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'[OT]: Thermocouples'
2000\10\20@095116 by Mark Skeels

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Does anyone know of a good source for thermocouples of the J or K variety?

I want to make an electronic temperature readout/alarm for my woodstove
chimney. Temps are normally room temp to 800F, with as much as 2500 F
possible (but _not_ recommended!)

Mark Skeels
Engineer
Competition Electronics
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Soli Deo Gloria!

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2000\10\20@110725 by Eric Chan

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Try omega.com for thermocouple supplies and they have a very nice theory
of operation that is given in their Temperature Measurement Handbook.
Analog Devices has a couple of ice-point reference ICs you might want to
look at when building the signal conditioning circuit.


Eric


Mark Skeels wrote:
{Quote hidden}

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2000\10\20@112353 by Mark Skeels

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----- Original Message -----
From: Eric Chan <birkospamKILLspamHOME.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Friday, October 20, 2000 10:02 AM
Subject: Re: [OT]: Thermocouples


> Try omega.com for thermocouple supplies and they have a very nice theory
> of operation that is given in their Temperature Measurement Handbook.

I checked Omega. They seem like the best, most obtainable source for my
purposes.


> Analog Devices has a couple of ice-point reference ICs you might want to
> look at when building the signal conditioning circuit.

I was thinking the AD594/595. This seems to be a complete solution.

But one question I do have: what's the best way to interface a thermoucouple
with a pcb? Any suggestions for connectors?

>
>
> Eric
>
>
> Mark Skeels wrote:
> >
> > Does anyone know of a good source for thermocouples of the J or K
variety?
{Quote hidden}

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2000\10\21@213027 by Eric Chan

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Mark,

If you would refer to the app. note given by Analog Devices it should
help. www.analog.com/techsupt/application_notes/AN369.pdf,
particularly on PCB layout.

My question, however, is how would you calibrate the device real life?

Good luck.

Eric


Mark Skeels wrote:

{Quote hidden}

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2000\10\22@093915 by Snail Instruments

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> I was thinking the AD594/595. This seems to be a complete solution.

Unless you are obsessive about cold junction compensation and
linearization, you could get along with an ordinary OA as well.

> But one question I do have: what's the best way to interface a thermocouple
> with a pcb? Any suggestions for connectors?

Generally any connector if cold junction compensation doesn't bother you.
Otherwise you'll find connectors and thermocouple wire at Omega site as well.

Josef


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2000\10\23@110637 by Mark Skeels

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>
> My question, however, is how would you calibrate the device real life?
>
I have a chimney thermometer now which has a probeon the back which goes
into the center of the chimney, right above the stove, where the
thermocouple would also be. I would calibrate it manually during use. Does
this sound OK?

Matrk

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2000\10\23@110640 by Mark Skeels

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----- Original Message -----
From: Snail Instruments <spamBeGonesnailspamBeGonespamIOL.CZ>
To: <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
Sent: Sunday, October 22, 2000 8:39 AM
Subject: Re: [OT]: Thermocouples


> Unless you are obsessive about cold junction compensation and
> linearization, you could get along with an ordinary OA as well.

I'm not sure exactly how much error the lack of cjc will introduce. Id like
to have, say, 10 degree f accuracy over room temp to 1500 degrees f. Is this
within the realm of possibility with a standard op amp?

Also, even if I use thermocouple wire and the connectors, I still
technically have another thermocouple junction wherever the metal in the
conductor changes types, isn't that correct? So using a "thermocouple
connector" still leaves me with a junction that will introduce error. Is
that correct? So, the object is to use the unavoidable second junction as a
compensation for the actual measurement thermocouple. Is _that_ correct?

Mark

Mark

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2000\10\27@144805 by Snail Instruments

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Hi Mark,

I had a busy week so the response is delayed.

>I'm not sure exactly how much error the lack of cjc will introduce. Id like
>to have, say, 10 degree f accuracy over room temp to 1500 degrees f. Is this
>within the realm of possibility with a standard op amp?

J type (chromel-alumel) thermocouple gives about 100uV for each 5 degree F
(but the curve is not perfectly linear), which is within reach of an op
amp. Beware that Pt-PtRh thermocouples have much lower slope. The
thermocouple is low output impedance, bipolar precision OA like OP27 will
probably exceed the requirements for drift and noise by an order of magnitue.

>Also, even if I use thermocouple wire and the connectors, I still
>technically have another thermocouple junction wherever the metal in the
>conductor changes types, isn't that correct?

Yes, exactly.

>So using a "thermocouple
>connector" still leaves me with a junction that will introduce error. Is
>that correct?

Right. The intention of same material connector is to bring the cold
junction at a well defined place, measure its temperature and make the
correction. The ADxxx will sense the cold junction temperature thru its leg.

>So, the object is to use the unavoidable second junction as a
>compensation for the actual measurement thermocouple. Is _that_ correct?

Perhaps I should say that first - the thermocouple signal is equal to the
difference of hot and cold junction voltages. This should clear up the the
cold junction error.

Josef

P.S. Microprocessor controlled chimney... Where are we going ;-)

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2000\10\27@150501 by Mark Skeels

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HI, Josef,

> P.S. Microprocessor controlled chimney... Where are we going ;-)
>

I used to say, "it's the 90's..." but it's awkward to say. "it's the
aught's...."

This will provide two very useful functions, though.

1.) It turns on a fan attached to the stove when the temp get's high enough,
and turns it off when the temp goes down due to consuming all of the wood.
The fan runs romm temp air through a channel on the back of the stove,
transferring heat to the air, which then blows out of the top and helps warm
the room.

2.) It will sound an alarm if the chimney temp exceeds a preset value. This
happens when the fire gets going "too good" and no-ones around to close down
the damper.

With this, we will be able to go to bed at night with a little higher
comfort level that we are not going to burn down the house.

I can see how someone would say that this defeats the whole purpose of a
wood stove; but we've got a saw mill in town and I can get a ton (2000lbs)
of slab-wood for $20.00. Heat your house all winter for under $2000.00. My
main furnace is also a dual fuel unit, wood and oil. I'm really glad I'm
burning wood this year! But it uses electricity, and the wood stove does not
(until now, that is.)

Mark

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2000\10\28@175554 by Andy Howard

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----- Original Message -----
From: "Mark Skeels" <RemoveMEmeskeelsEraseMEspamEraseMEEARTHLINK.NET>
To: <RemoveMEPICLISTspam_OUTspamKILLspamMITVMA.MIT.EDU>
Sent: Friday, October 27, 2000 8:02 PM
Subject: Re: [OT]: Thermocouples


> HI, Josef,
>
> > P.S. Microprocessor controlled chimney... Where are we going ;-)
> >
>
> I used to say, "it's the 90's..." but it's awkward to say. "it's the
> aught's...."

It's the "naughties"


.

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2000\10\30@043744 by Alan B. Pearce

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> P.S. Microprocessor controlled chimney... Where are we going ;-)

I remember a description of one of these built using a national Semiconductor Scamp microprocessor. There was a temperature sensor at the top of the chimney which was monitored, and the microprocessor controlled a motor drive on the flue damper to keep the temperature constant. When it got to a point that the flue was fully open and the temperature was below a set point it rang an alarm for someone to put some more wood on the fire. As the flame grew it then sensed the temperature rise and closed the damper and was all happy again. Apparently the reduction in wood consumption was worth the effort.
This was all back in the early to mid 1970's when the micro revolution had every company coming out with their own micro family, many of which have since disappeared from the market.

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2000\10\30@044622 by Alan B. Pearce

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> I used to say, "it's the 90's..." but it's awkward to say. "it's the
> aught's...."

>It's the "naughties"

Only for those who turned fourty this year :)

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2000\10\30@124605 by Dale Botkin

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On Mon, 30 Oct 2000, Alan B. Pearce wrote:

> > I used to say, "it's the 90's..." but it's awkward to say. "it's the
> > aught's...."
>
> >It's the "naughties"
>
> Only for those who turned fourty this year :)

Oo!  Oo!  That's me!!  Does that give me an excuse now?  Thanks!

Dale
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