Anaesthetic machine: Pressure regulator, pressure gauge, flowmeters Flashcards
Pressure gauges
Function, location, calibration, colour
- Measure pressures in the cylinder or pipeline supply - usually of oxygen, nitrous oxide and medical air
- Mounted on front-facing panel on anaesthetic machine. Some modern machines have digital display
- Each pressure gauge is labelled, colour coded and calibrated for a particular gas or vapour and for cylinder or pipeline supply
Oxygen: white
Nitrous oxide: blue
Air: black
Note: cylinders are kept under much higher pressures than pipeline gas supply, therefore using same pressure gauges for both can lead to inaccuracies and/or damage
Difference between oxygen and nitrous oxide gauges
Oxygen: pressure measured indicates contents available in the cylinder, because oxygen is stored as a gas and obeys Boyle’s gas law: Pressure * volume = constant
Nitrous oxide: stored as a liquid and vapour. Gas laws do not apply
Difference between cylinder and pipeline gauges
A pressure gauge designed for cylinders should not be used to measure pipeline pressure and vice versa.
Wrongly connected gauges can lead to inaccuracies and/or damage to the pressure gauge.
Image shows pipeline (left) and cylinder (right) oxygen rpessur gauges - note v diffferent pressure ranges
Pressure gauge structure, safety features
A pressure gauge is a Bourdon-type gauge
* Made of robust, flexible and coiled tube that is oval in cross-section
* Outer end of coiled tube is exposed to gas supply. Inner end is sealed and connected to a needle pointer which moves over a dial
* When gauge connected to cylinder/pipeline: high pressure gas -> tube uncoils -> movement of tube causes needle pointer to move on calibrated dial indicating the pressure
Safety features: designed to be able to withstand sudden high pressure when cylinder is switched on (e.g. 13700 kPa for oxygen)
* If coiled tube ruptures under excessively high pressure, gauge is designed to vent escaping gas through the back of the casing (to prevent other parts of anaesthetic machine from damage due to high pressures)
* Face of pressure gauge is made of heavy glass
Pressure regulators
What is, where found, function for anaesthetic machine (4)
Pressure regulators= devices that convert variable, high pressures to constant, lower output pressures
Used with gas cylinders
Usually positioned on back of anaesthetic machine or under tray
Functions
* Gases are stored under high pressure in cylinders -> reduce variable cylinder pressure to constant safe operating pressure of ~400 kPa
* Maintain constant flow despite changes in temperature and pressure of cylinder contents as gas is used up (without a regulator, constant adjustment of gas flow would be required)
* **Protect low-pressure components of anaesthetic machine **against pressure surges
* Allows low pressure piping and connectors to be used in the machine: reducing consequences of any gas leak
Safety features
* If diaphragm ruptures, valve will fail. Therefore relief valves (usually set at 700 kPa) fitted downstream of the regulators to allow escape of gas in event of regulator failure -> loud hissing sound
* Cylinder gases should be water-vapour-free to prevent ice formation in the low pressure chamber (otherwise may condense and freeze (->block gas flow) as a result of heat lost when gas expands on entry to low pressure chamber)
Pressure regulator: mechanism
- Gas from cylinder enters via high pressure inlet into high pressure chamber fitted with a valve
- As passes to low pressure chamber via the valve, exerts a force that works to close the valve
- An opposing force from the diaphragm and spring works to open th evalve.
- A balance between the two opposing forces maintains gas flow under a constant pressure of about 400 kPa from the oulet of the regulator -> flowmeters of anaesthetic machine
Flow restrictors
Simple constrictions between the pipeline supply and the rest of the anaesthetic machine
- Used with pipeline supply of gas - because pipeline supply pressure is not as high (400 kPa) and variable as that delivered by cylinders (c.f. cylinder supply uses pressure regulators)
- Function in similar way to pressure regulators: significantly reduce the pressure while maintaining a constant flow rate, despite any changes in pipeline pressure
- Lower pressures of 100-200kPa can be achieved
- Designed to protect the anaesthetic machine from pressure surges in the system
Pressure regulators vs flow restrictors
Both reduce the pressure while maintaining a constant flow rate, despite any changes in supply pressure
Pressure regulators are used with cylinder supply of gases. Reduce pressures to 400 kPa
Flow restrictors are used with pipeline supply of gases. Reduce pressures to 100-200kPa
Pressure regulators have more complex mechanism. Flow restrictors are simple constrictions between pipeline supply and anaesthetic machine
Flowmeters
Function, structure, calibration
Measure the flow rate of a gas passing through them
Structure
* Flow of gas is controlled by the flow control (needle) valve via manual adjustment.
* Gas then flows into tapered transparent plastic or glass tube that is wider at the top than the bottom
* Light-weight rotating bobbin or ball measures the flow. Bobbin held floating within the tube by gas flow. Higher the flow rate -> higher rises.
* Bobbin-stops at either end of the tube ensure that the bobbin is always visible (safety feature)
Calibration
* Each flowmeter is individually calibrated for each gas
* Calibration occurs at room temp and atmospheric pressure (sea level)
* Accuracy +/-2.5%
* Increments of 500ml/min for flows above 1L/min, and increments of 50ml/min below that
Flow control (needle) valves
Function, material, how to increase flow, safety feature
- Control the flow through flowmeters by manual adjustment
- Turn valve anticlockwise to increase flow
- Made of brass. Screws into base of flowmeter
- Control knobs are labelled and colour-coded. Oxygen control knob is designed to be larger with pronounced ridges to allow easier recognition
Differences in gas flow around bobbin at low and high flow rates
Note flowmeter tube is wider at the top than the bottom
**Low flow **rates: clearance around bobbin is longer and narrower, thus acting as a tube
* Flow is **laminar **
* Flow is a funciton of gas viscosity (Pouseuille’s Law)
High flow rates: clearance is shorter and wider, thus acting as an orifice
* Flow is turbulent
* Flow is a function of gas density
Bobbins and balls in flowmeter
How to take readings, different designs, most commonly used
Bobbin -> reading taken from top of bobbin
Ball -> reading taken from midpoint of ball
Most modern anaesthetic machines use a skirted bobbin with a dot. (3 in picture)
Bobbins may have slits (flutes) cut into the top: gas flowing through these slits causes the bobbin to rotate inside the flowmeter tube - indicate to the operator that bobbin is floating in gas flow and not stuck inside tube.
Flowmeter calibration, functionality at low flows
- Each flowmeter is individually calibrated at room temp and atmospheric pressure (sea level) for each gas
- Accuracy +/-2.5%
To facilitate measurement of very low flows: arrangement of two flowmeters in series is used
* Thinner flowmeter reads max 1L/min, calibrated in 50ml increments
* Other flowmeter reads up to max 10-15ml/min, calibrated in 500ml increments
* One flow control knob per gas needed for both flowmeters
Anti-hypoxic features in flowmeter design
(2)
Oxygen is last gas to be added to the mixture by design
* Oxygen control knob is situated on the left in the UK (right in USA and canada).
* Crack in adjacent downward flowmeter could therefore result in hypoxic micture, if oxygen flowed through least resistant path (out of cracked flowmeter)
* Therefore, oxygen is the last gas to be added to the mixture delivered to the back bar (although flowmeter retained on the left)
European standard EN 740:
* Anaesthetic machines must have means to prevent delivery of a gas mixture with an oxygen concentration <25%.
* In modern anaesthetic machines, oxygen and nitrous oxide have separate flow control knobs, but linked mechanically or pneumatically, so is impossible to deliver under a specified oxygen concentration (typically 25-30%)
Quantiflex anaesthetic machine
Mostly used in dental anaesthesia
Safety feature prevents delivery of hypoxic mixtures:
* Two flowmeters, one for oxygen and one for nitrous oxide, with one control knob for both flowmeters.
* The oxygen flowmeter is situated to the right, whereas the nitrous oxide flowmeter is situated to the left.
* Relative concentrations of oxygen and nitrous oxide are adjusted by a mixture control wheel. The oxygen concentration can be adjusted in 10% steps from 30% to 100%.
