Don't breathe tanks to zero?

[tplyons]

I both dive trimix and fill it, and I would never do either without an analyzer.

As for draining the tank, bleeding out $100 worth of helium for no reason is just plain ludicrous. Perhaps the course material was written before analyzers were generally available.

I agree. Since I don't participate in Trimix blending, I can only provide input based on my training.

The manual is telling the blender in that paragraph that he needs to know the residual mixture - analyze the residual gas, or start with an empty cylinder. Anyone blending trimix will have both oxygen and helium analyzers, so testing the residual mix is standard procedure.

Why not also bold the "In this case" part? It doesn't say you must start with an empty cylinder every single time like you wrote, it says you must if you don't have the equipment to establish O2 and He2. (Maybe this made sense when He cost a lot less and analyzers cost more?)

Last revision is 05/07. Not sure if He analyzers were affordable back then.

Personally I can't agree with how it's written at all. "If you did not personally mix"? What use is that? That almost sounds like "I mixed it so I know it's right. I didn't mix it so it must be wrong." Analzyze the contents and go from there. Head scratch....

Never said I agreed with the manual, but its what I was taught. I wonder what other agencies stance is on this? Is PADI/DSAT just being ultra-conservative?

 

[windapp]

However, it's also important not to blow off important things like safety stops just to get out with 500 psi in your tank. Yes, I've seen this happen and it's kind of scary. After all, those dive masters can be quite intimidating on the boat when they demand that you surface with no less than 500 psi. They want you to plan your dive in such a way that you come up with some reserve. However, if you have to go below that to do accomplish a safety stop, there should be no question: DO THE SAFETY STOP. Don't run out of air during the stop, but at this point, I would be willing to surface with only 50 psi or so. If your buddy is close and has plenty of gas, go on their octo for the duration. Just don't run them out of air.

I agree and disagree. Certainly, blowing off safety stops to meet the 500-psi-at-the-surface limit is simply nonsensical. Even worse, is racing to the surface after the stop in order to have 500 psi left in the tank. This really defeats the whole purpose of the stop. However, I think that 50 PSI is way low for finishing a dive. At that point, you are either out of air (gauge accuracy varies), or so low on air that your regulator is breathing very hard. The effect, again, tends to be a race to the surface that counteracts any good effect the safety stop may have had. I would blow off the rest of the safety stop as soon as my gauge reaches 300 PSI, personally.

 

[tplyons]

I agree, but that's not what tplyons wrote

The way I interpret the paragraph is the same way you did. If you analyze it, you're fine to start with.

HOWEVER, the only question I got wrong on the exam was something along the lines of "customer brings in a cylinder that you did not fill with a known blend of trimix, and asks for this, how do you fill?" and the options included adding topping off with appropriate gases (which I would do after analyzing and confirming) and emptying and starting over using more gas (which was the correct answer)

I'm still scratching my head on that one...

 

[redacted]

I agree and disagree. Certainly, blowing off safety stops to meet the 500-psi-at-the-surface limit is simply nonsensical. Even worse, is racing to the surface after the stop in order to have 500 psi left in the tank. This really defeats the whole purpose of the stop. However, I think that 50 PSI is way low for finishing a dive. At that point, you are either out of air (gauge accuracy varies), or so low on air that your regulator is breathing very hard. The effect, again, tends to be a race to the surface that counteracts any good effect the safety stop may have had. I would blow off the rest of the safety stop as soon as my gauge reaches 300 PSI, personally.

300 psi in an Al80 is almost 8 cf which is more than enough to complete a 5 minute safety stop and slow ascent to the surface. Seems unnecessary to forgo either of those extra safety safety measures.

 

[The Chairman]

I would blow off the rest of the safety stop as soon as my gauge reaches 300 PSI, personally.

Not me. As long as my gauge indicates that there is gas, I want to do my full five minute stop. No hurry, no worry and they aren't going to give me a discount on the gas I don't use.

 

[Abdullah]

In contrast, if you walk into most shops in Colorado and ask for a nitrox fill, you will leave empty handed. Nobody dives nitrox in Colorado--the lakes are too shallow to make it worthwhile--so most shops don't have the capacity to make it. The shops that do will have a few O2 industrial bottles in the back, and they will transfill a custom blend for you with no booster. That is a completely different situation, and it calls for completely different procedures and policies.

This is mostly what you see in this area. The equipment and consumable supplies for the banked and membrane setups are too expensive this far down the supply chain to make it profitable for an LDS to maintain such systems. The majority of shops are PP blending out of a rack full of oxygen cylinders in their mix room. The good news is that I can get any custom mix I want for the same price as a standard mix.

 

[windapp]

300 psi in an Al80 is almost 8 cf which is more than enough to complete a 5 minute safety stop and slow ascent to the surface. Seems unnecessary to forgo either of those extra safety safety measures.

First off, I was taught to do a 3 minute safety stop. Second, this is only 4 cf until you reach IP, the point at which your regulator will start breathing very hard.

 

[boulderjohn]

First off, I was taught to do a 3 minute safety stop. Second, this is only 4 cf until you reach IP, the point at which your regulator will start breathing very hard.

A 3 minute safety stop has been the norm for a long time, and that is what is usually taught in classes; however, there is a growing trend to do 5 minute stops instead. There is some research that supports this.

 

[chrpai]

The way I interpret the paragraph is the same way you did. If you analyze it, you're fine to start with.

HOWEVER, the only question I got wrong on the exam was something along the lines of "customer brings in a cylinder that you did not fill with a known blend of trimix, and asks for this, how do you fill?" and the options included adding topping off with appropriate gases (which I would do after analyzing and confirming) and emptying and starting over using more gas (which was the correct answer)

I'm still scratching my head on that one...

Sometimes you just have to smile and agree in order to appease the establishment. Hubris is common in this industry and it'll always be US who is wrong if we disagree. Some pretend to fight this but in reality they just defend a different world view.

 

[redacted]

First off, I was taught to do a 3 minute safety stop. Second, this is only 4 cf until you reach IP, the point at which your regulator will start breathing very hard.

I was not taught to do a safety stop but I learned to do 3 to 5 minutes (sometimes more if I have time and gas; and high N2 loading). I have breathed my regs well below IP just to make sure I knew how they would work. With a high quality balanced 2nd, You can easily feel the difference but it still provides plenty of gas to comfortably finish that final slow ascent if necessary with just a couple cf left.. I am just saying it is silly to forgo the extra safety precautions just to have gas in your tank when the dive is completed. Of course, anytime you finish a dive with less gas than you planned to have is a good time to review your dive/gas planning procedures. It is rare that I plan to essentially empy a tank. And I only do that in the safest of environments to test my equipment. I often plan on arriving at a safety stop with 500 psi which would leave me surfacing with 200 to 300 psi. I do fail to execute that plan quite often, but always on the safe side.