[xchcui]

Hi.
I have a standard thermocouple probe type-K
TP01 for air measurements(see photo)and the datasheet says
that it can measure until 400°C.
My question is:why does it limit to 400°C?
is it because the wire or wire's insulation can't handle temp.
above 400°C?(will it melt?),is it because that the junction can be damaged by the heat?or them all can actually stands higher temp
but the results won't be accurate?
Thanks.

[Borek]

Probably to some extent all of above. Plus it is quite possible the probe itself is soldered to the wires and can lose contact when the solder melts.

[xchcui]

Thanks Borek.
I asked this question since i was trying to measure
the air temperature that blow-out from an hot air gun.
The heat gun has two temp. options:300°C and 600°C.
Since my tp01 is not made to measure 600°C,i bought
a cheap TP02 probe(see photo)that made to measure until
750°C(at least,this is what was stated in the datasheet),
but the problem that when i was trying to measure the 600°C
option with the TP02,the reading didn't passed the ~400°C.
I though first,that the heat gun has a problem,but then i measured the 300°C
option with the TP02 and the TP01 and i found that,while the TP01
give me a reading of about ~330°C,the TP02 give me a hardly a reading of
220°C.What are you thinking?is this a problem with the TP02 probe?

[Borek]

a cheap TP02 probe

Probably you got what you paid for.

Try to calibrate both probes for several different temperatures to see if they show similar results. For absolute measurements 0°C and 100°C are good calibration points as they can be easily and quite accurately reproduced using just water and ice. That can already tell you if the probes are worth anything.

[xchcui]

The first thing that i did was testing the tp02 on ice and boiling water.
It gave me a correct reading for this test,but i bought this probe for
temp. above 400°C not 0-100°C.Why could be the reason that it can't
measure correct even on temperatures around 300°C(gives ~100°C less than
real temperature),while it made for temp. till 750°C?
BTW,the TP01 is my old probe and everything is fine with it,
it just wasn'tt made for temp. above 400°C.
My issue is with the new TP02 that i bought.

[Borek]

Absolutely everything can be wrong, starting with the probe (if it is cheap, it is probably of low quality), through the meter, up to the measurement technique. No way for us to guess what is wrong, the only thing you can do is to test the probe in a wide range of temperatures comparing the results with those measured with another probes.

[xchcui]

No need to guess.I assume that i need to clarify my intention and
add what i knew from the start.
The problem is in the TP02 probe,nothing else.
The TP02 is bad quality,it is not accurate at all
at temp. above 100°c,though it states to be till 750°c.
What i was trying to figure out is what cause the thermocouple
probe(not just my specific probe)to be accurate at 0-100°C,while
above 100°c,it isn't?
I know that the temperature on the junction
of two different metal creates a voltage on the other side,which
translate to temperature on the gauge.So what gets wrong at higher
temperature?what there is on expensive probe that gives it the ability
to be accurate at higher temperatures,since i don't see any particular
different at their construction.

[xchcui]

Okay,i assume that you don't know the answer
and as i can see,no one here is expert in that area.
Anyway thanks for your response.

[xchcui]

BTW,Borek or any other nice member that can help.
Is there any material(that can be find in the house)with a
melting point somewhere between 500°C-590°?
I made a deep search in the web,but i didn't find one.
Most plastics melt below 400°C.Aluminium above 600°C
etc.
Does anyone know the anser?
Thanks.

[pcm81]

Here is the lowdown on thermocouples:

You have different types that produce different voltage for a given temperature, but also have different temperature ranges. Lower temp range TC has higher mV/deg C output, so can be more accurate.
Furthermore the mV to deg C conversion is only linear for a certain range of temperatures. So if the measuring device does not have the look up table built in, and rather just relies on slope/intercept linear function, the device is limited to certain range of temperatures.

Good quality TCs are welded to form the junction. Lower quality can be silver soldered, hence creating junction temperature limited by solder used.

If you google search thermocouple graph or thermocouple output voltage you will see a graph with K type shown up to about 1300 degrees C as a straight line. Well, that is false. None of the thermocouple outputs are linear function of temperature. They are close to being linear within certain temperature ranges, but none are truly linear. And so it becomes a balancing act for the manufacturer between range of temperatures a device can read and accuracy it can provide due to linearization errors.

Naturally things like insulation used on TC wire is also a limiting factor, especially in cheaper units that rely on insulation alone to construct the tc harness/probe.

[xchcui]

Here is the lowdown on thermocouples:

You have different types that produce different voltage for a given temperature, but also have different temperature ranges. Lower temp range TC has higher mV/deg C output, so can be more accurate.
Furthermore the mV to deg C conversion is only linear for a certain range of temperatures. So if the measuring device does not have the look up table built in, and rather just relies on slope/intercept linear function, the device is limited to certain range of temperatures.

Good quality TCs are welded to form the junction. Lower quality can be silver soldered, hence creating junction temperature limited by solder used.

If you google search thermocouple graph or thermocouple output voltage you will see a graph with K type shown up to about 1300 degrees C as a straight line. Well, that is false. None of the thermocouple outputs are linear function of temperature. They are close to being linear within certain temperature ranges, but none are truly linear. And so it becomes a balancing act for the manufacturer between range of temperatures a device can read and accuracy it can provide due to linearization errors.

Naturally things like insulation used on TC wire is also a limiting factor, especially in cheaper units that rely on insulation alone to construct the tc harness/probe.

Hi pcm81.
Your explanation has covered all the aspect of my question.
Thanks a lot for your answer.