What they don't teach you in Nitrox class...

[PerroneFord]

That was going to be my question... what analyzers are you guys using? I've been looking at some to buy. I've used an OxySpy, Analox, and an Oxycheq. I've seen others. What do you guys like?

 

[PerroneFord]

Yea, Soggy, altitude might make all the difference. I know when I calibrate to o2 and check ambient I am at about 20.7 or so on the few times I've done it. But getting 100% oxygen is rare from what I hear. I'm generally only diving 32 or 36 anyway, and am nowhere NEAR MOD for either gas, so I just need it close. However, if I was getting into the differences Im hearing about here (ask for 32 get 38????!!!) I'd use another shop or blend it myself.

 

[in_cavediver]

Soggy, your suspicions were close, but I guess it's not the sensor, it's the user. LOL

I can't believe it either.

I believe if you think it through, it will make sense. Think about the fact that he mentioned that pressure was required to get the proper flow rate. Now think about ambient air movement with no pressure and how 02 sensors work (consuming O2 to generate a small voltage). Given all of that, I'd say the design of the sensor/sensor housing is such that air is somewhat trapped when not under pressure, thus generating a lower reading due to the consumption of 02 by the sensor. The balance point would be somewhere in the 17-18% range for ambient diffusion in his case. It could be verified by watching the sensor after being removed from a pressurized air system. If what I suggested is actually happening, it would be slow drop to 17% and then stabilize.

Mike

 

[Soggy]

That is a very nice scientific explanation, but it does not fit my empirical observations.

I believe if you think it through, it will make sense. Think about the fact that he mentioned that pressure was required to get the proper flow rate. Now think about ambient air movement with no pressure and how 02 sensors work (consuming O2 to generate a small voltage). Given all of that, I'd say the design of the sensor/sensor housing is such that air is somewhat trapped when not under pressure, thus generating a lower reading due to the consumption of 02 by the sensor. The balance point would be somewhere in the 17-18% range for ambient diffusion in his case. It could be verified by watching the sensor after being removed from a pressurized air system. If what I suggested is actually happening, it would be slow drop to 17% and then stabilize.

Mike

 

[Kevfin]

It is a new sensor.

If I connect it to a tank of air, and adjust to 20.9% then read O2, I read 100%. If I then expose the sensor to ambient air with no flow, it will read 17.7%.

If I calibrate it to 17.7% on ambient air, then connect it to a bottle of O2, I read 100% so I'd say the sensor is fine. It's all about the flow rate.

The hose that connects to the sensor, if you put it into your mouth you could not blow through the restriction, it takes more pressure to allow the correct flow than a humans lungs can create, hence the need for a first stage connected to the tank.

I was trained to calibrate off of ambient air, but it sounds like that may not be the best idea, depending on the type of meter you use and altitude.

It seems like a good idea to find out if the sensor one uses has a different reading for ambient air and tank air (under pressure) and if that number is always consistent.

If you don't have an air tank to calibrate off of, do you calibrate the meter to 17.7%? Then check your tank? If you haven't had to deal with that issue, do you think if I calibrated my sensor to an air tank and then measured ambient air, I could always calibrate the meter to that ambient air reading (at similar altitude) before measuring a tank? Would that be safe?

 

[Jason B]

Soggy, the sensor is from Analytical Industries. It's a PSR-11-39-JD

Ever hear of these before.
Any suggestion on a better one?

Steve, you don't have to believe it. I'm looking at it and have a hard time with it, that is why I asked Soggy.

Oh, and I'm not at altitude.

 

[Soggy]

Do It Easy had a good explanation of how to do it....check your air by calibrating to ambient, then recalibrate to your air if the analysis is close.

 

[Soggy]

Honestly, I don't know a lot about the different brands of sensors. The ones I have primarily used are the ones included with the Oxycheq El Cheapo, the Oxycheq Expedition, and the OMS Analyzer, but 3% sounds ridiculous.

http://www.oxycheq.com/sensors.html

Soggy, the sensor is from Analytical Industries. It's a PSR-11-39-JD

Ever hear of these before.
Any suggestion on a better one?

Steve, you don't have to believe it. I'm looking at it and have a hard time with it, that is why I asked Soggy.

Oh, and I'm not at altitude.

 

[do it easy]

Soggy, the sensor is from Analytical Industries. It's a PSR-11-39-JD

Does your sensor ever read above 100%. Maybe it's grossly off and when you measure mostly pure O2, it's just pegging the meter. I agree with the other guys that 3% between ambient and tank is HUGE. I could believe .3% as in 20.9% versus 20.6%

I've heard that every once in a while, you get a DOA sensor..

 

[do it easy]

That was going to be my question... what analyzers are you guys using? I've been looking at some to buy. I've used an OxySpy, Analox, and an Oxycheq. I've seen others. What do you guys like?

I have an OMS with the long life sensor- I bought it in 2004 and the cell is still going strong.

I think that most analyzers are good, as long as they have an in-line flow meter/restrictor. I don't like the ones where you push up to the tank valve because it's not fool proof, especially with DIN valves