1. FLOWMETERS & FLOW MEASUREMENT DEVICES
Dr S. Parthasarathy MD DA DNB PhD FICA ,IDRA
Dip. software based statistics

2. Flow meter –
measures flow
But we can change, adjust the flow
FLOW MEASUREMENT DEVICES –
it just measures the flow !!

3. Let the lecture go this way !
HISTORY
PRINCIPLES & TYPES
NEEDLE VALVE & THORPE TUBE

4. HISTORY 1868- Chameroy 1876- Sir Alfred Ewing’s Ball & Tube Flowmeter
1879- Joslin’s dry bobbin flowmeter
The development of flowmeters began in the late 19th century. In 1868, Chameroy described a flowmeter that was developed later into the Heidbrink meter. A “ball and tube” flowmeter was described by Sir Alfred Ewing in 1876, which consisted of a small ball in a tapered tube placed at angle to the horizontal. Gas was introduced into the tube through its lower end, and the height to which the ball was raised was an indicator of the gas flow. Joslin introduced the earliest version of the dry bobbin flowmeter in 1879.

5. HISTORY Coxeter Bobbin Meter
inaccuracies due to friction between the bobbin and the wall,
blockade of the exit holes by dirt or dust,
inability to measure fine changes in the gas flow as it could be measured only in steps from one hole to the next.
The Coxeter bobbin meter was a modification of Joslin’s flowmeter. It had a glass case enclosing three flowmeters to measure the rates of flow of oxygen, nitrous oxide and carbon dioxide. Each meter consisted of a glass tube of uniform bore, with a series of small holes. As gas entered the lower end of the tube, the bobbin was forced upwards, it rose to a certain height. At this point, a number of holes became available for escape of gas from the interior of the tube to the outer glass cylinder, which, in turn, was passed on to the patient. If the needle valve was opened to increase the flow of gas, the pressure to which the bobbin was exposed increased, and the bobbin rose further, permitting gas to escape through more holes, till the pressure underneath the gas counteracted its weight. The rate of gas flow was indicated by the height to which the bobbin was raised. Each flowmeter was calibrated for the particular gas it was meant for. The problems with this instrument were inaccuracies due to friction between the bobbin and the wall, blockade of the exit holes by dirt or dust, and inability to measure fine changes in the gas flow as it could be measured only in steps from one hole to the next.

6. HISTORY Heidbrink meter
The Heidbrink meter was a modification of Chameroy’s model, and it consisted of a metal container of compound bore, into which was inserted a black indicator rod. The topmost part of this rod was visible through a glass case with a calibrated scale, and the height of this rod determined the flow of gas through the metal container.

7. HISTORY 1908- Karl Kuppers 1910- M. Neu 1930- Connell
rota
1908- Karl Kuppers
1910- M. Neu
1930- Connell
1937- Richard Salt
The first flowmeter using a float was designed by Karl Kuppers in 1908, and this design has stood the test of time. Kuppers also described the principle of the rotameter. M. Neu first applied the rotameter in anaesthetic practice for the administration of nitrous oxide and oxygen, which was not a commercial success due to the high cost of nitrous oxide in Germany at that time. In 1930, the original ball and tube flowmeter underwent further modifications by Connell. He introduced a compound bore in order to enable measurement of a much wider range of gas flows and also replaced the single ball by two balls, to minimize disturbances and oscillations due to gas flows. Richard Salt employed a redesigned flowmeter using a float commercially in UK, which had originally been devised for use in Germany.
During the world war II, access to flowmeters was limited. After the war, UK machines regularly employed rotameters, although nonrotating floats were still used in US machines. The calibrations were changed from gallons/hr to l/min.
CONNELL METER

8. Boyle’s Water Sight Flowmeter
HISTORY
Boyle’s Water Sight Flowmeter
PRINCIPLE SCHEMATIC
While the dry bobbin flowmeter was undergoing its evolution, simultaneously, another flowmeter was introduced by Robert Boyle in his first anaesthetic machine, and underwent parallel development as his machine developed into newer versions. The problems associated with these flowmeters were leak of fluids, especially during high gas flows due to disturbance of water. Slowly, these became totally obsolete and were replaced by dry bobbin flowmeters almost entirely.
Each meter consisted of a metal tube. In practice, two or three tubes for measuring different gases were placed in the same bottle, which was filled with water till the level of the horizontal metal plate which bound the tubes together. The part of the tube beneath the water level was perforated by five small holes placed one below the other. As the gas flowed, it escaped through the holes, bubbled through the water, and was led away from the top of the bottle. The greater the rate of flow of gas, the greater the number of holes through which the gas would emerge into the water. Thus, a rough measure of the rate of flow of gas could be obtained. A tap on the nitrous oxide entry tube could bypass approximately 5 l/min of gas without it having to pass through the flowmeter.
Another historical flowmeter was the Foregger meter, which shall be discussed later.
As flowmeters became more and more advanced, a wide variety of safety features were incorporated, and there is still wide scope for further advancements.

9. RECENT ADVANCES & FUTURE TRENDS
ELECTRONIC CONTROL OF FLOW
Mass flow controller !! – solenoid valve and electric change
ELECTRONIC FLOW MEASUREMENT
Future anaesthetic machines are likely to have electronic flow control valves, as the conventional valves are not well suited for servo- control. The most likely electronic flow control to be applied is the mass flow controller. It has a continuously variable solenoid valve which varies the flow by varying the voltage applied across the solenoid. The flow is continuously monitored and this information is fedback to the solenoid valve. The flow is controlled either by splitting into a number of channels or by pulsing the flow. The volume of gas passing through the valve depends on how long the valve is open and on the frequency of opening.
In the next generation of machines, the gas flows will be electronically measured and displayed numerically on bar graphs or other video display as the total gas flow and the percentage of each gas.

10. CONSTANT ORIFICE, VARIABLE PRESSURE DIFFERENCE
PRINCIPLES & TYPES
CONSTANT ORIFICE, VARIABLE PRESSURE DIFFERENCE
TUBULAR
ORIFICE
Measurement of gas flow is based on two basic principles- by employing a constant orifice with a variable pressure difference across it, or utilizing the principle of a constant pressure difference across a variable orifice.
CONSTANT ORIFICE, VARIABLE PRESSURE DIFFERENCE
Gas is allowed to flow through a venturi, a tube or an orifice. A tube is an opening with a length much higher than the diameter, while in case of an orifice, the diameter is much larger than the length. A venturi has a variable cross- sectional area.
VENTURI

11. Its variable pressure difference and not variable pressure

12. CONSTANT ORIFICE, VARIABLE PRESSURE DIFFERENCE
PRINCIPLES & TYPES
CONSTANT ORIFICE, VARIABLE PRESSURE DIFFERENCE
Water depression meter
Bourdon’s pressure gauge
Foregger’s gas flow meter
Fleisch pneumotachograph
Various devices that utilize this principle are the water depression meter, Bourdon’s pressure gauge, Foregger’s gas flow meter and Fleisch pneumotachograph.

13. WATER DEPRESSION METER
Gas h
Water depression meter: This utilizes the law that gases always flow from a region of high pressure to one of low pressure. Hence, if a gas flows through a constriction, it generates a high pressure proximal to the constriction, which results in depression of the water level of in the proximal limb of a water manometer. The difference in the heights of water in the two limbs is a measure of the flow of gas through the orifice.

14. BOURDON’S PRESSURE GAUGE
Bourdon’s pressure gauge: The Bourdon’s pressure gauge is used to measure the cylinder or the pipeline pressure of gases. In this instrument, pressurized gas flows into a hollow malleable metal tube which straightens out to a variable extent, depending on the pressure of the gas in it. This in turn is attached via a gear-train mechanism to an indicator needle which rotates by an angle that depends on the pressure of gas proximally. This mechanism is enclosed in a glass case, which is calibrated for a particular gas.

15. FOREGGER FLOWMETER Only history
Foregger’s flowmeter: This is now of historical interest only. It was a differential pressure fixed area flowmeter. Gases were passed through an orifice, the pressure across which was measured by a water manometer, whose two limbs were of different diameters. The narrower limb was visible from the front of the meter, and the degree of depression of the water level indicated the rate of flow of gas.

16. FLEISCH PNEUMOTACHOGRAPH
. A bundle of parallel small- bore tubes is placed in the gas pathway, which provides a low resistance, and is called the Fleisch screen.
It is electrically heated to prevent condensation of water- vapour.
The pressure difference across the resistor is a measure of the flow through it.
This principle is employed in many of the modern ventilators for measuring respiratory flows.
Fleisch pneumotachograph: This instrument has a low resistance to flow and a fast response rate necessary for measurement of respiratory flows. A bundle of parallel small- bore tubes is placed in the gas pathway, which provides a low resistance, and is called the Fleisch screen. It is electrically heated to prevent condensation of water- vapour. The pressure difference across the resistor is a measure of the flow through it. This principle is employed in many of the modern ventilators for measuring respiratory flows.

17. D- lite flow sensor ( Pitot’s tube )
D- lite flow sensor : This uses a two- sided Pitot tube to make a differential pressure measurement between the total pressure (in the direction of flow) and the static pressure (in the opposite direction) in order to calculate the dynamic pressure, which is proportional to the square of the gas flow. The sensor is placed between the breathing circuit and the patient, preferably as close to the patient as possible, with a filter or heat and moisture exchanger on either side. Since the pressure tubings face on both sides, measurements can be made during inspiration as well as expiration. This monitors concentrations of carbon- dioxide, oxygen and anaesthetic agents, flow rates, airway pressures, and also calculates and displays tidal and minute volumes, compliance and resistance.
Static pressure – opposite
Dynamic pressure – same side – difference is measured

18. VARIABLE ORIFICE, CONSTANT PRESSURE DIFFERENCE
PRINCIPLES & TYPES
VARIABLE ORIFICE, CONSTANT PRESSURE DIFFERENCE
Thorpe tube
VARIABLE ORIFICE, CONSTANT PRESSURE DIFFERENCE
Thorpe’s tube is a tube that is tapered vertically with its smallest diameter at the bottom. It has an indicator that moves freely up and down inside the tube. When the flow control valve is open, gas enters enters the bottom of the tube and flows up, elevating the indicator. The gas then passes between the indicator and the inside wall of the tube, and finally, into the outlet at the top of the tube.
The indicator floats freely in the tube at a position where the downward force on it caused by gravity is exactly balanced by the upward force due to gas molecules hitting its bottom. In other words, the force due to the pressure difference across the annular orifice exactly balances the force of gravity onto the float.

19. THORPE TUBE

20. THORPE TUBE HIGH FLOWS LOW FLOWS Borosilicate glass – pyrex
Thus, the height of the float indicates the rate of gas flow through the tube.
Borosilicate glass – pyrex

23. PRINCIPLES & TYPES OTHERS Volumetric flowmeters
(Positive displacement flowmeters)
Diaphragm / bellows flowmeter
Drager volumeter
OTHERS
VOLUMETRIC FLOWMETERS. i.e. Positive Displacement Flowmeters.
DIiaphragm / Bellows flowmeter: This is used in physiological research for measuring minute voluume, and also in the domestic gas meter, which is a simple, reliable and inexpensive device.
Drager volumeter:The Drager volumeter is a large instrument, permanently mounted on the anaersthesia machine and incorporated into the circle system, and also sold as a separate component.

24. DRAGER VOLUMETER Cogwheel , large internal space ; cumbersome
The gas flow in the instrument is from top to bottom. It displays the minute volume and the tidal volume. Inside, it contains two hourglass- shaped rotors, which mesh as they spin. Gas flow actuates them, and a cog- wheel mechanism transmits this rotation to the pointer on the gauge. Advantage is easy reading, but it also has certain disadvantages like large and cumbersome size, large internal volume, and erroneously high readings at low flows and vice versa.
Cogwheel , large internal space ; cumbersome

25. Variable Orifice Sensor
Aisys
Variable Orifice Sensor: It is used with the Ohmeda 7900 ventilator and the circle system. A Milar flap is placed across the direction of gas flow, which causes the flap to bend and creates an orifice with a pressure drop across it. Tubes on either side of the orifice are connected to a differential pressure transducer inside the anaesthesia machine. Before use, the tubes should be checked to see that they are clear, pointed up, and free of kinks, and filters should be used. Calibration should be done weekly. It can measure flows from 1 to 120 l/min. The main advantage of this device is that it allows the ventilator to automatically make adjustments for changes in fresh gas flow. Disadvantages are the need of two sensors and filters.
Milar flap

26. WRIGHT’S RESPIROMETER (INFERENTIAL)
Wright’s respirometer (Inferential): It is supplied with adaptors to facilitate connection to a mask, endotracheal tube or breathing system. It has a sliding stud for on-off control and a spring- loaded reset button to set the hands of the scales to zero. Its dead space is 15 ml. An electronic version and a paediatric size are also available. Gas entering through the outer casing is directed through a series of tangential slots enclosed in a cylindrical housing and strikes a vane, causing it to rotate. This vane is connected to a mechanical gear system to the hands on a dial, so that a reading corresponding to the gas passing through the device is registered. It gives false high readings with high flows, pulsatile flows and when a mixture of nitrous oxide and oxygen is used. Advantages are light weight, small size and low dead space. Disadvantages are absence of alarms, difficulty in reading, need of a watch to determine minute volume, high expense in maintenance of optimum mechanical condition and measurement of flows only in one direction. The Wright’s respirometer is most accurate at moderate gas flows; its accuracy is less at extremes of gas flow.

27. WRIGHT’S RESPIROMETER
Modern versions of the Wright’s respirometer use a Hall effect detector or a light source and photodetector. The Hall effect detector is a semiconductor device that responds to very small changes in a magnetic field. These pulses are then converted electronically to indications of tidal and minute volume. Alarms for low volumes and respiratory rates may also be incorporated in these newer versions.

28. NEWER SPIROMETERS
The spiromed is an electronic respirometer employing a displacement rotating lobe impeller that generates electronic pulses in response to flow. It is used with North American Drager machines.

29. FLUIDIC FLOWMETERS
These rely on some dynamic instability in the gas which generates an oscillation whose frequency is proportional to the flow rate.
employed in some Ohmeda machines, in which a swirl is created in a series of fixed vanes due to rotation of a single vane induced by gas flow, and this is sensed optically.
Fluidic flowmeters: These rely on some dynamic instability in the gas which generates an oscillation whose frequency is proportional to the flow rate. This principle is employed in some Ohmeda machines, in which a swirl is created in a series of fixed vanes due to rotation of a single vane induced by gas flow, and this is sensed optically.

30. FLUIDIC FLOWMETERS Nitrous oxide VORTEX SHEDDING FLOWMETER
The vortex shedding flowmeter has a low resistance and minimal moving parts. A bluff body is placed in the path of a laminar flow of gas through a smooth- bore tube, creating a small obstruction and generating vortices of gas. The frequency of shedding is linearly proportional to the flow rate. The vortices are detected by their interruption of a narrow ultrasonic beam.
Nitrous oxide

31. ULTRASONIC FLOWMETERS
Principles:
Transit time.
Doppler principle.
Ultrasonic flowmeters: These can be of two types theoritically, employing either Doppler or transit time. However, doppler is not used for respiratory gas flow measurement as it requires particulate impurities for its measurements. The “transit time” or “time of flight” is used by introducing short pulses of gases into a gaseous medium and then detected at a fixed distance from the source. The time taken for this is proportional to the gas flow. This flowmeter has very fast response rates and low resistance to flow, and is easily sterilized. However, this is very expensive.
blood
flow

32. Ohmeda 5400 Volume monitors
Optical sensor:The monitor consists of a sensor that fits into the breathing system, a display unit, and a coiled electrical cord connecting the two. The sensor portion has two parts- a cartridge placed in the breathing system, and a clip- on optical coupler, which fits over the cartridge. The clip has arrows to indicate the direction of flow, and a small heater to help prevent condensation. The cartridge has an internal volume of 6 – 10 ml. As gas passes through the cartridge, it strikes the vanes of a rotor, causing it to spin. The coupler contains two light beam sources and an optical sensor. As the rotor spins, it interrupts the light beams shining through the cartridge and the sensor generates a pulse every time one of the light beams is blocked. The number of pulses is proportional to the volume of gas flowing through the sensor. A computer calculates tidal volume, minute volume and respiratory rate. These are shown on a liquid crystal display unit. Condensates or contaminants can block these light beams and cause errors in measurement. Strong ambient light on the transducer also led to errors.

33. ANEMOMETERS
Anemometer: In a heated wire anemometer, the gas flows around a thin wire heated to a constant temperature in a measuring tube. Heat is dissipated when gas flows past this wire. The greater the volume flowing past the wire per unit time, the greater the heat dissipated, so the current required to keep the wire at a constant temperature is an indicator is a measure of the gas flow.
The effect of various types of gases is compensated for by using a second heated wire. The heat dissipated by the second wire is determined when there is no gas flow.
Gas flow around a thin wire – heat dissipation related to flow

34. Electromagnetic flow meter
Faraday’s law
Flow of gases
Magnetic field
Potential difference

35. Electronic gas selector switch
Electronic gas selector switch. Either nitrous oxide or air can be selected by pushing the appropriate button (lower left). Total gas flow and oxygen percentage are set by pushing the hard keys and rotating

36. FACTORS AFFECTING RATE OF FLOW
PRESSURE DROP ACROSS THE CONSTRICTION
DIAMETER OF THE ANNULAR ORIFICE
PHYSICAL CHARACTERISTICS OF THE GAS
TEMPERATURE AND PRESSURE EFFECTS
FACTORS AFFECTING RATE OF FLOW:
1.PRESSURE DROP ACROSS THE CONSTRICTION: When gas flows between the indicator and the wall of the flowtube, there is a frictional resistance resulting in a turbulent flow. This results in loss of energy that is reflected as a pressure drop across the orifice. To keep the indicator floating in equilibrium, this fall in pressure exactly balances the gravitational force on the indicator, and equals the weight of the indicator divided by its cross- sectional area. Hence, these are named as “constant pressure difference” flow indicators.
2.DIAMETER OF THE ANNULAR ORIFICE: As the pressure drop is constant, the flow rate is directly proportional to the annular opening around the flow indicator. Greater the annular opening, greater is the flow of gases around the float. Increasing the flow of gas does not increase the pressure drop; instead causing the float to rise to a higher position in the tube, providing greater flow area for the gas. Thus, elevation of the float is a measure of the annular area available for gas flow, and therefore, of the gas flow itself.
3.PHYSICAL CHARACTERISTICS OF THE GAS: The physical property relating to gas flow to the pressure difference across a constriction varies with the form of constriction.
A) Tube: Hagen-Poiseuille’s equation: When the constriction is long and narrow as in low flows, the flow is laminar and is a function of the viscosity of the gas.
V/t = (Pr4)/(8l)
B) Orifice: Graham’s law: When the constriction is short and wide as in high flows, the flow becomes turbulent and is a function of the density of the gas.
V/t  (P  Area of orifice) / d
4.TEMPERATURE AND PRESSURE EFFECTS: Flowmeters are calibrated at atmospheric pressure (760 torr) and room temperature (20°C). Any change in temperature or pressure will affect the density and viscosity of the gas, and hence, the accuracy of the indicated flow rate.
Changes in temperature cause only insignificant changes in the flow rates, unlike pressure changes, which can result in gross inaccuracies in flow readings.
Eg. At high altitude, the barometric pressures are low. At low pressures, there will be little change in flow rates as viscosity is independent of pressure changes. However, at high flows, the density decreases significantly, and thus, the actual flow will be more than the flowmeter readings. At high barometric pressures, as in hyperbaric chambers, the actual flow will be less than that indicated on the flowmeter, especially at high flows.

37. In a hyperbaric chamber, a flowmeter will deliver less gas than indicated.
With decreased barometric pressure (increased altitude), the actual flow rate will be greater than that indicated

38. Flowmeter tubes are most accurate in the middle half of the tube.
Accuracy decreases at the bottom and top.
This is why separate tubes are often used for low flows

39. COMPONENTS FLOW CONTROL VALVE COMPONENTS
FLOW CONTROL VALVE: It comprises of a body made of brass that screws into the base of the flowmeter.A stem can move inside the body, and has fine threads, which allows it to move only a short distance when a full turn is made. A needle or pin valve is attached to the distal end of the stem; it is made of stainless steel and is conical in shape. This sits into a valve seat made of plastic or metal.

40. SCHEMATIC – VALVE CLOSED
VALVE SEAT
CONTROL KNOB
INLET
By turning the screw, one can increase or decrease the opening of the valve, and hence, control the flow of gas.
VALVE PIN

41. SCHEMATIC – VALVE OPEN VALVE SEAT CONTROL KNOB INLET VALVE PIN
When the valve is open, there is an annular opening between the pin and the seat, allowing gas to flow through the valve into the flowmeter.
There are stops for the “off” position (to prevent damage to the valve seat) and for the “maximum flow” position (to prevent the stem from getting disengaged from the body).
INLET
VALVE PIN

42. O2 CONTROL KNOB
CONTROL KNOB: This is joined to the stem, and should be large enough to be turned easily. According to the standards for anaesthesia machines, the knobs are touch and colour coded and labelled, and the knob for oxygen is larger and with a chatacteristic fluted profile. Turning the knob counterclockwise increases the gas flow, and vice versa. Excessive tightening should be avoided.
Close proximity of the flow control knobs can lead to errors. So, they should be as far apart as possible in order to prevent inadvertent changes in the flow settings.
Some machines have a bar or shield or protective barrier to prevent accidental changes from the preset position, and placing them high above the working surface lessens their contact with the objects on that surface.
Before using a machine, it should be checked that the flow control valves are closed. When the machine is not in use, the gas source should be closed or disconnected. The flow control valve should be opened till the gas escapes completely, and then closed fully.

43. FLOWMETER SUBASSEMBLY
TUBE
SINGLE- TAPERED
DUAL / DOUBLE
TAPERED
FLOWMETER SUBASSEMBLY:
TUBE:It is usually made of glass. It may be single-tapered, as in flowmeters using different tubes for high and low flows, in which the diameter increases gradually and uniformly from bottom to top, or dual/ double tapered. In dual tapering, two different tapers are present on the inside of the tube- one for low flows and another for high flows. The opening increases more rapidly above a flow of 1 l/min.

44. SINGLE -TAPERED DOUBLE -TAPERED
The taper of the tube is constructed so that it varies in order to elongate the lower part of the scale. This has the advantage that even with a short tube, accurate measurement of low flows is possible.

45. FLOWMETER SUBASSEMBLY
INDICATOR
ROTATING
NONROTATING
INDICATOR ( float / bobbin ) : It is a freely moving device within the tube. Different kinds of tubes are available- rotating and nonrotating.

46. Types of bobbins Plumb Bob Skirted Non rotating Ball - float H - Type
Rotating floats are made up of aluminium, have an upper rim and body. The rim is larger than the body, and has special slanted grooves or flutes cut into it- gas passing between the rim and the wall of the tube impinges onto the grooves and causes the float to rotate. The reading is taken from the rim, with the indicator in vertical position. The float is maintained in the centre of the tube due to its constant spinning. They are of two types- plumb-bob and skirted.
Nonrotating floats have a similar shape, but lack the flutes, and hence, do not rotate. Reading is taken from the upper rim. When the tube is in vertical position, the gas flow keeps the float in the centre of the tube. A special type of nonrotating float is the I/ spool/ H float.
H - Type

47. Where to read ?

48. FLOWMETER SUBASSEMBLY
STOP
SCALE
PLASTIC SHIELD
LIGHTS
ON – OFF SWITCH
STOP: This is present at the top of the flow indicator tube. It prevents the indicator from plugging the outlet, thus ensuring a continuous gas flow, and preventing increased pressure in the tube. It also prevents the float from rising to a point where it cannot be seen, as a hidden indicator looks similar to one that is turned off. Stops may break and fall onto the float, in which case, the float registers a lower flow rate.
SCALE: According to ASTM standards, the flow indicator scale should be marked either on the tube or located on the right side of the tube when viewed from front, is colour-coded for the gas it serves, and is calibrated in l/min (preferably in 100 ml/min till a flow of 1 l/min).
Flowmeter scales are individually calibrated with their floats at a specific temperature and pressure. They are not calibrated from zero, but only from the lowest accurate point, and this is the first mark on the scale. Hence, taking readings by extrapolation below this mark is unwise.
PLASTIC SHIELD: It protects the flowmeter assembly from dirt and dust from outside.
LIGHTS: These are optional; useful when the machine is used in a darkened room.
ON-OFF SWITCH: It is present in some machines for safety, convenience and economy. It must be turned on to obtain any flow through the flowmeters or the flush valves, and it also activates other machine components, such as alarms.

49. ARRANGEMENT OF FLOWMETER TUBES
PARALLEL
SERIES /
TANDEM
ARRANGEMENT OF FLOWMETER TUBES: The flow indicator tubes for different gases are arranged side-by-side. The various gas flows meet at the common mixing chamber at the top.
When there are two indicator tubes for the same gas- one for low flows and one for high flows, they can be arranged in parallel or in series.
PARALLEL: This has two complete complete flow indicator assemblies with separate flow control valves. I.e. two flow control valves for one single gas. The total gas flow is equal to the sum of flows on both the flow indicators.
SERIES / TANDEM: There is a single flow control valve for the two flow indicator tubes. Gas first passes through the tube for low flows and then through the tube for high flows. These tubes are accurate at both low and high flow rates. The total flow is read from the higher flow tube.

50. Series Skirted float indicators.
Note the stops at the top of the fl ow indicator tubes.
The flow indicator tubes are in series.
The total flow is that shown on the higher flow tube, not the sum of the two tubes.

51. Auxiliary flow meter
Upto ten litres
Need not be switched on !!

52. Danger
The common gas outlet should not be used to administer supplemental oxygen to a patient.
This will delay conversion to the breathing system if an emergency arises.
Another potential problem is that a vaporizer on the back bar may be accidentally left ON, leading to undesired administration of the agent.

53. SEQUENCE OF FLOWTUBES A,B- Oxygen upstream C,D- Oxygen downstream
E – Alternative arrangement
SEQUENCE OF GASES IN THE FLOWMETER BLOCK: ISO 5358 states-“…..the oxygen shall be delivered downstream of all other gases…..”. No international agreement has been reached regarding the sequence of flowmeter tubes.
In A & B, oxygen is located upstream. If a leak occurs, it becomes a potentially dangerously dangerous arrangement as a hypoxic mixture can be delivered. In C & D, oxygen is located downstream. Hence, anaesthetic gases rather than oxygen are lost preferentially in the event of a leak, and there is less danger of delivering a hypoxic mixture of gases. E is an alternate arrangement having the same outcome.
In the event of any leak upstream, placing oxygen flow indicator nearest to the outlet results in a loss of nitrous oxide and not oxygen. But even this arrangement does not guarantee that hypoxia will not occur. A leak in the oxygen flow indicator tube between the float and the manifold can still cause a selective loss of oxygen, though it is placed downstream. E O2
AIR
N20

54. PIN- INDEXING OF FLOWTUBES

55. SAFETY DEVICES CONTROL KNOBS ARRANGEMENT OF FLOWMETERS MISCELLANEOUS
( dots / two colours; fluorescent screen / lights; plastic screen; protective bar; pin- indexing; downstream flow resistor)
CO2 FLOWMETER
ALARMS
SAFETY DEVICES:
²    CONTROL KNOBS: -Colour coded
-Touch coded
-Labelled
²    ARRANGEMENT OF FLOWMETERS:-Oxygen downstream
²    MISCELLANEOUS:
X       Dots/ two colours on the indicator- to see the rotation and confirm that it is not stuck.
X       Fluorescent screen at the back for easy reading.
X       Lighting system- for easy visibility in a dark room.
X       Plastic screen- to prevent damage to the flowmeter tubes.
X       Protective bar in front- to prevent accidental changes in flows.
X    Flowmeter pin-indexing system- to prevent accidental interchange during installation/ maintenance.
X      Flow resistor- Pressure rises at the common gas outlet are transmitted back to the gas above the bobbin, thus causing inaccurate readings. Minute volume divider ventilators exert back- pressure as they cycle. A flow resistor is fitted downstream of the flowmeters to prevent this from happening.
²    ALARMS:
Some machines have alarms to alert the operator if oxygen ratio falls below a preset value, and these get activated only if the master switch is turned on. Pressures downstream of the flow control valves are transmitted to diaphragms, which are linked together. If the oxygen percentage is low, the diaphragms move to the right, causing a leaf- spring mechanism to close and activate an alarm.
²    CARBON DIOXIDE FLOWMETER:
If facilities are available for the use of carbon- dioxide on the machine, the flowmeter is designed to allow a maximum flow of 500 ml/min to be added to the gas flow, to ensure that dangerous levels of hypercarbia are avoided.

56. at low (laminar) flows, the flow rates of gases with similar viscosities would be
read identically (eg, oxygen and helium have viscosities of 202 and 194 micropoise, respectively)
at high flows gases of similar densities (eg, N2O and carbon dioxide, both of which have a molecular weight of 44 atomic mass units) would be read identically.

57. PROBLEMS INACCURACY PROBLEMS WITH FLOATS OBSTRUCTION LEAKS
( Tubes are leak free because of neoprene washers at both ends )
PROBLEMS WITH FLOWMETERS:
INACCURACY:
Flowmeters have the greatest accuracy at midscale readings (2%). Errors as high as 70% may occur when the float is in the lower portion of the flowtube. Restrictors are placed proximal to the flowtube to filter the gas and reduce its pressure to 15 psig for maintaining the accuracy of gas delivery.
Causes for inaccuracy :
-Improper assembly/ calibration.
-Wrong flowmeter.
-Tube not vertical- If the flowtube is tilted, the shape of the orifice gets distorted or asymmetrical; so the readings are inaccurate. If the tilt is increased so that the float touches the sides of the tube, the resulting friction can cause even more inaccurate reading.
-Static electricity-More in dry weather; the float may stick to the walls of the tube. This can be prevented by coating the inside of the tube with a thin, transparent gold film, or by spraying the outside of the tube with an antistatic agent like tin oxide or croxtene.
-Dirt on tube/ float can cause sticking; and also inaccurate readings due to change in the effective orifice diameter.
-Back pressure-Any increase in downstream pressure, either from the breathing system or the oxygen flush valve, causes the float to drop to a lower position than the actual flow. This effect can be lowered by the use of restrictors proximal to the common gas outlet.
FLOAT PROBLEMS: Damage by handling, or sudden upward push to the top by a rapid increase in flow rate may cause inaccuracies. The float may also get dislodged or break and rest on top of the float. An indicator at the top can get unnoticed and may cause problems.
OBSTRUCTION: The tube outlet may get blocked if the stop gets broken. The float may close the outlet so that no flow occurs even though the flowmeter registers a high reading; and this may lead to leak or even rupture of the flowtube.
LEAKS: A leak can be caused by a crack/ chip in the tube, or a problem with the connections of the tube. Obstruction to the gas flow downstream may cause the seal at the top of the flowtube to rupture. A leak may occur if the flowtube is left open and there is no cylinder or yoke plug in the yoke. The indicator at the bottom of the tube will not prevent backflow of gases. Leaks upstream of the common manifold but downstream of the gas flow indicator will result in a lower than expected concentration of that particular gas, leading to either a hypoxic mixture or a low anaesthetic gas delivery to the patient.

58. Dangers Sudden rise may cause to stuck it works like an assembly
Clean – service engineer
But one at a time
Don’t screw the control knob very tight
Feel the knob -- See the knob
Static electricity – stuck and croxtine spray

59. MAINTENANCE & CARE THANK YOU MAINTENANCE & CARE:
Flow indicators should be protected by turning the valves off when a cylinder valve is opened or the pipeline hose is connected to the machine. This prevents a sudden rise of the indicator to the top of the flowtube, which might damage the indicator or allow it to go unnoticed.
The flow indicator tube, scale, float are all calibrated as a unit. If any parts are broken or damaged, the entire assembly must be replaced by a new set.
The tube and float should be regularly dismantled and cleaned by a service engineer. Dirt may sometimes be removed by passing a high flow of gas through the tube. Only one flowmeter tube should be cleaned at a time, to prevent possible mixing of the indicator or tubes, which would result in inaccuracies.
The control knob should not be screwed in too tightly.