Is it Carbon

beeger

I had to fix the heater today because it was starting but not staying on. I called a friend (heating guy) to see what to do. He told me about a sensor that probably had build up on it and that was the problem. This sensor is a 1/16” diameter solid rod 4” long that is held directly in the natural gas flame. I took steel wool to clean off a very thin film on the rod. Works fine now.
What would the build up be made of? I am thinking Carbon because when Methane is burned it gives CO2,H20 and must be leaving Carbon atoms attatched to the metal rod.
It seems extrodenary to me that such a thin build up would stop heat transfer.I would like a bucket of this to paint my house!!!

pzona

Hmm, this seems strange. I know that diamond and graphite (two allotropes of carbon) are really poor insulators so this doesn't seem to make sense to say that it's just carbon. I can't think of what else it might be though. Maybe a different allotrope?

lightarrow

Diamond it's not a thermal insulator at all, actually, it's one of the best thermic conductors we know...
http://en.wikipedia.org/wiki/Thermal_conductivity
Graphite it's a good thermal conductor too.
http://physics.info/conduction/
Anyway, it's rather improbable the layer was one of those. Incomplete combustion of hydrocarbons leads to an enormous variety of chemicals, the solid part of which is generically called carbon; when in little particles is called soot; that material can have a very low thermal conductivity, even because it can be very porous.

beeger

OK,so Carbon probably not. Soot, yes but too general. To me it is like saying: Wet, as in; the serface is wet. Could be any liquid. Soot could be any solid,Right-Wrong?

So now, the Natural gas additives are more likely to cause the soot.

chemisttree

It is likely that the carbon acted as a very good IR absorber and the rod was actually hotter because of this rather than because of poor thermal conductivity. Similar reason that dark baking pans produce a darker baked product or crispier crusts.

beeger

Sorry I didn't explain myself too well, the rod was not getting the heat causing the sensor to turn off the gas even though it is directly in the flame. So it does not run the gas if there is no flame.

I am exploring Sulfites now,I am looking for the thermal conductivity.

russ_watters

(to chemisttree)....except that the sensor was actually in the flame, so radiation is irrelevant.

Here's an explanation that I found via google that matches the previous ones given: http://www.arnoldservice.com/furnace_flame_sensors.htm

beeger

Thanks Russ

Now I just have to paint my house with Dust and Dirt and I can keep it warm with a candle!! LOL! Wife will be happy she doesn't have to clean any more.
I am still going to look into this closer, if I can. I am going to find the componants of the Dust and Dirt and soot.

P.S. your much better searching Google than I!

chemisttree

Russ, I've seen that too. Do you really think that an extremely thin layer of carbonaceous material has that much insulating value? Can it keep a metal temp sensor cool enough to give a low range fault? Being in the flame is just where I would put an IR absorber to maximize its absorption. Remember that dark baking pans are actually in the oven.

Hmmm. Perhaps the sensor is a high temperature electrochemical sensor of some type like the O2 sensor in an engine. Fouling that would certainly give a bad reading. (edit: Not likely. Its reportedly a bimetallic strip)

Dr Gonzo

Carbon Aerogel is so insulating that one could put a cube of ice on top a 2mm thick slab and a blow torch on the bottom and not have the ice melt. so to say its made of carbon is not unimaginable.

lightarrow

When I wrote "carbon" I intended "coal" (or something like that) that is the black, solid material used to cook, for example.With "soot" I intended to say the very fine powder made of carbon and hydrocarbons (see coal) that comes from an incomplete combustion of hydrocarbons (as in the combustion of a candle).

IndieSci

Don't forget about the mercaptains... RSH... that are added to natural gas so we can smell gas leaks... this means that sulfur compounds such as sulfur, sulfate, etc could also be present...

russ_watters

Well note, the temperature of the flame is really hot so it doesn't take very much loss of conductivity to drop the temperature of the sensor by a lot. I'm considering an olympic run...

russ_watters

I don't know how thin it is, but if the sensor needs to get really hot to function, maybe it doesn't take much loss of conductivity. Dark pans in the oven don't get extra hot because they absorb heat from the air in the oven - the air in the oven isn't glowing in the infrared, it is transparent. The pans get extra hot because they are absorbing radiant heat from the heating element at the bottom of the oven. I thought of that - does the O2 sensor work by running electricity through the ionized flame?

But yeah, according to the goggling I did, the flame sensors are either photosensors (newer ones) or bimetallic strips (older ones).

beeger

Found this on my google search yesterday:

The bi metallic strip will output a very small voltage in the form of a mille volt, read as ".001 volts" or "1mv." When the tube enclosed strips come in contact with a heat source a voltage is generated. The greater the heat, the more voltage will be generated by these two electrically opposing pieces of metal.

This is the type I am talking about.

beeger

Updating my quest!
After spending a couple weeks of intensive searching the web for info I keep finding more and more directions to go. I am seeing that it may very well be Carbon build up(or soot) in the form of nano tubes or buckyballs,one area that sounds interesting is dopeyballs in which carbon atoms are replaced by other atoms.
Here is a link http://www.science.org.au/nova/024/024key.htm
Most of what I am finding is a little on the old side. Anyone have some links to some newer info?