Most of us take water for granted. We drink it, bathe in it, water the lawn with it, wash the dishes and car with it, swim and fish in it, and dive in it, and yet very few of us have ever taken the time to explore the intricacies of water. How much do you know about how water affects us in our everyday lives and, more specifically, as divers?
The two primary chemicals that make up water are hydrogen (H) and oxygen (O). Each molecule consists of two hydrogen atoms and one oxygen atom; that’s why water is commonly referred to as H2O.
Most of us think of water as a liquid, but it takes other forms as well. When water freezes it turns to ice (a solid), and when heated it turns to steam (a gas).
Water is the elixir of life, occupying an even higher rank in the pecking order than blood. Without constant replenishment of water, the human body becomes dehydrated, the blood supply dwindles, blood pressure decreases and, eventually, bodily functions cease. That’s why it’s recommended that adults drink at least eight glasses of water per day. In addition to drinking it, humans use water in other ways. How much water do you suppose the average American uses per day? An answer in the vicinity of 170 gallons/644 liters is close enough.
Water also affects the world’s weather. Its heating and cooling properties are responsible for tropical storms and eventual hurricanes. Look what El Niño did last winter. The continual barrage of torrential rains, high winds and damaging seas that battered the California coast and made diving dangerous (if not impossible) was credited to El Niño.
In the Galapagos the warmer-than-normal water currents this year caused a noticeable decline in hammerhead shark sightings. The cold water that normally wells up on these mid-ocean islands, bringing the hammerheads with it, was overridden by warm El Niño currents. Sharks were still there, but they were much deeper than normal.
Water and the Diver
You may care little about the more general properties of water, but as a diver it is important to understand and respect that water impacts how, when and where you dive. Let’s begin by looking at water temperature.
We all agree that divers get cold faster underwater than in the air. But why?
Water is a much more efficient conductor than air and actually robs a diver’s body of heat. The cold of the water doesn’t pass into the body; the body’s heat is conducted away from the skin into the water. As a result the heart and circulatory system work futilely to replenish the heat lost to the surrounding water. When this occurs, in an elaborate system of self-preservation, the body shunts blood from the extremities and directs it to the body core to provide thermal protection for vital organs.
This is why divers get cold even in warm water. In fact, when exposed for a prolonged duration, a diver will get cold in any water cooler than the normal body temperature of 98.6˚F/37˚C. If a diver remains in the water long enough, hypothermia — the decline of the body’s core temperature — will set in and eventually death could result. Divers should wear some type of thermal protection when diving in water cooler than 82˚F/28˚C and exit the water when they begin to experience numbness or shivering. (For more information, see “Cold-Water Diving,” Dive Training, September 1998.)
Have you ever experienced a thermocline? Inland bodies of water — especially lakes and quarries — and some ocean locations have thermoclines. A thermocline is an abrupt decrease in water temperature encountered as a diver descends. The change can be anywhere from 2 degrees (F, 1˚C) to 20 degrees (F, 8˚C).
A thermocline is created when sunlight heats the water nearest the surface and little water motion exists to mix it with the cooler water below. Several increasingly colder thermoclines may be encountered as you go deeper. Thermoclines occur at any depth, but are most common between 30 and 90 feet/9 and 27 m.
Buoyancy
What about buoyancy? How does water affect whether a diver floats or sinks, or is this simply a function of how much weight he or she wears? If you wear 18 pounds/8 kg of lead during your open-water training dives in a Texas quarry, will you need the same amount when diving in the Cayman Islands?
Your initial answer might be, “It depends on whether you are wearing the same wet suit.” It is correct that the thinner the neoprene you wear, the less weight you’ll need, but there is another factor. What about the water itself? The quarry is fresh water and the Caribbean is salt water. What difference does that make?
As you learned during Open Water class, an object (in this case, a diver) is buoyed up by a force equal to the weight of the water it displaces. Assuming you wore the same wet suit in Cayman as in Texas (that means that you are displacing the same volume of water), you’ll still need to increase the amount of weight worn.
This is because salt water (64 pounds per cubic foot) is heavier than fresh water (62.4 pounds per cubic foot). If you displace 2 cubic feet of water, you’ll be buoyed by a force of 124.8 pounds (62.4 x 2) in fresh water and 128 pounds (64 x 2) in salt water. Therefore, you’ll need to wear more weight when diving in salt water.
Pressure
The weight of the water also impacts the rate at which a diver’s body absorbs nitrogen while underwater — the primary contributor to decompression sickness (DCS).
Remember the concept of atmospheres? The Earth’s atmosphere — the vertical space between the planet’s surface and space — exerts a pressure (weight) of 14.7 pounds per square inch on the Earth’s surface (at sea level). During scuba training we learn that this surface pressure is referred to as 1 atmosphere. Every man, woman, child, plant and animal on Earth is exposed to atmospheric pressure 24 hours a day.
When a diver enters the water, he need only descend to a depth of 33 feet/10 m in salt water (fsw) before the pressure doubles — 14.7 psi x 2 = 29.4 psi. The pressure experienced at this depth is called 2 atmospheres. The pressure increases by 1 atmosphere (14.7 psi) with every 33 feet/10 m of depth. At 99 fsw/30 m the pressure equals 4 atmospheres. So why does it take over 50 miles/80 km of vertical air space to create the first atmosphere and only 33 fsw to create the second? The answer lies in the weight of the water — water is heavier than air.
The human body is designed to accommodate surface pressure. We don’t even notice that first atmosphere. But as we descend in the water, what happens? The pressure increases, but what else?
Upon descent we begin to feel squeezing, sometimes pain, in our air cavities — sinuses, ears and the space inside the mask (mask squeeze). That is why one of the first maneuvers learned in scuba training is equalization — how to make the pressure inside our body’s air spaces and mask equal to the increasing pressure around us (ambient pressure) as we descend.
Sinus and ear equalization involves a simple process of closing the mouth, pinching the nostrils shut (through the mask nose pocket, of course) and attempting to exhale gently. With the mouth and nose closed, the higher-
pressure air from the diver’s lungs detours into the middle ear and sinuses. Air spaces must be repeatedly equalized as a diver descends.
Mask squeeze can be avoided by occasionally exhaling gently through your nose. Similar to what happens with the body’s air spaces, this maneuver introduces ambient-pressure air inside the mask area, making it equal to the water pressure outside.
Other than in the air spaces, the increased pressure of depth is rarely felt by divers, since the human body is comprised primarily of incompressible fluids. However, at depth a diver’s body does absorb nitrogen more quickly than at the surface. The deeper he dives, the more nitrogen he accumulates. The longer he stays, the more nitrogen he accumulates. The potentially dangerous combination of diving too deep and staying too long is what causes DCS — the bends. So how does a diver keep from getting bent?
Dive tables and dive computers are tools that aid divers in planning their dives to avoid DCS. The tables display the maximum no-decompression limits for specific times and depths. Divers should not exceed these limits. In addition to providing planning data, dive computers maintain a running calculation of absorbed nitrogen, making it easy for a diver to know when he is approaching the limit. The data provided by both tools should be applied conservatively.
Seeing Underwater
In addition to cooling, buoying and squeezing us, water plays tricks on our eyes. Do you know how?
Underwater objects appear 25-percent larger and closer than they actually are. I still remember my first exposure to this optical phenomenon. While snorkeling in Bermuda many years ago, I spotted what appeared to be a hurricane lamp lying in 20 feet/6 m of water. Thinking it might be a remnant of an ancient shipwreck, I kicked like mad toward the bottom. As my outstretched hand neared what I thought was a lamp, the glass object suddenly shrank to a size that fit comfortably within my palm. My eyes had deceived me. Not only was the anticipated treasure smaller than it appeared underwater, it was a cocktail glass from a nearby hotel, not a hurricane lamp.
This size and distance phenomenon is caused by refraction — the bending of light rays as they pass from one medium to another. This is easily demonstrated in your own kitchen. Stand a pencil in a glass of water. From the side, the pencil appears to be broken where it passes through the surface of the water.
What about colors? Are they different underwater? The answer is a definite “yes.” Light waves are diffused and absorbed by the water, making colors disappear as we descend — first the reds, then orange and yellow. As the color disappears, these objects appear black. Below 100 feet/30 m, everything is blue and violet unless we use artificial lighting.
That is one thing that makes diving at night especially exciting. Dive lights not only restore the colors as they illuminate objects, they enhance the colors, making them even more brilliant than when viewed under natural light.
Water also affects visibility — how far a diver can see horizontally while underwater. Can you name two properties of water that impact visibility?
In its purest form, water is a naturally clear liquid, but unfortunately most of the water we dive in has at least a slight degree of turbidity. Turbidity is defined as the concentration of suspended particles. The greater the turbidity, the poorer the visibility.
Suspended particles are either organic, such as plankton — usually microscopic animal and plant life found floating or drifting in the ocean or bodies of fresh water — or inorganic, such as sediment and sand. Visibility is reduced substantially during plankton blooms, when extremely high concentrations of organic matter cloud the water.
It is not uncommon to encounter a layer or wall of turbidity. This may be caused by plankton bloom occurring in the warmer water near the surface or by turbid ground-water runoff forming a dense, murky 2- or 3-foot/1-m layer along the top of the water. Once a diver descends below this layer, visibility clears up.
A similar phenomenon may be experienced as a vertical wall of turbid water where streams dump into the ocean. The turbid freshwater runoff resists mixing with the sea water, creating a definite line of demarcation between the two. Venturing into a turbid wall of water should be avoided unless you are trained in low-visibility diving and properly equipped.
Divers often create their own poor visibility. They kick up the bottom or swim too close to heavily silted objects.
The definition of good visibility is relative to each diver’s experience. When visibility drops to 50 feet/15 m in destinations like Cayman and Bonaire, the divemasters tell visiting divers, “Viz isn’t too good today.” In contrast, divers in most cold-water locations would love to have constant 50-foot visibility.
Sound
Water is 800 times denser than air. Do you know what effect that has on how divers hear underwater?
Since water is denser, sound travels faster through water than through air — four times faster. So divers hear sounds sooner and from farther away. But that doesn’t necessarily mean that divers hear better underwater than on land. Yes, the sound waves do reach your ears more quickly — at 3,240 miles per hour/5,217 kph — but that exceptional speed itself creates a problem.
Humans determine the direction from which sound is coming by the time lapse between when the sound waves strike each eardrum. On land this is possible because sound travels slowly through air. But underwater, where the sound is traveling much faster, this natural instinct is ineffective. Underwater the human brain perceives the sound waves as reaching both eardrums at the same time, making the determination of direction nearly impossible.
Although we take it for granted, water is a critical resource, especially for divers. Protect it, conserve it, respect it and enjoy it.
A Review of the Properties of Water
The Properties of Water at a Glance*
• Water molecules consist of two atoms of hydrogen combined with one atom of oxygen (H2O).
• Water is 800 times denser than air.
• Salt water weighs 64 pounds per cubic foot.
• Fresh water weighs 62.4 pounds per cubic foot.
• Every foot of descent in salt water adds another .445 pounds per square inch of pressure.
• Every foot of descent in fresh water adds another .433 pounds per square inch of pressure.
• Every 33 feet/10 m of depth in salt water equals an additional atmosphere and another 14.7 pounds per square inch of pressure.
• Every 34 feet/10 m of depth in fresh water equals an additional atmosphere and another 14.7 pounds per square inch of pressure.
• Water buoys an object with a force equal to the weight of the water that the object displaces.
• The weight (density) of water causes a diver to absorb more nitrogen than when on land.
• Water is an efficient conductor of heat.
• Water’s heat capacity is thousands of times greater than air.
• Water absorbs light in a predictable manner.
• Water absorbs colors based upon their wavelength.
• Water transmits sound approximately four times faster than air.
• Sound travels at 3,240 miles per hour/5,217 kph underwater.
• Sound can travel over 15 miles/24 km underwater.
Roy G Biv
Do you know Roy G Biv? This is the acronym used by scuba instructors to help students remember the order in which colors disappear as they descend. From shallower to deeper, they are:
R = Red
O = Orange
Y = Yellow
G = Green
B = Blue
I = Indigo
V = Violet