Nitrox: How to calibrate/analyze

[padiscubapro]

When calibrating an analizer for 80% mix, I alway thought using 100% to calibrate the analizer instead or 20.9% gave a safer and more reliable reading.

It will gove you a better reading near that calibration point as long as you are getting 100%.. just because its called oxygen doesn;t mean you have 100%..

In areas where oxygen is made from fractional distillation you canb be fairly sure its probably very close to 100%, if its made by a psa system 95.5% is the best you will get.. The minimum % that can be legally called oxygen is 90 %.

If your sensors are good, they are pretty linear and calibrating in air (with proper value and technique) gives a pretty reliable reading..

 

[padiscubapro]

I heard the analyzers don't last long and have to be replaced--doesn't this turn into a huge expense over time?

The sensors are replaced.. depending on use and how it is stored, a new sensor every 1 to 2 years is common...

Retail pricing on many sensors is around $75

 

[Rick Murchison]

actually not entirely true..

it really depnds on the sampling chamber.. the sensor chamber is always slightly above ambient (otherwise there would be zero flow).. if the gas flows freely enough (and flow rate low enough) the pressure increase is small and the adjusted value will be much closer to actual near the calibration point, but the slope (of the output) is definately effected just a few tenths near the calibration point will generate a few % error at the top end.

I rather error on the side of caution.. The results will vary based on analyzer design (mostly chamber output), a 32% mix isn't going to be that far off from actual, but by the time you are checking an 80% mix, on most desings I would expect a few percent error.. The more restricted a chamber is or the higher the flow rate is the bigger the error will be..

if you keep a flow around 2lpm the pressure increase is very small and the error near the calibration point will be very small..

I did some tests a while ago ( for a ccr controller) to see how much of a pressure increase You could get by playing with flow rates in restricted spaces.. I was able to easily get changes of more than 3 psi (thats over 6k ft of elevation), it definately makes a difference..

Great day in the mornin'! We're talking about oxygen analysers for Nitrox here, using normal sampling procedures, not seeing "how much of a pressure increase" you can get with a CCR controller!
Don't confuse folks bringing in wierdness from left field.
I suppose if you had an instrument that was accurate to .0001% you might be able to measure a difference inside and outside a Nitrox analyser's sampling chamber at normal sampling flow rates, but for all practical purposes the pressure inside the sampling chamber is ambient pressure, and to suggest an altitude correction should be made in calibration of the sensor is completely wrong and potentially dangerous. Don't do it. To jack up the input flow to get 3 psi is crazy. Don't do it.
Rick

 

[Rick Murchison]

When calibrating an analizer for 80% mix, I alway thought using 100% to calibrate the analizer instead or 20.9% gave a safer and more reliable reading.

I think that's a fair statement. Especially if the sensor is less than pristine. But with a relatively new sensor I see a variance of less than 1% either way. As an added precaution when mixing deco gasses I always do the math (and use the compressibility charts) to see what the final mix should be. If the analysis is off my math by more than 1% then I dig up a couple more analysers before accepting the analysis. So far the math has always been confirmed in the "final analysis."
Rick

 

[padiscubapro]

Great day in the mornin'! We're talking about oxygen analysers for Nitrox here, using normal sampling procedures, not seeing "how much of a pressure increase" you can get with a CCR controller!

This was for doing research it for doing a calibration routine for a ccr controller.. Its not from left field it is definately on topic.. I was trying to see what was the most efficient way to calibrate the oxygen sensor.. if the flow was too high for the opening pressure rose pretty rapidly... on a small opening it doesn;t take much flow at all...
The test equipment was a metered valve into a chamber with a variable orifice.. the output was hooked to a digiital sampling scope to watch the sensor output.. The procedure was repeated over a wide range of gases.

If you are using something that keeps the flow restricted to 2lpm max for probably most of the analyzers out there it will should work with reason from ambient.. The whole pont was that people at altitude have to determine if they are effected with THEIR analyzer.. being off by .2 or .3 for gases near air, doesnt introduce that big of an error , the error does grow as oxygen content increases..

The person needs to determine it on his own, try it one way the test against a know gas.. I'm NOT suggesting ANYONE use the theoritical numbers.. it has to be determined for each analyzer/flow rate combination..

I was able to get a 3 psi rise in pressure with as little as 5 lpm flow rate.. I probably could have gotten it at a lower flow but I was at the limit of the nozzle I was using.

The principal between giving a reading for PO2 and for Fo2 is the same its strictly a voltage across a load resistor.. How you normalize the output will determine what you are reading..

 

[Rick Murchison]

This was for doing research it for doing a calibration routine for a ccr controller.. Its not from left field it is definately on topic.. I was trying to see what was the most efficient way to calibrate the oxygen sensor.. if the flow was too high for the opening pressure rose pretty rapidly... on a small opening it doesn;t take much flow at all...
The test equipment was a metered valve into a chamber with a variable orifice.. the output was hooked to a digiital sampling scope to watch the sensor output.. The procedure was repeated over a wide range of gases.

If you are using something that keeps the flow restricted to 2lpm max for probably most of the analyzers out there it will should work with reason from ambient.. The whole pont was that people at altitude have to determine if they are effected with THEIR analyzer.. being off by .2 or .3 for gases near air, doesnt introduce that big of an error , the error does grow as oxygen content increases..

The person needs to determine it on his own, try it one way the test against a know gas.. I'm NOT suggesting ANYONE use the theoritical numbers.. it has to be determined for each analyzer/flow rate combination..

I was able to get a 3 psi rise in pressure with as little as 5 lpm flow rate.. I probably could have gotten it at a lower flow but I was at the limit of the nozzle I was using.

The principal between giving a reading for PO2 and for Fo2 is the same its strictly a voltage across a load resistor.. How you normalize the output will determine what you are reading..

Your overpressures in the chamber are *still* altitude independent. If you're using a system that's so off the wall that you have a measurable positive pressure over ambient in the sampling chamber, it'll still be the same pressure over ambient regardless of altitude, and you still don't want to apply any altitude correction.
Remember, your LP hose is delivering its pressure over ambient to the flow restrictor too. The gas is thinner in the LP hose, through the flow restricter, in the analysis chamber and in the surrounding atmosphere.
Bottom line: Don't change your calibration numbers based on altitude, or you will get an inaccurate analysis.
DA Aquamaster's experience as related in his earlier post is perfectly normal and expected.
Rick

 

[padiscubapro]

it'll still be the same pressure over ambient regardless of altitude, and you still don't want to apply any altitude correction.

yes but if the sensor is calibrated by waving it out in air and the chosen flow rate cases a pressure rise, the measureg gas is wrong.. the greater the altitude the greater effect the pressure rise in the chamber has..

at sea level a 1psi rise has less of an effect on the measured value than at altitude. IF you keep flow rates low and keep the pressure rise very small then you are altitude /ambient pressure independent..

This is the ONLY difference in using a sensor for analyzing "fo2" or using it to monitor PO2.. in one way you are adjusting the output to match ambient pressure * FO2 of gas, in the other you are adjusting it so that the the fo2 is read and ambient is supposed to remain constant..

It doesn't take a bad design to cause a rise in pressure.. I have seen way too many people using flow meters that are set to high or flow restrictors that needed adjustment.. If the person can easily HEAR the has hissing its too high a flow rate, I can't count how many times I run acrss this.

 

[DA Aquamaster]

I must have missed where someone suggested soem type of fixed altitude correction be applied.

What I stated is that you WILL notice a difference in where the potentiometer is set when calibrating at different altitudes whether you do it open air or off a tank and through the restrictor. It's an empirical observation - not theoretical - as is the observation of the difference in readings when calibrating in open air and off a tank of 21%.

Personally, I don't buy the humidity argument as the sole cause of the difference as I live in a very dry areas where 10% humuidy is the norm. I have also not noted any variation on days when the humidy was higher. It may well contribute to things but it is a much smaller factor (Humidity is a factor in density altitude as well as is temperature but they are smaller factors than barometric pressure unless the differences in temp or humidity are extreme.) What is independent of either relative humidity or barometric pressure is the increase in pressure in the system itself when testing off a tank as oppsed to open air. A higher PO2 is logical, expected and consistent with the results on the analyser. My observations are also done with a 1 lpm flow rate, so excessive flow rate is also not the issue here.

 

[matt_unique]

this method is flawed because changes in atmospheric pressure alter your readings.. At altitude you will get a VERY wrong reading....

The best method is to use a take of air, using a reg with controlled flow, then set to 20.9, now use the same flow rate with a reg on an EAN cylinder..

if you calibrate based on ambient air at altitutude, then use a controlled flow to measure a tank
if you want to get it close you would have to adjust the meter for a different percentage
.., at 1000 ft the corect ambient "air" reading would be about 20.1, at 2000ft the corect ambient "air" reading would be about 19.4%, by 5000 ft its down to about 17.3%..

The cells don't actually measue oxygen percent they measure po2, we normalize this to a known value to trick the analyzer.., if you use a trapped gas using a controlled flow, you can still do it this way..

If you calibrate based on holding the cell in air the flow rate has to be low on the gas to be tested...

You should have been taught this in any good Nitrox class..

EDITED: Had type for value at 1000ft... corrected

This is an interesting thread. It is a bit of analysis paralysis though. I know we measure pressure of 02 and not %, hence the need for travel gas for trimix divers, etc. I analyze my fills at about 300' above sea level and I dive in the ocean. The difference you are referring to is negligible and less than the accuracy of the analyzer. In my opinion it is better to calibrate using ambient air than to trust what is in a tank. The risk of a tiny measurement difference is much less than the risk of what may or may not be in a tank at a dive shop.

--Matt

 

[DA Aquamaster]

Trusting what is in the tank is a valid point, but there are going to be warning signs if things are not as they should be.

If you analyze off what you think is really 21% and it is in fact someone's used 36% tank that was topped off and ended up around 25% then you are also not going to get the results you expect when you analyze your nitrox tank. If the calibration mix is too far off from 21%, you won't even be able to calibrate to 20.9 with many analysers as the potentiometers may not have the neccesary range.

If an odd and unexpected analysis result occurs after calibrating on a tank alledged to be 21%, I would first remove the sample chamber from the sensor an see what the open air reading is. I would expect it to be a few tenths to maybe 1 % lower than 21% and if so, it's a good indicator that the calibration tank was accurate and that the mix is wrong in the nitrox tank you were analysing. On the other hand, if the open air reading is significantly off from 21% then it would be a good indicator that the tank you calibrated from is not really 21%.

The important thing here is that the diver have a basic understanding of how the analyser works to be able to figure out where the problem lies and then recalibrate and retest accordingly.

On normal nitrox mixes of 36% or less that are used at a 1.4 PO2 or less on no deco dives with saftey stops, I would agree that the difference caused by analyzing with an open air calibration versus off a tank of 21% is most often not going to present a signficant risk.

But if I am checking a tank of 50% that I will be using for deco at a PO2 of 1.6, the size of the error is potentially greater and I am also much more concerned with knowing if it is fact 45% or 55% rather than 50% due to the effects this will have on MOD and PO2.