29.07.16

TOP TIPS: Getting your weight right

Okay, hands up, who's thinking about buying some new gear? A new undersuit perhaps, or maybe a seven-litre sidemount cylinder? It's a fair bet that if you get anything much bulkier than a dive computer or a mask and snorkel, then it will have an impact on your buoyancy. A few days ago, a diver I know said: "My weight was perfect in the sea, then I went to Vivian Quarry and went down like a brick!" He was trying to get to grips with a new drysuit at the time. You could quickly jump to the conclusion that he didn't remove 2kg of lead before jumping into freshwater, but it's not that simple. Did he wear the same undergarments for both dives? Was he overweighted on the sea dive but didn't really notice?

PADI has some simple advice about weighting: You are correctly weighted when you float at eye level, holding a normal breath, with no air in your BCD at the end of the dive. It's well worth looking at this in more detail if you really want to understand your weighting.

A key phrase is 'at the end of the dive', because your cylinder(s) may have 50bar or even less remaining in them. If you dive with a 12-litre cylinder filled to 232 bars and you are neutrally buoyant at the start of the dive, you will be about 2.6kg lighter by the end of the dive. That is the weight of air you have used. This could be enough to prevent you from being able to stay at your safety stop and control your ascent rate as you near the surface.

If you are a budding mixed-gas diver, this point is crucial to understand. For example, with twin 10-litre cylinders and two 7-litre sidemounts, you have the potential to be around 7kg lighter at the end of the dive (depending on the gases used). By the time you are into your shallowest (and longest stops), you may have dumped all the air, not just from your BCD but your drysuit as well, and still be struggling to stay down. Even if you can remain at the stop, you may be both cold and uncomfortable with your undersuit compressed. Just to add to the misery, drysuit valves can sometimes leak water when there is no air in the suit.

In order to be neutral at the end of the dive, you need to be slightly overweighted at the start of the dive. This is okay because your BCD can accommodate the extra air that you will need to get neutrally buoyant. If you use a drysuit then you need just enough air in the suit to relieve the feeling of squeeze - at all stages of the dive.

We also need to define 'holding a normal breath'. Most of the time we do our weight checks at the start of the dive, not at the end. Having just entered the water we may not be the picture of elegance and poise we would like to present. Cold water pouring down the back of a wetsuit is guaranteed to cause a sharp intake of breath, as will any anxiety or excitement. Every litre of air that you hold in your lungs produces 1kg of buoyancy.

If you have absolutely no idea how much weight you will need for a dive, then you need an educated guess to get you started. Assuming you weigh at least 60kg and have a single 12-litre steel cylinder:

            With a 3mm wetsuit                            -           4kg

            With a 5mm two piece wetsuit           -           6kg

            With a 7mm wetsuit or a drysuit         -           8kg

If you weigh less than 60kg or are very slim, remove 2kg from the above. If you are using an aluminium cylinder, then add 2kg.

Now get kitted up with a full cylinder and do a weight check:

  1. Enter standing-depth water. Have a few spare 1kg weights to hand in a bag.
  2. Relax for a couple of minutes, longer if you are wearing a wetsuit so that water gets into the suit and displaces the air.
  3. When your breathing has settled, try to kneel on the bottom by dumping all of the air from your BCD (and drysuit) and breathe out fully.
  4. Keeping a near-vertical position you should be able to sink. If you can't, double check that you have dumped all the air from your BCD/drysuit.
  5. Add more weight until you can sink slowly to a maximum depth of 1m. Lie on the bottom ready to try a fin pivot.
  6. Now take a full breath in. This full breath should provide just enough buoyancy to begin to lift you off the bottom. Add or remove weight until you can just achieve this.
  7. If you are using a drysuit, put just enough air into the suit to relieve the squeeze.
  8. Now relax your breathing and practice a fin pivot, putting air into your BCD if necessary to achieve neutral buoyancy.
  9. Now go for a dive. At the end of the dive, in standing depth, repeat the weight check. Hopefully you will find that you are more relaxed than you were at the start and not holding much air in your lungs - just enough for a 'normal' breath.
  10. Now look at where your weights are stowed. The distribution of weights is open to experimentation and can make a big difference to your comfort and trim.

If, after a few dives you still find that you can't get your weight right, book yourself onto a Peak Performance Buoyancy Course. I have never known anyone who regretted doing that course. Lastly, if you intend diving in freshwater quarries this summer, don't forget to add 2kg before you go back in the sea.

Text by Martin Sampson

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