hi all
A few more info to permit you to understand the o2 sensors working principles
first of all an o2 sensor is an electrochemical device containing a fuel that "burns" in presence of oxygen and that produce a small cc signal.
the output tension is directly proportionnal to the ppo2 in the mix to be analyzed.
so you have to mesure the output tension to have an idea of the ppo2 in the mix to be analyzed.
typicaly a r-22d teledyne sensor produces a 10-13 mv when new IN AIR AT ONE ATM (0.21 bar ppo2)
in pure o2 at one atm if get up to arround 63 mv
to mesure the tension and then the ppo2 one can use a basic lcd voltmeter that one can buy every where from his local radio shack as you say in your country.
those small voltmeters are usualy powered with 9 volts batteries and cost nothing.
the problem is that reading for example 45,5 mv on a lcd display will not give you one the ppo2 !!!!
so one have to calibrate the signal to get a directly readable data in ppo2.
to do this one can simply use a potentiometer , this simple device permit to divide the tension by a ratio that make it possible to read the ppo2 directly.
a pot has 3 pins , an input, a wipper, and an ouput, there is also a screw and when one screww or unscrew the wipper will move from the input pin to the output pot pin dividing more and more the tension intaken in the input pot pin
the sensor + output is simply connected the input pot pin , the output pot pin is connected to the "-" of the sensor and the middle pin of the pot (the wipper rw) is connected to the voltmeter to be mesured.
if one screw totaly the pot the signal getting out of the wipper wil be 0 volts and 0 volt will be displayed by the voltmeter
if one unscrew totally the pot the signal will be the full signal of the o2 sensor in air at one atm in our example it will be 10-12 mv
so using the pot one can calibrate the signal in order to read 002.1 mv or bar ppo2.
after the calibration if the sensor is put in pure o2 the voltmeter will magicaly read
010.0 mV or ppo2 (100% o2 in 1 atm) just because the output increases propotionnaly to the ppo2 (it is linear thank of god!!!)-
calibrating the sensor signal the operator have only divided the signal from 12Mv (in the r22D) to 2.1 mv
this is the easiest way of doing.
the only problem is that using this kind of sensor one don't obtain a decimal because the voltmeters displays ususaly 1999 points from 0 mV to 199,9 mv
as a result the display gives one a poor accuracy or rather a poor definition it will display 21% o2 22% o2 but not 22,3 % (for example)
to get a more accurate ppo2 with one decimal, one can use an op amp to amplify the signal for example in the 0 - 2 volts range (in this case the voltmeter will also have to be tuned in this range), the calibration is ther done changing the op amp gain (all of this is very simple , it is basical eletronics) , for that rail to rail precision op amp need to be emmployed rather then cassicle basic one like the famouse 741 which is not enough (i use the analog device OP90)
to do this the voltmeter chosen will be able to read a tension references to the - of the voltmeter supply battery instead of a floating voltmeter
there is 2 other ways to get decimals,
the first is to use a high output sensor that is in use in rebreather world like the teledyne R-22DHO
There the output is 25 MV in air at one atm so one can divide the output with a pot to get it down to 021.0 mV instead of 002,1 mv and to get one decimal more .
the second way of doing is to change a little bit the voltmeter tension reference with the help of a potentiometer.
in this case even a low output signal can provide decimals simply because one will force the voltmeter to display 021,0 mv even if only 12mv is input in the voltmeter mesuaring pin.
many of those circuits are displayed on the internet.
in this case it is necessary to find out where is the tension reference resistor and to replace it by the good pot.
I coud give all of you the shemes I have got.
here you will find complete literrature as for the teldyne diving sensors , use diving sensors not automovtive ones or medical ones for they are temperature compensated and protected from humidity : http://www.teledyne-ai.com/oem/diving.html
go there also http://www.ppo2.com/sensors.htm
I've taken the liberty of explaining all of that before any question from you cause if I had had this info before i would have saved mutch time
sorry for my poor french man english
here is what i know about it that permitted me to survived hundred hours rather deep ....
now comming in your world i'd like to be DRY at the bottom (((-:
regards
hope it will help
jean mi
---- Original Message -----
From----Message d'origine---- Date: Sun, 28 Aug 2005 17:08:17 -0400 De: Marie-Andrée et Pierre
Sujet: Re: Re: Re: [PSUBS-MAILIST] O2 sensors
A: personal_submersibles@psubs.org
AH!!!!! I get it!!!!
Thank you very much Jean Mi.
It all make sense now. So in a normal dive, I will not need to inject O2 before a long time if I go down to 10M. Because I will have plenty of O2 from ambient air from tanks...
So I need to track Partial Pressure of Oxygen which is given directly with the sensor (in volts).
Thanks again! I will start to make some maths now!
Pierre "more bright now" Poulin
----- Original Message -----
Sent: Sunday, August 28, 2005 4:41 PM
Subject: Re: Re: Re: [PSUBS-MAILIST] O2 sensors
Hi,
I honestly think that it would be better to get rid of the pressure data in your oxygen anyzing process.
if the gas analyzed is a one atmosphere gas of course the partial pressure displayed by the meter will be also the oxygen percentage.
for example air contains 0.21 of oxygen (21 %) so at one atm ppo2 is 0.21x1=0.21
but the percentage of o2 let's call it the fraction of oxygen is not a good data to lett you know you if you are breathing a good o2 mix or a bad one.
in any pressure condition if your partial pressure is between 0.16 bars and 1.6 bars you are breathing a good mix.
a simple o2 sensor a voltmeter and a potentiometer is enough to display the partial pressure.
if you really need to know the %tage of o2 you will have to use a microcontroller that will read both the partial pressure from the O2 sensor using an a/d converter and the pressure via a pressure sensor which is really difficult to master unless you are a good digital electronician, then the microcontroller will divide the partial pressure by the pressure in ATM and you'll get de fraction or the percentage of o2 displayed via an ascii lcd digital display
but keep in mind that the percentage of o2 is only a usefull data at one atmosphere but as soon as the pressure is going to change the o2 percentage is not a relyable data.
if you are breathing 21% of o2 in a 600 mb atmosphere you will loose collapse.
21x.6=12,6 bars hypoxic lethal
o the other hand if you are breathing 15% o2 in a 1500 mb atmos
15x1.5=22,5 bars normoxic safe
in both cases o2 fraction will be a bad friend for you
in your cabins pressure can go down when you breathe a part of the o2 without replacing it and getting rid of o2 via soda sorb scrubbers or it could increase in case of a leak of air or o2 supply in the cabin.
in bith case the only inportant data is the O2 partial pressure and NOT the fraction
so what is interresting for you is the partial pressure
in mix gas diving and in diving chambers we know :
- a mix at less then 0.16 bars of o2 is hypoxic blellow it get dangerous , we trimix divers all breathe 0.16 trimixes at surface it is ok but not bellow.
- for long periods in habitats of chambers the max ppo2 is arround 0.4 bars of o2 after there is an iritation of the lung (no serious but not pleasant)
- for short periods max partial pressure is 1.6 bars O2 short means arroud one hour , for 2 hours 1.5 is better and for 3 hours divers stay in general under 1,4.
for all thoses reasons i think that it would be mutch easier just to buit a simple analogic ppo2 analyser, that would be cheap , relyable well known .
hope it will help
regards
jean mi
----Message d'origine----
Date: Sun, 28 Aug 2005 15:56:46 -0400
De: Marie-Andrée et Pierre
Sujet: Re: Re: [PSUBS-MAILIST] O2 sensors
A: personal_submersibles@psubs.org
Thank you Al and Jean-Michel for the reply,
Both of you have home-made rebreather. I think that the application I want to do is very similar because both are ambient pressure.
So do you guys have a pressure sensor in combination with the O2 sensor? So the "computer" can divide the partial pressure of O2 by the ambient pressure and give a reading equivalent to 1atm? (0.21 being normal even if taken at 2ATM)
I can't see another way to do that. Otherwise the reading doesn't make sense if you don't take the ambient ATM into account.
Am I right in assuming this? And if so, what type of pressure sensors do you guys use?
Thanks again for the replies! It really help me out!
Pierre Poulin
----- Original Message -----
Sent: Sunday, August 28, 2005 2:54 PM
Subject: Re: Re: [PSUBS-MAILIST] O2 sensors
Yes as a home made close circuit rebreather diver i can say that a ppo2 meter is really simple to built.
basicly a simple 10 dollars lcd voltmeter and a pot are enough for the electronics
but with no alarm
a system with an audible alarm can also be built cheaply .
what ojne have to know is that an o2 sensor is a very fragile electrochemical device that can drift from a second to the other with no warning
so any vital ppo2 system need to be doubled or trippled to be abble for the operator to compare the readings
with 2 sensors one can know if the system works (if the reading are close)
with 3 sensors one can know witch sensor is bad in case one starts to drift .
a sensor life is usualy 12 to 24 monthes
as for the humidity problem don't care the r 22's have hydrophobic membranes and works well in hyper humid condensing conditions like rebreathers so i a bigger space like a sub cabin it will definitly be better.
I can help if necessarry.
regards
jean mi
----Message d'origine----
>Date: Sun, 28 Aug 2005 10:13:11 -0400
>De: Al Secor
>A: personal_submersibles@psubs.org
>Sujet: Re: [PSUBS-MAILIST] O2 sensors
>
>Pierre,
>
>O2 sensors measure partial pressure of O2, so if your sub cabin is at 2 atm, the
>sensor will read ..42 in air (assuming it's been calibrated to .21 at 1 atm).
>Be aware that condensation on the face of the sensor can lead to
erroneous
>readings so you want to try to avoid condensation directly on the face even
>though some are manufactured with hydrophobic membranes.
>
>A good place to get sensors is from Patrick at oxycheq.com. His prices are very
>reasonable and he provides good service. (I have no relation to Patrick, just
>a satisfied customer). I use Teledyne R-22D sensors in my homebuilt
>rebreather. A very simple O2 meter can be built from a recalibrated DVM
>
module. There are several sources on the web that describe how to do it and I
>think Patrick's webpage also describes it.
>
>Hope this helps.
>
>Al
>
>Al Secor ARS: WA3PWX Scuba Instructor SSI PDIC TDI
>http://www.geocities.com/SubDiverI
>
>
>
>Quoting Marie-Andrée et Pierre :
>
>> Hi guys,
>>
>> After seing Jon's presentation at the convention I decided to go
with a
>> scrubber and a O2 injection device.
>>
>> My question is: Will an oxygen sensor work for a dry-ambient sub? How does
>> the sensor work? I know it give a different voltage outpout depending on the
>>
>> amount of oxygen in the air. but what happened when put under pressure? Will
>>
>> it track PPM of oxygen in the air or the % of oxygen in the air? Will it
>> detect 21% at 1atm, 42% at 2atm, 176% at 5atm,
etc...
>>
>> Does anybody here have experience with a scrubber/O2 system in an ambient
>> sub?
>>
>> The reason I decided to go scrubber/O2 is that I have about 35-40 minutes
>> inside my sub before I feel the effect of too much CO2 in the cabine. If I
>> refresh the air inside the cabine by injecting fresh air from my tanks, it
>> is not very effective and takes alot of air.
>>
>> Thank you all!
>>
>> Pierre "O2" Poulin
>>
>>
>>
>>
>>
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