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.