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'[EE]: High temperature sensor'
2001\12\06@235035 by Zemin Liu

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Hi all,

I am trying to make a temperature controlled heat gun myself. Does anyone
have experience on making a homebrew high temperature (100 - 400 degree C)
sensor? Any suggestion is welcome.

Thank you in advance!

Zemin

{Original Message removed}

2001\12\07@044419 by David Venz

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Random idea: IR detector?  Dunno about accuracy or linearity...

Cheers,
-Dave.




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Hi all,

I am trying to make a temperature controlled heat gun myself. Does anyone
have experience on making a homebrew high temperature (100 - 400 degree C)
sensor? Any suggestion is welcome.

Thank you in advance!

Zemin

{Original Message removed}

2001\12\07@054916 by Vasile Surducan

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copper resistance but better is a platinium one, low wire diameter to have
as large resistance you can ( at least 50 ohm but better 1kohm )

On Thu, 6 Dec 2001, Zemin Liu wrote:

> Hi all,
>
> I am trying to make a temperature controlled heat gun myself. Does anyone
> have experience on making a homebrew high temperature (100 - 400 degree C)
> sensor? Any suggestion is welcome.
>
> Thank you in advance!
>
> Zemin
>
> {Original Message removed}

2001\12\07@055015 by Alan Shinn

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Zeman,
I would use a chromel/alumel thermocouple, as they are pretty
bulletproof and can easily go to much higher temperatures than 400C. The
biggest problem with them  is that they only measure the difference in
temperature between the hot end and the cold end. If you need more
accuracy than this allows, then you need an ambient temperature sensor
for compensation (LM35 etc.?). The other "problem" with thermocouples is
their low sensitivity but 100 to 400 C range shouldn't be too hard-- 75C
delta T gives (room temp ~25C so 100C -25 = 75C) about 3mV and 375C
delta T gives about 15mV. As you can see, it is pretty linear (at about
.04mV/deg C)
--
Looking forward:
Alan Shinn


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Date:    Thu, 6 Dec 2001 23:37:47 -0400
From:    Zemin Liu <.....zeminKILLspamspam@spam@IPOLINE.COM>
Subject: [EE]: High temperature sensor



>Hi all,
>
>I am trying to make a temperature controlled heat gun myself. Does anyone
>have experience on making a homebrew high temperature (100 - 400 degree C)
>sensor? Any suggestion is welcome.

>Thank you in advance!

>Zemin

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2001\12\07@104922 by Douglas Butler

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Platinum RTDs are cheap these days and are very stable, but they need
lots of amplification.
Thermocouples are very robust and new chips make compensating them easy,
but they don't have the high accuracy of platinum.

What accuracy and stability do you want?  Look at Analog Devices' web
page (http://www.analog.com) and search for "thermocouple".

Sherpa Doug

> {Original Message removed}

2001\12\07@134751 by Robert A. LaBudde

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At 09:01 AM 12/7/01 -0500, Sherpa wrote:
>Platinum RTDs are cheap these days and are very stable, but they need
>lots of amplification.
>Thermocouples are very robust and new chips make compensating them easy,
>but they don't have the high accuracy of platinum.
>
>What accuracy and stability do you want?  Look at Analog Devices' web
>page (http://www.analog.com) and search for "thermocouple".

All of these methods are overkill for a "temperature-controlled heat gun".

Why not just use the resistance of a nichrome wire in the exhaust? This is
an acceptable substitute for platinum.

A thermocouple would also work (any two metals, e.g., nichrome+copper,
copper works up to 400 C = 752 F).

Commercial thermocouples are accurate to +/- 1 C = 2 F, but absence of a
cold-junction compensation would expand that to +/- 10 C = 18 F perhaps.

Remember that 1% x 400 F = 4 F, so extreme accuracy isn't required.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: .....ralKILLspamspam.....lcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
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2001\12\07@203107 by Peter L. Peres

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Use a RTD or a thermocouple. A mechanical (bimetal) thermostat will also
work (see oven supplies etc). The last method is the least recommended
unless you use several thermostats and a tapped heater.

I once made a thermostated plate out of a cooker plate with 5 taps. I used
a double thermostat (washing machine) to switch 500W to 100W at 40C and
100W off at 55C. It keeps 0.5l of chemical solution at 55 +/- 0.5C after
30 minutes heatup. The total thermal mass is about 0.5l solution + 1kg
iron (heater cooking plate).

You don't really need rocket science for these things. Just adjusting the
power with a dimmer at constant air flow will probably be enough. A high
temperature thermometer can be bought for pennies in a hardware store
(cooking thermostat for oven). Most DMMs have a plug for K type
thermocouple which should cover your temperature range.

good luck,

Peter

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2001\12\07@223729 by Zemin Liu

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Thank everyone who replys! The ideas are really helpful. I don't need much
accuracy and linearity, so it seems nichrome wire can suit my need. I'll try
this way first.

Thanks again.

Zemin

{Original Message removed}

2001\12\08@080634 by Roman Black

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Peter L. Peres wrote:
>
> Use a RTD or a thermocouple. A mechanical (bimetal) thermostat will also
> work (see oven supplies etc). The last method is the least recommended
> unless you use several thermostats and a tapped heater.
>
> I once made a thermostated plate out of a cooker plate with 5 taps. I used
> a double thermostat (washing machine) to switch 500W to 100W at 40C and
> 100W off at 55C. It keeps 0.5l of chemical solution at 55 +/- 0.5C after
> 30 minutes heatup. The total thermal mass is about 0.5l solution + 1kg
> iron (heater cooking plate).


One really cheap method is to use a 5w resistor,
the white ceramic wire-wound type. They make a good
rugged thermosensor up to maybe 300'C if you
do a calibration table with the PIC. You won't
get real fine accuracy but since people are
talking thermocouples and stuff the resistor
option might be useful to hobby guys or people
on a tight budget. :o)
-Roman

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2001\12\09@132748 by Peter L. Peres

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> One really cheap method is to use a 5w resistor,
> the white ceramic wire-wound type.

Hm. there usually have 300 to 500ppm/deg temperature coefficient. This is
not so good. But there is an even cheaper method I've used (and still
use).  Use a tiny signal lamp rated maybe 30mA and 24V. This is around 800
Ohms when lit. When cold it is typically 1/2 to 1/10 of that. Use as a
RTD. It is good to almost 700C when the glass softens. I usually hold them
above a mild gas flame in tweezers until the bulb is drawn by the vacuum
and contacts the filament (on all sides). This reduces the time constant.
Use twiddled or compression connections or point weld. Solder won't work!

Peter

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2001\12\10@072123 by Roman Black

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Peter L. Peres wrote:
>
> > One really cheap method is to use a 5w resistor,
> > the white ceramic wire-wound type.
>
> Hm. there usually have 300 to 500ppm/deg temperature coefficient. This is
> not so good. But there is an even cheaper method I've used (and still
> use).  Use a tiny signal lamp rated maybe 30mA and 24V. This is around 800
> Ohms when lit. When cold it is typically 1/2 to 1/10 of that. Use as a
> RTD. It is good to almost 700C when the glass softens. I usually hold them
> above a mild gas flame in tweezers until the bulb is drawn by the vacuum
> and contacts the filament (on all sides). This reduces the time constant.
> Use twiddled or compression connections or point weld. Solder won't work!


Cool! An excellent idea, and with the interesting
"glass-blower" approach is very interesting. So you
modify the bulb to become a special purpose temp
sensor?
Does it matter if the bulb vacuum fails?

The larger resistance (10k+) wire wound resistors
use a very fine nichrome wire and over 100'C
they are quite usable for temp trip points and
some low res temp measuring.
:o)
-Roman

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2001\12\10@081619 by Vasile Surducan

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On Mon, 10 Dec 2001, Roman Black wrote:

{Quote hidden}

 of course ! The wolfram filament is covering with
oxide at high temperature and the resistance is changing
The method is good but have a little non-linear response.
Checked with small bulb ( those used inside an
electronic hand clock ). Another problem is aging;
But is a nice trick !
Vasile

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2001\12\11@165947 by Peter L. Peres

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> The method is good but have a little non-linear response. Checked with
> small bulb ( those used inside an electronic hand clock ). Another
> problem is aging; But is a nice trick ! Vasile

Yes. The small bulbs all have a vacuum in them (a pretty good one - 1.E-3
or better - unlike normal bulbs which are backfilled with Nitrogen or
Argon). If you collapse the bulb carefully from all sides using a gas
flame then the glass will touch the filament on all sides and there will
be no 'air' left in the bulb. The filament will be protected by the glass
as before. Aging is not a problem but a bakeout would be a good idea
(maybe 300C for 2 hours followed by very slow cooldown - another 2 hours
probably) to anneal the glass.

The oldest such sensor I have made (7 years old now) has unmeasurable
resistance deviation since then (<0.1Ohms from 100OHms which is 0.1%, and
this is due to the limits of my measurement probably).  This is to be
expected with glass cased or molded devices (which are the best aging-wise
afaik). Just use good quality signal lamps. The 7 year old one was by
Tungsram.

I tried to do hot wire anemometry (low speed) with a collapsed bulb but it
does not work right.

Peter

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2001\12\11@170005 by Peter L. Peres

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> Cool! An excellent idea, and with the interesting "glass-blower"
> approach is very interesting. So you modify the bulb to become a
> special purpose temp sensor? Does it matter if the bulb vacuum fails?

It does not matter if the vacuum fails since I do not intend to run the
bulb (electrically) hot. A tungsten filament is ok in air until it gets to
about 1200+ degrees C when it will burn up in the air oxygen (producing
white smoke).

> The larger resistance (10k+) wire wound resistors use a very fine
> nichrome wire and over 100'C they are quite usable for temp trip
> points and some low res temp measuring.

Roman, Nichrome, Constantan and other heater and resistor wires are
specially formulated for low tempco. You are better off using the
unadultered metal. Like Iron, Copper, Aluminium, Tungsten, Platinum, you
get it. The bare metal always has a higher temp. coefficient than the
alloys formulated for low tempco. A simple copper coil (high temp, rated
miniature choke etc) will work better probably. Also beyond a certain
degree of heat there are other choices (like molten salts and conducting
glass ;-). These have rare applications however. FYI molten glass is not
so bad a conductor, under the circumstances.

Peter

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