flow and pressure measurement Flashcards
how does a rotameter work?
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.
how accurate are rotameters?
+/- 2.5%
How are rotameters calibrated?
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
what is the calibration error of a rotameter?
+/- 2.5%
what are the different methods of measuring pressure?
may measure absolute or gauge pressure
absolute - barometer
gauge - manometer, aneroid gauge, electrical gauge
how does a manometer work?
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
what is the difference between using a mercury vs water manometer?
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
pros and cons of a manometer?
pros - easy, no electricity , simple, no calibration needed
cons - mercury poisoning, glass can break, impractical because big and hence limited to smaller pressures.
what is a barometer?
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
which barometers measure sub atmospheric pressure?
fortins and Goethe device
the height will fall rather than rise
what are the pros and cons of barometers?
simple
can measure absolute pressure
can measure subatmospheric pressure
however mercury poisoning, glass breaking, bulky , impractical
how does an aneroid gauge work?
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
pros and cons of aneroid gauge?
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
safety features of a bourdon gauge?
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.
when are low pressure aneroid gauges used?
measuring airway pressures quickly
however these are more sensitive to temp/ vibrations.
what are electrical gauges?
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
how does a pneumotochograph work?
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.
how do pilot tubes work? advantages
hgiher the flow, the bigger the pressure difference
no resistance mechanism
cheap and small
not much addition of dead space
how do flow meters work?
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.
how are rotameters calibrated?
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.
how does a wrights respirometer work?
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.
how does a wrights peak flow meter work?
- 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.
what is a benedict roth spirometer?
upside down bell attached to mouth pieces , rises and falls with ventilation
can be used to calculate BMR
