Flow and Flowmeter

Flow and Flowmeter by by Dr. Mohammad Abdeljawad

1. Flow and flowmeter Dr.Mohammad Abdeljawad Anesthesia Specialist AL- Bashir Hospital

2. Fluid Flow Definition of flow (F) Flow is defined as the quantity of a fluid, i.e. a gas or a liquid, passing a point in unit time. Therefore, the flow is the rate of the change of quantity which has a symbol (Q}. It is pronounced "Q dot” The small dot above the letter Q indicates the rate of change of quantity to differentiate the flow from the quantity (Q).

3. Flow of Fluids may occur through: A) A tube of a constant diameter. B)A tube of a variable (non-uniform) diameter. C) An orifice.

4. A) Flow of Fluids through a tube of a constant diameter. In order to drive a fluid through a tube, a pressure difference (~ P) must be present across both ends of - tube i.e., Pl-P2. LAMINAR FLOW In laminar flow (Fig. 2.1) a fluid moves in a steady manner and there are no eddies or turbulence

5. Characteristics: • It occurs when a flow passes through a smooth uniform tube • The fluid moves in a steady regular manner and there are no eddies. The fluid passes as if it is in concentric layers which are parallel to the sides of the tube. The flow is greatest in the central axial layer (twice the mean flow) and decreases gradually towards the periphery where it reaches zero in the layer touching the sides of the tube i.e., parabolic velocity profile. • The flow is silent. • It occurs when the flow moves with a velocity below a critical velocity.

6. The graph in Fig. 2.2 shows the result if various flows are passed through a tube and the resulting pressure drop across the ends is recorded. There is a Unear relationship so that flow is directly proportional to pressure under conditions of laminar flow. The ratio of pressure to flow is a constant known as the resistance R of the apparatus or tube concerned.

7. • The flow of fluids varies directly with the pressure difference (~ P) i.e., the relationship between the flow and the pressure is linear (figure 5-52) and the resistance (R) is constant. • The resistance is lower than that for the same turbulent flow.

8. Figure 2.3 shows how resistance can be measured. A known constant flow Q is passed through the apparatus concerned and the difference in pressure Pi—P2 between the ends of the apparatus is measured. By dividing pressure difference by the flow the resistance of the apparatus is obtained and, provided that flow is laminar, the resistance is independent of the flow. A technique such as this may be used to measure resistance with either gas or liquid flow

9. The resistance (R) is affected by: 1-Radius of the tube (r ): the resistance is inversely proportionate to the power of 4 of the radius i.e., R a 1/r4 Halving the diameter reduces the flow to onesixteenth of its original value if the pressure drop along the tube remains the same. In other words, the flow is proportional to the fourth power of the diameter. Consequently, a slight reduction of the diameter of an endotracheal tube can have an appreciable effect on resistance and therefore on flow Factors affecting the laminar flow:

10. 2- Length of the tube (l): the resistance is directly proportionate to the length i.e., R a l If the length of the tube is halved, the flow will double ,

11. 3-Viscosity of fluid (q): the resistance is directly proportionate to the viscosity i.e., R a I] Viscosity is a measure of the frictional forces acting between the layers of the fluid as it flows along the tube the viscosity of the fluid affects resistance to laminar flow in such a way that the higher the viscosity the slower is the flow .

12. The effects described may be summarized as follows:

13. All these factors are incorporated in an equation which can be derived theoretically and is known as the Hagen- Poiseuille equation: Hagen- Poiseuille equation : Laminar flow obeys Hagen- Poiseuille formula and depends on the viscositv .

14. resistance

15. clinical Applications : • To increase the rate of transfusion of fluids : Through an intravenous line, it is more important to increase the radius of the cannula inserted rather than increase the pressure, as flow is porportional to pressure i.e., an increase in the presure increases the flow by the same ratio, but flow is proportional to the power of 4 of the radius i.e., an increase in the radius produces an increase in the flow by 16 times (if r = 2 so, 24 = 16). Resistance to breathing is much greater when a tracheal tube of a small diameter is used as in pediatrics To determine.the peripheral vascular resistance As Q = deltaP /R or P= QxR – this relationship can be applied to the circulation as Main blood pressure (P) = Cardiac output (Q) x resistance (R)

16. TURBULENT FLOW laminar flow may change to turbulent flow if a constriction is reached which results in the fluid velocity increasing. In turbulent flow, fluid no longer flows in a smooth fashion but swirls in eddies and the resistance is higher than for the same laminar flow.

17. Characteristics: • It occurs when a flow passes through a smooth uniform tube with a constriction, an orifice, a sharp bend, or some other irregularity. • The fluid moves in an irregular manner and there are eddies (figure 5-51). The fluid passes in a haphazard manner and the lines of the flow are!'. parallel to the sides of the tube. There is no marke .: difference between the velocity in the center and the periphery i.e., flat flow profile.

18. • The flow is noisy i.e., it creates sounds e.g., carotid bruit and cardiac murmurs. • It occurs when the flow moves with a velocitv above a critical velocity. • The resistance is higher than that for the same laminar flow . . The variation of fluid velocity across the tube is different in turbulent flow from that which occurs in laminar flow.

19. • The flow in not directly proportional to the pressure difference (~ P), but AP is proportional to the square of the flow i.e., the relationship between the flow and the pressure is not linear (figure 5-52) and the resistance is not constant. in order to double the flow, the pressure must be increased by a factor of four.

20. Factors affecting the turbulent flow: Q a r4

21. i.e., turbulent flow does not obey Hagen- Poiseuille formula and depends on the density. Density represented by the Greek letter rho (p) and is equal to mass divided by volume (kgm~3).

22. ONSET OF TURBULENT FLOW turbulent flow may occur if there is a sharp increase in the flow through a tube, but there are several other factors influencing the type of flow, such as the viscosity and density of the fluid and the diameter of the tube. These factors may all be combined to give an index known as Reynolds number , which is calculated as follows:

23. Empirical measurements with cylindrical tubes show that, . if Reynolds number exceeds about 4000 , then turbulent flow is likely to be present. . If Reynolds number is below 2000, the flow is likely to be laminar . . If Reynolds number is between 2000 -4000, the flow is likely to be N.B.: Critical velocity occurs when the Reynold number is > 2000

24. clinical Applications : 1- During bronchial asthma , broncho -constriction occurs; therefore, the velocity of flow is increased above the critical velocity and the flow becomes turbulent where the resistance is very high. When helium is used (it has very low density) with oxygen, the density of the inhaled flow is decreased; therefore, the Reynolds number falls below 2000 and the flow returns back to the laminar flow with a low resistance 2- Flow of air in the respiratory tract is a mix between laminar and turbulent flow. • In the wider parts as the nose, nasopharynx , and trachea, the flow is more laminar. • In the branches of the bronchial tree, the flow is more turbulent. • In the lower respiratory tract as the surface area is large, the velocity is low so the flow is more laminar again. Any pathology as spasm or infection increases the turbulent flow

25. 3- During anesthesia , reduction of the resistance to flow can be achieved by avoiding the angle piece connector and making the internal surface of the breathing circuit smooth.

26. B) Flow of Fluids through a Tube of a Variable Diameter ‘when a fluid passes through a constriction in a tube, the velocity of the fluid increases and the pressure exerted on the wall of the tube falls. This is named after its discoverer Bernoulli. Bernoulli Effect EXPLANITION ~there are two types of energy when a fluid passes through a tube: 1-Potential energy : associated with the pressure exerted by the fluid on the wall of the tube. 2- Kinetic energy: associated with the velocity of the flowing fluid Both potential and kinetic energies are constant.

27. When a fluid passes through a constriction in a tube, the velocity of the fluid increases, which - associated with an increase in the kinetic energy. As the total energy is constant; therefore, an increase the kinetic energy is accompanied by a fall in the potential energy which is associated with a decrease the pressure exerted on the wall of the tube. The pressure exerted on the wall at the site of the constrict! may fall greatly to be sub-atmospheric.

28. Venturi Arrangement: When a cross-sectional area of a tube gradually decreases towards a constriction then gradually incre again, this is called a Venturi tube (figure 5-53).

29. The fluid passing through a Venturi shows the following changes: • At first the velocity increases gradually as the fluid passes through the gradually decreasing crosssectional area of the tube, and at the site of the constriction, sub-atmospheric pressure occurs (Bernoulli effect). • Then when the fluid passes through the gradually increasing cross-sectional area of the tube, its velocti decreases and the pressure increases gradually again until it reaches its original value

30. Clinical Application: . 3- Venturi facemask for oxygen therapy. The sum of the driven gas (100%oxygen) and the entrained exceeds the peak inspiratory flow rate; therefore, a constant oxygen percentage is delivered to the pt . 2- Jet ventilation e.g., Sander's injector during bronchoscopy .

31. 4- Nebulizers: the driven gas passes through the central tube, and the liquid (an inhaled drug entrained via the side tube where it is broken up into small droplets by a disc or an anvil. Gas-driven (pneumatic or jet) nebulizer is used for humidification of inspired air.

32. ENTRAINMENT RATIO The entrainment ratio is defined as the ratio of entrained flow to driving flow. Thus, a 9 to 1 entrainment ratio indicates that there are 9 litre min-1 being entrained by a driving gas of 1 litre min'1. In clinical practice, entrainment ratios may not be constant and obstruction at the outlet of the Venturi can result in a fall in the entrainment ratio. This gave rise to problems in ventilators worked by injectors in which oxygen was used to entrain air or nitrous oxide. In such cases, back pressure altered the flow and so the resulting oxygen concentration.

33. THE COANDA EFFECT Based on the Bernoulli and Venturi effects, when a gas passes through a constriction, a sub-atmospheric pressure is produced. If there are no holes at the site of constriction, the sub-atmospheric pressure will –hold the stream along the wall of the wide tube. If a narrow tube is connected to a Y-connection of a wider i.e., a tube with two Venturis , the flow which is held to the wall will tend to cling to one side of the y - ccnnection (i.e., the flow does not divide evenly between the two sides, but flows through only one of the 'Y'). This is called the Coanda effect

34. clinical Applications : Coanda effect explains: • the uneven distribution of gas flow to alveoli when there is a slight narrowing of the bronchiole before it divides, resulting in alveolar collapse. • the uneven distribution of coronary blood when there is a slight narrowing of the coronary artery before it branches, resulting in myocardial ischemia.

35. · a small tube is inserted perpendicularly at the exit of a narrow tube and little pressure is applied, asimple valve switch mechanism is produced where the flow can be directed from one exit tube to the (without any mechanical parts). This device is called fluid logic and can be used in gas-driven ventilators (called fluidic ventilators).

36. flow of Fluid through an Orifice In an orifice, the diameter of fluid (Dl) pathway exceeds the length ( Ll ), but in the tube, the length (L2) exceeds the diameter (D2) (figure 5-55). The flow through an orifice is turbulent .

37. Factors affecting flow of the fluid through an Orifice : 1- The square root of the pressure difference across the orifice i.e., 2- The square of the diameter ot the orifice i.e., • 3- The density of the fluid i.e., Q a 1/ density clinical Applications : In the bobbin flowmeter " Rotameter ®", at low flow rates, the narrow annular space between the bobbin and the wall mimics a tube . At high flow rates, the width of the annulus is large in relation to the height of the bobbin and the annular space forms an orifice . Thus at low rates, the viscosity of gas determines the ; poisition of the bobbin (as it is laminar flow), whereas at higher rates the effect of the density of the gas becomes more important (as it is turbulent flow).

38. Flowmeter

39. Flowmeter · Flowmeters on anesthesia machines are classified into: 1- “variable orifice (constant or fixed pressure drop) flowmeters ( Rotameter ): They are the conventional ones used. 2- Electronic flowmeters : They are available now in the recent anesthetic machines. In these new machines, there must be a back-up conventional (Thorpe) auxiliary oxygen flowmeter which is used in of in case failure of the electronic type Other models of anesthesia machines have the conventional flowmeters but measurement of the gas flow is done electronically along the Thorpe tube or there are digital/ graphic displays of the flow

40. Flowmeter in addition to delivering a controlled flow of oxygen, nitrous oxide, or air, they also reduce the pressure of these gases form 4.1 bar (in the pipelines) to 1 bar, which is delivered to patients variable orifice (constant or fixed pressure drop) flowmeters ( Rotameter ) Flowmeter Assemblies .- The flowmeter assembly consists of the flow control valves and the flowmeters , and its purposes are precise control and measurement of gas flow traveling to the common gas outlet.

41. “ The valves themselves represent an important anatomic landmark within the anesthesia workstation because they separate the intermediate-pressure section from the low-pressure section. . The operator regulates flow entering the lowpressure ircuit by adjusting the flow control valves . After leaving the flowmeters , the mixture of gases travels through a common manifold and may be directed through an anesthetic vaporizer if selected. The total fresh gas flow and the anesthetic vapor then travel toward the common gas outlet.

42. Flow Control Valves. The flow control valve assembly consists of a flow control knob, a tapered needle valve, a valve seat, and a pair of valve stops. The inlet pressure to the assembly is determined by the pressure characteristics of the intermediate-pressure segment of the respective machine secondary pressure regulators are often used before the flow control valves to provide stable input pressure despite fluctuations in hospital pipeline supply pressure. The location of the needle valve in the valve seat changes to establish different orifices when the flow control valve is adjusted Gas flow increases when the flow control valve is turned counterclockwise , and it decreases when the valve is turned clockwise.

43. Saeftey Feature: - Control knobs have the same color code as the gas cylinders (figure 6-20). - The oxygen knob is usually fluted, larger and protrudes further than the other knobs

44. Flow Tubes. Atapered glass or plastic tube (Thorpe tube), which is narrow at the bottom and wide at the apex like ·an inverted cone, is present vertically where a light metal alloy bobbin (its trade name is Rotameter ®)or ball present inside the tube The quantity of flow is indicated on a scale associated with the flow tube. Referred to as variable orifice area flow tubes or Thorpe tubes, these glass tubes are narrowest at the bottom and widen vertically

45. An indicator float is housed within the tube that is free to move vertically Opening the flow control valve allows gas to travel through the space between the float and the flow tube. This space is known as the annular space, and it varies in size depending on the position of the indicator in the tube

46. The indicator float hovers freely in an equilibrium position in the tube where the upward force resulting from gas flow equals the downward force on the float resulting from gravity at a given flow rate. These flowmeters are commonly referred to as constant-pressure flowmeters because the decrease in pressure across the float remains constant for all positions in the tube.

47. When the flow increases , the bobbin or the ball rises in the wider parts of the tube against the gravity and the annular orifice around it increases; therefore, the flow resistance decreases and the clearance around the bobbin or the ball increases. So, the effect of the gravity (weight of the bobbin or the ball) is balanced by the increased flow and so the pressure across the bobbin (or the ball) stays constant although the flow increases .

48. the reverse occurs when the flow decreases as the bobbin or the ball falls in the narrower parts of the tube and the annular orifice around it decreases; therefore, the flow resistance increases and the clearance around the bobbin or the ball decreases and so the pressure across the bobbin (or the ball) stays constant despite the flow decreases i.e., there is a variable orifice and fixed pressure drop around the bobbin (or the ball)

49. At low flow rates , the narrow annular space between the bobbin (or the ball) and the wall mimics a tube and the flow becomes laminar . At high flow rates , the width of the annulus is large relative to the _ higet of the bobbin (or the ball), the annular space forms an orifice and the flow becomes turbulent . at low rates , the viscosity of gas determines the position of the bobbin (as it is laminar flow), whereas at higher rates the effect of density of the gas becomes more important (as it is turbulent flow).

50. Recently electronic flowmeters have been available on the computer screen

51. Flows can be displayed numerically or sometimes graphically in the form of a virtual, digitalized flowmeter . Numerous types flow sensor technologies can be applied, such as hot-wire anemometers, a differential pressure transducer method, or mass flow sensors. Electronic Flow Sensors. An example of an electronic mass flow sensor : A device relies on the principle of specific heat to measure gas flow. As gas streams through a heated chamber of known volume, a specific amount of electricity is required to maintain the chamber temperature. The amount of energy required to maintain the temperature is proportional to the flow of the gas and the gas’s specific heat.

52. Factors Affecting the Performance of the Rotameter : 1- The viscosty and density : Because the flow in this flowmeter is a mixture of laminar (at low flow rates) and turbulent (at high flow rates) flow, so both the viscosity (in laminar) and density (in turbulen of the gas is important. Therefore, each rotameter has to be calibrated for a specific gas i.e., different gases can not be used in the same flowmeter except after recalibration or change of the scale written on the tapered tube . Although temperature and barometric pressure can influence gas density and viscosity, under normal clinical circumstances flow tube accuracy is not significantly affected by mild changes in temperature or pressure

53. 2- Sticking The bobbin may touch the wall of the tapered tube and stick to it. To avoid this: The flowmeter tube must be kept vertical to reduce the friction between the bobbin and the tube As electrostatic charges (which increase sticking) may build up on the bobbin and the wall of the tube it rubs against the wall of the tube; therefore, to conduct away the electrostatic charges: - Some tubes are coated from inside by a conductive transparent material (as gold or tin" stannous oxid "). A conductive strip is present from inside the tube. The plastic cover of the rotameter is sprayed with an antistatic spray such as Croxtine .

54. Small slots are placed round the top of the bobbin causing it to rotate centrally in the gas flow and a dois present in the body of the bobbin indicating its rotation (the dot is not used to indicate the level frorr which readings are made). The ball is used as sticking is less . Dust is prevented by incorporating a dust filter in the needle valve at the bottom of the tube becaus dust on the bobbin may cause sticking or even alteration in size of the annulus which causes inaccuracies

55. It is within+ 2-2.5%. To increase the accuracy: Accuracy: Avoid sticking, as above Readings are made from the upper surface of the bobbin ( more accurate as there is a well defined surface for reading ) or the central equator (the middle) of the ball (less accurate).

56. In recent anesthetic machines where very low flows are needed in closed circuits, two flowmeters , on for the low and one for high flows, are made in series and are still controlled by one valve (figure 6-16).

57. • Attachment of a vaporizer or a ventilator e.g., Manley, after the flowmeter produces back pressurwhich increases the resistance in front of the flowmeter . This in turn increases the pressure at the outlet ~the flowmeter . This increased pressure affects in turn the calibration of the flowmeter , due to affection the viscosity and density of the gases, which affect the accuracy as there may be as much as 10% more gas flow than that indicated on the flowmeter . Some flowmeters are now pressurized and calibrated to wor at a high pressure of several bars, which minimizes the effect of the relatively smaller pressure change the outlet.

58. To increase the safety: safety: The position of the flowmeters When there are flowmeters in series e.g., one for 02 and the other for N20, and a break in the junction between two flowmeters occurs e.g., in air flowmeter , the concentration of the gas mixture obtained from e flowmeters may be changed and become hypoxic as follows: - the 02 flowmeter is located at first and the N20 flowmeter is located after the air flowmeter , the 02 may w out of the break in the system as in (A) and a hypoxic gas mixture is obtained

59. To solve this problem: - The sequence of arrangement of the flowmeters is reversed, where the 02 flowmeter is located after the air flowmeter as in (B).This is the standard in North America. - Because the standard in most other countries is placement of the 02 flowmeter at first; therefore, a channel is present at the outlet of the 02 flowmeter to deliver it separately away from the N20 (C). This is the standard in the United Kingdom. A

60. A leak in the oxygen flow tube may result in creation of a hypoxic mixture even when oxygen is located in the downstream position (Fig. 29-10). Oxygen escapes through the leak, and nitrous oxide continues to flow toward the common outlet, particularly at high ratios of nitrous oxide to oxygen flow.

61. Oxygen/nitrous oxide ratio: In the modern anesthetic machines, there is a link between the oxygen flow controller and nitrous oxide controller to ensure administration of at least 25% 02 when the N20 flowmeter is turned on alone. When the N20 flowmeter is turned on alone, the 02 flowmeter is turned on _obligatorily to at least 25% of the total gas mixture; therefore, hypoxia is avoided. This is achieved by one - the following methods: A-mechanical method: where a chain link is present between the 02 and NzO flowmeter control knobs

62. Datex-Ohmeda Link-25 Proportion-Limiting Control System The system is based on a mechanical integration of the nitrous oxide and oxygen flow control valves and a difference in the taper of the needles of the oxygen and nitrous oxide flow control valves. It allows independent adjustment of either valve, yet it automatically intercedes to maintain a minimum oxygen concentration with a maximum nitrous oxide– oxygen flow ratio of 3:1. The Link-25 automatically increases oxygen flow when then nitrous oxide flow is increased to more than a certain point relative to oxygen flow to prevent delivery of a hypoxic mixture

63. b- A Pneumatic method: where a pneumatic mixing valve is present. The North American Dr ن ger sensitive oxygen ratio controller system (SORC) is a pneumaticmechanical , oxygen–nitrous oxide interlock system designed to maintain a ratio of no less than 25% oxygen to 75% nitrous oxide flow into the breathing circuit by limiting the nitrous oxide flow when necessary. The SORC is located between the flow control valves and the electronic flow sensors. The SORC consists of an oxygen chamber with a diaphragm, a nitrous oxide chamber with a diaphragm, and a nitrous oxide proportioning valve

64. All interconnected by a mobile horizontal shaft. Pneumatic input into the device comes from the oxygen and nitrous oxide flow control valves

65. As oxygen flows out of the SORC, it encounters a resistor that creates backpressure This backpressure is transmitted to the oxygen chamber diaphragm, which causes the diaphragm to move to the right, thereby opening the nitrous oxide proportioning valve. As the oxygen flow is increased, so too is the backpressure and the rightward motion of the shaft. If the nitrous oxide flow is now turned on, it will also flow into the SORC, through the proportioning valve, and past its resistor to create backpressure that will press on the diaphragm in its respective chamber. The counterbalance between the two gas flows (backpressures) determines the positioning of the nitrous oxide proportioning valve

66. If the oxygen is turned down too low (<25% of the nitrous oxide flow), the shaft will move to the left and thus limit the nitrous oxide flow. If the operator tries to turn up the nitrous oxide too high relative to the oxygen flow, the SORC will limit the nitrous oxide flow regardless of how far the flow control valve is opened. If the oxygen flow is decreased to less than 200 mL /minute, the proportioning valve will close completely

67. c- An electronic method. • Minimum oxygen flow: Some recent flowmeters allow minimum oxygen flow of 150 mL /min O_ when the anesthesia machine is turned on even when the oxygen flow valve is turned off. This safety feature helps ensure that some oxygen enters the breathing circuit even if the operator forgets to turn or the oxygen flow.

68. • The Quantiflex mixer flowmeter (figure 6-19) eliminates the possibility of reducing the oxygen supplinadvertently because: One dial is set to the desired % of oxygen, and it is adjusted first Then the total flow rate is adjusted independently by another dial (the black knob). Therefore, the percentage of ~ is fixed and there is no need to readjust the flow of 02 manually whenever the flow rate is changed.

69. Flow valves They constitute an important landmark of the anesthesia machine because they divide the machine into two gas circuits • The high-pressure circuit: is the part of the machine that is upstream from the flow control valves and consists of the pipeline system, the gas cylinders and the tubes connecting them to the machine • The low-pressure circuit: is the part of the machine that is downstream from the flow control valves and consists of the flowmeters , the vaporizers and the common gas outlet that receives all gases and vapors from the machine