One-hand or two-hand valve drills (back mount doubles)

[Nick_Radov]

I have heard such stories, but it is hard for me to figure out how one tank can be filled with helium and the other with something else. I have done a ton of partial blending in my life, and I can't figure out what kind of a sequence would be required for me to make such a mistake. I have certainly made mistakes that were caught when we checked the blend, but I am having trouble figuring out how I could make that one.

Start with empty tanks. Close isolator. Add oxygen to right tank. Get distracted by something and come back later. Add helium to left tank. Top off left tank with air to maximum pressure. Analyze left tank and notice that the mix is wrong, then "fix" it by venting some gas and then boosting or whatever. Now you have a full left tank that analyzes correctly, and a right tank containing only oxygen.

Start with empty tanks. Add helium. Add oxygen. Close isolator. Top off left tank with air to maximum pressure. Now you have a full left tank that analyzes correctly, and a right tank containing hyperoxic heliox.

Probably some other ways as well.

 

[boulderjohn]

Start with empty tanks. Close isolator. Add oxygen to right tank. Get distracted by something and come back later. Add helium to left tank. Top off left tank with air to maximum pressure. Analyze left tank and notice that the mix is wrong, then "fix" it by venting some gas and then boosting or whatever. Now you have a full left tank that analyzes correctly, and a right tank containing only oxygen.

Start with empty tanks. Add helium. Add oxygen. Close isolator. Top off left tank with air to maximum pressure. Now you have a full left tank that analyzes correctly, and a right tank containing hyperoxic heliox.

Probably some other ways as well.

Sure. I would rate the potential of doing those sequences at about nil.

 

[PfcAJ]

Sure. I would rate the potential of doing those sequences at about nil.

Eagle's nest accident report

By Rick Gershman from the St petersburg times Weeki Wachee___Deep below an algae coverd pond. Eagles Nest is considered one of the most breathtaking underwater cave systems in the world. Its intricacies have alternately been described as challenging and dangerous. Judi Bedard never made it...

scubaboard.com

 

[crofrog]

If you're into your dive and you find your manifold closed. I'd recommend against opening it and just terminate the dive then, considering once you open the manifold you no longer know what your mix is.

 

[kensuf]

Eagle's nest accident report

By Rick Gershman from the St petersburg times Weeki Wachee___Deep below an algae coverd pond. Eagles Nest is considered one of the most breathtaking underwater cave systems in the world. Its intricacies have alternately been described as challenging and dangerous. Judi Bedard never made it...

scubaboard.com

As an aside, Judy survived that incident. She and I got to dive together in Belize in 2015 or 2016.

 

[NothingClever]

@lukas_manthony

Below is a link to what I mean by an air gun. Instructors connect an LP hose to their first stage and then the air nozzle fitting to that. It allows them to simulate a gas leak at either post or even your short hose reg.

DGX Adapter: HQ Air Nozzle = Schrader QD Post

Use this high quality adapter to inflate marking buoys, beach toys, balloons, etc. from a SCUBA tank using a first stage regulator and BC inflator hose. Soft rubber nozzle is {3.5 in |8.8 cm} in length.

www.divegearexpress.com

 

[boulderjohn]

If you're into your dive and you find your manifold closed. I'd recommend against opening it and just terminate the dive then, considering once you open the manifold you no longer know what your mix is.

That depends in part upon the difference in volume in the tanks. The two will equalize with incredible speed, and that will create some mixing, and if there is a large difference in the volumes, there may be considerable mixing, but almost only in the cylinder getting the blast of gas from the one with the greater volume. When the cylinders are at equal volume, it takes a long, long time to mix.

Years ago someone loaned me two full cylinders to use for a coming sidemount trip. They had different mixes, and quick math showed they would be perfect if I could mix them. I put a transfill whip on them and left them alone for 3 days, after which they were both exactly where they were when I started. We have had at least one thread on this topic, and people have reported the same experience with doubles.

 

[NothingClever]

Did you ask them? There should be no reason to ever close that valve, especially as all the shop does is fill the tanks and for that the valve needs to be open.

No, I didn’t ask. It was in an area that doesn’t share the same sense of procedure or accountability as the West. Asking them would’ve resulted in zero progress. I voted by never going back to that shop and alerting other divers to avoid the shop or to closely supervise the compressor attendant. It wasn’t the shop I went to for anything beyond 21%.

 

[NothingClever]

The really dangerous part of this is that you were possibly breathing the wrong mix.

Air only dive (long, 6-9m exploratory DPV dive along the coast). Wouldn’t have gone to that shop for even Nitrox.

 

[crofrog]

That depends in part upon the difference in volume in the tanks. The two will equalize with incredible speed, and that will create some mixing, and if there is a large difference in the volumes, there may be considerable mixing, but almost only in the cylinder getting the blast of gas from the one with the greater volume. When the cylinders are at equal volume, it takes a long, long time to mix.

Years ago someone loaned me two full cylinders to use for a coming sidemount trip. They had different mixes, and quick math showed they would be perfect if I could mix them. I put a transfill whip on them and left them alone for 3 days, after which they were both exactly where they were when I started. We have had at least one thread on this topic, and people have reported the same experience with doubles.

You don't know for sure what the mix is. If you analyzed off the right post and then opened the isolator, it's going to mix, sure, but you no longer know what the contents of the gas is and, therefor, shouldn't be breathing it. You've already made multiple mistakes no point in compounding them into a fatality.

You missed the closed manifold when you did your equipment assembly and gas analysis; you missed it during your flow check during your pre-dive checks. Hopefully, you caught it during your first situational awareness check, and you're only 10-15 minutes into the dive.

At that point, it's time to end the dive and have your team ready to donate.