A dive that did not happen

[ClumsyCuttlefish]

It is going to be a long post, but it is hard to describe the situation without details.
My husband and I are not experienced divers. I have 43 dives and he has 20. When we were even less experienced than this (yeah, it is possible), we dove with the dive shop. With more than 10 dives from the boat, we thought we had a clear idea of how much weight we needed.
Now it was our next step in diving – a shore dive. We rented the tanks and arrived to the site. The water conditions were the same, there was not change in diving equipment (I thought so) and we estimated we will need the same amount of weights. Actually, we even took 2 extra lbs for each.
Now, it is our first completely independent dive. A beautiful diving day. The temperature is 21C (90F), the sun is shining. The entry to the water is near the tall pier where people usually fish. A lot of people. (What a great show they watched!) The sea was not outrageous, but there were some annoying waves, that created that back and forth drag. It was hard to stay in the water.
At 9:15 we were on the site. Bring all the equipment to the shore. Set the rigs. Review our simple dive plan. My husband says he will be responsible for navigation (very important since we don’t want to be killed by whale watchers on big boats). But he does not remember how to use the compass. Explain how to use the compass. I’ll carry the dive flag. Never used it before – well, should not be hard. Put on 7mm=5mm=hood=gloves. Hot! Put on the rigs. Hmm, where should we put on the fins: on the shore or in the water? I opt for the water and regret it – the waves. My husband opted for the shore and now walks towards the water like a penguin. Oops. I forgot to put weights in my back pockets. Yell to my penguin-husband who is still on the shore. He can’t hear me because of the wind. Yell louder. Now he hears me (together with fishing people) but can’t find the weights. Get out of the water and pick up the weights. Finally, we are ready to dive. I am already tired.
But we can’t sink at all. After numerous effects to control my heavy breathing and exhale properly, we give up and exit the water. Let’s sit and think. Aha! The tanks have changed. We used steal tanks on the boat dives and the ones we rented are the aluminium ones. I remember from my book that you need to add 5 lbs for an aluminum tank. We decided to play safe and added 10 lbs each. At this point we are so tired, so we just decided to see if our weighting is correct, do a 10 min dive and return.
We go back in the water, swim to where it is 3 m deep and give a signal down. My husband sinks, I don’t. I try really really hard to control my breathing, but it simply does not work. I float at the eye level, but exhaling does not bring me down. I just feel my lungs are the size of a big watermelon. I do the last desperate attempt and swim head down to the bottom. For a minute a managed to stay there, but it was a constant fighting with the force that pulled me up. I gave up and surfaced, followed by my husband. The people on the pier did not applaud but I am sure they wanted to.

Now I wonder, what haven't we consider when culculating weights? Wasn't 10 lbs enough to compensate for the aluminum tank? Or was it all my breathing? Any advice would be greatly appreciated.

 

[Malpaso]

Since I'm fairly new myself, and am still working on correcting my weight, I'll offer two possible problem areas.

First, make sure all the air is out of your BC. I know, very simple, and I'm not talking down to you, but sometimes on a shore dive not all of it is purged as easily as a boat dive. Squat down to your neck and make sure every last bit of air is out.

Second, flood your wetsuit. Air pockets get trapped there on shore dives as well. Plus, it's a good way to cool down on a hot day when you're overheated, tired and frustrated.

 

[vladimir]

Compare your two tanks' buoyancy in the last column of this chart:

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

The difference is what you needed to add to your rig to sink, assuming you were properly weighted before the tank change. Check your tank's buoyancy while empty (next-to-last column) to make sure you can stay sunk at your safety stop.

 

[Steve_C]

It is not just aluminum vs steel. There are different sizes and weights for each as shown in the charts above.

I am assuming there was no difference in your gear. Different wet suits etc can change buoyancy.

I am guessing it was just anxiousness but hard to tell.

-Couple years ago I had an instabuddy on an easy reef dive in the Keys. She had maybe 20 dives but had not had a dive in serveral months, maybe even a year. On dive one she tried to get down but had trouble, got frustrated, and she thumbed the dive. On dive 2 with a little coaching from the crew she got back in and was able to get down. Within a minute or two she was like a fish. Had a great dive and went on to dive several more times that week with no trouble.

-BCD. With my own BCD in usual trim I can quickly dump as much air as I want with just a tug on my inflator hose. I have found that with some rental BCDs it takes more effort to dump air. Had one that I had to actually pivot to 30 degrees and raise the inflator hose to dump air at all. Different tanks/equipment can alter your trim and how the BCD rides on you which can effect dumping air.