Nitrox: How to calibrate/analyze

[JS1scuba]

I recently passed a Nitrox course; at the shop I was taught to analyze by attaching the analyzer to a tank of regular air, adjusting the dial on the analyzer until it reads "20.9" or thereabouts, and then remove the analyzer and place it on the tank containing the EANx mix.

However my friend was telling me that you can just leave the analyer in the open air and calibrate it that way--would anyone be able to give me a step by step procedure for doing it that way? thanks!

Calibration of an analyzer without a microprocessor must be done at the working pressure of the gas you will be sampling, and never greater that 3 LPM. Here is an article that Dr. Hamilton and I wrote on this subject.

You can visit it at my other website

http://www.nitroxdiver.com/Library/analyze.html

The only analyzer specific for diving that does not need a specific flow to calibrate is the Oxyspy, which has an auto calibrate feature built into the electronics. You can find that item at: http://69.72.128.66/~scuba/tdl/store/index.php?main_page=product_info&cPath=10&products_id=224 and there is a link for the Users Manual as well at the bottom.

Regards,

 

[padiscubapro]

I never calibrate based on a tank because I don't know what's in it. I always calibrate with open air (turn on analyzer, adjust to 20.9, then connect to EAN tank and continue with the process specific to your analyzer). I prefer those with the flow control.

--Matt

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

 

[Upwelling]

How are you supposed to do this if you are supplied tanks on the boat? Use another diver's air tank?

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 19.1, 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..

 

[JS1scuba]

How are you supposed to do this if you are supplied tanks on the boat? Use another diver's air tank?

Bingo ---- if a boat is supplying nitrox cylinders then they are responsible and obligated to have available an analyzer and a means to calibrate it.

However, having used oxygen enriched air now for 15 years, if you are going to dive gas often you need to have your own oxygen analyzer. There are a variety of quliaty units on the market today, (ours included). Most are under $300. Well spent money.

Regards,

 

[SDAnderson]

If you're diving nitrox, you should have your own analyzer and use it - both at the shop when you pick up the gas and again, shortly before you hop in the water. Consistency is one of the first steps to help you avoid making mistakes and "shortly" would include before the tanks are loaded or before the boat left the dock, if you like.

Calibration normally requires a controlled flow of air over the sensor, so waving an analyzer around in the air won't give you an accurate reading. You need a control source, another diver's tank will do in a pinch - but make sure he isn't diving nitrox and doesn't have a nitrox band on his tank. I wasn't aware that the OxySpy doesn't require a controlled flow to calibrate, that's a nice feature and worth checking out.

 

[Charlie99]

My method is to do a rough calibration with room air, then check it with a tank of air.

Using room air will cause small errors every time. Using a tank usually is more accurate, except for dangerous, gross errors caused by using a tank that isn't air.

---------------------------

Advantages of calibrating with room air:
you know it is air

Disadvantages of using room air:
It is not at the same flow rate/slightly higher pressure that will be seen by the analyzer when checking the nitrox.
Humidity in air affects the reading.

The combination of these two effects may cause you to calibrate the analyzer such that it reads higher than actual value. In my experience with several different analyzers, the reading of the nitrox tank is about 1 percent high.

-----------

Advantages of calibrated with air from a tank:
More accurate because you are using dry air at the same flowrate/pressure as the tank to be analyzed.
Disadvantage of calibrating from a tank:
ARE YOU REALLY, REALLY, REALLY SURE IT IS AIR!
You have the potential for major errors either high or low if the tank isn't what you think it is.

 

[DA Aquamaster]

Padiscubapro hit it on the head. The speed of the chemical reaction in the O2 sensor that determines how much current is generated and sent to the analyzer is based on the partial pressure of the O2 in the sample, not the precentage of O2 present in the sample.

So calibration is required as the "correct" calibration needed to give an accurate reading will vary with barometic pressure. On my analyzer, where the potentiometer is adjusted for proper calibration at sea level is a lot different than where it is at 3000 ft where I normally fill my tanks, at 4000 ft where I live, or at 5000 ft where I usually dive.

Most quality analzers also use a restrictor to deliver a fixed flow rate into a small chamber at the sensor itself. This creates a slight (but very consistent) increase in pressure inside the senor above ambient pressure. If you calibrate with the sensor exposed to open air at ambient pressure, the increase in pressure that will occur when the analyzer is connected to the nitrox tank to be tested will affect the accuracy of your analysis.

This can be a problem if you don't have a known tank of 21% to calibrate with. However, at a given altitude with a given analyser the difference caused by the pressure difference inside the sensor is fairly constant. So if you calibrate with a known 21% sample at a given altitude and adjust the reading on the analyser to 20.9% and then, without changing the calibration, expose the sensor to open air for a few minutes, you will get a slightly lower reading. With my analyzer at 4000 ft I will normally get a reading of approx 20.5. So in a pinch, as long as the barometric pressure is the same, I can calibrate in open air and adjust the analyzer to 20.5 rather than 20.9 and expect to get a fairly accurate result. The particular difference in readings will vary depending on on barometirc pressure and the characteristics of your particular analyzer.

The calibration difference of .4% does not seem like a big deal, but I have noted that not accounting for this difference can produce a error of 3-4% with 40 to 50% nitrox.

If you are diving nitrox on a regular basis, you really should invest in your own analyzer. Oxycheck sells an O2 analyzer kit that is not overly hard to assemble (2-3 hours with some basic soldering required) for about $130 including the extra accessories you will need. They work well and are pretty cheap insurance and peace of mind.

 

[padiscubapro]

This can be a problem if you don't have a known tank of 21% to calibrate with. However, at a given altitude with a given analyser the difference caused by the pressure difference inside the sensor is fairly constant. So if you calibrate with a known 21% sample at a given altitude and adjust the reading on the analyser to 20.9% and then, without changing the calibration, expose the sensor to open air for a few minutes, you will get a slightly lower reading. With my analyzer at 4000 ft I will normally get a reading of approx 20.5. So in a pinch, as long as the barometric pressure is the same, I can calibrate in open air and adjust the analyzer to 20.5 rather than 20.9 and expect to get a fairly accurate result. The particular difference in readings will vary depending on on barometirc pressure and the characteristics of your particular analyzer.

The calibration difference of .4% does not seem like a big deal, but I have noted that not accounting for this difference can produce a error of 3-4% with 40 to 50% nitrox.

This is surprising.. at 4000ft your value if its at ambient should be somewhere around 17.9.. I'm guessing that the cell is not truely equalized to ambient... this can happen if the back side of the cell is sealed and there is a pressure imbalance between the front and rear of the cell..
I cant think of another way without having some stagnant other than having some stangant pressurized gas in the chamber..

But the error on the higher mix is close to expected..

 

[The Kraken]

Web Monkey wrote:
"The shop will typically have a calibration air cylinder next to the o2 checker just for this purpose."

. . . and who certifies that this cylinder is of correct mixture? Is it the responsibility for the LDS to supply a "calibration tank" every time a tank is provided?

 

[padiscubapro]

Web Monkey wrote:
"The shop will typically have a calibration air cylinder next to the o2 checker just for this purpose."

. . . and who certifies that this cylinder is of correct mixture? Is it the responsibility for the LDS to supply a "calibration tank" every time a tank is provided?

If a store is pumping Nitrox, there should be a calibration source available.. if they don't do this as practice find another shop, because you don't know what else they are lax in..

Unless the shop is really careless in how they bank their gases and do their fills, air is going to be air....
At sea level you can see if the meter is in the ballpark (after proper calibration) by "checking" it against ambient it should be a few tenths low, the exact value doesn't matter, its just an indicator to see if you are close..

Calibrating of a reg with a controlled flow, is making the claibration more accurate because its getting calibrated at the flow and "presssure" the unknown gas will be samples, plus the gas is DRY , humidity effects the calibration value..