HomeRoast Digest


Topic: aadjusting digital thermometer (7 msgs / 180 lines)
1) From: Kim G
Hello.
 I started roasting with popper last summer and have been lurking here =
since then. Just got Sweet Maria's Circuit Specialists Thermometer and =
finding it a few degrees low (but within it's +/-4F spec), opened it up =
hoping to find something to tweak. I did and I tweaked it and now at =
boiling it's right on and at room temp it's closer to agreeing with the =
other thermometers at home. It agrees well with the little Update =
International $5 dial unit I got from SM's, though it's hard to read =
that one to resolutions finer than 5F which is what prompted getting the =
digital unit.
To get inside the box, under the sticker on the back is a screw - remove =
it. Then pry the bottom open, then the top. Prying at the top is =
important because of two small thin flanges, that stradle the yellow =
jack for the temp probe, that you can't see until you get the back off. =
Once the back is off you see 12 small black screws that appear to hold =
the circuit board in place. Actually 4 of them are for the display =
mounting and 4 are for the switch mounting. Only remove the 4 corner =
most screws.
On the circuit board there are 4 blue trimpots. Likely they are 10 turn =
trimmers--gave one 2 turns and hit no stop. It's the one in the upper =
right above the display. A clockwise turn increased the displayed temp.
 Turning the other 3 one turn one at a time caused no change in the =
displayed temp. I returned them to their original setting. I'd really =
like to know what the other 3 trimpots do???? anybody???
I don't think a 3F increase in accuracy is terribly important, though I =
feel better knowing it's closer to correct. Most important are the =
repeatability of measurments. I like that it's easier to read and =
responds faser than the dial thermometer. Sometime ago I had decided =
that the desired roast was achieved about 5F (with the dial thermometer) =
sooner than Tom's descriptions in the visual roast aids ( =http://www.sweetmarias.com/roasting-VisualGuideV2.html). While the dial =
and the digital agree well at constant temps I've observed that the dial =
lags the digital by about 5F while the roast is progressing. They got =
close to the same during the first crack but split again after that.
Kim G.
Winona, MN 
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2) From: Jared Andersson
Welcome to delurk mode Kim.  I hope your posts keep comming. Winona is
a beutiful area to live in.  I went to graduate school at Saint Mary's
University (in Minneapolis).  It is always nice to hear of other
roasters in the area.
Jared
Chanhassen MN (Minneapolis)
On Mon, 31 Jan 2005 10:49:05 -0600, Kim G  wrote:
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3) From: Jason Molinari
You likely skewed your thermocouple for higher temps
now. Normally calibration is at least 2 point, on the
TC i have it is 0F and 1800F. Given that it is hard to
generate 1800F, there are ways to trick the
thermocouple into thinking it is at 1800F with
voltages...
Anyhow, my point is, your TC is now correct for room
and boiling temps, but most probably wrong for the end
limits of its range.
My $0.02
<Snip>

4) From: Kim G
You likely skewed your thermocouple for higher temps
now. Normally calibration is at least 2 point, on the
TC i have it is 0F and 1800F. Given that it is hard to
generate 1800F, there are ways to trick the
thermocouple into thinking it is at 1800F with
voltages...
Anyhow, my point is, your TC is now correct for room
and boiling temps, but most probably wrong for the end
limits of its range.
My $0.02
***************************
You're might be right about accuracy at the limits, but I'm not using it =
at the meter's limits.( I think the supplied thermocouple's longterm =
limit is something like 480F or so.) I'm thinking that a couple of those =
other 3 trimmers might be used for setting limit voltages and the one I =
adjusted might be intended for the tweak I performed???  Don't =
thermocouples vary somewhat from unit to unit? If I open it again I'll =
try to see what chips are being used and what's connected where. The =
display covers most of the pcb making it hard to see parts and values. =
Anyway, as far as I can see, it still works and it's definitely not =
worse and seemingly better. 
I did call Circuit Specialists (no help) and found they are manufactured =
by Mastech in China. Have emailed Mastech about the calibration =
procedure for the unit.........
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5) From: Jason Molinari
Yes, of course all thermocouples are different. mine
only has 2 pots and is an Omega...I called them and
they were SUPER helpful for the calibration procedure.
In fact mine is a super old model and he went and dug
up a calibration sheet for it, and faxed it to me!
Either way..i would try and test the reading closer to
the values you'll be using it for...maybe put it in an
oven set at 400, and see what it reads (assuming your
oven is close to correct!).
jason
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message.
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6) From: Les
Welcome Kim.  used to live 18 miles across the river from you in Wisconsin.
Les 
On Tue, 1 Feb 2005 11:21:41 -0600, Jared Andersson
 wrote:
<Snip>

7) From: Kim G
I'm prety sure my oven temps are off. And the Update Internation got from SM 
for $4.60 has plastic cover, over the dial, that melted a little in one spot 
while screwing around  before making a hole and mounting through the popper 
lid as suggested athttp://www.sweetmarias.com/airpoppermods.thermo.html.I 
think if I were to place the UI in the oven to verify the TC it might 
totally ruin it.
Kim


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