I like the direction your thoughts are headed. Control of the reaction zone begins with accurately measuring it’s temperature.
Another way might be to use the effect of electric resistance in that stainless wire. Resistance increases with temperature.
Instead of stainless wire perhaps we could use the type of wire used for making heating elements: Kanthal, or nichrome.
I like the idea of several loops because it will give a more accurate measurement of the total amount of thermal energy in the reaction zone.
I suspect that the oxide layer that forms on the surface of Kanthal, or nichrome wire will act as electric insulation so that contact with charcoal, which is electrically conductive, will not change the reading when electric resistance is measured.
Rindert
Been thinking more about the Kanthal wire. I’m thinking maybe stainless would be better. I wish I knew of some sort of insulation that would handle gasifier temps, ~1000C.
Rindert
Ceramic fiber insulation will handle the high heat. Some of the members have used in their firetube treated with waterglass. Al Flick used it in his gasifier. I used it for the shield on my gasifier inside the lower housing. You need to use a mask, gloves, and hasmat coveralls when cutting or handling it. It is nasty stuff. I wrap mine in a welding blanket and seal it up.
Perlite is starting to soften up a bit at 1000C but should work and isn’t dangerous.
Some cast refractory hard face on the hot inside and you should be good. Waterglass (Sodium Silicate) can be used as a binder for the perlite. 1000c is pushing it for waterglass as well but a couple of inches into the perlite matrix you’ll be well below those temps. It should all hold together and keep your lungs healthy.
If you are trying to measure temperatures, do you really want insulation? while some heat gets through, and you can measure it, it adds a bit of complexity to the calculation based on the thickness of the insulation, and you will have to calibrate each individual piece.
Silicon Carbide can withstand higher temperatures. I think they increased manufacturing capacity recently so I suspect the price is coming down. If you buy say a brick, you probably need to be careful, it doesn’t have a binder in it that breaks down at a lower temperature. It transfers heat, you can put the more sensitive wire on the backside of a brick, and get a decent thermal reading.
I didn’t check to see if it has a varying thermal resistance that could be measured directly.
That wouldn’t really be the same thing. @abreaks talked about twelve or eighteen loops of cheap stainless. I imagined those loops surrounding the entire reaction zone. This would give a good indication of the total thermal energy contained in the hearth and would give a good indication of how much water or EGR could be added. Right now I’m imagining an inch of ceramic wool against the shell of the gasifier. Inside this the 12-18 loops of stainless wire, not touching each other anywhere. Then a quarter inch of ceramic wool to cover the stainless wire and isolate it from the charcoal. The inner surface of the ceramic wool would be hardened with water glass cured with a torch.
Rindert
Insulation by definition does not transfer heat so it skews the readings depending on thickness, and really time which isn’t what you really want. If you turn on your oven to high, touch the door, it is cold for a while, then after a while it might warm up but still safe to touch. You have to calculate the rate of change over time.
The wire directly inside is a good cheap way to test the concept, and you can use standard resistance values.
If you use a conductive layer, you can protect the wire or a thermocouple, and still get an accurate reading.
Now that I think about it, it transfers heat if there was a ring of it, it would even out the heat in the reaction zone. I wonder how that would affect the gasification.
The main caveat to all this is ash is a refractory material any build up of ash is going to affect the readings…
If you have hot zone—insulation—thermocouple—more insulation (including heat-ex and whatever else)—outside world, you have the thermal equivalent of an electrical voltage divider. If things don’t change (and I bet they will), if you know the outside temperature, the thermocouple temperature, and the “R-value” of the two insulating layers, then you can calculate the hot zone temperature. Unfortunately, you will surely have some thermal capacitance (materials storing and releasing heat as they change temperature), so you really have the thermal equivalent of an electrical “R-C” circuit, with time delays. If you know the different characteristics of the thermal circuit, a microcontroller might be able to anticipate the internal temperature ahead of time.
From reading the DOW, I think most folks that have thermocouples place them a ways from the hot zone, and make the temperature divider calculations bases on experience. Probably good to move toward the reduction zone where there’s less oxygen to shorten thermocouple life.
It would be interesting to hear from the folks with thermocouples, instead of my guesses
I think you’re talking about electrical resistance that varies with temperature? or a thermal R-value that changes with temperture (less likely) ?
the electrical resistance. A thermocouple is just wire that varies it’s resistance in a known, measurable way because of temperature. Silicone dioxide conducts electric and heat, but it might be too good of a conductor of electric to get enough of a variance to get a measurement. They do use it as the outside protective layer of a thermocouple because it conducts heat and can handle high furnace temperatures.
Not to get pedantic but a true thermocouple isn’t measuring resistance. It uses a voltage differential between two different and bonded metals. The voltage varies with temperature. The signal is VERY weak and requires a simple but specialized high impedance amplifier to read. Some/better multi-meters have that amplifier built in. Thermocouples are very accurate and different types cover different temperature ranges, IE J-type, K-type, etc.
Silicon dioxide is generally considered a good electrical insulator. Compared with metal it is certainly an insulator. Its temperature conductivity depends on its form. It can be good (quartz crystal) or bad (fiberglass).
There are lots of ways to measure temperature. The best one is the one that works for you.
At the hottest gasifier temperatures… optical methods might be best because the “color” of the glowing hot zone correlates with temperature and optical methods can keep the sensor out of the heat.
I like to use my eye (optical sensor LOL) to measure temperature when I am doing foundry and black smith work, because my brain can look past chunks of slag and dirt on the surface and get an accurate reading. However, if I take apart one of those pistol type thermometers, aim the sensor down the nozzle, and somehow rig the output to control water or EGR I don’t think I’ll get a good result. This is because ashes build up around the nozzle over time and keep it cooler.
Rindert
I am quite okay with you being pendantic and correcting me. I should be called out for it.
A thermocouple works on the principle of the Seebeck effect (essentially a TEG module), where a voltage is generated when two dissimilar metal wires are joined at one end and exposed to a temperature difference. They usually have compensation for the cold junction. Then as you say gets amplified and run through an ADC (analog to digital converter). I did not see a standard TEG module that could handle 1000C.
Thermistors and resistance temperature detectors (RTDs) do use resistance to measure temperature.Neither go up to the temperature range needed. Neither Stainless steel wire or nichrome will work. They would need to be insulated.
And as far as a multimeter. The digital multimeters and oscilloscopes use ADCs and have amplifiers for millivolt readings. It isn’t free but pretty close to add it to a multimeter.
You are right about silicon dioxide. I meant Silicon Carbide which is a good thermal and electric conductor. Again my bad.
that was the other thought is like a laser or infrared sensor, if it is behind the Silicon carbide, it isn’t exposed to the tars, gases, etc. but it will still be exposed to heat.
You might need the insulation and calculations just to keep the cost down. The trickier part is making sure the theoretical matches the real world. However, since it is a one-off system, it can be relative, you don’t need to have an accurate temperature, you just need to know when it hits a certain point how much water to add. To me the larger obstacle is the time delay.
K-Type thermocouples can reach 1000C but not that much higher. K type are not exotic or very expensive. N-type is another alternative with a similar heat limit and cost.
