Shipwreckscanada
Contributor
When I took a Nitrox course a little while back, we tested the air at the shop and again at the dive site. Is the dive site test really necessary if I’m diving the same day I fill my cylinders?
Is a Nitrox test at the dive site necessary?
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CuzzA
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I don't own a personal analyzer. So, my answer is no. I have my tanks filled, "I" analylize at the shop after confirming calibration, label appropriately and dive.
If I owned a personal analyzer, I would double check the reading at either the fill station or dive site.
If I owned a personal analyzer, I would double check the reading at either the fill station or dive site.
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Assuming I analyzed and labeled the tanks, I would not feel obliged to reanalyze them at the dive site
Stoo
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I am one of those guys that thinks an analyzer should be included in the cost of any nitrox course, so yes, I think it's a great idea.
If you personally tested a tank at the dive shop, then there's probably not much chance anything will have changed, but it isn't impossible.
I dive doubles most of the time, and I find that the mix will change slightly over the few hours following a fill, and it's just the result of the manifold restricting the flow and mixing of the O2 and air as the tank is filling. The change is pretty minor however, and not something that would effect my dive.
I lost a friend a few years ago who neglected to test his gas at the dive site, and died as a result.
I have also had a shop insist that I had a "32" fill, and when I insisted we check it again in my presence, the tanks were actually at 70% (!). I'd have died had a I used this fill.
Testing a tank is about a 30 second operation... a small step to verify that you are breathing what you think you are.
Nitrox is a great tool to bring an additional margin of safety to our diving.
It can also kill you without warning.
The other important reason for testing the tank on-site is because your buddy will also know what's in the tank. After all, the gas in your tank, may end up being your buddy's gas in an emergency... and vice-versa.
In other words, a short round of gas testing for you and your buddy should form part of your suiting up procedures.
Nitrox is a great tool to make our diving safer.
It can also kill you without warning.
The cost of an analyzer is minimal and should be in your kit if you're diving the stuff.
If you personally tested a tank at the dive shop, then there's probably not much chance anything will have changed, but it isn't impossible.
I dive doubles most of the time, and I find that the mix will change slightly over the few hours following a fill, and it's just the result of the manifold restricting the flow and mixing of the O2 and air as the tank is filling. The change is pretty minor however, and not something that would effect my dive.
I lost a friend a few years ago who neglected to test his gas at the dive site, and died as a result.
I have also had a shop insist that I had a "32" fill, and when I insisted we check it again in my presence, the tanks were actually at 70% (!). I'd have died had a I used this fill.
Testing a tank is about a 30 second operation... a small step to verify that you are breathing what you think you are.
Nitrox is a great tool to bring an additional margin of safety to our diving.
It can also kill you without warning.
The other important reason for testing the tank on-site is because your buddy will also know what's in the tank. After all, the gas in your tank, may end up being your buddy's gas in an emergency... and vice-versa.
In other words, a short round of gas testing for you and your buddy should form part of your suiting up procedures.
Nitrox is a great tool to make our diving safer.
It can also kill you without warning.
The cost of an analyzer is minimal and should be in your kit if you're diving the stuff.
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not.
I live by myself, my tanks are stored in my room, after I get a fill, I analyze them. I write on them what's in there (easy, it's always 32-ish, because that's what I fill them with). What's gonna happen to them? Is a robber gonna come, steal the tanks, empty them, fill them on pure O2, put them back?
The testing is necessary if there's any way you have the wrong tanks in your hands. I wouldn't trust a label to be sufficient for that however (if there's other tanks and other people around the place), so yes, in your situation I would double test them.
I would not say it is absolute.
Let's take the case of my most recent dive. I picked up my tanks at the dive shop and analyzed them. I put tape on the tanks and put my initials and the results of the analysis on the tank. I carried them 50 feet to the boat and put them in their places. While the boat was en route, I set up my gear. When the boat arrived at the dive site, I did not analyze my tanks again. I had complete confidence that nothing had changed in the interim. I feel pretty safe in saying that the percentage of divers who would do the same thing in the same situation rounds off to 100%. I include Jax in that group, because I have seen it happen.
OK, that's extreme, but very common. Short of that, you have to use good judgment. What is the possibility that things are not correct? If there is any chance, by all means analyze. In my diving, I sometimes make nitrox at a shop and then analyze it. I then put all the tanks in a van and drive 6 hours and dive for several days. The chances that what it says on the tape is wrong are slim indeed, but those chances exist, so I analyze.
Now, let's raise those odds a little more. Several years ago a diver in south Florida was all set to do a deep dive when he broke his foot and could not dive for several months. When he was able to dive again, he scheduled a dive on a wreck at about 160 feet. He clearly remembered that the double tanks he had filled before breaking his foot were filed with air so he did not bother to analyze. After he died, the gas was analyzed at 36%.
In another case of memory trumping proper practice, a diver arrived at Ginnie Springs in Florida planning to dive to about 100 feet using a tank that clearly said "oxygen" on it.. His friends asked him about it, and he said that even though it said "oxygen," it was actually air--he had filled it himself. His friends said he should analyze it, but he refused. He knew damn well what he had put in his own tank. After his death, the analysis showed that the tank did indeed contain pure oxygen.
Let's take the case of my most recent dive. I picked up my tanks at the dive shop and analyzed them. I put tape on the tanks and put my initials and the results of the analysis on the tank. I carried them 50 feet to the boat and put them in their places. While the boat was en route, I set up my gear. When the boat arrived at the dive site, I did not analyze my tanks again. I had complete confidence that nothing had changed in the interim. I feel pretty safe in saying that the percentage of divers who would do the same thing in the same situation rounds off to 100%. I include Jax in that group, because I have seen it happen.
OK, that's extreme, but very common. Short of that, you have to use good judgment. What is the possibility that things are not correct? If there is any chance, by all means analyze. In my diving, I sometimes make nitrox at a shop and then analyze it. I then put all the tanks in a van and drive 6 hours and dive for several days. The chances that what it says on the tape is wrong are slim indeed, but those chances exist, so I analyze.
Now, let's raise those odds a little more. Several years ago a diver in south Florida was all set to do a deep dive when he broke his foot and could not dive for several months. When he was able to dive again, he scheduled a dive on a wreck at about 160 feet. He clearly remembered that the double tanks he had filled before breaking his foot were filed with air so he did not bother to analyze. After he died, the gas was analyzed at 36%.
In another case of memory trumping proper practice, a diver arrived at Ginnie Springs in Florida planning to dive to about 100 feet using a tank that clearly said "oxygen" on it.. His friends asked him about it, and he said that even though it said "oxygen," it was actually air--he had filled it himself. His friends said he should analyze it, but he refused. He knew damn well what he had put in his own tank. After his death, the analysis showed that the tank did indeed contain pure oxygen.
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I routinely analyze my tank as I set up my gear. There are indeed situations where such an analysis seems unnecessary, and then there are situations where an analysis on the site is prudent. I don't really trust myself when it comes to potentially deadly situations, so for me, it's easier to follow one single procedure: Always re-analyze at the site.
Even if you drag the tank from the filling station straight to the dive site, there's a risk for mixups: http://www.scubaboard.com/community/threads/mistaken-identity-travel-and-deco-gas-mixup.518087/
Even if you drag the tank from the filling station straight to the dive site, there's a risk for mixups: http://www.scubaboard.com/community/threads/mistaken-identity-travel-and-deco-gas-mixup.518087/
Chugwhump
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It depends on an individual trip by trip, and individual basis.
I am the tank bee-otch as a general rule for the large group of people that I dive with.
This is because I have a large truck and I work very near our local fill station of choice.
There are as many as 20 tanks at a time in my garage.
Essentially when we come back in from a boat trip,
everybody throws their tanks into the back of my truck,
and I get them filled during the work week.
I generally fill the tanks myself.
And generally we are just diving premixed gas that comes out of the bank.
And generally this mix ranges between 36 and 38%.
So when I analyze the tanks, I of course label them with a piece of tape.
And then I usually find myself transporting the tanks to whatever boat we are going out on.
Once in a while somebody will come in from out of town and use the tanks that we toss out there from my group, and they will very often analyze the tanks themselves.
Which I think is a very solid practice.
I would say if you are using RENTAL tanks it is a really really good idea (even if you tested your tank at the shop yourself, but somehow lost chain of custody even if it means for 15 seconds, you handed your tank to a dive master on the back of a boat and he/she stowed stowed it) to re-check that tank at your seat.
Rental tanks can look remarkably alike, with nearly identical scuff marks, peeling patches of paint, initials, dive shop decals, and every other dangerous similarity in the world, is a VERY REAL DANGER or other possible situation.
The other wild card to this is that if you are having your tanks filled at a partial pressure fill station,
there can be a 2% to as much as 4% range in the true percentage of oxygen that the sensor will register if you do not roll of the tanks around for a little bit immediately after a partial pressure fill.
There have been several conversations on Scubaboard in the past, about this process but here is the quick rundown on this potential variance in an accurate O2 reading.
If you put in 300 or 500 psi of oxygen, and then put air on top, and then test the tank without rolling around on the floor, there can be up to a 4% variance in the accurate reading of the gas that is in the tank.
4% would be the most extreme differential that I have ever personally heard of.
Some people are of the opinion that this only really happens if you test the tank immediately or very shortly after the fill being performed.
There is a body of thought that says if you leave the tank sitting there overnight that the gas will evenly distribute and you will get an accurate reading.
I am of the opinion that if a tank has been partial pressure filled, even if it has been sitting in the shop for a week waiting for pick up, you ought to at least roll it across the floor for 15 seconds before you put it on the analyzer.
My personal situation is this.
I very, very, very, rarely rent scuba tanks.
If I pick up my or my friends tanks at the shop that have been filled, I test them there.
They then go into my garage.
Then if I am going somewhere I throw them into my truck, and I drive to a boat, and I dive them without testing them.
But I have a certain laissez-faire attitude about this, because almost all of the tanks in my garage are within a point or two of one another.
Generally in the 35 to 38% range.
This is a whole different ballgame if we are doing some special deep wreck or reef or even decompression diving.
When that happens, I will test the tanks at the shop, and when we arrive at the boat,
not only are we testing the percentages again, but we are also checking to make sure that the tanks are full.
But the mistakes do happen.
As recently as three weeks ago, I ended up grabbing two empties that somebody slipped into my garage, and did not test them.
The rule with my group is upon depleting a tank you have to remove the percentage mix label/tape from the crown.
Somebody did not do that, and I did not know that the tanks were in the garage empty.
My buddy and I had to do two dives on one tank apiece.
Which was not the end of the world, because a low pressure 120 can really do nice things for you if you know how to fill them correctly.
We ended up diving one hour and 45 minutes instead of the two hours that was allowed for that particular trip.
So it was no big deal.
Chug
Often laissaiz fair but nosy at the same time.
I am the tank bee-otch as a general rule for the large group of people that I dive with.
This is because I have a large truck and I work very near our local fill station of choice.
There are as many as 20 tanks at a time in my garage.
Essentially when we come back in from a boat trip,
everybody throws their tanks into the back of my truck,
and I get them filled during the work week.
I generally fill the tanks myself.
And generally we are just diving premixed gas that comes out of the bank.
And generally this mix ranges between 36 and 38%.
So when I analyze the tanks, I of course label them with a piece of tape.
And then I usually find myself transporting the tanks to whatever boat we are going out on.
Once in a while somebody will come in from out of town and use the tanks that we toss out there from my group, and they will very often analyze the tanks themselves.
Which I think is a very solid practice.
I would say if you are using RENTAL tanks it is a really really good idea (even if you tested your tank at the shop yourself, but somehow lost chain of custody even if it means for 15 seconds, you handed your tank to a dive master on the back of a boat and he/she stowed stowed it) to re-check that tank at your seat.
Rental tanks can look remarkably alike, with nearly identical scuff marks, peeling patches of paint, initials, dive shop decals, and every other dangerous similarity in the world, is a VERY REAL DANGER or other possible situation.
The other wild card to this is that if you are having your tanks filled at a partial pressure fill station,
there can be a 2% to as much as 4% range in the true percentage of oxygen that the sensor will register if you do not roll of the tanks around for a little bit immediately after a partial pressure fill.
There have been several conversations on Scubaboard in the past, about this process but here is the quick rundown on this potential variance in an accurate O2 reading.
If you put in 300 or 500 psi of oxygen, and then put air on top, and then test the tank without rolling around on the floor, there can be up to a 4% variance in the accurate reading of the gas that is in the tank.
4% would be the most extreme differential that I have ever personally heard of.
Some people are of the opinion that this only really happens if you test the tank immediately or very shortly after the fill being performed.
There is a body of thought that says if you leave the tank sitting there overnight that the gas will evenly distribute and you will get an accurate reading.
I am of the opinion that if a tank has been partial pressure filled, even if it has been sitting in the shop for a week waiting for pick up, you ought to at least roll it across the floor for 15 seconds before you put it on the analyzer.
My personal situation is this.
I very, very, very, rarely rent scuba tanks.
If I pick up my or my friends tanks at the shop that have been filled, I test them there.
They then go into my garage.
Then if I am going somewhere I throw them into my truck, and I drive to a boat, and I dive them without testing them.
But I have a certain laissez-faire attitude about this, because almost all of the tanks in my garage are within a point or two of one another.
Generally in the 35 to 38% range.
This is a whole different ballgame if we are doing some special deep wreck or reef or even decompression diving.
When that happens, I will test the tanks at the shop, and when we arrive at the boat,
not only are we testing the percentages again, but we are also checking to make sure that the tanks are full.
But the mistakes do happen.
As recently as three weeks ago, I ended up grabbing two empties that somebody slipped into my garage, and did not test them.
The rule with my group is upon depleting a tank you have to remove the percentage mix label/tape from the crown.
Somebody did not do that, and I did not know that the tanks were in the garage empty.
My buddy and I had to do two dives on one tank apiece.
Which was not the end of the world, because a low pressure 120 can really do nice things for you if you know how to fill them correctly.
We ended up diving one hour and 45 minutes instead of the two hours that was allowed for that particular trip.
So it was no big deal.
Chug
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That sounds a bit impractical to me. I tear off the part with the pressure, date and initials to indicate that it's not full anymore. Also, the tape around the valve is gone, so I have two independent indicators that the tank has been used.
Since I usually finish my dive with some 40-100 bar left in my tank, I prefer to have the spent tank marked with the percentage mix. If I fill only 32%, it really doesn't matter, but if I'm switching mixes it does matter as to what mix I should fill to get the mix I want.
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