If the dive kit doesn't change we only need to find the change in buoyancy due to changes in body volume and weight. We know the dry weight changed from 210 lbs to 190 lbs. If we can find the volume we can calculate the buoyancy.
B = VD - W
According to the equation the buoyant force B is the difference between the weight of the water displaced by the object (VD) and the dry weight (W) of the object. If B is zero the object is neutrally buoyant and will neither sink nor float. If B is negative (-) the object will sink and if B is positive (+) the object will float.
Let's calculate the buoyancy (B) for both dry weights and see what the difference is. We can find the approximate volume by dividing the weight by the density of the human body. The density is 1 g/cm3 (from
How to Calculate the Volume of a Person | eHow). The procedure to find volume is:
1. Measure the weight in lbs.
2. Convert the weight to grams (W lbs x 453.6 g/lb).
3. Get volume (in cm3) by dividing weight by density (V = W/D).
4. Convert volume in cm3 to ft3 using 28,316.8 cm3/ft3.
W[210] = 210 lbs x 453.6 g/lb = 95,256 g
W[190] = 190 lbs x 453.6 g/lb = 86,184 g
V[210] = 95,256 g / 1 g/cm3 = 95,256 cm3 x 1 ft3 / 28,317 cm3 = 3.36 ft3
V[190] = 86,184 g / 1 g/cm3 = 86,184 cm3 x 1 ft3 / 28,317 cm3 = 3.04 ft3
Using the equation B = VD - W and 64 lbs/ft3 as the density of salt water the B for each weight is:
B[210] = 3.36(64) - 210 = 5.04 lbs.
B[190] = 3.04(64) - 190 = 4.56 lbs.
Dropping 20 lbs in body weight lowers your buoyancy by 0.48 lbs (a 9.5% change). This suggests reducing your lead weight by only 1 lb. If you are interested in the buoyancy in fresh water use 62 lbs/ft3 for the density of water. (Just for comparison B[190] in fresh water = -1.52 lbs).