Flame detection

IIRC the gas fridge I used some 30 years ago it's actually a thermal system. Bulb -> capillary -> diaphragm operated needle valve. System filled with some suitable fluid. As the bulb heats up the fluid expands, pushes on the diaphragm, diaphragm opens the valve. A push button to keep the valve open until bulb heats up. Sometimes the bulb may be omitted and the end of the capillary wound a turn or two around the flame.

- YD.

--
Remove HAT if replying by mail.

Those blue flames emit quite an amount of UV (hydrogen line). Don't know if there are any cheap and easy-to-get sensors for that. I have a sensor from the sixties I grabbed off a furnace controller. Gas-filled bulb with two electrodes, runs at 150V, operates like a geiger tube. Doesn't sense sunlight but makes quite a storm in the speaker if you flick your bic close to it.

- YD.

--
Remove HAT if replying by mail.

I used to work for Honeywell. The three methods usually employed for flame proving are: thermocouple, flame rectification and UV detection. Gas burners with no electric power available (e.g. water heaters) use thermocouples to enable the main gas valve. Small burners with electric spark ignition usually use flame rectification. Larger industrial burners use UV detection.

Your remote "don't modify the burner" app suggests either a simple fiber optic solution making the flame directly visible, or a UV detector. Another poster suggested a suitable sensor from Hamamatsu. If you can locate and obtain one of these, if you'd like to contact me by email I'd be willing to help you design a circuit to use it.

The datasheet may be seen at

This sensor will need a low-current source of about 325 VDC. No problem, a simple flyback converter from 12VDC would suffice.

The requirement to inspect the flame color for proper operation really tips the balance in favor of a fiber optic approach. Although many of the other systems will work to indicate the presence if a flame, in the end, one still must peer through a poorly located inspection window.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Fast wine, loose cars, old women.

I am in agreement with Glen Walpert on the flame colour, or rather, lack of colour. Since we are talking refrigeration for caravans, we are talking bottled LPG, not town gas, and the correct flame colour for LPG is blue with no yellow tip. I believe there is a high degree of UV in this flame and it is possible that one of the InfraTec sensors with a UV filter might do the job. Somehow, I think they might be expensive.

In message , dated Mon, 11 Sep 2006, Ross Herbert writes

But if you can SEE it, it must emit VISIBLE light. Why bother with UV?

For extra light, put a tiny piece of chalk in the flame. Limelight!

--
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 article , Ross Herbert wrote: [...]

If it is U enough V, perhaps just looking for a leakage current from a bit of zinc plated metal would be enough. Sunlight will discharge an electrometer with a zinc plate attached.

--
--
kensmith@rahul.net   forging knowledge

If you are inserting something in the flame, insert two electrodes. Then you can detect the flame and listen to the radio.

--
--
kensmith@rahul.net   forging knowledge

The problem with visible light is EMI from the sun leaking in through the air inlet vent or the flue gas exit vent or cracks between fridge and housing. This is normally much greater than the amount of light emitted by the flame.

UV is the standard method of detecting flame presence in small oil burners here, and the UV flame failure sensors are available fairly cheaply but they are designed for larger flames (more UV). The tiny flame of a fridge might not put out more UV than leaks in from sunlight either.

Good one. This is why I frequently enjoy your posts - your lame humor is unsurpassed by any other regular poster. (DNA is in another humor category entirely, not a direct competitor.)

I think omeone else suggested a temp switch attached to the flue pipe, before I suggested one above it. Hard to beat a temp switch for simplicity and no power use when open, if one can be found that closes on temp rise a bit above max expected ambient and also opens upon cooling to maximim ambient (small dead-band). Attaching to the hottest part of the flue pipe should be trivial with a dab of epoxy and/or or a bit of bailing wire.

An effective visible + IR sensor can be made with a pair of identical thermistors glued to small bits of black metal sheet separated by an insulator. A comparator can indicate when the one facing the flame (from a reasonable distance so as not to block air flow) is hotter than the one facing away from it. This will be largely insensitive to solar EMI and variations in ambient temp.

Or just attach one of the thermistors to the stack and put the other nearby. 2 thermistors, 2 bridge completion resistors (slightly different for some offset), one cheap op-amp as comparator, one LED with series resistor - almost as simple as the switch, but sucks a bit of power when off. I think someone else suggested this one also, but I am not paying much attention to this thread and don't recall who.

Of course none of these eliminate the need to regularly inspect the appearance of the flame per the manufacturers instructions, especially when re-lit after being off. It only takes one dead bug, bug nest or dust ball to screw up combustion in these tiny burners, making the flame turn yellow and sooty :-).

Charged which way? Quite a feat if the UV is kicking out positrons. ;-)

Checking a table I see zinc has a work function that isn't particularly impressive;

magnesium would be more suitable, at least as bare metal. However, zinc oxide (present on the surface of zinc metal) is a known semiconductor (band gap 3.2eV, which would respond to UV and blue).

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Yes, it would be markedly less sensitive if charged positive.

I don't think you can keep magnesium bare. I think it tries to turn into Brucite when left on its own.

Basicly it applies its own sunblock. :>

--
--
kensmith@rahul.net   forging knowledge

Because LPG flame, according to the physicists, is in the UV range. It may be visible under certain circumstances or if other impurities exist. If it is pure LPG flame then I wouldn't want to gamble on it being visible under all circumstances.

LPG flame spectra was measured between 415 - 440nM which puts it in the UV range so it is either totally or near invisible. In the gas detection and flame detection industry (eg. LPG/LNG storage) General Monitors FL3101 is a UV only flame detector but it is recommended for LNG/LPG flame detection according to the spec sheet

In message , dated Tue, 12 Sep 2006, Ross Herbert writes

So the little window provided by the fridge makers so you can see the flame is not only inconveniently-sited but fundamentally useless anyway.

I rather doubt it.

--
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

Yes, I agree with that logic. They do put the window there for a reason. The major reason is that it is cheap, and provided that everything is perfect, a healthy individual with 20/20 eyesight should be able to see if there is a flame or not. A more sophisticated electronic method would add considerable cost to the refrigerator and it would seem that all manufacturers consider this is unwarranted.

I agree that even a blue flame is visible - provided that you can get your head into the right position - something which often is not possible to achieve in a caravan - and your eyesight is good enough to allow recognition that it may be alight.

For domestic use, such as in refrigerators, LPG is Propane which should burn with a blue flame, as shown in the blurb for this jewellers LPG gas torch

If incomplete combustion takes place, or soot is present, then there will be a yellow colour to the flame and it will be highly visible. It may be that manufacturers of gas refrigerators have worked out that after settling in the flame will start to become "sooty" and therefore visible.

Bivar, and probably many others, make flexible lightpipes which might do the job if the OP is feeling in an experimental mood.

In message , dated Tue, 12 Sep 2006, Ross Herbert writes

My understanding is that only a small amount of heat is required and a 'with air' (Bunsen) flame is too hot as well. So the flame is more like a candle flame. Certainly, the flame in the fridge in my friend's caravan was quite visible if you didn't try in bright daylight with nothing to create a shadow,

--
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

I don't remember where I saw it, possibly in a piece of equipment burning hydrogen (invisible flame). A simple loop of nichome (or platinum or niconel, or whatever) in the flame. Heat of the flame causes the wire to glow brightly. No glow, no flame. Very simple, cheap, easy to implement.

But a propane flame will have _some_ UV, it's just not _all_ UV, which a hydrogen flame almost is. a hydrogen flame is almost impossible to see, which is why they use the UV sensors. The UV from propane is just a lucky co-winky-dink. :-)

Cheers! Rich

Not Available

415-440 nm is visible - from middle or bluish violet to violetish blue.

Meanwhile, I have tried making fluorescent objects fluoresce from a torch using propane (which LPG largely is), with a blue flame with various fuel/air ratios ranging from carburizing flame to oxidizing flame. The results appeared to show at least as much visible light as UV, although there was some significant UV.

I took a look:

The UV sensitivity is for wavelengths 185-260 nm.

I don't think flames could make much of those wavelengths because of temperatures only around 2000 Kelvin, but apparently they have to make something there or else nobody would make a detector on that basis. My guess is that a detector of these wavelengths would be good for detecting flames because most other light sources don't produce/deliver such wavelengths. Although I would suspect such a detector to detect bigtime electric arcs, germicidal lamps, and broken HID lamps.

- Don Klipstein ( snipped-for-privacy@misty.com)