Download Flow Meters - Ain Shams University

Flow Meters
Flow: Volume of a fluid passing certain point
over a given time
Flow Rate.
Flow meters:sets used to measure flow either
gas or liquid
Gas Flow Measurements
• Rotameter:variable orifice constant
pr.device to control the flow of fresh gas
from the anesth.machine to the
pt.breath.circ.
• Consists of :Flow tube_Float(bobbin)
_Control knob .
• Flow is affected by the viscosity&density
• At low flow :
viscosity
• At high flow:
density.
• So,each type of gas requires diff.
calibrated flow tube.
• The flow tubes designed for one gas
should not be interchanged with another
hypoxic mixtures.
• Flow tube :Conical glass tube.
• Low flow tube
graduated/100
ml/min. up to one liter
• High flow tube
graduated/1L/min.up to 10L/min.
• Control knob :when it is turned
needle valve is opened.
• Gas flows into the bottom of the flow tube
& moves upward around the bobbin
upward force pushes the bobbin up in the
tube .
Float (bobbin)
• It may be balls or cylindrical with slots in
its upper part & tapered base to make it
to rotate in the gas stream .
• Non spinning bobbin that may indicate O2
is flowing while the bobbin is stucked with
no adequate flow
hypoxic mixture.
• The density of a gas is dependent on the
•
•
•
ambient pr .
Large diff. in atmosph. Pr. alter the calibrat.of
flow tube .
At high altitudes with hypobaric condit.
Flow meter setting is less than the actual
At hyperbaric condition flow setting is more than
the actual
hypoxic mixture
• The spinning movement of the bobbin can be
•
•
•
affected by dust , dirt & static electric charges .
Static charges can be dealt with by bimetallic
strip .
Dirts & dusts by air filter proximal to the needle
valve.
Slots on the bobbin to spin resist the effect of
static charges & dust.
• The twist required to turn the knobs must
be sufficient to prevent accidental
readjustment from casual knock .
• If axial push _ pull forces are applied to
the needle valve spindle the flow change
must not exceed 10% .
O2/N2O Ratio Controllers
• In modern anesth. ,there is interconnect.
Of N2O & O2 flowmeters to ensure
delivery of a conc.of at least 25% O2 .
• There are 2 systems :1. Pneumatic syst.
ORMC (designed
by Drager).
2. Chain _link 25 proportion (by Ohmeda )
Safety Measures (additional)
1. The flow tube for oxygen has been on
the extreme rt. Side
O2 is the last
gas to be added to the fresh gas supply.
2. A color _coded flow control knob is
located under its respective flow tube.
3. A color_coded cylinder pr. gauge for the
gas entering the flowmeter & located
under its respective flowmeter.
4. A plastic screen used to prevent damage
to the flowmeters.
5. In some machines , protective bar placed
infront of the flow meters to protect from
accidental bumping.
6. Lighting system for the flow meter is
valuable when the op.room light is
dimmed .
7. Flow tube pin _indexing syst. to prevent
interchange of flow meter units
hypoxic mixture.
• O2 flow can not be completely turned off.
A minimum flow rate of 200_450 ml/min
is delivered to provide sufficient O2 for
the basal metabolic needs by :
1. Applicat. Of resistor in the oxygen
pipeline.
2. OR/stop screw on O2 control valve.
• The control knob of O2 has esp. shape:
 Octagonal
 Greater diameter
 Project beyond the other knobs
Other types of flow meters
1. Electronic flow meter
a. Oscillations of pr. Drop across an orifice
is proportional to the flow rate
b. Thermister flow meter :
Gas flow affects thermocouple attached
to either end of flow tube
2.
3.
4.
5.
6.
7.
Coxter flow meter
Bubble flow meter
Rotor meter
Wright’s spirometer
Pneumotachographs (Vitalograph)
Ultrasonic flowmeters
Electronic magnetic flowmeter