flow and pressure measurement

Question 1

how does a rotameter work?

Answer 1

variable orifice, fixed pressure device

has a needle valve controlling flow of gas through a taped transparent tube with a bobbin inside and a calibrated scale.

As the needle valve is opened, the flow will push the bobbin up the tube until the forces pushing upwards and down equilbriate at that particular flow. the user can then read off the calibrated scale the flow rate that corresponds to this position.

if the needle valve is opened further the increase in flow has more force to push the bobbin higher, until a new equilbrium is met.

Question 2

how accurate are rotameters?

Answer 2

+/- 2.5%

Question 3

How are rotameters calibrated?

Answer 3

calibration is specific to each gas this is because the visocity and density of each gas will vary.

At the bottom of the tube, the gap between bobbin and the tube is narrow - this promotes laminar flow. therefore flow is inversely proportional to viscosity and the tube will be calibrated in this way.

At the top of the tube, the gap gets wider and turbulent flow exists whereby flow is inversley propotional to root density so it is calibrated in this way.

the scale on the tube is therefore non-linear and gas specific

Question 4

what is the calibration error of a rotameter?

Answer 4

+/- 2.5%

Question 5

what are the different methods of measuring pressure?

Answer 5

may measure absolute or gauge pressure
absolute - barometer
gauge - manometer, aneroid gauge, electrical gauge

Question 6

how does a manometer work?

Answer 6

a manometer measures gauge pressure - i.e. relative to atmospheric pressure

consists of an open ended fluid filled column that is U shaped.
one end open to atmosphere and the other to a test pressure e.g. BP
without the test pressure - both sides open to atm and the fluid will be same height.
as pressure increases, the column is pushed around.
the height difference between both sides is used to measure the gauge pressure.

pressure = density x height x gravity

may be a water column or mercury column

Question 7

what is the difference between using a mercury vs water manometer?

Answer 7

mercury is more dense than water so requires more force to push the column. hence better for larger pressures as its more practical.

water is more sensitive to small changes but less practical

Question 8

pros and cons of a manometer?

Answer 8

pros - easy, no electricity , simple, no calibration needed

cons - mercury poisoning, glass can break, impractical because big and hence limited to smaller pressures.

Question 9

what is a barometer?

Answer 9

a method for measuring absolute pressure
fluid filled U shaped column
one side open to the test pressure
the other side closed of and contains a torricellian vaccuum - SVP of mercury.

height difference = absolute pressure

Question 10

which barometers measure sub atmospheric pressure?

Answer 10

fortins and Goethe device
the height will fall rather than rise

Question 11

what are the pros and cons of barometers?

Answer 11

simple
can measure absolute pressure
can measure subatmospheric pressure

however mercury poisoning, glass breaking, bulky , impractical

Question 12

how does an aneroid gauge work?

Answer 12

e.g. bourdon gauge

coiled metal tube
fixed volume
exerting pressure causes it to unwind
linked to levers and a pointer
calibrated scale

measures gauge pressure

Question 13

pros and cons of aneroid gauge?

Answer 13

pros - simple, compact, no electrics, can measure much higher pressures as not requiring column of liquid. used in pipelines and cylinders. can measure low pressures and amplify by altering compliance of lever.
accurate and sensitive

however sensitive to temp change , vibrations
demonstrates hysteresis

Question 14

safety features of a bourdon gauge?

Answer 14

colour coded in anaesthesia
if it breaks the gas will vent to protect other parts of the machine
face of the gauge is made of heavy glass.

Question 15

when are low pressure aneroid gauges used?

Answer 15

measuring airway pressures quickly

however these are more sensitive to temp/ vibrations.

Question 16

what are electrical gauges?

Answer 16

Electrical pressure gauges measure pressure by converting the physical force of the pressure into an electrical signal

e.g strain gauges or piezo electric sensor

strain gauge - stretches, resistance changes
piezoelectric - mechanical stress causes crystals to vibrate and produce current

Question 17

how can flow be measured?

Answer 17

flow is the volume of fluid passing a point per unit time.
measurement methods can be categorised as

variable pressure, fixed orifice devices = pneumotacograph
variable orifice, fixed pressure devices = rotameter, wrigths peak flow meter.

wrights respirometer - measures volume which can then measure flow

Question 18

why is flow measured?

Answer 18

monitoring patient - gas flows inspiration and expiration
monitoring performance of anaesthetic equiptment

Question 19

how does a pneumotochograph work?

Answer 19

constant orifice variable pressure device
relies on hagen poiseulle law and thus laminar flow.

flow proportional to pressure difference and inversely proportional to resistance. since resistance is constant, flow can be measured via pressure difference.

pressure measured by 2 pressure transducers either side.
used to measure respiratory flows on anaesthetic machine.

Question 20

what are the different types of pneumotacographs?

Answer 20

screen - mesh made of gauze
Fleisch - fine hollow tubes
hot wire - measures flow by chnage in resistance as the gas cool the wire the faster they flow.
pilot tube

Question 21

what are the problems with a fleish pneumotachograph?

Answer 21

can become clogged with sputum, water
larger dead space
larger and more costly as difficylt to clean
good for short term monitoring

Question 22

what are the causes of inaccuracies of pneumtachographs?

Answer 22

viscosity and temperature - affects flow
the viscosity can be altered by different anaesthetic gases and different proportions
usually contains heating element to maintain a constant temp

Question 23

how do pilot tubes work? advantages

Answer 23

hgiher the flow, the bigger the pressure difference
no resistance mechanism

cheap and small
not much addition of dead space

Question 24

overall pros and cons of pneumotacographs..

Answer 24

pros - no moving parts so reliable and accurate, rapid. relatively low resistance to gas flow.

cons - risk of water condensation and blockage, can heat to reduce this. flow must be laminar for accuracy - temp and velocity can effect this.

Question 25

how do flow meters work?

Answer 25

constant pressure, variable orifice device. needle valve connical transparent tube bobbin gas in - upward force on bobbin, gravity bobbin down tappered tube allows gas to escape so forces are better balanced calibrated scale - specific to that gas.

Question 26

how are rotameters calibrated?

Answer 26

flow is proportional to viscosity at the bottom and density at the top bottom = tube = laminar =hagen poielle = viscosity top = orifice = turbulent = density = grahams law hence calibrated scale specific for each gas also depends on tube size and weight of bobbin calibration occurs at room temp and atmospheric pressure.

Question 27

why do rotameter use tapered tubes?

Answer 27

improves accuracy and sensitivity without this the force of gas would push bobbin very high by allowing some gas to escape it balances the forces better so a more convienient scale can be used and more sensitive and accurate measurements

Question 28

where are rotameters found?

Answer 28

wall O2 supply many anaesthetic machines for O2, air, N20

Question 29

what is the difference between a tube and an orifice?

Answer 29

tube - length > diameter - laminar orifice other way round - turbulent

Question 30

pros and cons of rotameters?

Answer 30

Advantages: - Cheap and simple - No electricity needed - Quick and easy – the dial and flow measurement are the same - Accurate for flows used clinically Disadvantages - Unique to each gas at set temperature and pressure – can’t be interchangeable - Bobbin can get stuck leading to inaccurate measurements. - Temperature changes and viscosity alters accuracy - Not as accurate at low flows - Pressure changes – if the pressure increases above the bobbin it will lead to inaccurate reading

Question 31

what can cause the bobbin to get stuck and how is this reduced?

Answer 31

Due to static forces – this is reduced by using antistatic materials on the lining of cylinder and bobbin. Also use of anti-static sprays to remove any charges. Due to contamination with dirt - more at low flow rate when gap between bobbin is small. Filter can be used before gas enters flowmeter.

Question 32

what are the different bobbin shapes

Answer 32

flat top - read above spherical - read in centre with grooves - to cause rotation to show not stuck

Question 33

tell me about a flow control valve..

Answer 33

manual adjustment of knob controls opening of a valve at base of rotameter increase flow by rotating anticlockwise also reduces from 4 bar to 1bar pressure

Question 34

what are the safety features of rotameters?

Answer 34

* Bobbin has vanes/grooves to make it spins - shows user it is not stuck * Anti-static coating to prevent bobbin sticking * Downstream flow restrictor prevent back pressure from breathing system pushing bobbin back down * Hypoxic guard – mechanical link between N20 and O2 * Oxygen rotameter is last in the row to be added, this prevents hypoxic mixtures if there was an upstream leak. * Colour coded * O2 control know - designed to have a different shape in order to allow identification and recognition by touch.

Question 35

how does a wrights respirometer work?

Answer 35

Measures volumes of gas which can be converted to flow measurement by dividing by unit time. The device consists of a vane (blade on a rotating axis) surrounded by housing containing angled slits. -as enters one end and causes the angled slits to rotate which causes the vane at the centre of the device to rotate and spin. moves the pointer over the calibrated dial so that the volume can be read.

Question 36

when is wrights respirometer used?

Answer 36

measures volumes e.g. tidal volume

Question 37

pros and cons of wrights respirometer?

Answer 37

- Advantages: o Simple, small and convenient o Cheap o Mechanical so no electricity needed. - Disadvantages: o Inaccurate for continuous flow o Over reads at high flow and under reads at low flows – due to inertia of the vane itself. o Can become contaminated by condensation / dirt – causes resistance to movement of pointer and parts to erode – inaccuracies o Only measures flow in one direction.

Question 38

how has wrights respirometer design improved?

Answer 38

- Use electronics to detect the vane rotation - Reducing inaccuracies due to inertia and water condensation. - Also can now measure bidirectional flow - This movement is measured by infra-red beams positioned across the vane.

Question 39

how does a wrights peak flow meter work?

Answer 39

- A simple low-cost device which records the maximal expired flow. - It is a variable orifice, fixed pressure device - Patient is asked to take a deep breath in and then then exhale as fast and hard as possible into the flow meter. - The air flow pushes against a diaphragm which moves in direction of air flow down a tube. * The movement of the diaphragm is opposed by a spring – this is to balance the forces more otherwise the diaphragm would go all the way to the end even at low flows – improves accuracy. * As the diaphragm is pushed, it exposes an orifice for air to escape along the sides – the further the diaphragm travels, the more air can escape as the orifice increases in size. * A sliding marker indicating the peak flow rate is pushed by the diaphragm, which continues to move along the tube until the spring force overcomes the air pressure. * Mini wright peak flow meter is more common now – the classic peak flow device.

Question 40

what is a vitalograph?

Answer 40

- A set of bellows connected to a pen draws a trace on a piece of paper as the patient breathes in and out through the bellows. Can be used to measure and represent volumes. - How spirometry works.

Question 41

how can gas volume be measured

Answer 41

wrights respirometer benedict roth spirometer vitalograph

Question 42

what is a benedict roth spirometer?

Answer 42

upside down bell attached to mouth pieces , rises and falls with ventilation can be used to calculate BMR