Physics of Diving Part 1 PDF

PHYSICS OF DIVING PART 1  Objectives and Lesson Goals The 3 classic states of matter The basic properties of water and how they affect light and sound beneath the surface The Archimedes Principle, relative density, the 3 states of buoyancy, and how to calculate app...

PHYSICS OF DIVING PART 1  Objectives and Lesson Goals The 3 classic states of matter The basic properties of water and how they affect light and sound beneath the surface The Archimedes Principle, relative density, the 3 states of buoyancy, and how to calculate approximate lifting needs The different types of pressure including atmospheric, hydrostatic, gauge, absolute, and partial pressure Gasses in diving and the Ideal Gas Law Boyle's Law - Pressure and Volume relationships and how to calculate volumes under varying conditions of these variables The importance of Boyle's law to air consumption Amontons' Law - Pressure and Temperature relationships, absolute temperature scales, and how to calculate Scuba cylinder pressures at varying temperatures Dalton's Law of partial pressures, how to calculate partial pressures of a variety of breathing gas mixes, how this applies to breathing Nitrox, and increases the risk of breathing contaminated air  Constants and Useful terms Feet Sea Water = fsw or ' Feet Fresh Water = ffw Meters Sea Water = msw or m Meters Fresh Water = mfw or m Weight of 1 cubic foot of seawater: 64 lbs Weight of 1 cubic foot of fresh water: 62.4 lbs Weight of 1 liter of seawater: 1.03 kg Weight of 1 liter of freshwater: 1.0 kg Cubic Feet = cf or ft3 Liter = L 1 atm (atmosphere) = 14.7 PSI (Pounds per square inch)  States of Matter Solid A substance that is structurally rigid and resistant to applied forces. Example: Ice Liquid A substance that flows freely but is of constant volume and is nearly in-compressible. Example: Water Gas A highly flexible collection of molecules with greater separation than in the liquid and solid forms. Example: Air  Density Density is the degree of compactness of a substance and often referred to as the "heaviness" of matter. The formula is: density = mass/volume or d = m/v We spend most of our lives after birth immersed in air, but most of our time in scientific diving immersed in water. The high density of water in comparison to air is responsible for the many differences we experience underwater. Let's examine water more closely.  Water Water has the chemical formula of H2O, meaning a molecule (or smallest possible piece) of water contains 2 hydrogen atoms bonded to 1 oxygen atom. H2O can exist in several states of matter, but usually when we say "water" we are referring to the liquid state.  Hydrogen Bonds H2O molecules are arranged in a "Polar" configuration, meaning that different sides of the molecules have different charges. As you'll see illustrated below, near the oxygen atoms (the big red balls), there is a slight negative charge. Near the hydrogen atoms (little white balls), there is a slight positive charge. These slight charges mean water molecules attract each other and form bonds that give it particular properties compared to other liquids.  Water Properties of water: High surface tension High boiling point and heat capacity Pure water is tasteless, odorless, and nearly non-compressible It boils at 212°F (100ºC) and freezes at 32°F (0ºC). Water is approximately 800 times denser than air. Salt Water/Sea Water Most scientific dives occur in saltwater, usually seawater. Seawater contains various substances, one of the prominent ones being sodium chloride. The presence of these substances alters the properties of seawater compared to pure freshwater, including making seawater denser than freshwater.  Pressure Pressure can be exerted by: weight of air weight of water weight other substances. P = F/A.  Types of Pressure in Diving Atmospheric Hydrostatic Partial Absolute Gauge  Atmospheric Pressure While you may not actively feel it, the several miles of atmosphere currently above you (unless you are an astronaut in space right now) are exerting pressure on you. In 1644 a student of Galileo named Evangelista Torricelli said "We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight". One experiment to which he refers is the first mercury barometer. It showed : that the pressure at sea level pushing down on an area of 1 cm x 1 cm is equal to 760 mm Hg (Mercury) In the English system of units (lbs/inches, etc..) this converts to 14.7 psi (pounds per square inch), or 1 atm (or atmosphere). In the metric system, this is 1013 mbar, or 1.03bar.. If you go up to altitude in the mountains, the pressure will be less, which is why there are special procedures for altitude diving.  Hydrostatic Pressure This is the pressure generated by water weight, and not surprisingly, as you go deeper, the pressure increases. Water pressure increases at a rate of: seawater - 1 atmosphere (14.7 psi) per 33 feet (10 meters) freshwater - 1 atmosphere (14.7psi) per 34 feet (10.4 meters) Weight of 1 cubic foot of seawater: 64 lbs Weight of 1 cubic foot of fresh water: 62.4 lbs Weight of 1 liter of seawater: 1.03 kg Weight of 1 liter of freshwater: 1.0 kg  Gauge Pressure a measurement of pressure that uses 1 atm or 1 Bar (the ambient atmospheric pressure at sea-level) as the zero point. Depth gauges and dive computers don’t actually measure depth, but instead measure: PRESSURE On the surface, your depth gauge or dive computer should read zero (0 m/ 0 ft), but as we previously learned, the pressure at the surface isn’t actually zero. Using sea-level as our baseline, we know that the pressure at the surface is actually: 1 atm or 1 bar or 14.7 psi  Absolute Pressure Equals what? the total pressure exerted on a diver by both atmospheric and hydrostatic pressure ATA = atmospheric pressure + hydrostatic pressure Depth Absolute Pressure Gauge Pressure 1 ATA 1 ATM 0' / 0m 2 ATA 2 ATM 33' / 10m 3 ATA 3 ATM 66' / 20m 4 ATA 4 ATM 99' / 30m 5 ATA 5 ATM 130' / 40m  Partial Pressure What is it and why is it important? the amount of pressure generated by a particular gas in a mixture of gases. A SCUBA tank resembles a perfect mixture of gases  Pressure calculations This table uses intervals of only 1ATA (33fsw or 34ffw), which makes it relatively straightforward to estimate pressure at depth, however; what if we want to be more precise in our calculations of pressure? The following equation can be used to achieve a more precise calculation of absolute pressure at any given depth: Absolute Pressure (ATA) = (Depth / Hydrostatic Pressure Constant) + Atmospheric Pressure Depth Absolute Pressure 1 ATA @ 33fsw this equation becomes: 0' / 0m 2 ATA 33' / 10m ATA = (33/ 33) + 1 = 2 ATA 3 ATA 66' / 20m 99' / 30m 4 ATA Note: 5 ATA Hydrostatic pressure constant = 33 fsw or 34 ffw 130' / 40m  Pressure calculations Using the equation ATA = (Depth/ 33) + 1 we can now calculate a precise pressure at any given depth in seawater. Let’s do this for a diver at 17 ft/ 5.2m. ATA = (17/ 33) + 1 = 1.51 ATA (@ 17 fsw) Depth Absolute Pressure 1 ATA 0' / 0m 2 ATA Calculate the Absolute pressure at 74ft/22.4m in freshwater: 33' / 10m 3 ATA 66' / 20m 99' / 30m 4 ATA ATA = (74/ 34) + 1 = 3.18 ATA (@ 74 ffw) 5 ATA 130' / 40m  Pressure calculations review Total pressure (ATA) is a combination of both Hydrostatic Pressure and Atmospheric Pressure ATA = Hydrostatic Pressure + Atmospheric Pressure Hydrostatic pressure is calculated by dividing the depth of water by its hydrostatic pressure constant. Hydrostatic pressure (saltwater) = Depth / 33 Hydrostatic pressure (freshwater) = Depth / 34 Atmospheric pressure for divers at sea-level, is assumed to be 1 atm So, our simplified equations for calculating precise pressures at depth becomes: ATA = (Depth/ 33) +1 in seawater ATA = (Depth /34) +1 in freshwater  ATA vs PSI Reminder: ATA = Hydrostatic Pressure + Atmospheric Pressure To express Absolute pressures in psi instead of ATA, we must convert using our known constant of 1ATA = 14.7 psi. To convert Absolute Pressure (ATA) to psi, simply multiply by 14.7 as in the example below: 2 ATA x 14.7 psi/ATA = 29.4 psi To express Hydrostatic pressures alone in psi instead of ATA, we must convert using our known constants of 0.445 psi/fsw and 0.432 psi/ffw. For example: At 66fsw the hydrostatic pressure is: 2atm = 33fsw x 0.445 psi/fsw = 29.36 psi At 68ffw the hydrostatic pressure is: 2atm = 34ffw x 0.432 psi/ffw = 29.36 psi  Helpful table posted to blackboard: Units PSIG PSIA ATM ATA FSW FSWA FFW FFWA PSIG Add 14.7 Divide by 14.7 Add 14.7, divide 14.7 Divide by 0.445 Divide by 0.445, add 33 Divide by 0.432 Divide by 0.432, add 34 PSIA minus 14.7 minus 14.7, divide 14.7 Divide by 14.7 minus 14.7, divide 0.445 Divide by 0.445 minus 14.7, divide 0.432 Divide by 0.432 ATM times 14.7 times 14.7, add 14.7 add 1 times 33 times 33, add 33 times 34 times 34, add 34 ATA minus 1, times 14.7 times 14.7 minus 1 times33, minus 33 times 33 times 34, minus 34 times 34. FSW times 0.445 Times 0.445, add 14.7 divide by 33 add 33, divide 33 add 33 times 1.03 times 1.03, add 34 FSWA minus 33, times 0.445 Times 0.445 minus33, divide 33 divide by 33 minus 33 minus 33, times 1.03 times 1.03 FFW times 0.432 Times 0.432, add 14.7 divide by 34 add 34, divide 34 times 0.97 add 34, times 0.97 add 34 FFWA minus 34, times 0.432 Times 0.432 minus 34, divide 34 divide by 34 minus 34, times 0.97 times 0.97 minus 34  Light Light traveling through water doesn't act the same as it does through air... Why is that? Why is that relevant to our discussion here?  Light Refraction Firstly, as you may have discovered if you've ever opened your eyes underwater without a mask or goggles, we see quite poorly underwater. Our eyes are specialized for vision through air. Light refracts as it passes through different materials. Underwater light waves (which are electromagnetic) are refracted or bent as water slows the light down.  Light How does this affect diving? Even with a mask, which provides an air pocket to allow our eyes to work more like they do on land, our vision underwater is affected by light refracting through the water. How we estimate size and distance is not nearly as accurate. In clear water at close distances, everything looks closer than it is. You may have experienced this if you've ever reached over to tap your dive buddy on the shoulder, only to realize they are out of arms reach. This is caused by refraction, and light traveling from the water through the mask to our eyes. Additionally, the mask magnifies our visual landscape and items will look approximately… 25 % closer and 33 % bigger than they actually are.  Light There are instances when it will appear the opposite however, especially in turbid water. (Turbidity is the cloudiness or haziness of the water.) When our brains process an image, one of the clues it uses to estimate distance is how hazy it appears. Our brains are accustomed to measuring this in air. Water can be much more turbid over a smaller distance, causing our brains to mistake this haziness as greater distance underwater. This is called Visual Reversal.  Color It is not just size and distance that can be affected. You may have noticed that colors disappear underwater, especially those in the red spectrum, even in shallow waters. This image shows light penetration in a couple of environments. You'll notice that reds go away very quickly in both. This contributes to everything looking blueish or greenish at depth.  Underwater Sound The properties of water greatly affect the transmission of sound. In the denser medium of water, sound waves transmit approximately 4 X (faster/slower) than when travelling through air. The exact speed will differ based on the temperature and composition of water.  Underwater Sound The  Underwater Sound How does this affect diving? To determine the distance of sounds, our brains use the gap between sound hitting one ear and the other. However, because sound travels so much faster underwater, the sound hits both ears of a diver almost simultaneously, making it difficult for the brain to determine sound direction and distance underwater. This results in sounds, such as boat engines or tapping, to sound louder, appear closer than they actually are, and be a poor indicator of direction.  Underwater Sound Additionally, equipment interference can affect our hearing. Wearing a hood can negatively affect your hearing underwater. In warm waters with no hood, you have a better chance of hearing rattles, tank clanging, or boats. Likewise, it is very difficult to 'talk' to your dive buddy through a standard scuba regulator with the sound of the bubbles in your ears. Divers need to take into account both the water's effect on sound waves and their diminished ability to hear underwater as sound is often used for both communications and for emergency recall.  Heat and Water The mechanisms of heat transfer that mainly affect us as divers are: Conduction heat transfer through direct contact Convection transfer of heat from one place to another by the movement of fluids Radiation Transfer of heat by the movement of waves  Heat and Water Water conducts heat approximately 25 times faster / slower than air. Water has a very high heat capacity (ability to hold heat). The heat capacity of water is approximately 3,200 larger than that of air. How does this affect us in diving?  Heat and Water Water temperature is almost always significantly cooler than a diver's body temperature. Water conducts heat away from the diver's body at a much faster rate than in air. This heat doesn't warm the water much because of high heat capacity, but this minimally warmed water is moved away by convection, and replaced by colder water. Additionally, as divers descend through the water column, water can drop in temperature quickly as we move away from the radiant heat of the sun, and convection works to pull warmer waters higher in the water column. Divers need to adequately insulate for diving, even in 'warm' water. It is important to wear appropriate exposure protection, in other words, a wetsuit or drysuit, for the water in which you are planning your dive.  Buoyancy Archimedes’ Principle An object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. Examples?  Buoyancy and diving How does this play into diving? This principle is engaged any time you take a breathe underwater, or add/remove air from an artificial airspace such as your BCD, drysuit, or a lift bag. When you breathe or add air, you increase how much water you displace, and therefore increase the buoyant force upward. Every time you exhale or vent air, you decrease the volume displaced, and therefore decrease the buoyancy force upwards.  Specific Gravity Specific gravity is the ratio of the density of a substance to that of a standard (generally water). The specific gravity of pure water is 1, because it is a 1:1 ratio. The specific gravity of saltwater varies depending on salt content but is usually approximately 1.03 g/ml (8.553ppg).  Buoyancy Calculations Why would a diver want to fuss with buoyancy calculations?  Buoyancy Calculations How to sink equipment to the ocean floor (making it negatively buoyant) is intuitive to most, just make it heavier (add weight or downward force). But this is not an ideal approach. Since we are elite scientific divers - How much weight do we need? Recall that: Upwards force of buoyancy = weight of the liquid displaced The weight of seawater = ~ 64 lbs per ft3/1.03kilograms per liter, and freshwater = ~ 62.4 lbs per ft3/1kg/L. In other words, if an object displaces 1 ft3 of seawater, the upwards force is 64 lbs. If an object displaces 1 liter of seawater, the upwards force is 1.03kg. The downward force (force of gravity) = weight of the object  Buoyancy Calculations (Imperial units) A small sealed box that weighs 100 lbs displaces 2 ft3 of water. Will it float or sink in saltwater? The upwards force: 2 ft3 (displaced water volume) X 64 lbs (the weight of 1 ft3 of saltwater)=128lbs. The downwards force: -100 lbs (weight of object). Will it sink or float? Positive force minus negative force 128 - 100 = 28 The box is positively buoyant and thus floats!  Buoyancy Calculations (Imperial units) A solid metal box that weighs 200 lbs displaces 2 ft3 of water. Will it sink or float in saltwater? The upwards force: 2 ft3 (displaced water volume) X 64 lbs (the weight of 1 ft3 of saltwater)=128lbs. The downwards force: -200 lbs (weight of object). Will it sink or float? Positive force minus negative force 128 - 200 = -72 lbs The box is negatively buoyant and thus sinks!  Buoyancy Calculations (Metric units) A small sealed box that weighs 40 kg displaces 39 liters (L) of water. Will it float or sink in saltwater? The upwards force: 39 L (displaced water volume) X 1.03kg (the weight of 1 L of saltwater) = 40.17kg. The downwards force: -40kgs (weight of the box). Will it sink or float? Positive force minus negative force 40.17 – 40 = 0.17 lbs The box is positively buoyant and thus floats!  Buoyancy Calculations and adjustments Back to the second example… A solid metal box that weighs 200 lbs displaces 2 ft3 of water. Will it sink or float in saltwater? 128 - 200 = -72 lbs. (Sinks) What if we wanted to get it to the surface? What piece of equipment would we use? Using a lift bag we would need to apply at least 72 lbs of lift to the object How much air would we need? 72lbs/64lbs per cf saltwater = 1.16 ft3  Buoyancy and your diving The ultimate goal of an HPU scientific diver is…. To achieve neutral buoyancy! It is not simple to predict the relative density of a human body because we are made up of tissues with a range of specific gravities. Fat, muscle, bone, etc... are all different, and we generally don't know our exact displacement. As a result, we don't make exact calculations of our relative density when diving. Instead, we make an educated guess, do a weight/buoyancy check, and then adjust during the dive using our BCD, lungs, and/or Drysuit.  Buoyancy and your diving Weight check in your beginner dive class for proper weighting: Estimating your weight needs Floating at eye level with an empty BCD and a normal breath of air Upon exhale, you should sink It is helpful not only to do a weight check at the beginning of the dive but also at the end. When using an aluminum cylinder, it will become lighter throughout the dive and therefore to dial in your buoyancy, do an additional post-dive weight check. You should be able to remain neutrally buoyant comfortably with little to no air in your BCD at a shallow depth at the end of the dive when the tank is low on gas.  Buoyancy and your diving Check your weight often. Especially if you are carrying different equipment for gathering data or working underwater, wearing a different scuba cylinder, exposure protection, or even subtle differences such as layers of drysuit thermals. Sometimes divers fall into the mindset of just adding more weight because it is 'better to have too much than not enough'. This is not true, it is best to have the right amount! Being over-weighted can increase your air consumption by making you work much harder than needed, make achieving neutral buoyancy difficult or impossible, and compromise your ability to ascend in a controlled manner (without dropping weights).  Gases What’s in our air? Most commonly, Scuba cylinders are filled with a similar mixture to what is found naturally in our air. Nitrox or Enriched Air (EAN) is a mixture where there is more Oxygen and less Nitrogen than standard air, resulting in longer no Decompression Limits. Trimix and Heliox are helium based breathing gases used for deeper diving to reduce or eliminate the narcotic effects of nitrogen and to ease the work of breathing by reducing the density of the breathing mixture. Additional information on these breathing gasses will be covered in more advanced aspects of your scientific diver training.  Ideal Gas Law *** explains the state of a hypothetical ideal gas. Equation? PV = nRT P = pressure V = volume T = temperature and (these are the ones we won't deal with as much) n = amount of substance R = ideal gas constant  Gases In our calculations of gas behaviors, neither the amount of substance (n), or the ideal gas constant (R) will change. Therefore, we can express how we compare a gas under two different conditions (1 and 2) as:  Boyle’s Law – Pressure and Volume "For any gas at a constant temperature, the volume of the gas will vary inversely with the pressure.” …if the pressure increases, the volume must decrease. You've already encountered this while diving. Remember how as you descend you have to add air to your bcd to remain neutrally buoyant? Also that you have to equalize the pressure in your ears by adding air pressure? This is all due to the decrease in gas volumes caused by increase in pressure upon descent. Boyle's law predicts as we ascend air volumes will expand. This is the law behind your training from day 1 in diving to never hold your breath! As you ascend the air in your lungs would expand, and as we'll discuss in the physiology module, this can cause serious injury in the form of pulmonary barotrauma. P1 V 1 = P2 V 2  Boyle’s Law – 5 ways … from the cheat sheet posted to blackboard  Boyle’s Law practice We take a flexible and/or open container with a 10 unit volume of gas from the surface down to 33'/10m of depth in the ocean. What is the new volume? (Note that 'unit volume' is a placeholder for cubic feet in English system calculations or liters in metric) P1 = pressure at the first depth Equation - P1V1 = P2V2 V1 = volume at the first depth P2 = pressure at the second depth V2 = volume at the second depth Pressure must be described in atmospheres absolute or ata. What does that mean? (add 1 ata for atmospheric pressure) P1 = 1 ata V1 = 10 unit volume P2 = 2 ata V2 = 5 unit volume V2 = ?  Boyle’s Law practice A gas volume of 4.5 volume units in a flexible container at 39'/11.8m of seawater is taken to 102'/30.9m. What will the new volume be? P1 = pressure at the first depth Equation - P1V1 = P2V2 V1 = volume at the first depth P2 = pressure at the second depth V2 = volume at the second depth Pressure must be described in atmospheres absolute or ata. What does that mean? (add 1 ata for atmospheric pressure) P1 = 39/33 +1 = 2.18 ata V1 = 4.5 unit volume P2 = 102/33 + 1 = 4.09 ata V2 = 2.39 unit volume V2 = ?  Boyle’s Law Got it? What is a direct way we can apply this to diving? Air (Breathing Gas) Consumption As you descend, it takes more molecules of air to fill your lungs to the same volume as they usually fill to at the surface. It takes twice as much at 33'/10m compared to the surface, 3 times as much at 66'/20m etc.. It is crucially important that you become familiar with your personal air consumption, and account for increases due to depth in your dive planning, to end the dive safely for you and your buddy AND account for the increase in task loading that is generally present when diving. More to come once we get into Dive Planning on this…  Amontons’ Law Pressure and Temperature (Sometimes also called Gay-Lussac's Law) "The pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant." Remember back to the ideal gas law equation P1V1/T1=P2V2/T2. Amontons decided to keep volume constant, meaning that V1=V2. Because they are the same, they cancel from both sides of the equation and we are left with P1/T1=P2/T2.  Amontons’ Law How does this apply to diving? Scuba cylinders! With a Scuba cylinder, we are dealing with a container in which the volume cannot change as external environmental pressure changes, but temperature and internal pressure changes will have noticeable effects. A cylinder placed in the hot sun will heat up and have higher pressure than it contained sitting in the shade. Moved back into the shade or cold water, the pressure in the cylinder will drop. Likewise, the act of filling a cylinder causes the temperature of the container to increase, and draining the cylinder causes the temperature of the container to fall. The pressure change will be approximately 5-6psi per degree F.  Amontons’ Law You may be used to using Fahrenheit or Celsius as your measures of temperature, but for these calculations, you need to use Rankine or Kelvin. Fahrenheit, use the Rankine scale and conversions for Gay- Lussac's calculations Celsius, use the Kelvin scale and conversions Conversions Rankine = Fahrenheit + 459.67 Kelvin = Celsius + 273.15 Amontons’ / Gay-Lussac’s Law … from the cheat sheet posted to blackboard  Amontons’ Law Apply It - Scuba Cylinder Temperature A scuba cylinder stored at 70°F and at a pressure of 3000psi is moved to a beach where the cylinder warms to 95°F. What affect does this have on the pressure of the cylinder? Reminder: Rankine = Fahrenheit + 459.67 First, we need to convert our temperatures to 70+459.67 =529.67 (T1) 95+459.67= 554.67 (T2) Rankine: Second, we need to convert our pressure to atm (this is the pressure in the cylinder): For this you'll need to remember that 1 atm = 14.7 psi 3000psi/14.7psi/atm=204.08 atm (P1) P2 = 214.46𝑎𝑡𝑚 3152 psi  Charles Law Volume and Temperature "When the pressure on a sample of gas is held constant, the Kelvin or Rankine temperature and the volume will be in direct proportion." As temperature increases, so does volume (and vice versa). We do not typically feel or experience this underwater, but it is included here for completeness.  Dalton’s Law Partial Pressures “The total pressure exerted by a mixture of gases is equal to the sum of the pressures of each of the different gases making up the mixture, with each gas acting as if it alone was present and occupied the total volume” Ptotal = PA + PB + PC  Practice with Dalton’s Law Ptotal = PA + PB + PC What are the partial pressures at 33fsw/10msw contributed by Nitrogen and Oxygen and 'other' for a diver breathing air (assuming 21% O2, 78% N2, 1% other). Ptotal = absolute pressure = (depth of 33'/33' per atm) + 1 atmosphere of pressure for the air above us = 2.00 ata of pressure P(O2) = 21% X 2 atm = 0.42 ata P(N2) = 78% x 2 atm = 1.56 ata P(other) = 1% x 2 atm = 0.02 ata Add PO2+PN2+Pother = 0.42 + 1.56 + 0.02 = 2.00 ata Total  Practice with Dalton’s Law What are the partial pressures at 33'/10 meters contributed by each gas for a diver breathing 40% Nitrox/Enriched Air? Nitrox or Enriched air is where a larger percentage of O2 is mixed into the breathing gas than atmospheric air. In this case, 40% O2, and we will calculate based on the rest (60%) being Nitrogen. Ptotal = absolute pressure = (depth of 33'/33' per atm) + 1 atmosphere of pressure for the air above us = 2.00 ata of pressure PO2 = 40% X 2 atm = 0.80 ata PN2 = 60% x 2 atm = 1.20 ata Add PO2+PN2 = 0.8 + 1.2 = 2.00 ata total  Henry’s Law – Gas Solubility This law forms the foundation for decompression theory and a number of other physiological effects of diving on humans. You'll recall that liquids have molecules that are closer together than gasses, but are still able to move with some space in between (unlike solids). This space allows gas molecules to 'dissolve' into the liquid, in a similar fashion to how sugar might dissolve into a cup of coffee or tea. The difference when gasses dissolve is that they retain their intrinsic properties. The dissolved gas continues to exert pressure, just as it would outside. This type of pressure is called gas tension, and will be relevant to physiology and decompression theory in later modules. Henry's law states: “The amount of any given gas that will dissolve in a liquid at a given temperature is proportional to the partial pressure of that gas in equilibrium with the liquid and the solubility coefficient of the gas in the particular liquid”  Henry’s Law – Gas Solubility If we look at this graphically, we see that the greater the pressure of the gas in contact with the liquid, the more gas enters the liquid. At some point, the pressure of the gas in contact and the gas in the liquid are equal, and this is known as saturation, or dynamic equilibrium. If the pressure is increased again, more gas molecules will flow into the liquid, and the gas tension will increase. If the pressure in contact with the liquid is decreased, gas molecules will flow out