MEASUREMENT OF FLOW

TYPES OF FLOW

Two types of flow measurement

1. Gas Flow

2. Liquid Flow

FEW BASIC CONCEPTS

GAS FLOW IN BODY

Mechanical ventilation and

breathing monitoring are very

important practice to mechanically

assist patients or to perform

essential diagnoses. During both

artificial ventilation and function

respiratory monitoring the use of

flowmeters is required to perform

an accurate and continuous

monitoring of gas exchange.

GASES

• Gas volumes (and associated flows) can be measured directly using a

system with a calibrated chamber, but in clinical practice,

measurement is usually made indirectly, using a property of the gas

that changes in parallel to flow or volume and which can be more

easily determined.

GASES: SCIENTIFIC PRINCIPLES

Flow by definition is the quantity of the gas or fluid which passes a point in

unit time.

Relationship between flow and volume: for gas delivery systems (e.g.

oxygen flowmeters), flow is generally constant and the relationship

between flow and volume is expressed by

F = Q / t

F is equal to the mean flow, Q = the quantity (mass or volume) and T =

time.

TYPES

Flow can be laminar or turbulent. Several factors determine which

type of flow predominates, and these are combined into Reynolds’

number (Re), a dimensionless value that can be calculated for a

smooth parallel-sided tube by:

where v is linear velocity, p is density, l is diameter of tube and μ is

viscosity. At an Re less than 2000 flow will probably be laminar; an

Re over 2000 indicates flow is likely to be turbulent.

TYPES

Reynolds numbers for some real-life examples

• Blood flow in brain ~ 100

• Blood flow in aorta ~ 1000

• Typical pitch in major league baseball ~ 200000

• Blue whale swimming ~ 300000000

TYPES

Laminar flow is efficient, with layers passing

smoothly over each other producing a

parabolic (bullet-shaped) flow profile, with the

greatest velocity centrally. It is determined by

the Hagen-Poiseuille formula:

where: P is pressure drop, r is the radius of the tube and l is the length of the tube.

Important features are that flow is:

• directly proportional to the pressure drop

• proportional to the fourth power of the radius

• related to the viscosity but not the density of the gas.

TYPES

Turbulent flow is less efficient, with multiple eddy currents

occurring in the overall direction of flow. Because of the variable

nature of turbulence, there is no precise and comprehensive

equation to calculate flow, but turbulent flow is related to the:

• square root of the pressure drop

• density of the gas rather than its viscosity.

If flow is being calculated by measuring one of the above variables,

unforeseen changes in any of the others will compromise accuracy.

Changes could occur directly, such as by alteration in gas

composition, or indirectly, such as temperature variation causing a

change in viscosity.

BERNOULLI'S PRINCIPLE

pressure on the wider/slower side P1 has to be larger than the pressure on

the narrow/faster side.

"For incompressible, inviscid flow, Static Pressure + Dynamic Pressure + Hydrostatic

Pressure = CONSTANT along the STREAMLINE".

Assume that there is no change in the height of the fluid,

the pgh terms cancel if we subtract them from both sides.

This formula highlights Bernoulli's principle since if the speed v of a fluid is

larger in a given region of streamline flow, the pressure P must be smaller in

that region (which is Bernoulli's principle). An increase in speed v must be

accompanied by a simultaneous decrease in the pressure P in order for the

sum to always add up to the same constant number.

BERNOULLI'S PRINCIPLE

• This inverse relationship between the pressure and speed at a

point in a fluid is called Bernoulli's principle.

• Bernoulli's principle: At points along a horizontal streamline,

higher pressure regions have lower fluid speed and lower

pressure regions have higher fluid speed.

• It is the basics of most flow meters

PRINCIPALS FOR FLUID FLOW

METERING

PRINCIPALS FOR FLUID FLOW METERING

• Direct Measurement: gas volumes (and associated

flows) can be measured directly using bulk filling of an

enclosed space of known volume. Instruments using

direct measurement include the gas meter, and water-

displacement spirometer. Because of the logistical

problems of such devices, their use in clinical practice

is limited.

PRINCIPALS FOR FLUID FLOW METERING

Indirect measurement: for clinical use, measurement is usually made

indirectly, using a property of the gas that changes in parallel to flow or volume

and which can be more easily determined.

1. Pressure drop across a resistance – as flow occurs through a resistance,

a pressure change occurs. This effect can be used to calculate flow by:

keeping the resistance constant and measuring the pressure change as the

flow varies, as in a pneumotachograph having a constant pressure drop and

varying the resistance in a measurable way (e.g. bobbin rotameter).

2. Mechanical movement – flowing gas has kinetic energy related to its

velocity, which can be converted into a measurable value by: rotation of a

vane (e.g. a spirometer) bending a flexible obstruction, transducing this to

produce an electrical signal.

3. Heat transfer – gas flowing past a heated element acts to cool it, as in a

hot-wire anemometer.

4. Ultrasound interference – the velocity of an ultrasound signal is

increased by a gas flowing alongside it in the same direction, and

decreased if the gas is flowing against it.

PRINCIPALS FOR FLUID FLOW METERING

IDEAL FEATURES OF GAS FLOWMETER

Ideal features of a device used for gas flow or volume

measurement in clinical practice include:

• accurate across a wide range of flows

• unaffected by changes in gas temperature or composition

• low resistance so that it can be used in a spontaneously

breathing patient

• minimal impairment of performance with prolonged use.

MEASUREMENT SITE

For measuring tidal volume, it is important that the characteristics

of the breathing system are taken into consideration when deciding

where to measure.

e.g. Intensive care ventilators are non-rebreathing systems, and

measurements can be made accurately in the inspiratory or

expiratory parts of the system. In practice, both inspiratory and

expiratory tidal volumes are usually measured, because clinically

important discrepancies between them can occur, for example a

leak caused by lung pathology or the use of an uncuffed tube.

FLOWMETER TYPES:

1. Differential Pressure Flowmeters

In a differential pressure drop device the flow is calculated by measuring

the pressure drop over an obstructions inserted in the flow. The differential

pressure flowmeter is based on the Bernoullis Equation, where the

pressure drop and the further measured signal is a function of the

square flow speed.

Bernoulli Equation

FLOWMETER TYPES:

Differential Pressure Flowmeters: Types

Orifice Plate: The orifice plate is the simplest and cheapest. It is simply a plate with a hole of specified size and position cut in it, which can then clamped between flanges in a pipelineThe increase that occurs in the velocity of a fluid as it passes through the hole in the plate results in a pressure drop being developed across the plate.

Their accuracy are poor at low flow rates. A high accuracy depend on an orifice plate in good shape,

with a sharp edge to the upstream side. Wear reduces the accuracy.

THE ORIFICE PLATE

According to Bernoulli’s Principle, if you increase the speed of a flow you decrease its pressure

- a fact that we can exploit to measure flow using DP elements such as orifice plates. An orifice

plate is a precisely machined obstruction. As shown in the picture below, when your media

reaches the orifice, it is forced through a narrow hole in the center of the plate. By reducing

your flow path like this, you see changes in the speed and pressure of your media - on one side

of the plate the pressure is high, while on the other side the pressure is comparatively low. The

faster your media moves, the greater the difference in pressure.

ORIFICE PLATE

where phi is the volumetric flow rate,A1 is the inlet area of the flowmeter,

and A2 is the passage area of flow obstruction.

FLOWMETER TYPES:

Differential Pressure Flowmeters: Types

I. Orifice Plate

II. Venturi Tube

In the Venturi Tube the fluid flowrate is measured by reducing the cross

sectional flow area in the flow path, generating a pressure difference. After the

constricted area, the fluid is passes through a pressure recovery exit section,

where up to 80% of the differential pressure generated at the constricted area,

is recovered..

the Venturi tube flowmeter is often used

in applications where it's necessary with

lower pressure drops than the orifice

plate can provide.

Venturimeter: is a device used for measuring the rate of flow of a fluid flowing through a pipe. It

consists of three parts: • A short converging part • Throat • Diverging part

It consists of a cylindrical inlet section

equal to the pipe diameter; a converging

conical section in which the cross sectional

area decreases causing the velocity to

increase and the pressure head to

decrease;

a cylindrical throat section where the

velocity is constant so the decrease

pressure head can be measured; and a

diverging recovery cone where the velocity

decreased almost all of the original

pressure head is recovered.

1. The fluid enters the straight inlet section and a pressuretransmitter measures the static pressure at the high-pressure tap.

2. The fluid continues to the convergent section where the venturicross section reduces gradually. This causes the velocity of thefluid to increase and the static pressure to decrease.

3. A second tap allows pressure measurement at the center of thethroat, the narrowest section of the venturi. The flow rate of thefluid can be calculated from the static pressure drop between thehigh and the low pressure taps.

4. The divergent section of the venturi allows the fluid velocity todecrease and the pressure to recover most of its initial level.There is only a small permanent pressure loss over the venturi.

Venturi tubes, like all differential pressure flow producers, are based upon Bernoulli'stheory. They operate on the principal of a high pressure in the low velocity, largediameter inlet section.

Advantages and limitations of venturi tubes

Venturi tubes are highly accurate; they recover most of the pressure drop

they produce, and they are less susceptible to erosion than orifice plates

because of their smoother contour. However, venturi tubes are relatively

expensive and they require the use of a differential-pressure transmitter,

which contributes to the total cost of the flow measurement set up. They

tend to be voluminous and they may be difficult to install. Venturi tubes

also require a certain length of straight pipe both upstream and

downstream to ensure a flow that is undisturbed by fittings, valves, or

other equipment. However, the required pipe lengths are shorter than

those required for orifice plates.

FLOWMETER TYPES:

Differential Pressure Flowmeters: Types

I. Orifice Plate

II. Venturi Tube

III. Rotameters

Rotameters are widely used in gas delivery systems for

continuous flow measurement. Several designs have been

used, though now almost all use a vertical tapered tube

containing a bobbin or ball which is supported by the gas

flow as it passes upwards through the tube. The weight of

the ball (and thus the pressure drop required to support it) is

constant, but as the flow increases its position in the tube

rises, lowering the resistance as a larger pathway is created

alongside the ball.

• Rotameter continue….

Rotameters are known as gravity-type flowmeters because they

are based on the opposition between the downward force of

gravity and the upward force of the flowing fluid/Gas. When the

flow is constant, the float stays in one position that can be related

to the volumetric flow rate. That position is indicated on a

graduated scale.

Q = volumetric flow rate, e.g., liters per minute

k = a constant

A = annular area between the float and the tube wall

g = force of gravity

h = pressure drop (head) across the float

FLOWMETER TYPES:

Vane meters

The most common vane meter is the Wright’s

spirometer, in which the gas flow is directed

tangentially to strike a rotating vane in the gas

pathway. Originally this rotation was linked

mechanically to a needle and the volume read from

the adjacent dial, but modern versions use a light

source and photodetector positioned across the

vane to count its rotation. The Wright’s

spirometer tends to under-read at low flows

(because of friction) and to over-read at high

flows (because of momentum).Its clinical use is to assess conditions such as asthma, where the problem is largely confined to

airway resistance, which limits the expiratory flow rate. Although its use is limited in this respect,

it is a very simple and reliable bedside test.

• Vane meter: To convert rotations in voltage output

MECHANICAL FLOW TRANSDUCERS

3. Mechanical flow transducers

Another device using mechanical movement is the

flow transducer used in the Siemens intensive care

ventilators. The gas flow is split so that

measurement is made in a small side channel using

a thin metal disc supported on a flexible pin. The

disc is placed in the measuring channel at right

angles to the direction of gas flow, which results in it

being bent backwards by the flow. A strain gauge

situated immediately behind the pin is compressed

as it is bent, with a force dependent on the flow. The

resulting electrical signal is processed to calculate

the flow rate with a high degree of accuracy.

4. Hot wire anemometer

In this device an electrically

heated wire is placed in the

gas pathway, which is

cooled by the gas flow. The

degree of cooling depends

on the gas flow rate, which

can thus be calculated.

PRINCIPLES OF OPERATION

• Consider a thin wire mounted to supports and exposed to a velocity U. When a current is passed through wire, heat is generated ( I2Rw). In equilibrium, this must be balanced by heat loss (primarily convective) to the surroundings.

• If velocity changes,

convective heat transfer

coefficient will change, wire

temperature will change

and eventually reach a new

equilibrium.

Velocity U

Current I

Sensor (thin wire)

Sensor dimensions:length ~1 mmdiameter ~5 micrometer

Wire supports (St.St. needles)

PRINCIPLE OF OPERATION

HOT-WIRE STATIC TRANSFER FUNCTION

2

3

4

5

5 10 15 20 25 30 35 40

U m/sd

U/d

E/U

vo

lts^

-1

1,6

1,8

2

2,2

2,4

5 10 15 20 25 30 35 40

U m/s

E v

olts

Output voltage as fct. of velocity Voltage derivative as fct. of velocity

PROBE TYPES

1. Hot film , which is used in regions where a hot wire probe would quickly break such as in water flow measurements.

2. Hot wire , This is the type of hot wire that has been used for such measurements as turbulence levels in wind tunnels, flow patterns around models and blade wakes in radial compressors.

HOT WIRE SENSOR

10/48

HOT FILM SENSOR

11/48

MATERIAL USED IN MAKING SENSOR

• High Temperature Coefficient of resistance

• High Specific Resistance

• High Mechanical Strength

• Good Oxidation Resistance

• Low Thermal Conductivity

• Availability in small diameters

Tungsten : good strength, poor oxidation resistance

Platinum: good oxidation resistance, weak

Tungsten with thin platinum coating is generally used.

At high temperatures – Platinum‐iridium alloys, Platinumrhodium

alloys are used.

MODES OF ANEMOMETER OPERATION

1. Constant Current (CCA)

2. Constant Temperature (CTA)

INFUSION PUMPS

Infusion Pumps are an effective pathway for delivering fluid, blood, and

medicants to a patient's vital organs. Since the entire blood supply in a human

body completely circulates within 60 seconds, substances introduced into the

circulatory system are distributed rapidly.

An infusion device typically consists of three major components:

1. the fluid reservoir,

2. a catheter system for transferring the fluids into the body,

3. a device that combines electronics with a mechanism to generate

and regulate flow.

INFUSION PUMPS

Why are they used?

• To provide accurate and controllable flow over a prescribed period or on demand

What are they used for?

Wide range of drugs and therapies including

• Chemotherapy

• Pain management

• Total parental nutrition

• Anaesthesia/sedation

• Etc. etc.

Fluid container

Drip chamber

Roller Clamp

BASIC INFUSION SYSTEM

• Flow by gravity

• Flow controlled by roller clamp

• Difficult to set and control infusion

rate

AN ERROR WAITING TO HAPPEN!

• The infusion of drugs requires high flow rate accuracy and flow uniformity.

Sensors can be used to count the number of drops passing through the

drip chamber to provide flow feedback for automatic rate adjustment and

to detect any downstream occlusions directly below the pumping

mechanism. However, flow rate accuracy remains limited by the rate and

viscosity of the drip as well as improper angulation if in motion. Flow

uniformity can also suffer at low flow rates from the discrete nature of the

drop detector. Despite these limitations, a processor with an advanced

graphical user interface, smart and real-time physiological processing, and

wired and wireless connectivity options for patient monitoring and data

logging applications provides an additional level of safety by quickly

detecting complications and generating an alarm.

• One alternative to the drop sensor is a volumetric metering chamber. A

pump with a stepper or servo-controlled dc motor can be used to provide

the driving force for the fluid by mechanized displacement of the contents

in the volumetric chamber. The stepping resolution along with the

chamber elasticity can influence the flow uniformity. When the volume is

not uniform over the mechanism's cycle, software control can be used to

compensate for the variation.

INFUSION PUMPS

TWO BASIC TYPES

• Syringe Pumps

• Volumetric Pumps

SYRINGE PUMP

SYRINGE PUMPS

• Generally used for low volume, low flow rate infusions

• Good short term accuracy

• Long start up time at low flow rates

• Prime and purge line before connecting to patient

• Alarms: End/near end of infusion; drive disengaged, occlusion, battery low

• Specialised syringe pumps for ambulatory use, PCA, sedation, insulin etc

VOLUMETRIC PUMPS (PERISTALTIC PUMP)

Latch

Cam followers (fingers)

Pressure sensor

Air in line detector

VOLUMETRIC PUMPS

• Preferred for medium and high flow rates and large volumes

• Generally not suitable for rates < 5ml/h

• Variable short term accuracy

• Alarms: Latch/door open, set out, occlusion, battery low, air-in-line

• Specialised volumetric pumps for ambulatory use, epidural infusions etc.