question of "emissivity" (or, rather, the 
question of whether emissivity is suppressed  at the wavelengths observed).
<Snip>
If grinding causes some change in the surface emissivity . . . which it *might* 
if, for example, the beans were oily or "shiny" on the surface but the grounds 
were not, the reading would change.  The "error" would not . . . (in this case 
"error" means "don't know what we're seeing").  When I compare room temperature 
beans of differing state of roast I'll "look" at grinds, too.
Deward
Ps . . . quick "first look" (with Raytek MiniTemp) . . . Sumatra Iskandar, 
"kitchen cutting board equilibrated" 
green		22.5C	every third or fourth reading 22.0
Full City	22.5C
ground		23.5C	still cooling from grinder, but wife wants her coffee 
in free air at cutting board
Kodak type 3 (mercury)	22.9C	"process" thermometer, +/-0.125 at 20 and 25C
platinum RTD 		23.1C	Extech "Oyster" PT100
"k" thermocouple	22.1C	Extech EA-15
"k" thermocouple	20C	Mastech 6500 (same thermocouple as above)
				(this is the $19.95 box we talked about a while ago,					 "good enough", 
for the price)
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

2) From: Tom Gramila
Deward,
Wow, -- DATA!  this looks pretty neat.
If this is a quick first look, I wonder what a real measurement is...!
I think that these measurements indicate that there shouldnt be any
great error caused by changes in emissivity caused by roasting.  Lets
assume the worst case for the sake of argument: that green measured as
22.0 C, so the difference with roasted was 0.5C.  If this difference were
due to emissivity changes, the ratios of the two emissivities would
be:
	[(273+22.5)/(273+22)]**4 = 1.0067  or a difference of 2/3%
So at roasting temperature, if one of these indicated 232C, the other
would indicate only 0.85 degrees C different from the other!  (This
assumes that no roasting chemistry happens, and that the emissivity is
not temperature dependent) That sounds great!
I'm delighted to hear!  
	Tom G.
PS -- a quick web search on window materials was not so encouraging.
NaCl makes a good IR window: sounds great in the winter but not in a
muggy summer...  I will look in reference books tomorrow in the lab...
<Snip>
							 at 20 and 25C
<Snip>
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast


HomeRoast Digest