1. The Physics of Diving NOAA Diving Manual Fourth Edition

2. The Physics of Diving Types of pressure Buoyancy Density Gases used in diving Specific Gravity Gas Laws pH Humidity Properties of water Light Units of measurement Sound

3. Overview Physics is the field of science dealing with matter and energy and their interactions. This presentation explores physical laws and principles that pertain to the diving environment and its influence on the diver. The principles of physics provide the keystone for understanding the reasons for employing various diving procedures.

4. Student Performance After reviewing this presentation you will be able to: Define the different types of pressure measurements associated with diving Recognize the affect of a variety of environmental factors and explain how they impact the diver. List the Gas Laws associated with scuba diving. Explain each of the Gas Laws; explaining how each pertains to scuba diving. Demonstrate your understanding of each of the Gas Laws by being able to solve problems germane to a scuba diving related scenarios.

5. Pressure “Pressure is force acting on a unit area” Pressure = force / area P = F/A In the USA pressure is typically measured in pounds per square inch (psi). Underwater a diver is affected by 2 types of pressure –The pressure exerted by the atmosphere –The pressure exerted by the surrounding water A diver, at any depth, must be in pressure balance (equilibrium) with the forces at that depth.

6. Atmospheric Pressure This is the pressure exerted by the earth’s atmosphere. At sea level it is equal to 14.7 psi, or one atmosphere (atm). It decreases with altitude above sea level. For example, at 18,000 ft. atmospheric pressure is 7.35 psi or half that at sea level. An pressure inside an individual’s lungs at sea level are at equilibrium with the surrounding pressure – 1 atm

7. Hydrostatic Pressure This pressure is created by the weight of water - called “hydrostatic pressure”. This pressure is cumulative. The deeper the dive, the more water above the diver and the greater the weight of the water. Hydrostatic pressure affects the diver from all sides equally.

8. Hydrostatic Pressure In seawater: –hydrostatic pressure increases at a rate of.445 psi per foot you descend. –One ata (14.7 psi) of hydrostatic pressure is reached at a depth of 33 feet sea water (fsw) & increases one atm for every additional 33 fsw thereafter. In freshwater: –hydrostatic pressure increases at a rate of.432 psi per foot you descend. –One ata (14.7 psi) of hydrostatic pressure is reached at a depth of 34 feet fresh water (ffw) & increases one atm for every additional 34 ffw thereafter.

9. Absolute Pressure The sum of atmospheric pressure plus hydrostatic pressure is called “absolute pressure”. It can be expressed as: “psia” (pounds per square inch absolute), “ata” (atmospheres absolute), “fswa” (feet of seawater absolute), “ffwa” (feet of freshwater absolute), or “mmHga” (millimeters of mercury absolute.

10. Gauge Pressure The difference between atmospheric pressure and the pressure being measured is “gauge pressure”. The “zero psi” reading on a scuba cylinder pressure gauge at sea level is actually equal to 14.7 psi. Gauge pressure + 14.7 = ata

11. Partial Pressure Dalton’s Law In mixture of gases, the proportion of the total pressure contributed by each gas in the mixture is called the “partial pressure”. For our purposes air is composed of 21% oxygen and 79% nitrogen.

12. Density “Density can be defined as weight per unit volume” Density = Weight / Volume or D = W / V Density is expressed in lbs/ft 3 or g/cm 3 Gas density is related to absolute pressure. Density is directly proportional to pressure –As depth increases, the density of the breathing gas and becomes heavier per unit volume.

13. Density Seawater has a density of 64 pounds per cubic foot. Freshwater has a density of 62.4 pounds per cubic foot. As a result, freshwater floats on top of seawater –a diver is more buoyant, given the same conditions, in seawater than in freshwater.

14. Specific Gravity Specific gravity is the ratio of the weight of a given volume of a substance (density) to that of an equal volume of another substance. –Water is the standard for liquids and solids. –Air is the standard for gases. Freshwater has a specific gravity of 1.0 –Substances that are more dense than freshwater have a specific gravity greater than 1.0. The specific gravity of seawater is 64/62.4 = 1.026

15. Water Freshwater (H 2 O): –is odorless, tasteless and very slightly compressible. –It freezes at 32 o F (0C), and boils at 212 o F (100C). –In its purest form, it is a poor conductor of electricity.

16. Water Seawater: –Contains almost every substance known. –The most abundant chemical is sodium chloride (common table salt). –Seawater is a good conductor of electricity.

17. pH The pH of an aqueous solution expresses the level of acids or alkalis present. The pH of a liquid can range from 0 (strongly acidic) to 14 (strongly alkaline). A pH of 7 is considered neutral The pH level in the blood is what signals the brain the need to breathe. –Too much CO 2 causes the blood to become acidic. One way the body reduces the acidity is to increase ventilations

18. Units of Measurement There are two systems for specifying force, length and time: the English system and the International System of Units (SI). –also known as Metric. The English System is based on the pound, the foot, and the second. –Primarily use in the United States The International System of Units is based on the kilogram, the meter, and the second. –Used everywhere else

19. Length 1 meter = 39.37 in = 3.28 ft To convert 10 feet to meters: –10 ft / 3.28 ft/m = 3.05 m Convert 10 meters to feet: –10 m X 3.28 ft/m = 32.8 ft

20. Area In both the English and IS system, area is expressed as a length squared. For example: –A room that is 12 feet by 10 feet would have an area of 120 square feet (12 ft x 10 ft). –A room that is 3.66 m by 3.05 m would have an area of 11.16 square meters.

21. Volume Volume is expressed in units of length cubed. Length x Width x Height = cubic feet (ft 3 ) or cubic meters (m 3 ) –The English System, in addition to ft 3, uses other units of volume such as gallons. –The SI uses the liter (l). A liter = 1000 cubic centimeters (cm 3 ) or.001 cubic meters (m 3 ), which is one milliliter (ml).

22. Weight The “pound” (lb) is the standard measure of weight in the English System. The “kilogram” (kg) is the standard measure of weight in the International System of Units. One liter of water at 4C weighs 1 kg or 2.2 lbs. –1liter (l) = 1 kg = 2.2 lbs

23. Weight (conversions) Convert 180 pounds to kilograms: –180 lbs / 2.2 lbs/kg = 81.8 kg Convert 82 kilograms to pounds: –82 kg X 2.2 lbs/kg = 180.4 lbs

24. Temperature Heat is associated with the motion of molecules. The more rapidly the molecules move, the higher the temperature. Temperature is usually measured either with the Fahrenheit (°F) scale or with the Celsius (centigrade) scale (C).

25. Temperature Temperature must be converted to absolute when the gas laws are used. The absolute temperature scales, which use Rankine (R) or Kelvin (K), are based on absolute zero (the lowest temperature that can possibly be reached). Note that the degree symbol (°) is only used with Fahrenheit temperatures.

26. Temperature (conversions) The Fahrenheit (°F) and Rankine (R) temperature scales are used in the English System. –To convert Fahrenheit to absolute temperature Rankine o F + 460 = R The Celsius (C) and Kelvin (K) temperature scales are used in the International System of Units. –To convert Celsius to absolute temperature Kelvin C + 273 = K

27. Temperature (conversions) To convert from Fahrenheit to Celsius –C = 5/9 X ( o F – 32) To convert from Celsius to Fahrenheit – o F = (9/5 X C) + 32

28. Heat An often forgotten but extremely important consideration in diving Humans can only function effectively in a very narrow range of internal temperatures. Maintaining the proper body core temperature is critical –This can be don’t by utilizing the proper exposure protection suit

29. Buoyancy Archimedes’ Principle “Any 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”

30. Buoyancy “Positive” Buoyancy is achieved if the weight of the displaced water (total displacement) is greater than the weight of the submerged object. – Object floats “Negative” Buoyancy is achieved if the weight of the displaced water (total displacement) is less than the weight of the submerged object. –Object sinks “Neutral” Buoyancy is achieved if the weight of the displaced water (total displacement) is equal to the weight of the water. –Object is suspended)

31. Buoyancy Buoyancy is dependent upon the density of the surrounding liquid. Remember: –Seawater has a density of 64 pounds per cubic foot. –Freshwater has a density of 62.4 pounds per cubic foot.

32. Gases Associated with Diving Atmospheric Air - 21% O 2 + 79% N 2 Oxygen - O 2 –The most important of all gases. –Usually used for decompression gas Nitrogen – N 2 Helium – He –Used extensively in deep diving Carbon Dioxide – CO 2 –A natural by-product of metabolism Carbon Monoxide – CO –A poisonous gas – produced by the incomplete combustion of fuels Argon – Ar –Not very common in diving Neon – Ne –Not very common in diving Hydrogen – H 2 –Not very common in diving

33. Gas Laws Boyle’s Law Charles’/Gay-Lussac’s Law Dalton’s Law Henry’s Law General Gas Law

34. Gas Laws Boyle’s Law “For any gas at a constant temperature, the volume of the gas will vary inversely with the pressure”

35. Gas Laws Boyle’s Law P 1 V 1 = P 2 V 2 P 1 = initial pressure surface absolute V 1 = initial volume in cubic feet P 2 =final pressure absolute V 2 =final volume in cubic feet

36. Gas Laws Boyle’s Law Determine the volume (V 2 ) of a 24 ft 3 open bottom diving bell with at 66 fsw: P 1 = 1 ata V 1 = 24 ft 3 P 2 = 3 ata V 2 = (P 1 V 1 ) / P 2 V 2 = (1 ata x 24 ft 3 ) / 3 ata V 2 = 8 ft 3

37. Gas Laws Charles’/Gay-Lussac’s Law “For any gas at a constant pressure, the volume of the gas will vary directly with the absolute temperature or for any gas at a constant volume, the pressure of the gas will vary directly with the absolute temperature.”

38. Gas Laws Charles’ Law Volume Change (pressure remains constant) V 1 / V 2 = T 1 / T 2 V 1 = volume initial T 1 = temperature initial V 2 = volume final T 2 = temperature final

39. Gas Laws Charles’ Law Volume Change A balloon with 24 ft 3 of air is lowered in water to a depth of 99 fsw. Surface temperature is 80 o F; temperature at depth is 45 o F. What will the volume of the balloon be at 99 fsw? V 1 / V 2 = T 1 / T 2 (pressure remains constant) V 1 = volume initial T 1 = temperature initial V 2 = volume finalT 2 = temperature final

40. Gas Laws Charles’ Law Volume Change Boyle’s law determines that the balloon will have a volume of 6 ft 3 at 99 fsw. Determine how temperature affects this volume. V 2 = (V 1 T 2 )/T 1 V 1 = Vol. @ 99 fsw = 6 fswV 2 = unknown T 1 = 80 o F + 460 = 540 RankineT 2 = 45 o F + 460 = 505 Rankine V 2 = (6 ft 3 X 505 R) / 540 R = 5.61 ft 3

41. Gas Laws Gay-Lussac’s Law Pressure Change (volume remains constant) P 1 / P 2 = T 1 / T 2 P 1 = pressure initial T 1 = temperature initial P 2 = pressure final T 2 = temperature final

42. Gas Laws Gay-Lussac’s Law Pressure Change A scuba cylinder contains 3000 psig (3014.7 psia) at 64 o F. After being left in the Sun the reaches 102 o F. What is the new pressure? P 1 / P 2 = T 1 / T 2 (volume remains constant) P 1 = pressure initial = 3014.7 psia T 1 = temperature initial = 64 o F + 460 = 524 Rankine P 2 = pressure final = unknown T 2 = temperature final = 102 o F + 460 = 562 Rankine

43. Gas Laws Gay-Lussac’s Law Pressure Change P 2 = (P 1 T 2 ) / T 1 P 2 = (3014.7 psia X 562 R) / 524 R P 2 = 3233.3 psia To convert to gauge pressure 3233.3 psia – 14.7 psi = 3218.6 psig

44. Gas Laws Dalton’s Law “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.”

45. Gas Laws Dalton’s Law P t = PP 1 + PP 2 + PP 3, etc P t = Total Pressure PP 1 = Partial Pressure of first gas PP 2 = Partial Pressure of second gas PP 3 = Partial Pressure of third gas, etc

46. Gas Laws Dalton’s Law What is the partial pressure of the nitrogen in a scuba cylinder filled with air to a pressure of 2000 psig? PN 2 = Gas % (decimal) X P t PN 2 =.78 X 2000 psig PN 2 = 1561.6 psig

47. Gas Laws Henry’s Law “The amount of any 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.”

48. Gas Laws Henry’s Law VG / V L = oc P 1 VG = Volume of a gas dissolved at STPD (standard temperature pressure dry) VL = Volume of gas in the liquid oc = Solubility coefficient at specific temperature P 1 = Partial pressure of that gas above the gas

49. Gas Laws Henry’s Law When a gas free liquid is exposed to a gas mixture, gas molecules diffuse into the solution, pushed by the partial pressure of each individual gas. “Gas Tension” – as the gas molecules enter the liquid they add to a state of gas tension, a way of identifying the partial pressure of the gas in the liquid. “Pressure Gradient” - The difference between the gas tension and the partial pressure of the gas outside the liquid.

50. Gas Laws Henry’s Law When the gradient for diffusion is high combined with low tension and high partial pressure the rate of absorption into the liquid is high. As the number of gas molecules in the liquid increases, the gas tension increases until it reaches an equilibrium value equal to the outside partial pressure. “Saturated” - the point of equilibrium described above.

51. Gas Laws General Gas Law Commonly called the “Ideal Gas Law” Used to predict the behavior of a given quantity of gas when changes may be expected in any or all of the variables Combines –Charles’ Law –Boyle’s Law

52. Gas Laws General Gas Law (P 1 V 1 ) / T 1 = (P 2 V 2 ) / T 2 P 1 = initial pressure (absolute) V 1 = initial volume T 1 = initial temperature (absolute) P 2 = final pressure (absolute) V 2 = final volume T 2 = final temperature (absolute)

53. Gas Laws General Gas Law A open diving bell with an air volume of 24 ft 3 is lowered to 99 fsw. Surface temperature is 80 o F and temperature at depth is 45 o F. What will be the air volume of the bell at depth?

54. Gas Laws General Gas Law (P 1 V 1 ) / T 1 = (P 2 V 2 ) / T 2 or V 2 = (P 1 V 1 T 2 ) / (T 1 P 2 ) P 1 = initial pressure (absolute) = 14.7 psia V 1 = initial volume = 24 ft 3 T 1 = initial temperature (absolute) = 80 o F + 460 = 540 Rankine P 2 = final pressure (absolute) = 4 ata x 14.7 psia = 58.8 psia V 2 = unknown T 2 = final temperature (absolute) = 45 o F + 460 = 505 Rankine

55. Gas Laws General Gas Law V 2 = (14.7 psia)(24 ft 3 )(505 R) (540 R)(58.8 psia) V 2 = 5.61 ft 3 The volume was reduced, due to the drop in temperature and the increase in ambient pressure.

56. Humidity Water vapor (a gas) behaves in accordance with the gas laws. The water vapor condenses at temperatures we are likely to encounter while diving, hence humidity is an important consideration –Mask fogging

57. Light Human eyes can only perceive a very narrow range of wave lengths (visible light) Water slows the speed at which light travels. –This causes the light rays to bend or refract –with a mask on the light rays are bent twice Objects appear 25 % larger. Turbidity can also effect vision by making objects appear farther than it really is.

58. Light Colors –Water absorbs light according to its wavelength Red is the first color lost Blue eventually become the dominant color at deeper depths –As depth increases the ability to discern colors decreases until visible objects are distinguishable only by differences in brightness. Contrast becomes the most important factor.

59. Sound Sound is produced by pressure waves triggered by vibration The more dense the medium through which sound travels, the faster the speed of sound. Sound travels roughly 4 times faster in water than in air –This makes detecting the origin of the sound very difficult.

60. The Physics of Diving Review of the chapter: Hydrostatic and atmospheric pressure both influence the diver. The density of the water, fresh vs. salt, affects the buoyancy of a diver. Depth affects the density of the gas being breathed thus affecting the rate of consumption. Measurements can be given in either the English System or the International System of Units. There are 5 different gas laws which explain the behavior of gases in a diving environment. Humidity, Light and Sound are all affected by the density of water