Monday, April 28, 2014

Flame Sensor Update

This is a follow up to the post on Automated Home Brewing.  There were several good ideas in the comments to that post about how to create a flame sensor.  Some require an analog input, which I describe in this post.  I have experimented now with a few methods for detecting a flame and here is what I found.

Plasma Conductance

Cool Fact - flame is a plasma and will conduct current.  I tried using two wires stuck into the flame.  The flame has a fair amount of resistance.  One of the wires was connected to 5V.  The other was connected to a voltmeter.  There were dozens of mV present when the flame was on and zero when it was not.
I think this could be used with an analog input as a flame sensor.

Thermocouple 

Thermocouple
I got a type K thermocouple and tested that directly with a voltmeter.  It only show a couple of mV with a flame on the tip.  I added a simple 100X amplification circuit and connected that to the analog input.  Now I see a range around 300-400mV and see a clear increase when the flame is on it.  One problem I saw is that the sensor has a fair amount of mass, mostly stainless steel.  Once it heats up, it takes a while for it to cool off.  As long as there is a distinct increase when the flame comes on, this could also be used for the flame sensor.
100X Amplification Circuit for Thermocouple

Digital IR Flame Detector

IR Flame Detector

Sensors are available that detect the IR signature of a flame.  Most of these provide analog output only, but I found some that also include a digital output.  This is provided by a built in level compare circuit with an adjustable level.  Because this method is so simple to interface with, this is what I decided to use.  One big problem quickly became apparent - the sensor is triggered by incandescent lights and sunlight.   It worked fine under florescent lights.




I was successful in using this configuration for brewing.  The IR sensor works, but has the false trigger problem.  It works for me but won't work in many environments.  My plan is to rebuild the brewing interface with analog input capability and try one of the other methods.

IR sensor mounted on a small block of wood and placed under the flame.




5 comments:

  1. Where you able to get any further with this?

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    1. I haven't had time to rebuild my interface yet, but I have made progress testing with a breadboard. I re-used an analog input board that I built a couple of years ago to monitor the input from a thermocouple. With the proper gain setting on the amplifier circuit I was able to get good readings that allowed me to detect a flame. This method should be far more reliable that the IR sensor I was previously using.

      I am hoping to brew a batch of Scottish Ale this weekend, so I will have to finish the interface soon and mount the thermocouple on the burner. If that all goes as planned, I will have an update to this blog by next week.

      Note: I think the resistors are reversed on the amplifier circuit shown above. I will verify that when I am back at the workbench.

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  2. Hello! Your post is quite useful for me! I am in the process to build a signal conditioner but still I have doubts. I wonder following the tables of thermocouples since there it is a negative signal for mostly negative range and some frame of the positive temperature. How can I overcome it. Is the scheme provided enough for it. I am a nuke in electronics. So please help if possible. Thanks a lot! Happy new year!

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  3. Hello! Your post is quite useful for me! I am in the process to build a signal conditioner but still I have doubts. I wonder following the tables of thermocouples since there it is a negative signal for mostly negative range and some frame of the positive temperature. How can I overcome it. Is the scheme provided enough for it. I am a nuke in electronics. So please help if possible. Thanks a lot! Happy new year!

    ReplyDelete
    Replies
    1. You can use a table to compute thermocouple temperatures (interpolating between the table entries) but the most common method is to use a polynomial formula. For instance, to get the temperature from the millivolt output of a type-K thermocouple.
      See this page for details.
      http://srdata.nist.gov/its90/type_k/kcoefficients_inverse.html

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