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"On" sensor for wood gun

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by emesine, May 20, 2010.

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  1. emesine

    emesine Member

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    In order to get my system to run right, I really need a probe, sensor, etc. that will tell me if my boiler is firing, or if it has burned out. This may sound simple, but I am having difficulties. With the way I have designed things, my storage tank will keep my boiler warm even after it is out of wood, so a simple temp sensor won't work. Does anyone have any ideas?

    1. Oxygen sensor in the flue? Don't know much about how these work.

    2. Delta T thermostat that measures the boiler temp as compared to the water entering the boiler? Theoretically, the boiler should always be hotter than the water entering it, as long as it is firing.

    3. High temp thermocouple somewhere in the firebox (sounds like a serious pain).

    Any other ideas? I'm stumped!

    Andrew

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  2. Nofossil

    Nofossil Moderator Emeritus

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    I use a thermocouple in the flue.
  3. emesine

    emesine Member

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    At times I expect my boiler to idle, but I don't want that to be an "off" signal. I want it to remain on as long as there is a charge of wood in the boiler..... On the other hand, I certainly see benefits to a sensor in that area.... It would be very simple to do. Probably a thermocouple taped to the outside of the flue under insulation would do it.

    Specifically what thermocouple do you use?

    Andrew
  4. Nofossil

    Nofossil Moderator Emeritus

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    It's a bit too simplistic to say 'I use a thermocouple' though it's certainly true. I use an Omega stainless jacketed type K. I also use combustion temp (inconel jacketed type K), outlet water temp, and some timer logic to deduce what phase the fire is in.
  5. jebatty

    jebatty Minister of Fire

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    Tarm uses a sensor in the smoke box (area above the hx tubes before the gases exit to the flue). I have mine set for about 210F. When temp drops to this point, the boiler draft fan shuts down, as the wood is burned out.
  6. pybyr

    pybyr Minister of Fire

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    Jim-- is there some sort of on-timer function that the Tarm initiates at the start of a burn to override the sensor that you mention for the time that the fan needs to run when a new fire is not yet up to temperature? [I still need to plan and build my controls for the overall system (the Econoburn has a good internal controller for the fan, but it does not include provision for shutting the fan off after the fire has gone completely out) and I'm definitely open to 'imitation as flattery' when I find simple and effective ways of doing things]. Also, is the probe that you mention on your Tarm solo actually inserted into the smoke chamber, or is it in some kind of "well" to shield the probe from combustion temperatures and chemicals?

    Thanks
  7. jebatty

    jebatty Minister of Fire

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    I would describe it a manual reset low-limit control. On a cold start, I press a reset momentary contact switch which allows the fan to come on for fire starting and burn. Temperature rises, and then on burn out as temperature falls to the setting, the fan goes off. A bulb is inserted into the smoke chamber inside of a metal tube, open ended, so no direct contact with hot gases exiting from the hx tubes.
  8. emesine

    emesine Member

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    I can get a thermistor (resistor that changes based on temperature) for about $1, and it is rated to 300C. That sounds like it would work pretty well to me.

    Andrew
  9. Nofossil

    Nofossil Moderator Emeritus

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    I originally used thermistors for pretty near everything, and I still use them even now. They're tiny, inexpensive and provide a strong signal that can be read with very low noise over very long wires. They do have a few issues:

    1) They're HIGHLY non-linear. That has two consequences. First, you have to linearize them in some way (usually software) if you want the readings to mean something. Second, it means that they have great resolution at low temperatures, but relatively poor resolution at higher temperatures.

    2) They're very sensitive to moisture and/or condensation. You have to package them carefully and protect them or they'll fail more easily than you might think.

    3) They change their response with age, especially at higher temperatures. If you want real accuracy, you'll need to recalibrate them periodically for the first year or two.

    I've been using semiconductor sensors a lot lately - the MCP700 and MCP701 as well as the venerable LM61. They provide a clean and linear signal in the 10 to 20 mv/degree C range and work over a wide range of temps. Not high enough for flue temp, though :-(
  10. emesine

    emesine Member

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    My favorite temp sensor IC is the 234 or 235. It has a current output, so it is reliable across any length of wire, bad connection, whatever. Again, though -60C to 100C.

    Can you recommend an off-the-shelf thermocouple that would work well in a flue? I have worked with copper-constantan thermocouples in the past.

    Andrew
  11. Nofossil

    Nofossil Moderator Emeritus

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    I've had good luck with the Omega Type K thermocouple probes with the stainless jacket. You'll need a thermocouple amplifier near the thermocouple, as the signal is measured in microvolts. Omega makes a little battery powered one for $100 or so (I don't remember exactly). If you had an NFCS, I make a two channel thermocouple amplifier for it as well.
  12. rwh442

    rwh442 Member

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    The type K thermocouple with a stainless jacket placed in the stack is the way to go as recommended previously.

    Check Ebay for a "temperature controller" and you will find all you need.

    You're local so PM me if you want to see my panel meters and thermocouples. I'd like to see that Wood Gun too!

    Rob
  13. emesine

    emesine Member

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    Thanks, it looks like I can find something useful there. I saw a few items available that looked promising.

    I put water in my system for the first time today. Ah, what a proud and wonderful moment. I powered up the mixing valve and SR506, turned my thermostats on, and plugged all the holes. With my daughter standing proudly at my side, I opened the valve to allow water to flow into the system...... and enjoyed a nice shower in the spray from a faulty solder in the main line. Ah, back to the drawing board. I thought my plumber had pressure tested it, but I guess not. This one is going to be particularly fun to fix, as it is in a low spot on the line and will be almost impossible to get the water out of it.

    Rob, you're welcome anytime to check out the system. Hopefully it will be up and running in the next few weeks.

    Andrew
  14. emesine

    emesine Member

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    OK.

    I did a little ebay window shopping. I need something VERY simple. The controller I need will:

    1. Close a 110VAC relay when the temperature at my thermocouple is GREATER than about 200F flue temperature at the K type thermocouple.

    2. Open a 110VAC relay when the temperature at my thermocouple is LESS than 200F (meaning the fire has gone out).

    Will the TET612 coltroller do this?


    http://cgi.ebay.com/Dual-Digital-F-...s=63&clkid=8349563525961254825#ht_2047wt_1137
  15. rwh442

    rwh442 Member

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    That controller looks like it has two alarm outputs (contacts) as well as the main PID solid state relay control. It should do what you want plus some.

    You will want to add some hysteresis to your alarm so you don't get hundreds of them around 200F. For instance, have your alarm go at 200F but not off again until the flue is at 250F. Most controllers have that functionality - it's just a matter of figuring out what they call it in the manual!
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