Thermocouple Problem...

[Joshua Guthrie]

I have an thermocouple problem.

Background:
I have two test/measurement setups that are measuring the same
phenomona. I am using the same thermocouple harness in both setups.
This harness comprises of 8 K-type thermocouples paralleled for
averaging. By shooting a signal into the connector for the harness, we
calibrated the system. The harness was verified (for function)
earlier. It checks out to w/in tenths of a degree F. The
thermocouples typically read between 800-1500F. The connector heats up
to about 400F (give or take about a 100F).

Problem:
I am loosing about 8 degrees F from one setup to another. I tend to
believe the 8 degrees difference.

Any ideas?

Also, any recomendations for a good book or something (in addition to
Omega's stuff) for learning about thermocouples. I understand the
basics about Seebeck, but it is still voodoo chemistry to me. How do
you model such circuits. How would you model a short to a known medal
kinda stuff. That 8 TC harness in parallel would be nice to
understand.

thanks in advance
-josh

 

Try
Practical Thermocouple Thermometry
by T.W. Kerlin
available from www.ISA.org

 

[Bob Eldred]

"Joshua Guthrie" <josh@westmouthbay.com> wrote in message
news:1111177243.431212.91600@l41g2000cwc.googlegroups.com...

I have an thermocouple problem.

Background:
I have two test/measurement setups that are measuring the same
phenomona. I am using the same thermocouple harness in both setups.
This harness comprises of 8 K-type thermocouples paralleled for
averaging. By shooting a signal into the connector for the harness, we
calibrated the system. The harness was verified (for function)
earlier. It checks out to w/in tenths of a degree F. The
thermocouples typically read between 800-1500F. The connector heats up
to about 400F (give or take about a 100F).

Problem:
I am loosing about 8 degrees F from one setup to another. I tend to
believe the 8 degrees difference.

Any ideas?

Also, any recomendations for a good book or something (in addition to
Omega's stuff) for learning about thermocouples. I understand the
basics about Seebeck, but it is still voodoo chemistry to me. How do
you model such circuits. How would you model a short to a known medal
kinda stuff. That 8 TC harness in parallel would be nice to
understand.

thanks in advance
-josh

I am troubled by the idea of "averaging" the thermocouples by putting them
in parallel. Thermocoules are voltage sources and you "never" average
voltage sources by placing them in parallel. For them to give an assemblance
of a true average, the junction resistance, lead length, termination
resistance, etc. would have to be exactly equal on all devices for a proper
average to occur, otherwise they rob from each other in unintended ways. I
suspect your proplem is related to this un-equal resistance issue.
Thermocouples should be averaged by placing them in series then dividing the
resulting millivoltage by the number of couples, but this requires more
advance signal conditioning. Usually multiple thermocouples are arranged
with each on it own separate channel then any required averaging done by a
computer that's also doing data logging.
I am also concerned about the connector being at 400 deg. Is this system
properly copper compensated? The cold junction has to be known and either
controlled in temperature or compensated for electronically to get accurate
readings. If the connector is a true chromel-alumel block and wired properly
this may not be a problem, but if it's copper or brass, it could be.
I think I would look at the averaging scheme and answer why the connector
is so hot. Needless to say that "any" thermocouple placed in the same
environment should give nearly the same result and not be off by eight
degrees. Good Luck.
Bob

 

[Glen Walpert]

On Sat, 19 Mar 2005 03:05:12 GMT, "Bob Eldred" <nsmontassoc@yahoo.com>
wrote:

"Joshua Guthrie" <josh@westmouthbay.com> wrote in message
news:1111177243.431212.91600@l41g2000cwc.googlegroups.com...
I have an thermocouple problem.

Background:
I have two test/measurement setups that are measuring the same
phenomona. I am using the same thermocouple harness in both setups.
This harness comprises of 8 K-type thermocouples paralleled for
averaging. By shooting a signal into the connector for the harness, we
calibrated the system. The harness was verified (for function)
earlier. It checks out to w/in tenths of a degree F. The
thermocouples typically read between 800-1500F. The connector heats up
to about 400F (give or take about a 100F).

Problem:
I am loosing about 8 degrees F from one setup to another. I tend to
believe the 8 degrees difference.

Any ideas?

Also, any recomendations for a good book or something (in addition to
Omega's stuff) for learning about thermocouples. I understand the
basics about Seebeck, but it is still voodoo chemistry to me. How do
you model such circuits. How would you model a short to a known medal
kinda stuff. That 8 TC harness in parallel would be nice to
understand.

thanks in advance
-josh

I am troubled by the idea of "averaging" the thermocouples by putting them
in parallel. Thermocoules are voltage sources and you "never" average
voltage sources by placing them in parallel. For them to give an assemblance
of a true average, the junction resistance, lead length, termination
resistance, etc. would have to be exactly equal on all devices for a proper
average to occur, otherwise they rob from each other in unintended ways. I
suspect your proplem is related to this un-equal resistance issue.
Thermocouples should be averaged by placing them in series then dividing the
resulting millivoltage by the number of couples, but this requires more
advance signal conditioning. Usually multiple thermocouples are arranged
with each on it own separate channel then any required averaging done by a
computer that's also doing data logging.
I am also concerned about the connector being at 400 deg. Is this system
properly copper compensated? The cold junction has to be known and either
controlled in temperature or compensated for electronically to get accurate
readings. If the connector is a true chromel-alumel block and wired properly
this may not be a problem, but if it's copper or brass, it could be.
I think I would look at the averaging scheme and answer why the connector
is so hot. Needless to say that "any" thermocouple placed in the same
environment should give nearly the same result and not be off by eight
degrees. Good Luck.
Bob

Bob is right on the mark here; your paralleled thermocouples force
current through each other with any temperature differences,
eliminating any possibility of accuracy. The hot connector may also
be a problem unless it is at a uniform temperature and has identical
thermocouple grade wire (not extension grade) on both sides.

A few references from my old notes, hope they still work:

http://uhavax.hartford.edu/~biomed/gateway/Thermocouple.html

http://www.temperatures.com/tcs.html

http://www.electronics-cooling.com/Resources/EC_Articles/JAN97/jan97_01.htm

http://www.engr.orst.edu/~aristopo/temper.html

http://www.linear.com/pdf/an28.pdf

http://www.chem.cornell.edu/fjd3/thermo/intro.html

Regards, Glen

 

[Joshua Guthrie]

I really don't suspect the harness. Someone a whole lot sharper than
me came up with it and it was tested prior to our runs. In fact, I
don't honestly know much about the harness. All I could dig up on it
was an elementry schematic where it appeard to be 8 TC's in ||. It
looked like they were doing the averaging through choosen reistances on
each leg. But the schematic was only an overview and really told me
nothing. However, the design has been in use for about 20 years. And
the same harness was used in both tests.

I would like to learn more about the harness and plan to do so Monday
(esp in light of this discussion).

It looks like they are using K type connectors. I saw an Al stamp on
one side (the other one was crimped in an unfortunate place). They are
not copper or gold. I did notice that they look to be using a steel
nut on top of the connector. I assume it would make a junction, but
would it affect the overall circuit? I would think the steel side
would just be on open circuit and just floa --that it would just add
noise, if anything.

thanks for the ideas!!
-josh

 

Paralleling thermocouples means that the average is weighted by the
resistance of the individual thermocouples (including the contact
resistance of any connectors). The legs of the thermocouples have TCRs
so you may be messing things up fairly seriously if you're running the
legs at high temperatures. A few modest precision resistors >> the
thermocouple loop resistance (at room temperature, and with no
gradients nearby) could work wonders in swamping those effects if you
keep them cool and with the junctions at each end at the same
temperature. You should NOT run the connectors at anything like 400°F
if you expect to see reasonable accuracy. Finally, CRIMPED? That's very
bad. It's possible to crimp gas-tight connections with the right kind
of connectors and a very expensive crimper, but generally speaking
that's anathema to accuracy. Welded connections for high or low
temperature and tight screw terminal connections for around room
temperature. Oh, and it should go without saying that the junctions
must be isolated from each other for this to work, otherwise you'll
have an unholy mess indeed. Even to achieve 5°C accuracy can require a
bit of care in thermocouple systems.

Best regards,
Spehro Pefhany (temporararily in Li4 jiang1)

 

Thanks... You guys are increasing my education by leaps and bounds.

Out of curiosity, why is it bad to crimp relatively high temp
connectors? I wouldn't have thought 400F that hot. Corrosion making
excess junctions? To make you feel better, the harness isn't crimped,
just the connector to my system. But it is in a hot location.

 

[Bill]

Joshua Guthrie wrote:

I really don't suspect the harness. Someone a whole lot sharper than
me came up with it and it was tested prior to our runs. In fact, I
don't honestly know much about the harness. All I could dig up on it
was an elementry schematic where it appeard to be 8 TC's in ||. It
looked like they were doing the averaging through choosen reistances
on
each leg. But the schematic was only an overview and really told me
nothing. However, the design has been in use for about 20 years.
And
the same harness was used in both tests.

I would like to learn more about the harness and plan to do so Monday
(esp in light of this discussion).

It looks like they are using K type connectors. I saw an Al stamp on
one side (the other one was crimped in an unfortunate place). They
are
not copper or gold. I did notice that they look to be using a steel
nut on top of the connector. I assume it would make a junction, but
would it affect the overall circuit? I would think the steel side
would just be on open circuit and just floa --that it would just add
noise, if anything.

thanks for the ideas!!
-josh

The responses are correct about averaging is weighted by loop
resistance. But the engine exhaust people have beeen using exhaust
thermocouple rakes for years. They typically use sensors all of the
same length, style, geometry, etc. so that the individual loop
resistances are the same.

You can easily check to see if your loops are the same with a DMM. If
they are not then that could be your problem.

As already mentioned, another problem source could be your
interconnections. Especially if you are making connections in thermal
gradients. Try seeing if local heating of a connection alters the
output of the sensor by passing a heat gun or torch across the
connection to give a sharp gradient across the hardware. You shouldn't
see a change in the output but if you do then you may have a problem at
that connection.

You can do this same test all along the length of a thermocouple to
look for thermoelectric inhomogeneities away from the junction. It's
not a precise test but it can highlight trouble spots. Type K can have
quite a bit of calibration shift due to short range ordering, grain
growth, alloy diffusion, and cold work in bent areas. Have the 2
setups you are comparing been through the same processes or is one new
and the other used?

Also note that standard limits of error for type K at 1500°F is 11°F
so consider that when you are trying to reconcile the 8°F difference.
And saying that 2 averaging thermocouple rakes calibrate to within a
few tenths of a degree for type K is stretching it. I would imagine
the calibration uncertainty is much larger than a few tenths. Check
out NIST's website for calibration uncertainty for type K.

Someone has already recommended Kerlin's book and he had another with
Bob Shepherd from NIST called Industrial Temperature Measurement.
Professor Kerlin is at U of Tennessee and he holds the copyright now to
the book. There's also ASTM's "Manual on the Use of Thermocouples in
Temperature Measurement". NIST also maintains a good site and the
people there are very helpful. Bentley from Australia CSIRO has a more
recent set of 3 books called "Industrial Temperature Measurement" or
Thermoemetry or something like that.

Bill Schuh
Watlow

 

Because the crimp joint will probably loosen and will likely not remain
gas-tight due to thermal cycling. This will result in a bad connection-
in your case one of the thermocouples might stop contributing to the
average, in more typical systems the break protection circuitry will
cause an error in the reading.

Best regards,
Spehro Pefhany (temporararily in Li4 jiang1)

 

[Tony Williams]

In article <1111573655.984085.237720@f14g2000cwb.googlegroups.com>,
<spehro@gmail.com> wrote:

Spehro Pefhany (temporararily in Li4 jiang1)

Two questions Speff.....

Q1. Where is Li4 jiang1?

Q2. How on earth did you get early parole from sed?

--
Tony Williams.

 

[Bill]

You can get some nominal information on the thermoelectric power of
metal elements in some reference books. Goodfellow's catalog has a
good listing for elements and some alloys but this is nominal
information. There's a little bit of information on binary alloys if
you search around. Try Kinzie's book "Thermocouple Temperature
Measurement". You might also look into the Symposium series,
"Temperature, Its Measurement and Control in Science and Industry". As
well as IMEKO and TEMPMEKO.

Finally the National Metrology Labs like NIST and CSIRO are a good
place to start or to send questions.

But you are asking for information that may not be there. For example,
300 series stainless steels weren't developed for thermoelectric
properties and so there's little motivation to characterize those
properties. Also slight chemical or metallurgical variations could
change the properties so it would be difficult to establish a standard
reference table or function.

Bill Schuh
Watlow

 

[John Woodgate]

I read in sci.electronics.design that josh@westmouthbay.com wrote (in
<1111673845.815866.130890@l41g2000cwc.googlegroups.com&gt about
'Thermocouple Problem...', on Thu, 24 Mar 2005:

I have another question. Somewhat unrelated. A good portion of the
literature I've seen so far relates thermocouple EMF's to the
substances Seebeck Coeficents. However, I've not seen anyone allude to
where a person can find a good list of these tables. I've found lists
for the normal TC types. But when you're using Seebeck Coefs. wouldn't
you normally be interested in something weird, like a jucntion between
Chromel and Steel? Any ideas. Perhaps I'm still missing something.

Google with 'Seebeck coefficients' gave 5600 hits.

To understand thermocouples, you need to look at the metals and the

temperatures all round the whole current loop. You also need to
understand that the thermal voltages are generated in **the wires that
have a different temperature at each end**, not at the junctions between
dissimilar metals.

In a simple case, with one chromel-alumel junction feeding a moving-coil
meter with a Manganin swamp resistor, the loop looks like (use Courier
font):

(------COLD------)
Chromel----Brass---Copper--+
HOT!! | |
Alumel-----Brass--Manganin-+

If the whole Brass-Copper-Managanin-Brass part is all at the same
temperature, the only voltages left are those generated in the Chromel
and Alumel wires, because they are the only ones that have a temperature
gradient between their ends.

You can put a piece of any metal anywhere in the loop and it has no
effect IF it is all at the same temperature, so you don't need to know
its thermoelectric properties.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Spehro Pefhany]

On Wed, 23 Mar 2005 10:45:14 +0000 (GMT), the renowned Tony Williams
<tonyw@ledelec.demon.co.uk> wrote:

In article <1111573655.984085.237720@f14g2000cwb.googlegroups.com>,
spehro@gmail.com> wrote:

Spehro Pefhany (temporararily in Li4 jiang1)

Two questions Speff.....

Hi, Tony. Google seems to have eaten my reply, so here's a short and
late version.

Q1. Where is Li4 jiang1?

About 10 hours north-west of Kunming by road (Southwest China).
Beautiful place with great food.

Q2. How on earth did you get early parole from sed?

Just a temporary break, but if I end up going to Yunnan University for
a semester in the fall, I'll probably have to take a longer one-- high
speed ADSL internet connections there generally are barely usable, IME
(though domestic sites seem to load quite fast). At 1500+ messages a
week, SED is pretty high traffic.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Winfield Hill]

Spehro Pefhany wrote...

if I end up going to Yunnan University for a semester in the fall...

What would you be taking, or teaching, perhaps?


--
Thanks,
- Win

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote (in
<d2mctl02i33@drn.newsguy.com&gt about 'Thermocouple Problem...', on Sat,
2 Apr 2005:

Spehro Pefhany wrote...

if I end up going to Yunnan University for a semester in the fall...

What would you be taking, or teaching, perhaps?


Yunnan is famous for its ham.....(;-)

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Spehro Pefhany]

On Sat, 2 Apr 2005 17:15:20 +0100, the renowned John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Winfield Hill
hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote (in
d2mctl02i33@drn.newsguy.com&gt about 'Thermocouple Problem...', on Sat,
2 Apr 2005:
Spehro Pefhany wrote...

if I end up going to Yunnan University for a semester in the fall...

What would you be taking, or teaching, perhaps?

A set of rather intensive courses in Mandarin mainly. I know a fellow
who taught some courses in China in a very esoteric branch of control
theory. He was amazed at the size of the audiences for his lectures-
and that they all seemed to be nodding appropriately, indicating at
least some level of understanding.

Yunnan is famous for its ham.....(;-)

Yes, it is. How did you come in contact with it? You see the
prosciutto-like hams all over the place (in high end venues). Like the
Parma version, it is not cheap. I didn't try it. Their coffee is
pretty decent, they make an okay sherry-like red wine and apparently
their tobacco is good. Interesting pictographs among the Naxi
minority. BTW, the level of abundance in Chinese supermarkets has
increased tremendously. Imagine having 5 different kinds of cooked
chicken feet to choose from! Gasoline is roughly at US prices at the
pump (about Y4.5 per litre).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Spehro Pefhany]

On Sat, 02 Apr 2005 13:33:51 -0500, the renowned Mike Monett
<no@spam.com> wrote:

Speff, that would be great if you can take class there. You might do
better than you think for web access. I do all my surfing with Win 3.1 on
28.8k dialup as this is available in most parts of the world. Many
hotels offer dialup connections.

For example, I used the high speed connections right at the main
high-rise telco office in central Kunming (a fair-sized city and
provincial capital)-- plenty fast for local sites such I'm familiar
with such as elong or Google China (these were demos for selling ADSL
to locals), but extremely slow to essentially non functional (half an
hour and the home page had not loaded) for *some* Western sites that I
use often. Some DNS problems as well. This problem does not exist a
mere two hours flight away in Hong Kong or up in Beijing. It might be
possible to improve things radically by using a proxy, I'm told. It
appears to be some kind of technical issue, I had no problems with my
usual news sites (Reuters, CNN, NYT, Bloomberg, London FT, etc.)
Internet cafes used by locals are typically uncontrolled (you don't
have to show photo ID like you would typically have to in North
America, just hand over a bit of money and sit down with 100 other
people, many of the males chain-smoking, and mostly playing online
games, chatting, watching movies and doing other time-wasting
entertainment stuff.

The foreigner's residence building (a high-rise) on the campus is set
up for ADSL and cable modems don't seem to be an option. Dial-up (ugh)
would be, but I'm not sure what options are available for ISPs. I used
a business center dial-up last time that was acceptably fast, but of
course business centers grossly overcharge for their services (about
30 times the going rate outside-- 1Y per minute vs. 2Y per hour).
Another I used this time (ADSL) was identical in problems but 30:1 in
price.

It's plenty fast enough. I have no problems keeping up with news sites,
slashdot, and all the newsgroups and mailing lists. Very often the
connect time dominates. Most of the time, I turn images off. The eye
candy adds little to the information content. The only problem is Acrobat
files. Newer formats can be unreadable.

My secure webmail works about as badly with text-only or with frames
and graphics-- part of the problem seems to be related to the secure
connection rather than the raw amount of information transfer. It
hasn't gotten much better in this area of the country from a couple of
years ago, so I don't think it's just a temporary glitch-- there
really are some issues with the high speed connections from "there" to
"here".

Maybe NSA's computers are faster at breaking the encryption than the
Chinese government ones. ;-)

The old software is immune to viruses and trojans. Yes, it crashes
regularly, but reloading the offending program ususally fixes it. I never
have to reformat and reinstall. All the config files are plain ascii text
and easy to modify. I never have to mess with the registry.

Good luck with your plans!
Mike Monett

Thanks!


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Ken Smith]

In article <je1u41pgug9pk9hbvvvkvogbbcrv2u2ag2@4ax.com>,
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:
[...]

Maybe NSA's computers are faster at breaking the encryption than the
Chinese government ones. ;-)

... or: The Chinese have to pick the lock and the NSA already has a key.

[....]

The old software is immune to viruses and trojans. Yes, it crashes
regularly, but reloading the offending program ususally fixes it. I never
have to reformat and reinstall. All the config files are plain ascii text
and easy to modify. I never have to mess with the registry.

The world owuld be a lot safer if everyone used "pine" for their e-mail.

--
--
kensmith@rahul.net forging knowledge