K-type thermocouple

[Taylor]

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

 

[Bob Parker]

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

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?

 

[Den]

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?

....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.

 

[Davo]

...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.

Or hold it tightly in your hand, should be about 32 degrees C.

 

[Mr.T]

...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.

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.

 

[Eeyore]

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

That's how I ALWAYS calibrate them.

Graham

 

[Eeyore]

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

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

Graham

 

[Eeyore]

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?

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

 

[Taylor]

Bob Parker wrote:

<< 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.

 

[Taylor]

Den wrote:

<<...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

 

[Taylor]

Mr. T wrote:

<<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.

 

[Taylor]

<<Why worry if it cooks OK? >>

Because it doesn't.

 

[Phil Allison]

"Taylor"

I was attempting to verify my oven

** Gas or electric ?

Element at the top or bottom ?

Putting the probe

** 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

 

[David]

That's how I ALWAYS calibrate them.

Graham

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

 

[Taylor]

<<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. ;-)

 

[-=Spudley=-]

Den wrote:

<<...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.

The tip of the thermocouple is generally the only place that is effected by
heat, this is because it is the junction of the two dissimilar metals that
forms the thermocouple that produces the voltage that is being read by the
meter.

 

[Taylor]

-=Spudley=- wrote:

<<The tip of the thermocouple is generally the only place that is
effected by
heat, this is because it is the junction of the two dissimilar metals
that
forms the thermocouple that produces the voltage that is being read by
the
meter. >>

Ding! All makes sense now; that corresponds to the link that Bob
Parker showed on thermocouple theory.

Thanks!

 

[Eeyore]

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.

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

Graham

 

[Bob Parker]

-=Spudley=- wrote:

<<The tip of the thermocouple is generally the only place that is
effected by
heat, this is because it is the junction of the two dissimilar metals
that
forms the thermocouple that produces the voltage that is being read by
the
meter. >>

Ding! All makes sense now; that corresponds to the link that Bob
Parker showed on thermocouple theory.

Thanks!

From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The
misconception is that the hot junction is the source of the output
voltage. This is wrong. The voltage is generated across the length of
the wire. Hence, if the entire wire length is at the same temperature no
voltage would be generated. If this were not true we connect a
resistive load to a uniformly heated thermocouple inside an oven and use
additional heat from the resistor to make a perpetual motion machine of
the first kind.

The erroneous model also claims that junction voltages are generated at
the cold end between the special thermocouple wire and the copper
circuit, hence, a cold junction temperature measurement is required.
This concept is wrong. The cold-end temperature is the reference point
for measuring the temperature difference across the length of the
thermocouple circuit."

 

[Davo]

From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The
misconception is that the hot junction is the source of the output
voltage. This is wrong. The voltage is generated across the length of
the wire. Hence, if the entire wire length is at the same temperature no
voltage would be generated. If this were not true we connect a
resistive load to a uniformly heated thermocouple inside an oven and use
additional heat from the resistor to make a perpetual motion machine of
the first kind.

I don't understand this at all, it seems like a lot of gobbledegook.
The voltage generated is related to the difference between the hot and
cold junctions. In most modern measuring devices the cold junction is
compensated for within the device. A long thermocouple may have some
resistance but with modern high impedance measuring devices the effect
is minimal. The voltage is generated at the junctions.