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joshur

How To Make An Oxygen Sensor For Your Kiln.

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Is there any interest here on how to make and use oxygen sensor for a  gas kiln, using an automotive oxygen sensor? or has this been done already? I built one a year or so ago and after working out the kinks, have had good results, If there is enough interest I will share what I have done and learned.

 Josh

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Joshur,

  I am so very interested.  I fire gas reduction and would like to purchase an O2 reading device, but the cost is too high for me.  If you had an alternative that I could make I would so be all over that.

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To start with I will show a  a picture of the basic parts of the setup and discribe their use and function. The parts are, a digital multi meter, $2.99 on sale at harbor freight, new automotive oxygen sensor, $25, about $10 in plumbing parts, and a long enough piece of 5/8 od ceramic tube to go through the wall of your kiln,$15.

 The adaptor from the ceramic tube to the iron tee is a 5/8" compression fitting to pipe.

 The function, and results of the store bought $1200. O2 sensor and this home made one are essentially the same, I started using the fancy one and was able to study and test it against the home made one, to compare the components, data and results.

The store bought one, samples the air directly in the kiln, while  the home made one has to siphon the gas out though the ceramic tube to the sensor. Both kinds of sensors use the same basic kind of system and both give their readings in DC volts, so all you are reading is an increase in voltage  as the O2 level decreases at certain tempratures.

 

 I am a slow at typing and need to get some work done, and will break this up into several parts, so this is all for now.

 

 Josh

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Alirobinson likes this

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I am converting an old electric to gas kiln. I'm interested and look forward to your next post. It looks like you use the long galvanized pipe as a small chimney that draws chamber gas past the sensor?..... great idea!

 

Jed

BRL likes this

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The tricky part.

 

Is to get a consistent flow of kiln gas to the sensor and at the proper temprature. As different kiln designs have very different air flow and internal pressure, the sensor assembly must be set up to the individual kiln.

  A down draft kiln that has enough back pressure/ positive pressure is the easiest to set up and operate, all you need to do is drill a hole in the kiln wall, about 3/4 of the way up the wall, twards the back of the kiln, and away from peep holes, you really do not need much flow by the sensor, but it must be consistent.

 The tougher one is the updraft kiln, the updraft is often operating in a negative pressure mode, not only that but the kiln air flowing past the ceramic tube on the inside of the kiln wants to pull air in through the tube from the outside of the kiln, like the venturi effect in a carburetor.

 I did two things to solve these problems on an updraft kiln, one was to create a gas catcher inside the kiln to assist in getting the flow in the right direction, the other is a long  metal chiminey, heated  by the exhaust from the kiln.

 The first two pictures are of the gas catcher,  just a piece of soft brick that is partly hollowed out,  then cemented and pinned over the intake end of the ceramic tube, to funnel the gas into the tube. The third picture is the sensor assembly on the kiln,  the next two are the kiln with the sensor and chiminey pipe attached, and the last is just the chiminey pipe, which is just a section of 3/4" metal electric conduit that slips onto the metal pipe that comes out of the tee. The last picture is of the complete setup. Feel free to ask questions, or ask for more pictures if things are not clear.

 

 Next: Operating the sensor.

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I'm converting an old electric to gas, making a downdraft. I think I will add a 3/4 to 1 inch coupling then extend the chimney with 1 inch conduit. It should draw very well.

 

What type of analysis is possible? Is the sensor an "all or nothing" current reading? Can it be titrated to the O2 level?

 

Jed

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Jed,

 

 The readings from the sensor  seem to be quite sensitive and cover a  broad range,

 The attached chart shows what is going on with the readings you will see on the meter,  the 0.1 to 0.8 readings on the left of the chart are volts,  when the meter is set to read in the say 2 to 20 volt range, your reading in the reduction phase might look like 0.712 volts and drop down to 0.345 for a few seconds after opening a peep hole on an updraft that would draw in extra air. Even when the kiln is very stable the reading will bounce around 0.020 or 0.030 volts. These readings seem to be very similar using either the commercial kiln sensor or the automotive one, same with the meters, fancy or cheap have similar readings, in fact some of the commercial units just use a relabeled common multimeter, as a readout.

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RobertS likes this

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Using the O2 sensor,

 

One of the major advantages of the store bought sensor, is that it needs little or no maintenance, on the other hand, our home made version, should be cleaned and inspected prior to each use, turned on at a specific point, and closed up immediately upon shutting down the kiln. 

The first step to setting up the kiln to fire, is to clean the sensor and the intake pipe and chimney.  Picture one shows the sensor in the storage mode, pic two shows the sensor after a firing, and three  is  the cleaning of the intake tube, it is best to clean the sensor and tube first as some debris, soot, spiders, etc.  may be blown into the kiln, I use compressed air to clean the sensor, and tubes. The threads on the sensor are slightly smaller than the thread of the 1/2" iron pipe, so a couple wraps of aluminum foil on the sensor threads, takes care of this and makes the sensor easy to unscrew for cleaning, you could find a proper adapter, but the foil works fine for me.

 The sensor exhaust  should be kept covered as pictured in #1 and the chimney not connected until the kiln is close to reaching the temperature for starting reduction, there are several reasons for this, one is both type of sensors, do not give accurate readings until  they reach a certain temperature, but most importantly our sensor operates at lower temperatures than a internal sensor and during the kiln temp rise from start until it is at least 1000F there is moisture, soot and other stuff being given off by the kiln and this can condense on the  sensor, and the temperature at the sensor may not be high enough to burn it clean.

 So, I clean the sensor, put it back in, close it up, clean and stack the kiln, then start the kiln. When the cone 011 guard cone for reduction starts to go down, I  remove the cover, see picture 1,  put on the chimney, pic 4, and hook up the meter, 5,  at this point the kiln is warm enough to provide hot dry air to the sensor, and quickly heat the chimney to establish a good kiln gas flow past the sensor, carefully check that the chimney near the sensor is getting warm or hot to confirm a good flow of gas. The portion of the chimney pipe that is in the kiln exhaust, may get hot enough to glow red.

 When you turn the meter on at this point, you should start to see a gradual rise in the voltage, and by the time the kiln is hot enough to need reduction, the sensor assembly should be giving an accurate reading.

This kiln is a little touchy on getting the gas air mix just right to get the reduction I want and sufficient temperature rise, but once set it requires little adjustment.

With a sensor to establish the proper carbon monoxide level, you may not see some of the usual indicators of reduction, especially as the kiln gets hotter, such as smoke, soot on the peep plugs, and a cloudy or slightly obscured view of the ware through the peep, as the kiln should be operating more efficiently while still maintaining the required carbon monoxide level.

 As soon as I shut off the kiln I disconnect the chimney and cover the sensor exhaust  to prevent outside air from going through the pipe into the kiln, for the usual reasons not to have an opening on the side of a hot kiln. 

 

 Next: Results

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Results:

 

First, a picture of the kiln and sensor in operation, this is right at the end of the firing, giving the kiln a short heavy reduction soak prior to shutdown.

This kiln is probably 45 years old, it has an interior dimension of  37" x 37" x 42" and a stack area of 24" x 28" x 37" it uses about 30 gallons of propane for a cone 11 firing and takes 7 to 9 hours.

Next are a couple pictures of my work at my current show at the Mendocino Art Center,

 

Most all of these pieces were fired using this kiln and sensor and almost all this year.

 

 Josh

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Hi joshur,

Hi all..   I'm back.. I tried out the sensors with U-shaped exhaust manifold that i used to capture the kiln exhaust gazes and then feed them past the sensor..  one sensor I bought was used and the reading just continued to climb as if the reading was changing due to the sensor heating up.. in the 45 mV range and kept increasing..  another sensor also used didn't give me any reading..  I was using the digital multimeter from Harbor Freight I think that was the one you said you bought..  

I'd love to get this working to at least see just how much reduction I'm getting early on in the cycle..  I know it's quite heavy as I see the carbon deposits on the peep plugs..  but wonder if controlling toward the high side just prior to shut down..  

any further recommendation are welcome..  

Joe..  

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Joe,

 look at some of the pictures I posted, you can see the sensor, the box it came in and the part number on the box, a new sensor is about $25-$40 at the discount auto parts stores, if you are in doubt about the accuracy of the one you have. Also  for one thing the reading does not start until the kiln gets up to about 1500F then you will see the reading climb with the rise in temp, that is if you are in reduction. You can see in another picture of the meter that the reading in heavy reduction at about cone 10 is 900mV, I try to keep the reading in the 500mV to 700mV range for most of the reduction portion of the firing.

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gents,

the millivolt for a given kiln condition is related to the oxygen level through the Nerst equation which essentially says that the mV reading is proportional to the sensor temperature (in Kelvins) and to the logarithm of the ratio of the kiln oxygen level at the sensor to oxygen in the ambient air.

 

Thus if the temperature of the sensor is climbing the reading is changing even if the oxygen level is constant.

 

It should be easy to calibrate just by changing the air inlet to the burners while holding the fuel flow constant while firing the kiln empty.  The gas temperature at the sensor should become steady quickly after making adjustments if the kiln is empty.

 

LT

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Hi Joe,

Thanks for the very helpful information and images. I'm planning on firing a small Bailey gas kiln (cone 10) soon and will be reduction cooling it, so I am primarily interested in keeping track of the atmosphere as the kiln cools. One burner will be on low and the other other burner port will be bricked up. The damper will be all the way in. I'm aiming to keep it in reduction until about 1600F.

 

Two quick questions:

 

1. You mentioned removing the sensor as soon as temp is reached. Do you forsee any problems leaving it in as the kiln cools - the whole reason that I'm using it is to keep the kiln in reduction down to 1600. Would it be too much thermal shock to the unit to insert it at the end of the firing?

 

2. I'm going to be renting a kiln at a local craft school and will not be able to drill a whole in the back wall. Do you see any problem with inserting it through one of the spies, as long as i make sure that it is airtight? I will have the other one plugged up completely once the cones are down.

 

thanks again!

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Hi Joe,

Thanks for the very helpful information and images. I'm planning on firing a small Bailey gas kiln (cone 10) soon and will be reduction cooling it, so I am primarily interested in keeping track of the atmosphere as the kiln cools. One burner will be on low and the other other burner port will be bricked up. The damper will be all the way in. I'm aiming to keep it in reduction until about 1600F.

 

Two quick questions:

 

1. You mentioned removing the sensor as soon as temp is reached. Do you forsee any problems leaving it in as the kiln cools - the whole reason that I'm using it is to keep the kiln in reduction down to 1600. Would it be too much thermal shock to the unit to insert it at the end of the firing?

 

2. I'm going to be renting a kiln at a local craft school and will not be able to drill a whole in the back wall. Do you see any problem with inserting it through one of the spies, as long as i make sure that it is airtight? I will have the other one plugged up completely once the cones are down.

 

thanks again!

 

You may find that your burner keeps the reduction level too high, or that having the port open allows for too much oxygen to enter the kiln. If I remember correctly, the reduction cooling kiln I built in grad school (long time ago!) had a sleeve over each burner that could be slid forward over the end of the burner pipe to seal off the ports. There was a small slot in the sleeve that allowed the flame from the pilot burner to provide the reduction.

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My kiln reduction gas kilns go to neutral the minute they are turned off-You will just need a pilot light to alter the on a closed kiln. I think a burner is to much even on low. 

(Would it be too much thermal shock to the unit to insert it at the end of the firing?)

 

Putting anything into a kiln at that temp will shock it unless its just metal not ceramic .

 

One thing to consider is learning what reduction looks like and not bothering with any device.

I say this as an owner of two Oxy probes but I did do 30 years of firing by eye and feel that worked out just fine. I think its best to actually know what back pressure and flame looks and feels like.

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I used to do reduction cooling by throwing sticks of wood into the kiln every so often for 2 hours after shutting down. I think I am very old school as I have never used an foxy probe. I never did reduction with heavy billowing smoke, just relied on the flame colors coming from the top peep holes achieved by tampering down and adjusting the air on the burners.

 

Marcia

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just to clarify - the ceramic tube is open on both sides, correct? I can't see how it would work otherwise but want to double check. Have you just chopped the end of a standard thermocouple sheath?

Yes the tube is open on both ends, one end cut off and cut to length at the same time, they are very hard to cut, nick or score the tube with a diamond blade.

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