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Topic: Speaking of temperature probes... (13 msgs / 341 lines)
1) From: Randall Nortman
There's been a lot of talk of temperature probes recently.  One thing
that's been bugging me is that most folks seem to use thermocouples
rather than thermistors.  Is there a particular reason for that?
Faster response time?  In general, given equally priced meters to read
the signal, you'll get a more accurate reading out of a thermistor,
since the signal is easier to read, assuming good tolerances on the
thermistor itself.  Accurate thermocouple amplifiers are more
expensive because you're amplifying a very small signal.
Case in point, the thermistor-based probe I use (found via a
CoffeeGeek forum post):http://indoorhealthproducts.com/912.htm$20 (free shipping, even) buys you +/-8F absolute accuracy in the
critical coffee-roasting temperature range, with 1F resolution (which
is actually sometimes 2F -- the meter skips certain steps, presumably
those which fall in between steps on the analog-to-digital convertor).
To get +/-8F out of a thermocouple amplifier is a bit more expensive.
Your typical digital multimeter with thermocouple input will not do
that well, even if it's a high end $120 meter.  Granted, dedicated
thermocouple meters in that price range can do as well or better, but
not for $20.
So is it the response time that makes people prefer thermocouples?
Seems that would mostly be a function of probe size and material,
forget whether it's a thermistor or thermocouple inside the probe.
I design circuit boards as part of my job (despite the complete lack
of formal training for that sort of thing), and I've been planning to
build myself a computer interface for profile monitoring and control.
I'm just wondering if I should build it for thermocouples or
thermistors.

2) From: Dan Bollinger
Randall, it is easy to snake a TC into the bean mass, and they also come as 
handy rigid probes, which are easy to mount. Another reason they are often used 
is that they are connected to a PID controller.  Dan

3) From: Randall Nortman
Nevermind, I just went shopping for thermistors a bit -- you can get
accurate thermistors, and you can get thermistors that work in the
temperature ranges required for roasting, but one that is accurate and
can also take the heat is hard to find and probably pretty expensive.
So I guess that answers it.
On Sat, Jun 30, 2007 at 02:36:14PM -0400, Randall Nortman wrote:
<Snip>

4) From: Alchemist John
And that is basically what I was going to say.  I would find +/- 8F 
accuracy was to inaccurate for my taste.  I have tested the one Tom 
carries ( I compared against a couple NIST calibrated thermometers) 
and they are accurate from 32 to 600 F within 1 F at the lower ranger 
(under 212 F) and within 2 F at roasting range.  Worth the extra $10 
in my book.
At 12:27 6/30/2007, you wrote:
<Snip>
John Nanci
AlChemist at large
Zen Roasting , Blending & Espresso pulling by Gestalthttp://www.chocolatealchemy.com/

5) From: raymanowen
There is this to say about that:
 If you're after absolute precision, SCRAP the thermistors,and ditto the RTD
sensors.
Think it through- Thermistors and RTD's have temperature sensing elements
that change resistance with changing temperatures. They also have leadwires
that do the same thing, so spin the wheel to determine exactly where you're
reading minute temperature variations.
The readout device has to have a very stable reference current source, that
feeds the thermistor or RTD through the leadwires. The sensor could be a NTC
device, and the leadwires PTC, or you could have a three or four wire
system. What a debacle, and it's still based on the assumption that leadwire
errors cancel.
Reasoning like that brought down the Challenger. Good in theory; won't hold
water.
Thermocouples develop a temperature-dependent potential at one point- the
junction. You could read a thermocouple from any distance, since it's a very
low impedance source, Direct Short, and all.
I hope you aren't greatly worried about building a high gain instrument
amplifier to read the thermocouple and drive the readout device. Can you say
"Operational Amplifier?" The 741comes to mind, and you couldn't hold $5
worth in both hands. High gain, high input impedance, precision- 8th grade
science fair project.
Cheers -RayO, aka Opa!
Why reinvent the wheel?

6) From: Randall Nortman
I suspect the one I have is much better than +/-8F, but that's the
spec.  I think I looked up the specs on the one Tom carries, and it
certainly wasn't +/-2F.  I could be mistaken, but the point is that
you usually get somewhat better than the specs, which give you the
worst case.  (Or at least they're supposed to.)
Anyway, it seems to be very consistent from roast to roast as long as
I don't move it, which is more important than absolute accuracy.  When
I get around to building my computer interface, I will probably go
with a thermocouple, as appropriate probes are readily available.  It
will be a little extra expense on the signal amplifier, but probably
worth it.
On Sat, Jun 30, 2007 at 02:00:44PM -0700, Alchemist John wrote:
<Snip>

7) From: Randall Nortman
On Sat, Jun 30, 2007 at 03:30:28PM -0600, raymanowen wrote:
<Snip>
Thermocouples have a similar problem, in that there are at least two
other junctions of dissimilar metals in the system -- where the
thermocouple wires connect to something else.  So you need a
compensated connector, and ideally you need to know what temperature
the connector is at and all that.  And don't forget that it will pick
up noise along the transmission lines, and if you add a little bit of
noise to a millivolt signal you have problems.
<Snip>
Nah, you don't need a current source -- a stable voltage and a
resistor bridge (i.e., Wheatstone bridge) will do the trick.  If you
can get by with less resolution, you don't even need a full brige --
just put the thermistor in a voltage divider circuit with one other
resistor.  That assumes you have a digital processor at the other end
that can do all the calculations necessary to turn that into a
temperature reading.
But anyway, I have convinced myself to go with a thermocouple simply
because of the lack of readily available high-temperature thermistor
probes that are also reasonably accurate (without calibration).
<Snip>
The classic 741 has too much offset error to amplify a millivolt
signal reliably, without calibration.  There are opamps that will do
the job, sure, though the good ones can run around $5 each in small
quantity, or even more for the really good ones.  You have to get one
that has low offset and good linearity even at really high gain -- if
you want a really good, accurate signal.  Better is to buy an
integrated instrumentation amplifier -- decent ones again can be had
around $5 in small quantity, but running up to $20 for the really
high-end ones.  I could probably get by with a $5 model.
<Snip>
Because existing wheels all have a critical design flaw: Somebody else
built them, and they're going to charge me if I want one.
How much for a good, accurate, dual thermocouple amplifier/digitizer
that outputs to a PC in real time?  How about with a couple of 15A
SSRs, also controlled by the PC?  What if I want to download my own
control program to the thing so I can untether it from the PC?  OK,
but now let's stick a wireless interface on there so I can have it
talk to the PC without the obnoxious tether?  Sure, I can buy
something like that, but not as cheaply as I can build it myself.  And
that's building just one.  If I can find half a dozen other coffee
freaks who want PC control of their roaster and/or espresso machine, I
can build seven of them for barely more than what it costs to build
one, and everybody wins.
Not that I'm trying to build a cottage industry here.  I just want it
for myself.  But you can bet that when I get around to building it,
I'm going to try to find others here or on coffeegeek.com who want the
same thing, so I can bump up the volume and lower the per-unit cost.
For now it's a pipe dream.  Actual work has me far too busy to have
time for hobbies.  Maybe in a few months...

8) From: Rick Copple
Randall Nortman wrote:
<Snip>
Hum, well, don't know about most, but any probe would get in the way of 
wok roasting. :) So, I don't use either. Sight, smell, sound guy here. 
Seems to work every time.
That El Salvadorian I ordered, Tom, is really good. Been sipping on it 
all day.
-- 
Rick Copple

9) From: Homeroaster
The Hearthware Precision air roaster uses a thermistor to regulate the heat 
output into the roast chamber.
I wonder what spec they use for that one?  The air temp is supposed to be 
500F entering the roast chamber according to a Hearthware technician several 
years back.
*********************
Ed Needham
"to absurdity and beyond!"http://www.homeroaster.com*********************

10) From: Pete
Randall,
You might be interested in this if you're thinking of
setting up you're own amp to read TC voltages.  
Dallas/Maxim makes a DS2760 chip that was designed to
monitor laptop batteries.  It measures voltages to +/-
15 microvolts (from memory).  The voltage data can be
read via a 1-wire interface.  I'm using this module to
monitor the bean mass temperature using a K-type TC.  
Personally I use TCs just because I have used them for
years at work,
-Pete
-------------------------------------------------------
There's been a lot of talk of temperature probes
recently.  One thing that's been bugging me is that
most folks seem to use thermocouples rather than
thermistors.  Is there a particular reason for that?
Faster response time?  In general, given equally
priced meters to read the signal, you'll get a more
accurate reading out of a thermistor, since the signal
is easier to read, assuming good tolerances on the
thermistor  itself.  Accurate thermocouple amplifiers
are more
expensive because you're amplifying a very small
signal.
I design circuit boards as part of my job (despite the
complete lack of formal training for that sort of
thing), and I've been planning to build myself a
computer interface for profile monitoring and control.
I'm just wondering if I should build it for
thermocouples or thermistors.
Finding fabulous fares is fun.  
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11) From: John Moody
I tossed mine so long ago; my memory says it looked like it could have been
a diode?
John

12) From: raymanowen
"...it could have been a diode?"
Some reverse biased junctions are extremely temperature sensitive. Diode?
Yes!! -ro

13) From: Larry Johnson
On 6/30/07, raymanowen  wrote (among other
things):
"....Thermistors and RTD's have temperature sensing elements that change
resistance with changing temperatures. They also have leadwires that do the
same thing,"
3-lead RTD's and a self-calibrating monitor (i.e. a GE Multilin) use the
third lead and the common lead to keep track of lead resistance and
compensate for same. I think.
-- 
Larry J
Whatever you do will be insignificant, but it is very important that you do
it.
  - Mahatma Gandhi


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