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Author Topic: Thermocouple vs. Gas phase device  (Read 347 times)

nikol.todorova

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Thermocouple vs. Gas phase device
« on: December 03, 2020, 01:45:24 am »

Hello

I am struggling to understand why there is such a big difference between the temperature recorded from a thermocouple and the one from a gas phase devise.
I have a simple fire model of a 35000 kW fire. that burns in a 2000m² rum and I want to measure the temperature above it.
The target is to record a temperature of 260°C under the ceiling in order to burn a tent.

I use a gas face device to record the temperature under the roof and control the appearance of the roof plate. When the gas phase device reaches 260°C the roof plate disappears. I use a gas phase device, because I couldn't find a control that can uses thermocouples to control elements. The problem is that if I place a thermocouple on the same place as the gas phase devise, it gives a different, lower temperature. I would expect that the thermocouple will have a higher temperature, since it accounts also for the radiation from the fire. But it is exactly the opposite.

I have read the user manual and the tech. paper, but I can't understand why the difference between the devices is so big (around 80 to 150°C) for the different sensors.

Is there someone that can help me and explain me the difference.

I am sorry if there is another topic with the same problem. I just couldn't find it.

Thank you in advance for your help.

Regards,
Nikol
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elchanan

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Re: Thermocouple vs. Gas phase device
« Reply #1 on: December 03, 2020, 06:31:43 am »

Hello Nikol,
A gas phase device records the actual quantity (temperature in your case) calculated in a mesh cell.
A thermocouple is a device that measures temperature by changes in electrical current that is passed through a metal bead. FDS calculates the temperature of a thermocouple with heat transfer equations.
that is why a thermocouple does not give the same values as a gas phase device.
in steady state, the two curves should be close.
In your case, a difference of 80 degrees is significant. the only way I can see that happening is if the fire ramps up very quickly and the thermocouple is lagging at the beginning due to the dampening and delaying effect of the heat transfer through the bead.
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nikol.todorova

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Re: Thermocouple vs. Gas phase device
« Reply #2 on: December 04, 2020, 07:54:23 am »

Hello Elchanan

Thank you for your fast response.
I do have a fast developing fire with a factor of 0,047kW/s².
So it takes around 461s to reach the 35000kW.

Which device would you recommend to use when doing a simulation with a focus on temperature in the developing phase of the fire? Is it enough with a gas-phase device?
I am also looking at the temperature of steel beams, placed at the ceiling over and close to the fire, so i can conclude if they will be exposed to critical conditions.

Thank you in advance for your help.

Regards,
Nikol
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elchanan

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Re: Thermocouple vs. Gas phase device
« Reply #3 on: December 06, 2020, 01:16:52 am »

Hello Nikol,

the device you use depends on your needs.
for example, if we are talking about a door opening after a smoke detector goes off, you should use a smoke detector.
if you are interested in a window breaking due to temperature, use a solid phase device for the window (don't forget materials).
as a rule, start by defining the actual fire scenario and build your model accordingly.

for the steel beams you can use a solid phase device. also, in ooutput-boundary condotions, you can define visualization of the wall (or beam) temperature.
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