My wife won't sink

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Using a buoyancy calculator that I find to be pretty accurate, I ran her weight, suit, fresh water, and assumed an aluminum 80 for the tank. It can up with 17lbs. You really don’t want to be duck diving and kicking to get down at the start of the dive - by the end of the dive an AL 80 will be 5 lbs more buoyant, which risks not being able to maintain a safety stop or even control ascent.

Sounds like she needs to add lead.
 
is she down there after all ? 😂😂😂
 
I'm female and the same size and in a 7mm in fresh water I use 18 lbs. I hate the cold so I suck it up and add the weight. Warm water diving in a 2mm in salt and I get down with 10.
18 !!!! way too much
 
Well you need to add as much weight as you need. The base line should be to be able to easily comfortably at 3m depth with an empty(!) Bcd with an almost empty tank like 40 bar left. That should be in (more or less... its recreational) neutral trim. The pressure differences are huge and so if you are upright that changes the buyoncy of the 7mm wetsuit for different body parts.
If you have more, you need to inflate your bcd more, so use more gas and have more resistence so have a higher force needed and that leads to increase of gas consume / average volume in lung.
Too much of lead, which you cannot release in emergemcy is dangerous! A 7mm looses quiet a lot of buyoncy between surface and 30m.

I believe its a combination of upright descent and finning, not exhaling enough so having a higher average lung volume, which can easily make 2 to 3kg of lead.
Also maybe at the beginning some air is trapped in the suit or bcd adding some more buyoncy. Sometimes you are not able to get all air out of the bcd if the geometry does not fit.
pyschologically it helps for me to breath out fully through the nose, equalizing the mask. Not sure if it has to do with my hood as well ...
Also to accept that descent is actually slow with a 7mm and you should breath easily is something. If you expect to descent fast, you may think you are not descanding at all, thinking it through -> chain reaction.

Some things you can try out and work on.
 
I can't add much here after all the expert advice given. Other than being in a 7 mil farmer john wetsuit I too must hold the BC inflator high over my head to empty it enough to start to sink, especially if it's my first dive that day and the suit is staring out dry. My wife is not a diver but she can float vertically in a fresh water pool with no limb movement and will not sink one bit (people think she is standing on the bottom). Anyway, I agree with all the advice about relaxing (that's not hard to do is it?), maybe duck diving down a bit, etc. But some people just need a ton of weight. In my farmer john I wear 40 pounds. When I took P.P. Buoyancy years ago the instructor also said I was way overweighted, until he saw I would not get down with any less. One fine instructor where I used to work wore 38 in his 7 mil.
 
I suspect she has above average tidal volume and has been instructed to take full deep breathes from the diaphragm. Here’s a video I just posted that might be of help to her.
 
Adding weight just to get down is not good. You should adjust weights to be properly weighted at the end of the dive (ie she can hold without moving a 3m stop at the end of a dive on close to empty tanks at 30bar/400psi).

The best solution I found to get the first few meters underwater is to stay vertical facing your team, fully deflate all buoyancy devices (wing, dry suit), do a couple of back kicks with legs pointing down and then drop horizontal while still facing your team.
 
It isn't a contest, she needs as much lead as she needs.
As you're trying to improve on your wifes buoyancy performance, obviously you are concerned to not overweight her, as some instructors tend to do, so that's good. But still, to me 6/7 kg seem very little wearing a 7mm.
If some people need a little more weight to get down comfortably, take them. For fine-tuning you two can still try and remove some kg in time. More comfort and calm will come with experience in that configuration, meaning not necessarily 1-3 dives but 15-20 dives with same conditions and equipment. Best of luck, enjoy and Let us know how it goes!
 

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