Wiv: Measurement of Flow of Gases
Definition
FLOW = the quantity of fluid passing a point per unit time
Units
L/min
Measurement
Flowmeter
Use
Gas flow through anaesthetic machine, gas cylinder, pipeline
Mechanism
Vertical conical measuring tube, which gets wider as you go up
Bobbin which can move up and down
The bobbin and conical tube create a variable ring orifice
The higher the bobbin, the larger ring orifice
When there is no flow, gravity keeps bobbin at bottom of conical measuring tube
As gas flow commences, the bobbin rises, until the opposing forces are in balance:
3 forces:
gravity – downwards
buoyancy – upward
drag (flow force) – upward
Gravity and Buoyancy are constant
Only flow force varies
Position of bobbin indicates the change in flow
Adv
Cheap and simple
No electricity
Accurate for clinically used flows
Disadv
Bobbin can get stuck