Many certification agencies instruct students to float at eye level with an empty BCD and a normal breath to determine proper weighting. Here is a bit more detail on that process to help new divers better understand the process.
If you were wondering why you are supposed to float at eye level for proper weighting
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John Miller
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Great video for understanding the concept. No one is doing a buoyancy check with a tank that is not full however. So for that first dive of the day, dive that LPI hose a hard pull and dump the BCD air. We most often balance the distribution of our weight so the concept of adding 5 lps isn't the best advice, add 4 and distribute it. You should slowly sink at this point, not drop like a rock. Or skip the step altogether and use your hands to back peddle down a few feet and pressure will kick in and down you will go for that first dive of the day. Here is the point I wish to make, you will have 500-600 PSI while hanging out at that safety stop at the end of the first dive. Do a buoyancy check there. With no air in the BCD, holding a normal breath and without kicking or sculling you should be neutrally buoyant if properly weighted. If you sink, depending on how fast, remove some weight (once out of the water) and if you are a cork, then you will need to add a couple of pounds. Now you are set for the rest of your dives.
If you trained in fresh water and now diving in salt without that wetsuit then you'll be very close to what weight you had. I am 204 lbs and n fresh water, in my standard dive kit and no wetsuit I use four pounds. If I am wearing a 3mm then I use 8-10. In my 7mm semi-dry I am using 22-24lbs. In salt water with a .5-1mm full length I usually have 8-10 lbs or the same as fresh in a 3mm.
Keep your log book and track the weight used in different conditions and configurations. Your ultimate goal should be neutrally buoyant with no air in your BCD at the end of the dive safety stop. Accomplish that and you are set.
Safe diving.
I use another method for doing the weight check with a full tank and accounting for the weight of the air that will be breathed.
Before you get in the water:
1. Ascertain the lift capacity of your bcd.
2. Count how many breaths it takes to completely fill the bcd using oral inflation.
3. Calculate how many breaths it takes to create 5 pounds of lift.
Example: Bcd has 29 pounds of lift. It takes seven full breaths to fill it by oral inflation. Each breath adds about 4 pounds of lift. It takes about 1-1/4 breaths to add 5 pounds of lift.
4. Once in the water and beginning the weight check, empty the bcd and then orally inflate the number of breaths calculated in step 3.
This method will simulate the buoyant force of an empty tank while letting the student do a weight check with a full tank.
Before you get in the water:
1. Ascertain the lift capacity of your bcd.
2. Count how many breaths it takes to completely fill the bcd using oral inflation.
3. Calculate how many breaths it takes to create 5 pounds of lift.
Example: Bcd has 29 pounds of lift. It takes seven full breaths to fill it by oral inflation. Each breath adds about 4 pounds of lift. It takes about 1-1/4 breaths to add 5 pounds of lift.
4. Once in the water and beginning the weight check, empty the bcd and then orally inflate the number of breaths calculated in step 3.
This method will simulate the buoyant force of an empty tank while letting the student do a weight check with a full tank.
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