K-type thermocouple

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I was attempting to verify my oven thermometer using a k-type
thermocouple attached to my multimeter.  Putting the probe into the
oven, the displayed temperature quickly rose to match my oven
thermometer, then slowly kept climbing until it was well over 100
degrees above the thermometer.  I know the oven wasn't really that
hot.  I'm not certain I'm using the thermocouple correctly.  Does
having a significant length of it exposed to the temperature affect
its reading?

Thanks

Re: K-type thermocouple
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     According to the thermocouple theory at
http://www.capgo.com/Resources/Temperature/Thermocouple/Thermocouple.html
  it shouldn't make any difference if a fair bit of the thermocouple
wire's exposed to the high temperature.
     Does it correctly read 100C when you measure the temperature of
boiling water?



Re: K-type thermocouple

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...was the sensor in the middle of the oven or near the element / flame /
fan inlet?

I imagine the temp may be a lot higher near the element/flame/fan inlet than
say in the middle of the oven.

The boiling water test suggested by Bob is a good idea, also maybe iceblocks
in water continuously stirred should give close to 0C.





Re: K-type thermocouple
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Or hold it tightly in your hand, should be about 32 degrees C.

Re: K-type thermocouple

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than
iceblocks


A crushed ice "slushie" is the usual method for zero C. Ice blocks in water
will be "close" as you suggest.
In both cases the water should be as pure as possible.

However calibrating at 0c and 100C, will not guarantee accuracy at 300C oven
temperature.
In any case the position of the sensor in the oven is likely to be the
biggest problem. Shielding from any direct radiated heat may help.

MrT.



Re: K-type thermocouple

<<Shielding from any direct radiated heat may help. >>

This may have been an issue.  The oven thermometer is shiny metal, but
the coating on the wire, other than the tip, was yellow.

Re: K-type thermocouple



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Would indeed have to be continuously stirred. Ice itself may well be much below
0C.

Graham



Re: K-type thermocouple
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Grahams right (for once) but if the bath is continuously stirred so
that the water is moving all the way through the ice (or a least
through the ice in the immediate vicinity of the sensor) and the water
is pure (or at least de-ionised) the temperature is 0C plus or minus a
few thousandths of a degree Celcius/Kelvin.

If you dig through the American National Bureau of Standards web pages
you could find a more detailed exposition (I did the last time I
looked but that was about five years ago).

--
Bill Sloman, Nijmegen

Re: K-type thermocouple

<<...was the sensor in the middle of the oven or near the element /
flame /
fan inlet? I imagine the temp may be a lot higher near the element/
flame/fan inlet than
say in the middle of the oven. >>

I wanted the sensor near where the oven thermometer was, so that was
in the middle of the oven.  I made sure the wire didn't run any closer
to the element than the oven thermometer.  However, the meter was
obviously outside the oven, so the wire had to run through the door,
where it was in contact with the metal of the oven.  This perhaps
affected the temperature measured.

Thanks

Re: K-type thermocouple



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That's how I ALWAYS calibrate them.

Graham



Re: K-type thermocouple
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Water only boils at 100C when the atmospheric pressure is 101.3kPa. If
you are above sea level, water will boil at a lower temperature. At
1000m above sea level, water boils at only approx 95C.

David

Re: K-type thermocouple



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Yes thank you. It's close enough for me though on an average day at close
to sea level.

Graham


Re: K-type thermocouple

<< Does it correctly read 100C when you measure the temperature of
boiling water?>>

I didn't use boiling water, but I did use an ice cube.  It was within
1 degree.  I'm fairly convinced that it reads accurately, at least
when using the tip, but I wasn't sure what effect exposing the whole
length of wire to the temperature being measured would have.

I'll read up on the theory, thanks.

Re: K-type thermocouple



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I have found K-type thermocouples to be very reliable. Even with those
cheap Asian multimeters. Maybe a degree or two out at most, but good
enough for general stuff.

I commend them to you.

Graham


Re: K-type thermocouple

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After all the previous responses, FWIW... It's likely that the reading
you're getting corresponds to the actual temperature of the t/c. The main
error is likely to be radiative heating of the sensor, if you can shield it
from that you're likely to get a good reading at the measurement point you
choose. Oven thermostats and controllers  are likely to be pretty rough, and
as has already been pointed out, the temperature's unlikely to be uniform
throughout the oven.

Why worry if it cooks OK?



Re: K-type thermocouple
<<Why worry if it cooks OK? >>

Because it doesn't.  :-)



Re: K-type thermocouple

"Taylor"

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** Gas or electric ?

 Element at the top or bottom ?


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**  Bead or metal rod ??

Does help a tad to be specific.

I suppose it's completely  out of the question for you to reveal the make
and model of your  " probe ".

Why spoil a good troll  - eh  ?




....   Phil






Re: K-type thermocouple
<<Gas or electric ?>>

Electric.

<<Element at the top or bottom ?>>

Bottom.

<<Bead or metal rod ??>>

Bead.

<<I suppose it's completely  out of the question for you to reveal the
make
and model of your  " probe ".>>

Whatever came with a MeterMan by Wavetek.  Something like

http://www.tequipment.net/Wavetek34XR.html

<<Why spoil a good troll  - eh  ?>>

I don't think you're a troll.  ;-)



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