the calibration needs to be done only once a year.
hook up 100% oxy : start the cal routine , wait for it to finish ( takes about 30 seconds ) . then switch to 21% ( air ) . hit the button , wait for completion ( another 30 seconds ) and done.
The cpu calculates the baseline , and the gainstep ( cpu uses a window integration algorithm with offset/gain compensation ) and updates the cal parameters in its memory ( nonvolatile :it gets written to the internal flash memory of the CPU )
operation is simple.
the unit has two buttons. You need to press and HOLD both buttons to turn it on ( prevents accidental turn-on that would run down batteries ). CPU boots and shows you the battery gauge for 2 seconds. once the battery guage disapperars you can release the buttons. if you release buttons before the unit switches off again.
hook up op LPI and look at the display. it take s15 to 20 seconds to settle.
the O2 fraction is displayed in big digtits and the smaller digits show your MOD ( in feet or Meter depending on your settings ) for the given PPO ( adjustable from 1.2 to 1.6 in steps of 0.1 )
top button is backlight ( turns on and automatically turns off after 10 seconds )
bottom button is menu access (you need to hold it for 2 seconds before menu is entered. )
menu is simple
PPO setting - feet/meter setting - aut timeout setting - cal - exit
the second button cycles through the menu the first button select an item
here are some images.
the box is made by BOX corporation ( funnybut appropriate name for a company that makes boxes. its a half shell design held together with 2 screws. the rubber bootr snaps over the box making it pretty showck proff.
the front windo is made from 2 mm lexan an dmilled down , attached to the case using tec-3 compound ( black glue silicone )
you see the standard LPF connector in the third image with the two control buttons ( yellwo and blue )
the overmold rubber is available in 7 colors ( so you can easily tell them apart : mine , the wife's , my buddy's etc )
the unit NEEDs to be connected to your LPf to give a correct reading. if left in normal air it will read low. ( the overpressure created in the sampling cavity is also taken into accoutn during the calibration )
every device has this problem. the flow of gas creates a pressure diffrential between the sampling cavitry and the sourrounding athmosphere ( however small ). the calibration measures this as well and nulls it out.
the reason i designed it the way it is is that i was not satisfied with all the stuff out there.
some of the things out there look like Mcgyver contraptions ( a box , a sensorcable , a sensor , a smpling piece ( t piece , a flow divert , a pice of tubing.... and there is always a piece missing or broken.. or the swtch was left on and the batteyr has run down.
Other design you have to hold in front of the tank and open the valve.. try getting a good reading with that. open the tank a bit too much and you are off by 2 or 3 % plus you are wasting gas like crazy.
The voltmeter style designs ( like el-cheapo ) tend to drift and you need to 'null them using a potmeter ever so often. additionally a one point calibration is no good. the sensor has a baseline ( null ofsset : if you give it no oxygen there is a residual voltage ) with onyl one trimpoint you can not get rid of that , you nly compensate the gain at one spot. that tilt of the curve may still be wrong. that was also a frustration for me
Other frustration: those sensors are expensive : 60$ and after one year you can kiss them bye bye.
I do NOT use the teledyne sensors , but a Figaro sensor. The drawback is that they are a bit slower to respond AND the electronics to read them is more complex singe you are dealing with very small signals. You are not allowed to load the sensor in any way either. even most digital voltmeter have too big a load for the sensor. ( you are not allowed to draw more then 100 picoampere : thats 10 to the power -12 ). The advantage is : the cost is the same , the sensor is smaller and it is guaranteed to work for 5 years. check it out at the manufacturers website
http://www.figarosensor.com/gaslist.html and look at the ke-25. that is what i am using.
the ke-50 is usable too but the reading is -very- slow ( it takes 4 to 5 minutes to stabilise). the only difference between the ke25 and ke50 is the gas absorbtion rate of the membrane inside. The fuel composition and amount are identical.
even though they only need one minute to 90% response they need another minute to close half of the last 10% , and then another minute for half of that. so to get a reading withing 1% : you are looking at 5 minutes. ( half times : 5,2.5,1.25,0.75 : so 4 steps to reacc below 1% line
The other sensor is much faster ( 12 seconds : same step count : 60 seconds for full readout resolution )
the KE sensors are medical grade and typically used for narcose equipment to measure oxygen absorbton of a patient under surgery.
anyway. i'll take some pictures of the guts and post them tomorrow
