Measuring O2 in enriched air

[hroark2112]

Altitude also matters. If I'm at home, without any correction applied, my rebreather thinks I'm gonna die.

How does your rebreather know you’re diving split fins??

 

[hroark2112]

I personally bring my tanks to the nearest CrossFit class and tell them it is the latest Keto-approved exercise!

 

[TheGraveyardDiver]

Shake them tanks, boy!

Yeah we had a similar initiation procedure in the Army.
We'd have the new guy who just arrived to the unit get up on top of the tank turret and jump up and down while we "checked the shock absorbers to see if they needed replacing."
It was funny as heck until the Sargent Major walked by the motor pool one Monday morning and saw the cherry jumping up and down on the turret flapping his arms around.

 

[HKGuns]

Direct from the PADI Nitrox course book:

"EANx that was mixed using partial pressure blending in your cylinder may need some time to mix evenly to get an accurate reading. Rolling the cylinder back and forth speeds this up, but most divers simply let it sit for 30 minutes to an hour before analyzing."

 

[W W Meixner]

I just recently completed the PADI enriched air course. My coursework said that it was important to calibrate the O2 measuring device from a compressed air source having the same flow rate as the enriched air source you would later test on. My instructor disagreed, and said that it wouldn't make any difference if you calibrate based on ambient air and then tested on enriched air which is under pressure. After calibrating on ambient air, he then proceeded to test several enriched air containers, and came up at least 2 percent shy of the O2 that had been marked on each of the cylinders by the person who previously filled and tested them. I suggested that perhaps it was a result of him not calibrating on compressed air, and he dismissed me. Since I don't own an O2 measuring device of my own, I can't really test if this is true or not, but I'm just curious -- does it make a difference like PADI says, or is my instructor correct?

MN...

I have an AI Palm...calibrates in ambient air...always within a few tenths of a point of desired mix...if the mix is correctly blended...the analyzer will indicate that...

In a shop that does a lot of blending and has multiple analyzers...its alway prudent to double check your compressed calibration gas with two different analyzers...as you have no idea who filled the cylinder...or with what...

If you ask for 32%...and your analyzer indicates 32.7%...calculate your MOD from 32.7...not 32...calculate your MOD to the higher number

Your MOD at 1.4 ATA for 32% is 111.3 ft...for 32.7% your MOD is 108.2 ft...

108.2 ft...staying with 1.4 ATA/NDL...is the safer depth...

Best

Warren...

 

[KenGordon]

Shaking a tank of gas has no effect on how the molecules mix. Gas molecules move very fast. Any gas mix is effectively mixed, with the cylinder just sitting, rather quickly. Although it would be rather amusing if you could get some video of your instructor shaking his scuba cylinders to mix the gas. Sounds like the kind of prank an instructor would play on his DM candidates.

Unfortunately, people tend to want to understand new things by trying to relate them to other things they already understand. Two liquids in a jar, like salad dressing, can separate due to their different densities (and the fact they are liquids.) And you can see them mix when you shake the jar.

But gases are different. Because they are not liquids.

I used to think that people were making it up about needing to shake cylinders. School physics tells us it should all be mixed pretty much instantly. However I have had this happen, completely mad o2 and He readings fixed by a bit of shaking about or waiting.

 

[Billy Northrup]

I've worked several fill stations, I use handhelds to verify tank mix, but when mixing I use a flow re-stricter and a more robust HE/O2 analyser.

A few handhelds I've seen have a port for a hose, I've been wanting to try to keep the gas flows consistent.

Continuous blending solves many problems for the average Nitrox shop.

 

[loosenit2]

Measure with a micrometer, mark with chalk, cut with an axe, hammer to fit and paint to cover it up.

There is no science that supports a ppO2 and its associated MOD of 1.2 vs 1.4 vs 1.6 vs 1.8

Yes, you want to make sure some bozo hasn't put 80% O2 in your tank (or you're planning on 32% and get plain-old-air), but beyond that the algorithm your computer uses for NDL is a bigger factor than measuring your tank mix to 0.1%

I don't disagree with you that there is not definitive scientific proof that PPO2 limits should be imposed as hard and fast rules, but there are solid reasons that agencies have adopted a 1.6 limit for recreational diving. Would strongly recommend reading the discussion that starts on page 88 in the 2001 DAN Nitrox Workshop, walks through the points and counterpoints in a very compelling way.

DAN Nitrox Workshop Proceedings.

 

[tarponchik]

The rate of air flow should not, and does not have any effect on the readings. What has an effect, is the temperature. You open the valve, the pressurized gas quickly expands and cools down.

 

[Storker]

The rate of air flow should not, and does not have any effect on the readings.

...provided the flow rate is low enough that you don't get an increased pressure at the sensor.

An O2 sensor is an electrochemical cell. Its voltage output is proportional to the PO2 close to it, not the FO2. So if the flow is so high that the cell experiences a pressure significantly above ambient, it will give a too high reading.