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I've just been doing some web searching about variations of oxygen proportion around the planet without much success. Only one said they were significantly lower in cities but that had some other aspects which seemed doubtful so I'm not sure about this. Still, if there really was a 2 or 3% decrease then you'd expect a 1% or so increase in CO₂ which would be remarkably high.

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certainly electrochemical cells for CO and O2 can be "drifty". Also some systems are subtractive, taking the sum of gasses sensed and taking them away from 20.9% to display % O2.

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Posted (edited)
4 hours ago, jack said:

 

But there isn't going to be a 2-3% difference in oxygen levels between places in the UK. It's more likely to do with machine calibration.

no these machines are self  calibrating every time you switch them on .has to be accurate  for  the emission test for the MOT  .

 the oxygen sensor is a part that is changed regularly , if it needs it when the machine is serviced 

so yes  there is that much difference in air quality  due to  pollution between city and countryside

wonder how low it is in inner  london !!

its not as though  i have seen it just the one time but when i was going around tuning cars for other people i would always look,as it also effected the bhp an engine could develop and accounted for some of the variance in test readings from different dyno,s

,that and inlet air temps  and amount of wheelspin during the test .

air temp would be the biggest factor if dyno was calibrated correctly .

raise inlet temp by 10c and you loose 3% of power --by pure physics ,in real world it is always more probably more like 5%--so keeping inlet temps low  is always good for power

I never got to find an actual  figure i could quote for less oxygen in atmosphere  not something i could change

,but I know cold day and tested in countryside dyno  was always good for big power output

Edited by scottishjohn

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Of course the absolute amount of oxygen (g/m³) will change with the air density, as the pressure or the temperature changes, but it'd be interesting to see reliable measurements that the proportion vs the amount of nitrogen or the total amount of air changes much other than as a result of changes in water vapour.

 

The take off performances of aircraft change quite dramatically with air density (mostly temperature) for the reasons you mention wrt to engine performance plus similar effects on the propeller/turbine and the wings, both for light aeroplanes and airliners. Take off performance calculations are done quite carefully for airliners but AFAIK they don't take proportion of oxygen into account, and I've discussed it in some detail with different airline pilots at various times; I'm sure somebody would have mentioned if it was a factor. E.g., London City vs Heathrow.

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1 hour ago, scottishjohn said:

air temp would be the biggest factor if dyno was calibrated correctly .

raise inlet temp by 10c and you loose 3% of power --by pure physics ,in real world it is always more probably more like 5%--so keeping inlet temps low  is always good for power

 

Sure, but no-ones disputing that changes in air density/temperature cause changes in power output. That's notoriously well known to anyone who's read car mags for any length of time, or been involved with flying planes 

 

However, everything I've read says that the proportion of oxygen doesn't change much out of doors anywhere on the planet. I can't find anything to support the idea that as soon as you move from a built-up area to the country the oxygen level consistently increases 2-3% as you suggest.

 

I'm not questioning what you've seen, but there could well be all sorts of other factors involved.

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I,m not talking about air density 

but the amount of burnable oxygen ,which does effect power 

i don't really want to start giving you lessons on the internal combustion engine  and the ideal air fuel ratio for best power or economy 

just accept that more oxygen means more of a bang and more power by the fact that you can add more fuel to burn to keep the air fuel ratio correct .

on n/a engines max power is usually attained at around 13-1 air fuel mix  on a modern 4 valve engine -older 2 valve types could be more like 12-1

I have no experience of jet aircraft only piston engined ones - and max engine performance on a piston aircraft will be at sea level 

it declines as you go higher due to the thinning of the air --and above 8000ft you will start to see a performance drop ,

from 5000 ft you normally start to lean off the mixture because of the lack of oxygen in the the thinner air  makes it run rich and is neither good for power  or the life of the engine 

temp will also compound this .

this is getting way off the topic  

 

 

 

 

 

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7 minutes ago, scottishjohn said:

I,m not talking about air density 

but the amount of burnable oxygen ,which does effect power 

 

We really need one of those emojis where the head actually rolls as it laughs.

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13 minutes ago, jack said:

 

Sure, but no-ones disputing that changes in air density/temperature cause changes in power output. That's notoriously well known to anyone who's read car mags for any length of time, or been involved with flying planes 

 

However, everything I've read says that the proportion of oxygen doesn't change much out of doors anywhere on the planet. I can't find anything to support the idea that as soon as you move from a built-up area to the country the oxygen level consistently increases 2-3% as you suggest.

 

I'm not questioning what you've seen, but there could well be all sorts of other factors involved.

it will probably be a local effect of city pollution on the air,

alot of the polllutants are heavier than oxygen and probably are displacing it to a higher altitude.

I really have no idea -as its  something I could  not do anything about  ,so pointless to think to much about it with what i was doing

,enough other  variables to contend with when tuning performance engines or fitting total stand alone ecu systems.

so yes all modern ecus systems use air density and air temp to adjust fuelling to get best performance/emission control ,they do this in real time and also when you

stop

It will make adjustments to  base ecu map adding the corrections it has had to make while you were running ,so when you start it will work from the last base map + that days corrections 

example --you are in south africa at coast then drive to up table top mountain and stop -it  will recalibrate ecu settings as though that 6000ft was sea level for next time you start up .

this is a simplification of what happens every day you drive a modern car .

all that whirring noise you sometimes hear after you turned it off is it checking sensors and calibrating them for next start

 

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7 minutes ago, scottishjohn said:

it will probably be a local effect of city pollution on the air,

alot of the polllutants are heavier than oxygen and probably are displacing it to a higher altitude.

 

I've had a very quick look and can find only a single source that suggests there are significantly lower oxygen levels in cities: https://www.theguardian.com/commentisfree/2008/aug/13/carbonemissions.climatechange

 

Maybe you're right, but everything else I found talks about the dangers of all the other crap in city air, and the hypoxic effects of high absolute humidity and temperature, but these don't affect the proportion of the oxygen in the air, just its effective availability to humans.

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29 minutes ago, jack said:

as you move from a built-up area to the country the oxygen level consistently increases 2-3% as you suggest.

 

 

you say "suggest" like i,m making it up.

i have reported what i have  seen on different 4 gas analyisers in multiple locations 

I've spent 40 years + building and tuning race car engines

been fitting stand alones ecus since 1992 

building super charged race enignes since Rotrex came to this country in 1998

I know what i,m talking about when it comes to internal combustion engines and what effects their performance and how atmospheric conditions effect dyno results

I also have over 300 hours logged flying home builts and other piston engined aircraft ,

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2 minutes ago, jack said:

 

I've had a very quick look and can find only a single source that suggests there are significantly lower oxygen levels in cities: https://www.theguardian.com/commentisfree/2008/aug/13/carbonemissions.climatechange

 

Maybe you're right, but everything else I found talks about the dangers of all the other crap in city air, and the hypoxic effects of high absolute humidity and temperature, but these don't affect the proportion of the oxygen in the air, just its effective availability to humans.

It really quite simple you can only have 100% of anything ,so if there are greater percentages of some gases there must be less of others

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3 hours ago, dpmiller said:

certainly electrochemical cells for CO and O2 can be "drifty". Also some systems are subtractive, taking the sum of gasses sensed and taking them away from 20.9% to display % O2.

which is why 4gas analaiser do an auto re-calibrate every now and then  when left turned on -you can hear the flow  pump  speed up when it does it

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6 hours ago, scottishjohn said:

which is why 4gas analaiser do an auto re-calibrate every now and then  when left turned on -you can hear the flow  pump  speed up when it does it

What calibration gas is used?

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2 hours ago, dpmiller said:

What calibration gas is used?

there is  no calibration gas cylinder attached  all the time ,

the service engineer brings that  twice a year  when he does his calibration tests ,used to be 4 times a year ,until they proved it was over kill

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10 hours ago, scottishjohn said:

I,m not talking about air density 

but the amount of burnable oxygen ,which does effect power 

 

Yes, we know that. You were originally talking about the proportion of oxygen in the air. We're sceptical that the proportions measured by exhaust analysers are a reliable measurement and wondered about other sources of information. It's you that brought up the irrelevant experiences based on air density from temperature and altitude effects.

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3 minutes ago, Ed Davies said:

 

Yes, we know that. You were originally talking about the proportion of oxygen in the air. We're sceptical that the proportions measured by exhaust analysers are a reliable measurement and wondered about other sources of information. It's you that brought up the irrelevant experiences based on air density from temperature and altitude effects.

 

FWIW, vehicle gas analysers are only required to be accurate to +/- 5% of the reading, so a reading of 21% O2 could be anything from ~20% to ~22% in reality.  These things aren't by any stretch of the imagination lab quality measurement instruments, they are pretty rough and ready garage tools that give a good enough figure for the purpose for which they were designed.

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5 minutes ago, JSHarris said:

 

FWIW, vehicle gas analysers are only required to be accurate to +/- 5% of the reading, so a reading of 21% O2 could be anything from ~20% to ~22% in reality.  These things aren't by any stretch of the imagination lab quality measurement instruments, they are pretty rough and ready garage tools that give a good enough figure for the purpose for which they were designed.

accepted 

but on multiple visits over a period of a few years the same differences were seen  every time 

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18 minutes ago, scottishjohn said:

accepted 

but on multiple visits over a period of a few years the same differences were seen  every time 

 

 

Just shows a constant error, though.

 

Climate scientists have been monitoring atmospheric gas composition changes for decades now, and no one has ever recorded a significant variation in O2 concentration from urban to rural areas as far as I can find, from an hour or two scanning the literature. 

 

There are tiny variations as a consequence of displacement by variations in atmospheric pollutants, but these are really tiny.  For example, doubling the atmospheric CO2 concentration causes a displacement that reduces the proportion of all other gases in the atmosphere by roughly 0.04%.  The pro rata reduction in O2 concentration from this would be less than 0.01%, a change that a vehicle gas analyser wouldn't be able to resolve.

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8 minutes ago, JSHarris said:

 

 

Just shows a constant error, though.

 

Climate scientists have been monitoring atmospheric gas composition changes for decades now, and no one has ever recorded a significant variation in O2 concentration from urban to rural areas as far as I can find, from an hour or two scanning the literature. 

 

There are tiny variations as a consequence of displacement by variations in atmospheric pollutants, but these are really tiny.  For example, doubling the atmospheric CO2 concentration causes a displacement that reduces the proportion of all other gases in the atmosphere by roughly 0.04%.  The pro rata reduction in O2 concentration from this would be less than 0.01%, a change that a vehicle gas analyser wouldn't be able to resolve.

the chances of getting same  constant error on 4 different units indifferent locations   is very small ,and never did the country analysers show lower than city ones

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19 minutes ago, scottishjohn said:

the chances of getting same  constant error on 4 different units indifferent locations   is very small ,and never did the country analysers show lower than city ones

 

The fact remains that the best atmospheric scientists in the world are reporting data that's at odds with your ad hoc observations on a garage gas analyser, so which is most likely to be accurate?

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10 hours ago, scottishjohn said:

It really quite simple you can only have 100% of anything ,so if there are greater percentages of some gases there must be less of others

 

Exactly. But none of the things you mention could possibly contribute to a 10-15% reduction (21% to 18-19%) in the amount of oxygen in the air.

 

No-one's calling you a liar, but clearly something else is going on. You're the one making the extraordinary claims.

 

 

 

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maybe there are pollutants that are displacing oxygen in cities - I cannot say for sure 

but i can say for sure that the same car dynoed in the city will showmore power when same test @same ambient temp / baro pressure  is done on  a dyno of the same make  outside a city .

this discussion started regarding MVHR+air quality + need for it  if occupants open windows on a daily basis, as my wife will do no matter if MVHR is fitted or not 

 ,and for sure air quality is very much better outside major conurbations .

 it is strange that all air quality survey equipment -which you can get live data from show neither co2 or o2  contents,especially when the big cry about saving the planet is about lowering co2 

 (extraordinary claims ) --Not calling me a liar? -

i am reporting what was observed  on lots of occasions that's all

 

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29 minutes ago, scottishjohn said:

it is strange that all air quality survey equipment -which you can get live data from show neither co2 or o2  contents,especially when the big cry about saving the planet is about lowering co2 

 

It'd only be strange if actual changes in outdoor CO₂ or O₂ proportions directly affect air quality (i.e., human health). It's well understood that indoor CO₂ concentrations can have such an effect but I know of no reliable evidence that the relatively small variations in outdoor CO₂ or even smaller changes in O₂ levels matter. The fact that air quality survey equipment does not routinely report O₂ levels is at least an indication that those responsible for the measurements don't think O₂ level changes matter. They could be wrong, of course, but I think more than a small set of measurements from equipment not intended for that purpose would be needed to be at all convincing.

 

(The steady increase in overall outdoor CO₂ levels does have indirect effects on human health via climate change and ocean acidification. However, measuring levels on a day-to-day basis in city centres, etc, won't tell us much interesting about that.)

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44 minutes ago, scottishjohn said:

maybe there are pollutants that are displacing oxygen in cities - I cannot say for sure 

but i can say for sure that the same car dynoed in the city will showmore power when same test @same ambient temp / baro pressure  is done on  a dyno of the same make  outside a city .

this discussion started regarding MVHR+air quality + need for it  if occupants open windows on a daily basis, as my wife will do no matter if MVHR is fitted or not 

 ,and for sure air quality is very much better outside major conurbations .

 it is strange that all air quality survey equipment -which you can get live data from show neither co2 or o2  contents,especially when the big cry about saving the planet is about lowering co2 

 (extraordinary claims ) --Not calling me a liar? -

i am reporting what was observed  on lots of occasions that's all

 

 

As I mentioned earlier, doubling the concentration of CO2 (chosen because it's the greatest single pollutant in urban areas) displaces less than 0.01% of atmospheric O2, below the capability of a vehicle gas analyser to detect reliably.

 

The biggest single difference between dyno readings I used to see on my old hillclimb/sprint cars was down to variations from one dyno to another (these could easily be around 5% to 10% alone).  The next greatest variation was from temperature density, as even with the dyno fan running flat out the air temperature at the intake would vary in line with the weather and with how many runs had been done that day. 

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35 minutes ago, scottishjohn said:

 (extraordinary claims ) --Not calling me a liar? -

 

I called your claim "extraordinary" because it contradicts basic physics (or at least what I understand of it). I didn't say you were a liar, I said that your arguments as to why there might be variations in measured O2 don't hold water, so "clearly something else is going on". Nothing you've said changes my opinion.

 

I'm done with this thread. Apologies to the original poster for contributing to it drifting so far from topic.

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