Working out your correct weighting when Scuba Diving
Why correct weighting for Scuba is important and how to achieve it.
Do you know what your correct weighting should be when Scuba Diving?
During a scuba dive we all want to achieve the great feeling of “Neutral Buoyancy”.
That feeling we get when our trim is perfectly horizontal. Our fins are working in the space created in the water by our body and we are effortlessly gliding along.
To achieve neutral buoyancy easily we need to be carrying just the right amount of weight, not too little and not too much. Neutral buoyancy makes for a relaxed dive, using less air and being totally in control.
A correctly weighted scuba diver finds it easy to achieve the basic safe diving skills:
- A Controlled Descent.
- Good body position during the entire dive.
- Less air used due to lack of drag.
- A controlled ascent with an easily controlled safety stop
An over-weighted scuba diver is likely to have the following problems:
- A risk of a too fast descent that could result in a mask squeeze or an ear Barotrauma.
- Difficulty controlling depth on the dive, maybe dropping below their planned maximum depth.
- BCD (rather than lungs) being used much more often for buoyancy control.
- Poor trim underwater resulting in a “head up, feet down” diving position.
- Shorter dive time as more energy is used simply to propel the diver through the water. Extra air is needed to provide this energy, along with all the air needed for the BCD.
An under-weighted scuba diver will also have issues:
- Problems descending.
- The potentially dangerous issue of not being able to stay down once the diver’s cylinder is less than ½ full.
- Inability to hold a safety stop.
How do I know what is the right amount of weight?
Use your Logbook
During your Open Water course a good diving instructor will have spent time with you doing weight checks;,they will have ensured you recorded these in your logbook.
You should have also recorded the relevant factors that affected your weight requirement. (Tank size and type / exposure suit type).
This logbook is always your starting point and you should record your weighting here each time your dive conditions change and you take a different weight requirement.
These logbook notes are a really useful resource for you particularly if you don’t dive that often.
Not keeping a logbook? Read my post on diver communication.
Know the factors that affect your weight requirements when scuba diving
- A change from an Aluminum 80 cu ft tank to a steel tank of the same size can be as much as 4 lbs / 2 kg either way (Aluminum when at 500 psi / 50 bar is much more buoyant than steel).
- A change from fresh water to salt water can change your weight requirements as much as 7 lbs / 4 kgs. (Not allowing for any wetsuit change).
- A change from a 3 mm shorty to a 3 mm full suit is plus at least 1 kg / 2 lbs.
- A change from a 3 mm full suit to a 5 mm full suit is plus around 2 kg / 5 lbs.
- Different BCD’s have different buoyancy characteristics, some BCD’s are quite negative, some (especially travel BCD’s) are neutrally buoyant. This especially becomes a problem with holiday divers who do not own their own BCD.
If you keep track of your weighting on different dives, then all this becomes easier to handle.
Here is some advice on overcoming BCD problems when diving.
Complete a weight check before scuba diving.
Scuba diving weight check with a full cylinder:
You should float at eye level, upright, on the surface while holding a normal breath with no air in your BCD, when you release that breath you should start to descend slowly. Keep your limbs perfectly still while doing this, finning and waving arms is going to mess up the results.
If you don’t descend then first check that your BCD is empty AND that your lungs are empty (you need to have proper breathing control). Then add a couple of pounds and try again.
Once you have this good and you are descending slowly then you just need to add enough weight to counteract the buoyancy characteristics of the particular tank you are using.
Scuba diving weight check with an empty cylinder:
You need to be at the end of your dive with 500 psi / 50 bar in your tank (or whatever is your normal reserve).
At the safety stop depth of 15 ft / 5 m you should be able to hold a neutrally buoyant stop with absolutely NO AIR in your BCD.
At this point of the dive you are at your “most buoyant” so if you still need air in your BCD then you have been carrying too much weight around for the whole dive.
The correct weighting for you as a scuba diver is an evolving process:
Part of a Scuba Divers skill set is the ability to repeat and fine tune skills until they become second nature.
The skill of weighting is just like this, you will need to change your weight when your wetsuit changes or if you buy a new one, when you go from salt to freshwater or dive with a different tank.
For new divers, you will slowly learn to relax more and when you do, you will start to feel “heavy”. This is because you now have better lung control, time to do some weight checks! You will probably find you need a little less weight.
Where divers get it wrong:
“But I can always put more air in my BCD to compensate for the extra weight!”
Yes, of course you can BUT:
- The extra weight drags down the lower half of your body.
- The extra air collects at the top of the BCD and pulls up the upper half of your body.
- You are now in an upright position, trying to “walk through water”, this trim is going to be very, very wasteful of air and your scuba dive is going to be much shorter than it should be.
“But the online Scuba Diving Weight Calculator says I need XX pounds!”
I just Googled 3 different online calculators, my weight recommendation came out as 12 lbs , 14 lbs and 16 lbs. So 3 different starting points and not a substitution for a proper weight check.
The best calculator I know for doing this is the one in the Padi Manual (sorry copyright prevents me publishing it here). In my experience however, for warm water diving, it nearly always comes out a little heavy, but as a starting point it is a very useful tool as it gets very close.
“I cant get off the surface unless I have XX pounds, then I am heavy underwater!”
This is all down to breathing control, one of the key skills of Scuba Diving.
You should not be “out of breath” or breathing heavily at the surface, if you are then something is wrong and you need to rethink your entry procedure.
Start by being vertical at the surface, empty your lungs (and your BCD) completely. As you descend take only short breaths in and breath them out slowly. No need to fill your lungs with every breath. Do this and once you pass about 2 m / 6 ft you will be descending slowly and completely in control.
This is the time to flip into horizontal trim for the rest of the descent.
The other factor here is when descending your body should be still. I often see legs still kicking when a diver is upright and leaving the surface, how are you going to go down if your fins are propelling you up?