Flame detection

In message , dated Fri, 8 Sep 2006, Ross Herbert writes

It is positively illegal.

The OP wants only to be able to see if the flame is on without using the inaccessible viewing window. He's told us that there IS a flame-failure device (without realising what it was).

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John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Yes, I see that post now. I should have seen it earlier but didn't.

I can well understand the inconvenience of having to get yourself into a position to see through the viewing window in the cramped conditions often found in caravans. It would seem to me that the manufacturers of these appliances should have thought to add the convenience of a visual display of some kind without having to contort one's body, even if it had to be battery powered.

While the idea of using fibre-optics would be an ideal passive method it might be difficult to see because the normal flame is blue with no yellow tip. It would be better to use the output from the thermocouple via some isolated interface to achieve the desired output if possible, but that requires a modification of the electronics and would be illegal.

In message , dated Fri, 8 Sep 2006, Ross Herbert writes

Not in this case; the flame is a 'no air' flame, yellow and a bit smoky.

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John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Basicaly there are three (or maybe more) easy flame detection circuits:

1. the optical methode which is susceptible to the flame wavelenght, I don't recommend it for standard methane (even buthane) gases, because the maximum peak response must be near UV and all semiconductor photodiodes have a poor response in that area (beside the dark current problem which is increasing with the temperature, so the sensor must be mounted at some distance from the flame)
2. the thermocouple mechanism closing a low level relay (there are plenty on the market) but has quite long response time (could be

20seconds or more) and need a also mechanichal push...

1. ionisation detector, that's the easiest one because it needs two wire in the flame and a high voltage (around 200V it's enoug) supply. It require only a simple FET based comparator which could be hooked directly into a microcontroller or other logic device.

greetings, Vasile

A series of Electrolux 2/3-way van fridges had a "remote" flame viewing window on the front panel, but it still involved gymnastics to line the Mk1 eyeball up with its axis to actually determine anything, and then it was only possible under favourable light conditions.

I suspect therein lies the reason it didn't become a widespread feature.

I am grateful for the deluge of responses. John W was first to understand exactly what I am after and he has summarised it nicely above. I am not planning /any/ modification to the fridge, just proposing an add-on, that may not even be bolted to the fridge, that tells me whether a flame is there or not (so I know when to stop pressing the ignitor button!).

The flame is actually blue, there is a jet that controls the gas flow, an air input port and the flame sits on a gauze grid at the end of a J-shaped tube. The whole fridge back is enclosed so I doubt that I will have too much problem with ambient light interfering with the flame as primary light source.

I am now completely convinced that John's photo-transistor is the way to go but would value help with the actual circuit if anyone can spare the time.

Thanks again.

Peter

[snip]

OK, is this what I need

(top diagram)? That circuit says "led is on when transistor is dark". Do I swap Q1 and R1 to get "on when light"? Presumably a trimmer pot in place of R1 will let me tweak the behaviour?

Again, thoughts welcome.

Peter

IANAL but from my reading of the Gas Safety (Installation and Use) Regulations (the statutory instrument regulating gas work in the UK) installations in caravans for private use are NOT covered:

Nothing in these Regulations shall apply in relation to the supply of gas to, or anything done in respect of a gas fitting on - ... (e) a caravan used for touring otherwise than when hired out in the course of a business

From the instruction label inside my Dometic gas fridge:

IMPORTANT ... 2. Inspect the flame for colour, it should be blue throughout - no yellow or other discoloration. ...

A yellow smokey flame is a very bad idea because the soot will foul the heat exchanger. I expect you have seen a fridge with the burner air holes blocked, or a fridge designed by someone completely clueless; I have seen quite a few gas fridges and none were designed to operate with a yellow flame.

If you don't think the OP should mess with the burner, how about a thermistor an inch or so above the top of the stack, where it cannot possibly interfere with flue gas flow? Or possibly a temperature switch, if the flue gas temperature is always well above maximum expected ambient?

My Dometic has a light pipe with a prism on the end for flame observation, which works well and can be seen while standing up although the door must be opened as it is located inside next to the thermostat. The far end of the light pipe, which is basically a piece of 1/4" or so acrylic rod, is well back from the flame so as not to interfere with it.

In message , dated Fri, 8 Sep 2006, Glen Walpert writes

No, they used to have yellow flames, those running from town gas. I didn't take into account that this fridge runs from bottled gas; these fridges have a BLUE flame, not a COLOURLESS one which some posters thought. The visible light is easily detected with a phototransistor.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

In message , dated Fri, 8 Sep 2006, John Stumbles writes

Oh, that's all right then. The OP can blow himself up or gas his family without breaking the law.

Have you checked that there isn't ANOTHER regulation that applies?

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
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John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

I think that comprised a mirror on a flap which you flipped down to 45 degrees from the vertical so you could look down onto the mirror instead of squinting at floor level.

A review of our caravan (1978 Abbey Cambridge) made a point that the siting of the fridge at waist level was convenient from the POV of viewing the flame.

Our fridge is just 2-way (gas and 12V) but I've a transformer that runs the 12V heater off themains when we're on a hookup. It actually delivers about 18V which gives far better cooling than the gas does (or native 12V would do). However it weighs a ton and I'd love to replace it with a lightweight smps if I could get one at a reasonable price. If it were OK with 12V a low voltage lighting 'transformer' (actually SMPS) would be ideal but I'd need to replace the heater element with a lower R one to make that work :-(

It shouldn't be: suggests the air ways are blocked with fluff. Mine is blue (light a pilot light flame on an old-fashioned boiler) and I'd expect any other one to be too, since a yellow flame is liable to soot up everything.

Again, the old town gas fridges had a yellow flame with a blue bottom, like a candle flame. Minimal smoke. But the bottled gas ones do have a BLUE flame, not a colourless one.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Basically yes. Unfortunately that could be insensitive to "blue" flame (about 380-400nm wavelenght)

Do you plan to cook alwys in dark environement ? :)

Do I swap Q1 and

In place of R1 no. Instead of one of 10K maybe. Note there is no positive hysteresis on the comparator (no reaction between output and noninverting input). The missing of that will create an oscillation on the comparator output at some light levels. If you have a relay instead of the output LED, will make a lot of noise when switching....

Vasile

P.S. Bad ideea to play with your own electronics (mostly if you have low electronics skills) on such dangerous equipments. I believe you have some laws in England about using gases on cooking machines...like the rest of the Europe.

The practice of using town gas in caravans never made it to this side of the pond :-). Besides, what you sais was "Not in this case; the flame is a 'no air' flame, yellow and a bit smoky.", which is decidedly wrong even for town gas fired absorption refrigerators, where there may be a yellow tip on the blue flame even with proper air, but never all yellow or smokey.

BTW there is a nice simple diagram and explanation of how absorption refrigerators work at:

use a IR Photo detector.., with the dark lens.

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use a "klixon" type self-resetting thermal fuse place it somewhere that the flame will warm it.

it won't give immediate response but it will draw zero current when it's hot

Bye. Jasen

ammonia adsorber fridge... an interesting cycle that seemingly turns 'hot' into 'cold'.

Bye. Jasen