diy high temp ir fibre optic?

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uxhamby
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diy high temp ir fibre optic?

#1 Post by uxhamby » 22 Jan 2021, 19:22

Hi,

I have a few flame sensors at hand, like this.

I have extended the mounting of the actual ir sensor component, away from the pc board with a length of wire, to facilitate installation.

With this setup, I can easily & reliably detect a candle flame at 8" distance.

Image

I have an application for gas flame sensing, in a tight space, where there is a standing pilot flame and a main burner.


I want to sense the main burner flame, ignore the standing pilot and yet be far enough away from the flames that radiant heat doesn't damage the ir sensor itself.

This combo is proving impossible to satisfy in the small space of this burner. Within the combustion chamber, heat is excessive. Mounting the sensor outside the combustion area looking in through the access hole used for gas pipes and thermopile tubing etc, does not seem to get the sensor close enough to, or afford a sufficient view of the flame, even with max sensitivity dialled into the electronics.

Some kind of high temp diy (ie free / cheap) ir capable fibre optic solution appeals to me but is beyond my expertise. This way perhaps I could shorten the apparent distance between / improve the view of flame & sensor, through the access hole.

I note glass tubes filled with mineral oil are reputed to work as effective light pipes, but nothing like that comes easily to hand and I wonder about the oil flame combination and ir wavelengths for this method in general.

Any thoughts / suggestions / ideas come to mind?

Thanks,

Brian H.
Uxbridge Ont.

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Re: diy high temp ir fibre optic?

#2 Post by Ath » 22 Jan 2021, 20:42

Would it be possible to measure the temperature with a thermocouple, instead of IR light, or would that be too slow in detecting the on/off state?
/Ton (PayPal.me)

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Re: diy high temp ir fibre optic?

#3 Post by TD-er » 22 Jan 2021, 21:52

What is your goal with this monitoring?
Is it to make sure no gas will be left open if the flame is no longer burning? (or burning inefficient producing CO)

When playing with those IR thermometers, you may have noticed it is quite hard to get a good reading for temperature.
This is caused by reflections of IR light which we cannot see and thus are not aware of.
For example if some specular surface is in view of this IR thermometer, you may actually measure your own body heat instead of the object you try to measure.

When looking at the lenses of those thermometers, you may have noticed you cannot see anything through it yourself.
This is because the material used (e.g. germanium) is blocking visible light, but may pass IR light in the needed wave lengths.
So I guess finding optical fiber for these kinds of wave lengths is not going to be easy, nor cheap.

Do you know what wave length your sensor can process?

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uxhamby
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Re: diy high temp ir fibre optic?

#4 Post by uxhamby » 23 Jan 2021, 00:11

Hi,

& thank you both for your contributions.

To clarify, I don't want to measure heat, I just want to know when the flame is lit or unlit, just for the pure joy of accumulating, transmitting and managing the data over time. A thermocouple solution does not appeal to me, due to the subjectiveness of interpreting flame on / off from temperature data. (. . . and I don't have one.)

Lets be clear, this is busy work for an old locked down, retired guy with time on his hands, a few bits and pieces laying around, (more to be had from ebay / banggood etc) and a controls back ground.

I have a similar system working in a different burner, with more space and no standing pilot, where I have lots of space to mount the ir sensor in the nice cool up draft below the flame. The challenge here is to adapt to the space constraints of this smaller burner.

This main burner is a honking big flame and it really surprises me that, if I can sense a candle flame from 8" away in free space, that I can't detect the burner flame looking thru the service entry port. I am wondering if this is more of a field of view issue, rather than a line of sight issue. That rounded end on the ir sensor component is a lens after all.

Thanks!

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Re: diy high temp ir fibre optic?

#5 Post by TD-er » 23 Jan 2021, 00:22

"Line of sight" or "blocking wave length" ?
Remember most glass is blocking IR light we call "heat".
It is also a very good mirror for IR light (every shiny surface is), so you're probably looking at somewhere else regarding IR light.

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Re: diy high temp ir fibre optic?

#6 Post by uxhamby » 24 Jan 2021, 22:04

A side issue, but for interest sake:
Remember most glass is blocking IR light we call "heat".
I borrowed a sheet of glass from a small ~140mm x ~80mm picture frame. Thickness about 2 - 3 mm.

I lit my candle and set the ir sensor ~8" from the flame, at about its max sensing distance. That is close to the threshold where any more distance, and the flame was no longer detected. I surmise that in this setup, even a slight flame view degradation by whatever means, would analogously trigger a loss of flame detection.

I then held the glass between the flame and the ir sensor. I angled the glass backwards and forwards & side to side. I put the glass close to the flame and I put the glass close to the ir sensor and everywhere in between. It made no difference at all and the flame was continuously sensed at all times.

I conclude that glass is transparent to ir or whatever this sensor is detecting.

BTW, & fwiw, I believe that the ir sensor component itself is encapsulated in plastic (/ epoxy whatever) that is dark in colour, not because that is necessary for ir transmission but as an ir filter, to filter out visible light from triggering the sensor and admit only the ir.

thoughts?

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Re: diy high temp ir fibre optic?

#7 Post by TD-er » 24 Jan 2021, 22:34

My first thought is... what were you measuring?
Do you need to "point" the sensor to the flame to let it register it?
As an analogy, take the remote control of your TV as an example.
Unless the remote is really weak, using low batteries, etc. you don't need to point the remote to the TV to control it.
Just point it somewhere what can reflect the IR signal is enough.

The same for your flame.
If there is a (reflective) path possible around your glass, would the sensor still be able to see the flame?
If you place your sensor in a longer cylinder to create some "tunnel vision" (literally), is it still able to see through the glass to detect the flame?

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Re: diy high temp ir fibre optic?

#8 Post by uxhamby » 27 Feb 2021, 17:49

Would it be possible to measure the temperature with a thermocouple, instead of IR light, or would that be too slow in detecting the on/off state?
This has actually turned out to be the most practical & fruitful approach. I gave up on IR, after burning up a couple of the sensors with no better prospect in sight. I acquired a thermocouple (& MAX6675 amp), and mounted it in the combustion exhaust gas stream of the burner. Response time is surprisingly good, in both heat up and cool down phases.

When just the pilot is running, temp settles at ~50C. When the main burner kicks on, temp rapidly exceeds 100C and tops out at about 285C on an extended run, cooling down again quite quickly when the burner shuts off.

I put in an ESPEASY rule that:

Code: Select all

on Celcius#Corrected do 
  if %eventvalue% > 100
    if [var#2]>=[var#3] or [var#1]>=[var#2]
      Publish,EnzoTopic/%sysname%/,"%lcltime% T=%eventvalue%C"
    endif

    Let,3,[var#2]
    Let,2,[var#1]
    Let,1,%eventvalue%
  endif

endon
. . . .publishes the temp, when it is above 100C and stops publishing when 3 successive readings are each one lower than the previous, indicating that the flame has shut off.

Here is example published data, from the end of one run and the beginning of the next:

Code: Select all

1614428527005,2021-02-27_07:22:07,2021-02-27 07:22:05 T=279.71C
1614428541863,2021-02-27_07:22:21,2021-02-27 07:22:20 T=279.71C
1614428556962,2021-02-27_07:22:36,2021-02-27 07:22:35 T=279.71C
1614428571957,2021-02-27_07:22:51,2021-02-27 07:22:50 T=276.42C
1614428586868,2021-02-27_07:23:06,2021-02-27 07:23:05 T=257.84C

1614433686823,2021-02-27_08:48:06,2021-02-27 08:48:05 T=131.44C
1614433701798,2021-02-27_08:48:21,2021-02-27 08:48:20 T=154.66C
1614433716807,2021-02-27_08:48:36,2021-02-27 08:48:35 T=174.02C
1614433732059,2021-02-27_08:48:52,2021-02-27 08:48:50 T=192.02C
1614433746796,2021-02-27_08:49:06,2021-02-27 08:49:05 T=206.54C
Seems to work a treat! I plan to edit my rule to stop publishing the actual temperatures and just publish 'burner on' and 'burner off' messages, at the beginning and end of the temperature runs instead.

Hope this is of interest. Thanks,

Brian H.

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Re: diy high temp ir fibre optic?

#9 Post by Ath » 27 Feb 2021, 18:53

The good news here is that you solved your quest. Congrats!

Thanks for reporting the, very detailed, result. ;)
/Ton (PayPal.me)

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