Nitrox Questions

[stever2002]

Just a few of Nitrox queries:

1. Do you always round the analyzed PO2 value UP to the nearest whole value when entering it into your computer? I.e. if your tank was read at 35.1, do you enter "35" or "36"?

2. Assuming you don't have your own, do you have the operator calibrate the analyzer IN FRONT OF YOU prior to reading your nitrox tank?

3. Assuming you DO have your own, do you ask the operator for a tank of air to calibrate your analyzer prior to reading your (nitrox) tank?

An analyzer is quickly moving up my list of "must haves".

Thanks for the advise,
Dive Safe,
...steve...

 

[stoddu]

The manual for my Suunto computer says to enter the lower number, in this case 35.

 

[Damselfish]

you enter the lower number because it is much more likely NDL will be the controlling factor, not O2 exposure. Rounding up would result in the computer giving you more bottom time than is correct for the real mix. (Not that it's all that precise, but it's just the more conservative thing to do.)

Most places have an analyzer but you do it yourself. If it's a place where they do it for you, you watch them calibrate and analyze. You don't always need a tank to calibrate depends on the analyzer but there should be one available if you do. It's better when you don't - you know for sure what's in the air around you but not another tank.

 

[harleythedog]

Agree with the suunto comment, thats what I use as well.

I usually ask the operator when the last time the analyzer was calibrated, if its more than a few hours or get an fluffy answer, I will calibrate it myself. I grab a tank nearby, usually one of their rentals, and use that to calibrate. I will ask them what tank I can use to calibrate and will cordially tell them I am calibrating the analyzer. I have taken the dont ask - just do approach being that its my nitrox and my right to get what I believe is an accurate reading. If a mistake is made, I made it, not someone else. I have never had a problem with this approach, if I did, I wouldnt visit the LDS again.

I have been in some shops where the operator will analyze for me. In this case, I ask them to please calibrate and they do the calibration and reading in front of me.

 

[stever2002]

I had guessed it was the lower value - that would be the more conservative value to use. I.e. Always round down. I was wondering if anyone rounded down between .1 and .5 and rounded up at .6 and above. I'm probably picking nits here. Just curious.

My LDS is very by-the-book when it comes to calibrating, reading and recording PO2 values. Since they taught me, thats the way I am as well. However, in just a few visits to other diver operations, I have seen some rather cavalier approaches to this procedure.

I have heard one story where some friends went on a trip and didn't like the way that the opperation was handling the readings and calibration. So they just dove air for that trip. Better safe then sorry.

Thanks,
...steve...

 

[JKSteger]

I've been in shops where I wondered if they even knew what they were doing with their nitrox. I went to one that was out of banked nitrox so they had to to a partial pressure blend (thank goodness my tanks were 02 clean). They didn't even check my tanks to see if they were 02 cleaned! Second, the fill guy couldn't even get an accurate reading when analyzing the tanks. Third I find out the fill station's 02 analyzer has a bad 02 sensor.
Finally had to get the shop to break out the 02 analyzer that was for sale and use it to check my tanks.
My next step was to just dump the tanks and get a good old AIR fill.

Oh and what topped it off was that everyone at the shop thought I was over reacting and they approced not knowing exactly the 02 in the tank as not being that big of deal. (I'll never use them again for a fill)!

Jeremy

 

[HowardE]

Just a few of Nitrox queries:

1. Do you always round the analyzed PO2 value UP to the nearest whole value when entering it into your computer? I.e. if your tank was read at 35.1, do you enter "35" or "36"?

2. Assuming you don't have your own, do you have the operator calibrate the analyzer IN FRONT OF YOU prior to reading your nitrox tank?

3. Assuming you DO have your own, do you ask the operator for a tank of air to calibrate your analyzer prior to reading your (nitrox) tank?

An analyzer is quickly moving up my list of "must haves".

Thanks for the advise,
Dive Safe,
...steve...

1. Personally - I round down for NDL planning, and round up for PPO2 exposure for additional safety margin. For example, if I get a reading of 35.4% I plan my NDL's based on a mix of 35% and I calculate my MOD based on a mix of 36% - this gives an added safety margin on both ends.

2. I calibrate the analyzer myself. Just be aware, that some analyzers like

This one in the photo - the base O2% of air vary according to humidity, so make sure you take that into account. If you use my method for calculating your dive plan in question 1, then a few 10ths of a percent aren't going to matter that much.

3. I don't have my own (I never really felt the need to get one - everywhere I dive has one, and the shop I get fills at has one), but a tank of air, or just THE AIR we breathe should be adequate for calibration of your analyzer. Just don't blow ON the O2 sensor. Have air pass over it.

If I was in a place where nitrox wasn't so readily available, or the shop where I got fills, wasn't one of the most reputable nitrox shops in the country, I would probably consider an analyzer more... but for now, I don't see the need for one (FOR ME)

 

[Blackwood]

3. Assuming you DO have your own, do you ask the operator for a tank of air to calibrate your analyzer prior to reading your (nitrox) tank?

If possible (can't easily be done with all analyzers), I calibrate with the air in the room. Otherwise, since I can't know what's in a tank, I'd be calibrating to an unknown.

 

[wedivebc]

Just be aware, that some analyzers like

This one in the photo - the base O2% of air vary according to humidity, so make sure you take that into account.

I would imagine all analyzers are affected by humidity in the same way since they all use the same method of measuring oxygen.

 

[HowardE]

I would imagine all analyzers are affected by humidity in the same way since they all use the same method of measuring oxygen.

I'm not sure myself, but that's the only analyzer I've seen with a relative humidity and temperature chart to calculate the starting point. Although it is all within .5% so a relatively small margin of error either way, as long as you calculate your MODs and NDL's within safe margins.