Density & Specific Gravity

Specific gravity is the ratio of the density of a material to the density of water. Materials with a specific gravity greater than 1.000 ( 1.035 in the ocean ) sink; materials with a specific gravity less than 1.000 ( 1.035 in the ocean ) float. Lead has the highest specific gravity - 11.35, except for gold. Gases like air also have weight, density, and specific gravity.

These figures are approximate and should be used only as general indicators.

Material

Density
( lb / cu-ft )

Specific
Gravity

Liquids

Water - pure (@ 39°F)

62.4 1.000 (reference)
Seawater (@ 39°F)

64.1 - 64.9
( 64.6 std )
1.027 - 1.040
( 1.035 std )
Ice (@ 32°F)

56.2 0.90
Petroleum Oil ~ 51.2 ~ 0.92

Seawater varies in salinity from place to place. High evaporation levels cause noticeably saltier surface water in the tropics. Freshwater runoff in some enclosed northern areas like the Baltic Sea dilutes the seawater to almost fresh. Locally, the specific gravity averages about 1.031.

Gases

Air - moist 0.075 0.0012
Air - dry 0.076 0.0012
Air - 500 psi 2.66 0.04201
Air - 3000 psi 15.59 0.2461

Helium

0.0111

0.0002
Nitrogen 0.0781 0.0013
Oxygen 0.0892 0.0014

compressible gases @ 1 Atmosphere ( 14.696 psia ) ~ 32°F
unless otherwise specified

1 cu-ft = 7.48 gallons

Note: a full 80 cubic foot scuba cylinder holds 77 x 0.076 = 5.85 pounds of air.

Metals

Aluminum * 168 2.70
Titanium 283 4.54
Rust ~ 330 ~ 5.3
Iron * 437 - 491 7.00 - 7.87
Steel * 490 7.85
Bronze * 518 8.30
Brass * 524 8.40
Monel * 552 8.8
Copper 559 8.96
Silver 655 10.5
Lead 708 11.35
Uranium 1184 19.0
Gold 1206 19.3

* depends on alloy and/or preparation

Plastics

Styrofoam (uncrushed) ~2.3 ~0.04
Rubber / Neoprene (foam) 8.0 - 12.0 0.13 - 0.19
PE (polyethylene) 58.7 0.95
ABS 65.7 1.05
Nylon 68.7 1.10
Rubber / Neoprene (solid) ~68.7 1.10
PVC (polyvinylchloride) 81.1 1.30
Fiberglass ~124.8 ~2.0


Woods

Softwoods:
Balsa * 6.9 - 8.7 0.11 - 0.14
Cork * 12.5 0.20
White Pine * 17.5 - 26.2 0.28 - 0.42
Cedar * 30.6 - 35.6 0.49 - 0.57
Hardwoods:
Red Oak * 28.1 - 41.8 0.45 - .067
White Ash * 30.0 - 44.9 0.48 - 0.72
Lignum vitae * 80.0 1.28

Wood - waterlogged * (all)

93.6

1.5

* depends on moisture content and other factors

Miscellaneous

Coal ( solid ) 84 - 94 1.3 - 1.5
Limestone ( coral ) 125 2.0
Ceramic 125 2.0
Brick 131 2.1
Concrete 144 2.3
Granite 162 2.6
Glass 162 2.6

Muscle

64.8

1.04
Fat 57.4 0.92
Bone 115.2 1.85
Blood 66.0 1.06 *
Brains ** **

* so blood is thicker than water
** depends on the individual


Sea Bass
A normal 3/4 pound male Sea Bass
on the right, and an enormous
18", 4-pound "Super-Male" on
the left. State record is 8 lbs 2 oz.

Spearfishing is really a solo activity. The presence of other divers in the water certainly does not preclude spearfishing, but the last thing you need is a buddy following you around in close proximity while you hunt, especially if he is spearfishing as well. This is in direct opposition to the conventionally accepted "buddy system" of diving and therefore makes spearfishing an activity only for more experienced divers who are capable of self-sufficiency in the water.

For every shot, assume you are going to miss. The resulting overshoot must never be a danger to other divers. This means either having visibility well in excess of the range of your weapon ( seldom ) or having a good backstop ( often the bottom is the best backstop. )

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