Ktype thermocouple used to measure the hot air temperature near a flange. Air started leaking from the flange gasket and insulation pads on the air piping deflected the leaking air at such an angle that air started hitting the T/C head JB. Now the air temperature started increasing than the temperature value it was before air leakage through the flange. After the insulation pads were placed in such a way hot air was not touching the T/C head JB, temperature dropped to normal value.
Can anybody help me to understand the phenomenon of temperature rise of hot air than the actual when air touched the T/C head JB.

 

Looks to me like the Law of Intermediate metals went pear shaped on you with your thermocople and compensation cable connections to the terminal metal at different temperatures.

Gray

 

Very basic explanation. The "join" where the two pieces of thermocouple wire are welded together is irrelavant. It is only a junction and takes no further part in the measurement of temperature. The thermocouple/extension cable is excited by temperature and, due to the fact that there are two dissimilar metals involved, a voltage is generated along the whole length of the cable. The temperature controller/sensor then receives thia voltage and converts it to temperature.
If the head got hot, it has thermocouple wire inside. If the remainder of the wire was cooler than the the head, the thermocouple wire in the head would generate a voltage high enough to give you the reading you received. This was not a false reading as the head was at the temperature shown on your controller.
An RTD, by contrast, is a Resistance Temperature Device. The resistor is platinum and the controller measures resistanc to determine the temperature. The temperature measured is at the point where the resistor is placed.

 

Hi.
Keywords: Seebeck effect, Cold Junction Compensation
Expl.:bad CJC,overcompensation
On the basis of similar phenomenon the only method to display temperatures below -273grdC was developed

siju

 

I am not an expert, but i have this theory: if you are not using appropiate (compensated) wires to conect the K t/c to your indicating device, then you have a "second t/c" in the JB, since the materials could be different. So, you have the K t/c in series with this second "parasite t/c" in the JB. (speaking more accurately, you should have two parasites t/c, one in each JB terminal)

When hot air hit the JB, the parasites t/c generates an additional mV signal, and you observe an increase in reading.

(still you are using compensated cables, type K cables made well his job up to a determinated temp, then an error is introduced)

I hope somebody could give you not a theory but an explantation!

Regards.
Daniel M.

 

Dear all,

Normally, heating the Cold Junction result in decreased (displayed) temperature values.

Now, the theoretically Cold Junction you should have away from the temperature gradients. Also, the temperature of zone box (head) shouldn't have any influence.

Practically,there is a lot of imperfections and/or bugs (incorrect or reversed compensation cable, broken or inhomogenous wires, oxidized connections, shortcuts, incorrect electrical loading etc).

If-as 'abc' asserts- the (displayed) temperature INCREASED with presence of leakage, a most probable cause may be the forced COOLING effect by massive air entraitment.

szilagyi