Manchester 1994

In European countries the circle absorber system is the most used breathing system. If the share of rebreathing is depicted by the percentage of CO2 really passing the absorber, the following can be shown:

only if the flow is reduced to a value of 1 1/min or lower, the share of rebreathing will increase to 50% or more (Fig. 1). To really use the advantages of the rebreathing technique, in 1952 Foldes recommended to use a fresh gas flow of 1.0 1/min (Low Flow Anaesthesia). Furthermore, in 1974 Virtue inaugurated an anaesthetic technique being performed with a flow of only 0.5 1/min (Minimal Flow Anaesthesia). Assuming a constant gas composition within the breathing system, the patient's total gas uptake will follow a power function. The uptake is high and declines sharply during an initial phase of about 15 to 20 minutes, but is comparatively low and decreases only slowly during the following time course of anaesthesia (Fig. 2). This exponential characteristic of the gas uptake results from the fact that the partial pressure difference of anaesthetic gases between the alveolar space and the blood, being initially high, decreases continuously with increasing saturation of the blood and tissues . In daily practice, using available manually controlled gas delivery modules, it will be very difficult to perform Closed System Anaesthesia, as that requires frequent adaption of the fresh gas flow according to the continuous alteration of the uptake. Contrarily, low flow techniques like Minimal and Low Flow Anaesthesia can be safely performed with already available anaesthetic equipment, if simple and standardized dosing schemes are used.

Premedication and induction of the anaesthetic procedure is performed according to the usually used induction scheme. After intravenous injection of a hypnotic, pure oxygen is delivered to the patient via a face mask for pre-oxygenation. After neuromuscular relaxation, an endotracheal tube is inserted and connected to the breathing system. According to own experiences, in about 85% of all cases also the gas tightness of the laryngeal mask will be enough to use fresh gas flows of 0.5 1/min, even in controlled ventilation.

According to the recommendations of Foldes and Virtue, during the first initial phase a comparatively high fresh gas flow is used : The oxygen flow is set to 1.4 1/min, the nitrous oxide flow to 3.0 1/min. This fresh gas composition guarantees in most of all cases an inspiratory oxygen concentration of at least 30%, meeting the recommendations of Barton and Nunn. If enflurane is used, the vaporizer is set usually to 2.5 Vol %, if Isoflurane is used, to 1.5 Vol %. If setting these values, during the following 10 to 15 minutes an expiratory concentration will be reached being about 0.7 to 0.8 times the MAC of the chosen volatile agent.

Additionally to the MAC of the nitrous oxide, assumed to be about 0.6, this will result in a common MAC of nitrous oxide, assumed to be about 0.6, this will result in a common MAC of 1.3, the AD₉₅, being enough for 95% of all patients to tolerate the skin incision without any muscular reaction. Furthermore, an initial high fresh gas flow is indispensable for sufficient denitrogenisadon and wash in of the inhalational agent into the whole gas containing space. If the flow is reduced too early to low values, gas volume deficiency may result compromising adequate ventilation.

According to the Low Flow Anaesthesia scheme, after 10 minutes the fresh gas flow can be reduced to 1.0 L/min. Always it has to be considered, that the oxygen concentration of the fresh gas must be the higher the lower the flow.

Thus, to maintain a safe inspiratory oxygen concentration in Low Flow Anaesthesia, the fresh gas oxygen concentration has to be increased to 50%, but at least to 40%. Furthermore, when using enflurane or isoflurane, being more soluble than desflurane, the fresh gas concentration of the anaesthetic agent has to be increased to keep constant the anaesthetic agent's concentration within the breathing system (Fig. 3). If switching to a flow of 1.0 L/min, the fresh gas enflurane concentration is increased to 3.0 Vol%, if using isoflurane to 2.0 Vol%. Although there can be seen a slight decrease of the agent's in and expiratory concentration, the expiratory nominal value will be kept in a range between 0.8 to 0.7 times the MAC of the chosen volatile anaesthetic.

If Minimal Flow Anaesthesia is performed, the initial high flow phase should last at least 15 minutes, but if an athletic young patient is anaesthetized, preferably 20 minutes. Accidental gas volume deficiency will be avoided if the duration of the initial high flow phase is sufficiently long. With the flow reduction to 0.5 L/min the fresh gas oxygen concentration has to be increased to 60%, but at least to 50%, to maintain a safe inspiratory oxygen concentration higher than 30%. At the same time the anaesthetic's concentration in the fresh gas is increased, if using enflurane to 3.5 Vol%, is using isoflurane to 2.5 Vol%. Again, although the vaporizer's setting in increased, a slight decrease of the agent's concentration within the breathing system can be seen, but the inspired concentration will be maintained.

The specific characteristics of low flow anaesthetic techniques can be shown most impressively by clinical experiences with Minimal Flow Anaesthesia. This is the low flow technique in which the greatest flow reduction is gained whilst being practical in daily routine.

oxygen and nitrous oxide uptake. It will be more pronounced in small and elderly patients with low oxygen uptake than in young athletic patients. After this initial increase, the inspiratory oxygen concentration tends towards lower values again slowly but continuously. Whenever the lower alarm limit is reached, which should be carefully adjusted to 30%, the oxygen flow has to be increased by 10% of the total fresh gas flow. Simultaneously the nitrous oxide flow has to be decreased by the same amount (Fig. 4). Thus, performing Minimal Flow Anaesthesia according to the standardised scheme, the 0^ flow has to be increased by 50 ml/min, and the N^0 flow reduced by 50 ml/min. Again an initial slight increase in the FIO,, followed by its slow but continuous decrease can be seen. Whenever the lower alarm threshold is reached anew, the oxygen flow again has to be increased by 10% of the total fresh gas flow, and the Np flow to be reduced by the same amount.

Although the fresh gas concentration of the volatile anaesthetic is increased according to the given standardized scheme, a slight decrease of the anaesthetic's inspiratory and expiratory concentration can be observed. As the vaporizers of nearly all modem anaesthetic apparatus are switched into the fresh gas flow (VOC), reduction of the flow will result in a corresponding decrease of the amount of anaesthetic vapour begin fed into the system. The breathing system, the ventilator, the connecting tubes and the patient hose assembly contain about 6 L in the different machines. This apparatus volume in addition to the gas volume contained in the lung of the patient (about 2.5 L in an adult) is the space for distribution of the anaesthetic vapour being fed into the system together with the fresh gas. Assuming a flow of 500 ml, a change of the vaporizer's setting from zero to its maximum output of 5 Vol% will result only in a 25 ml/min increase of vapour being fed into the system - a very small volume if compared with its distribution space. Thus, during performance of Minimal Flow Anaesthesia, one has to consider that there will be always a great difference between the vaporizer's setting and the agent's concentration being gained within the breathing system. If the inspiratory concentration of the chosen volatile anaesthetic needs to be increased or decreased, the vaporizer has to be adjusted to a concentration considerably exceeding the target value (Fig. 5).

Accidental under or over dosage of the inhalation anaesthetic with a flow as low as 500 ml/min will be nearly impossible.

The time constant is a measure of the time taken for alteration of the fresh gas composition to lead to corresponding alteration of the gas composition within the breathing system. According to the formula given by Conway the time constant (T) can be calculated by dividing the system's volume (Vs) by the difference between the fresh gas flow (Vf) and the gas uptake (Vu):

Assuming a given volume of the system and a given gas uptake, the time constant is inversely proportional to the fresh gas flow.

The increase of the time constant has to be considered when switching from high to low fresh gas flow. Whenever the gas composition within the breathing system has to be changed rapidly, the fresh gas flow has to be increased for accelerating the wash in of the intended gas composition.

According to the long rime constant characterising low flow anaesthesia, the vaporizer can be closed about 15 to 30 minutes before the end of the surgical procedure. If the low flow is maintained, the decrease of the anaesthetic's concentration is delayed and slow. During that time recovery of spontaneously breathing patients can be induced by using the SIMV ventilation mode or by manual assistance of the ventilation. Not until about five minutes before extubation the anaesthetic gases are washed out by switching to high flow of pure oxygen. Care is taken of the recovering patient in the usual manner.

As the difference between the gas concentrations in the breathing system and the fresh gas increases with the extend of flow reduction, the anaesthetic gas composition cannot be assessed from the composition of the fresh gas. Thus, continuous monitoring of the inspiratory oxygen concentration is absolutely indispensable. The same applies to the anaesthetic's concentration, if a fresh gas flow lower than 1 L/min is used. The fresh gas flow has always to be set to a value sufficiently compensating the gas loss via individual uptake and leakages. As gas volume deficiency will lead inevitably to an alteration of the ventilation, continuous monitoring of the airway pressure and the minute volume is likewise indispensable. The disconnection alarm should be set to a value 5 mbar lower than the peak pressure, the lower alarm limit of the minute volume to a value 0.5 L/min lower than the nominal value, the lower limit of inspiratory oxygen concentration to 30%, and the upper limit of the inspiratory anaesthetic concentration to 2.0 - 2.5 Vol%.

The technical features of the anaesthetic apparatus have to comply with the following requirements :

The machine, of course, must have a circle absorption system, the flow meters must be calibrated in the low flow range, and the vaporizer must be flow-compensated. The breathing system has to be sufficiently gas tight. The leakage must not exceed 100 ml/min at a pressure of 20mbar so to meet the requirements of Minimal Flow Anaesthesia. The performance of low flow techniques is considerably facilitated by the availability of an anaesthetic gas reservoir, by which a small accidental volume deficiencies

can be balanced. A gas reservoir can be provided by the end inspiratory volume of a bag-in-bottle ventilator or of a ventilator with standing bellows;

or the manual ventilation bag in fresh gas-decoupled machines. Whenever an anaesthetic apparatus is used featuring continuous flow of the fresh gas into the breathing system, one must consider that every alteration of the fresh gas flow will lead to a corresponding alteration of the tidal volume. Only in the fresh gas-decoupled machines the fresh gas is fed into the system discontinuously, only during the expiratory phase, while during inspiration the fresh gas is stored intermediately in the manual bag.

The advantages of the rebreathing technique were emphasized comprehensively already in 1924 by Ralph Waters :-

and considerable reduction in costs, which apply the more for expensive inhalation anaesthetics.

All these advantages can be gained simply by the judicious use of modem anaesthetic equipment. Former reservations against .low flow anaesthetic techniques are not justified any longer, as the performance of the gas delivery systems and of modem vaporizers, and the gas tightness of modem compact breathing systems meet all technical requirements for safe use of even the lowest flows. Furthermore, the monitoring required will be part of he standard safety equipment being stipulated by the new common European Standard for Anaesthetic Workstations And Their Modules EN740.

Peculiarities resulting from the application of simple dosing schemes for the performance of low flow anaesthesia

Low and Minimal Flow Anaesthesia are low flow techniques in which the fresh gas flow is reduced to its utmost extent if using available anaesthetic machines. In both of these techniques a small amount of excess gas is used:

The performance of Low and Minimal Flow Anaesthesia becomes very simple and practical, even in daily routine practice, if standardized schemes are used for controlling the fresh gas flow and its composition.

However, if such simplifying schemes are applied, which require occasional adjustments at the gas flow controls, the anaesthetist has to accept a specific feature of the resulting anaesthetic procedure. The gas concentrations within the breathing system cannot be kept constant exactly at the intended concentrations but rather will oscillate slowly but continuously around these values.

Last, but not least, standardized schemes for the performance of low flow anaesthesia are only guidelines. The fresh gas flow and its composition must always be adapted to the individual patient's reaction and the current requirement of the surgical procedure.

R M Jones

St. Mary's Hospital Medical School,

London

The fluorinated ether, desflurane, is the latest of a series of halogenated ethers to enter clinical practice in the UK. Substitution of fluorine (at. wt. 18.9984) for chlorine (at. wt. 35.453) in isoflurane produces a molecule of lower molecular weight and therefore lower solubility. This small change in molecular configuration also confers a remarkable degree of stability on the molecule and desflurane is, for all intents and purposes, not metabolised (< 0.1 % of an inhaled dose),

Low solubility in blood and tissues indicates that desflurane is eliminated quickly, allowing for rapid recovery or change of depth of anaesthesia. However, a lower solubility agent will also be less potent because of the association of lipid solubility with potency. The MAC value of desflurane is around 8% in young adults. Although recovery from anaesthesia is rapid the vapour is irritant to the airway and inhaled induction of anaesthesia is associated with a high incidence of coughing and breath-holding. The irritant nature of desflurane may be associated with its lower potency i.e. the airway is exposed to appreciably higher concentrations of the agent when is used in clinically appropriate amounts. Desflurane's physical characteristics also differ from isoflurane and other agents in one other important respect. The boiling point is 22.8°C and this precludes its safe use in traditional vaporizers. At temperatures above 22.8°C it would vaporize uncontrollably delivering high and unpredictable amounts to the patient. A heated, pressurized (Tec 6™) vaporizer has been developed with features which allow for safe delivery irrespective of temperature.

Clinical trials

Initial clinical trials suggested that the cardiovascular effects of desflurane were similar to isoflurane (1,2). Recently, however, there has been some concern about the potential of desflurane to cause activation of the sympathetic nervous system leading to a transient tachycardia and increase in blood pressure (3, 4). It would appear that the vapour causes irritation when its concentration is increased above about 7% and this leads to sympathetic stimulation. In fact, a similar phenomenon is known to occur with isoflurane (4) but to a lesser extent. This is probably because isoflurane is used in much lower inhaled concentrations. As long as this phenomenon is kept in mind the side effect is of little clinical significance in the majority of patients. Low solubility is a particular feature of desflurane and there have been a number of clinical trials investigating the rate of recovery from anaesthesia. In general, these have shown that emergence is more rapid following desflurane anaesthesia compared with isoflurane (5) or propofol (6). It can be predicted, with some confidence, that emergence will also be more rapid compared with sevoflurane but in this instance the difference should be less marked than with isoflurane.

Clinical trials have been undertaken in a number of special patient groups, such as the elderly (7), and in specialised surgery such as cardiothoracic surgery (8). In general these studies have shown desflurane to be a safe anaesthetic for general clinical use. At present there has been insufficient data in some patient groups, such as the neurosurgical (9) and obstetric (10) patient, and desflurane is not currently licensed for use in these circumstances in the UK.

The structural integrity of desflurane associated with a relatively low potency indicate that the agent can be used with economic benefit in a low flow breathing system.

References

1. Jones RM, Cashman JN, Mant TOK. Br J Anae.sth 1990; 64: 11-15.

2. Cahalan MK, Weiskopf KB, Eger El et al. Anesth Analg 1991; 73: 157-164.

3. Ebert TJ, Muzi M. Anc.sthesiology 1993; 79: 444-453.

4. Wciskopf KB, Moore MA, Eger El et al. Anesthesiology 1994; 80: 1035-1045.

5. Ghouri AF, Bodncr M, White PF et al. Anesthesiology 1991; 74: 419-424.

6. Wrigley SR, Fairfield JE, Jones KM ct al. Anesthesia 1991; 46: 615-622.

7. Ruist M. Personal communication.

8. Parsons RS, Jones RM, Wrigley SR et al. Br J Anae.sth 1994; 72: 430-438.

9. Muxzi DA, Daltner C, Losa.sso T. Anesthesiology 1991; 75: A167 (abstract).

10. Abboud TK, Zhu J, Richardson M. Anesthesiology 1992; 77: A996 (abstract).

THE CHALLENGE OF DESIGNING A VAPORIZER FOR DESFLURANE

Mr Lucian Galbenu

Introduction

The main function of a vaporizer is to deliver a controlled v/v concentration of anaesthetic agent to the face of a patient during a surgical operation. The output concentration must not be affected by the fresh gas flow and composition, temperature, fluid level, down stream pressure, altitude or the back pressure.

Desflurane

In the 1960's, researchers at Ohio Medical Products synthesised and tested more than 1000 methyl ethyl compounds. These compounds tended to be stable and non flammable and were effective anaesthetics. Two compounds from this series that have become clinically important are enflurane and isoflurane, which are isomers differing only in their structure. Desflurane is another compound in this group, but its development was not initially pursued because of difficulties in its synthesis and its high vapour pressure (close to one atmosphere at room temperature). However desflurane has a low solubility and its possible use in out patients motivated Anaquest to reconsider its development. Anaquest began clinical trials in 198S, and in 1993, desflurane was released by Ohmeda Pharmaceutical Products Division under the trade name Suprane.

Desflurane is highly volatile. Its boiling point is just above room temperature and its vapour pressure is near ambient pressure. This volatility combined with its moderate potency, makes it unsuitable for use with conventional vaporizers.

Operation of Conventional Vaporizers

In a conventional vaporizer or a variable bypass vaporizer, the total background gas flow that enters the unit is split in two streams. The smaller stream, which acts as the carrier gas, passes through the vaporizer chamber containing the anaesthetic agent and becomes saturated with agent vapour. The remainder of the gas bypasses this chamber. A wick may be used in the vaporising chamber to provide increased surface area for efficient evaporation of the drug and saturation of the carrier gas. The saturated carrier gas leaves the chamber and mixes with the bypass gas. One adjustment is made to set the required concentration. This adjustment simultaneously balances the carrier and bypass flows to produce the blend required for the set concentration.

Evaporation of the liquid agent is driven by heat absorbed from the walls of the vaporizer. In consequence the vaporizer and its contents cool. A temperature sensitive mechanism is used to automatically adjust the carrier and bypass flows to compensate for temperature changes.

Requirements for Vaporising Desflurane

The boiling point of desflurane (22.9°C) at atmospheric pressure is just above room temperature. In consequence, small increases in ambient temperature or decreases in atmospheric pressure can cause it to boil. The change in vapour pressure of desflurane per change in temperature is as much as three times that for other volatile agents at sea level atmospheric pressure. These profound effects of temperature and ambient pressure on the vapour pressure of desflurane make stabilising the delivered concentration at a set point extremely difficult in a passive mechanical system such as a variable bypass vaporizer. As a result the variable bypass design was abandoned for desflurane and Ohmeda developed a new vaporizer, Tec 6 based on a different design.

Alternative designs considered

Measure and Inject Design:

A microprocessor based control system injects in a vaporising chamber an amount of anaesthetic liquid dependent of the fresh gas flow and of the required concentration

- difficult to measure the fresh gas flow and to design a reliable alarm system due to open loop control circuit.

Cooled Conventional Design:

The sump is cooled in order to be well below desflurane boiling point, allowing the use of the conventional bypass design.

- difficult cooling system, dangerous failure mode in case that the cooling stops working.

Balanced Regulator Design:

The sump is heated in order to obtain the working pressure, a mechanical balanced regulator keeps the pressure on the drug side equal with the pressure on the fresh gas side in order to allow setting of concentration independent of the fresh gas flow. - the mechanical regulator offers only proportional action (integral action is required), dangerous failure mode and non practical physical size.

Tec 6 Chosen Design

Desflurane Vaporizer Key requirements:

The vaporizer must be suitable for background flows between 0.2 and 10 LPM.

The concentration setting on the dial must be between 1% and 18%.

The vaporizer must be designed in order to be mounted on a Select-a-tec backbar.

The vaporizer requires electrical power to heat the desflurane to 39°C, producing a stable, saturated vapour pressure of approximately two atmosphere absolute. No wick is used and no carrier gas enters the sump chamber. Instead a steam of vapour under pressure flows out of the sump. This stream blends with the background gas stream, which originates from the anaesthesia machine's flowmeters, to achieve the desired calibrated concentration.

The background gas stream passes through a fixed flow restrictor, producing a back pressure upstream of this restrictor that is proportional to the background gas flow. The desired desflurane concentration is set on the dial of a variable restrictor. The pressure in the vapour upstream of this variable restrictor and the back pressure in the background gas stream are continually

sensed by a differential pressure transducer. The transducer controls a pressure regulated valve between the sump and the variable restrictor. The pressure regulated valve permits only that flow from the sump necessary to cause the pressure upstream of the variable restrictor to equal the back pressure in the background gas stream. In this way the ratio of the variable restrictor to the fixed restrictor determines the ratio of the flows in each stream and therefore the concentration of the vapours at the output of the vaporizer. If the background gas flow is altered, the flow of vapours from the sump is automatically adjusted so that the pressure at the two monitoring points remain equal, the flow ratio does not change, and the output concentration continues to match its setting.

The control circuits and heating elements in the vaporizer are turned on by the act of connecting the vaporizer to electrical power. The unit then heats to and remains at the operating temperature as long as it receives power, whether it is delivering agent or is in the standby mode.

Tec 6 characteristics

Tec 6 is a heated, constant temperature, pressurised, concentration calibrated gas vapour blender. It weighs about 9Kg and although it is larger than other vaporizers in Ohmeda Select-a-tec's Series, it fits the Select-a-tec manifold (back bar) and it is compatible with its interlock system.

Effect of flow rate on output concentration

The output concentration is very dependent on the fresh gas flow. However, for high dial settings, the concentration will decrease slightly when the fresh gas flow increases towards 10 LPM due to non linearity of the fixed and variable restrictor and due to the high demand of drug vapour from the sump.

Electrical Safety

Tec 6 vaporizer conforms to IEC 601-1 -

Electrical Safety for Medical Devices. IEC (International Electrical Commission) Tec 6 vaporizer conforms to IEC 601-2-13

Electrical Safety for Anaesthesia Machines.

Tec 6 vaporizer meets UL Recommendations - USA version of IEC. UL (Underwriters Laboratories).

Vaporizer Safety

Delivering desflurane using Tec 6 is similar to other agents using conventional vaporizers. However, because electricity is required for the heaters and control circuits in the Tec 6, it is also available to power indicators and alarm systems (other vaporizers do not have alarms). These systems sense and display the level of anaesthetic agent (the high operating pressure does not permit the usage of a sight glass to show the level), the temperature and the status of other functions. If the vaporizer is tilted while operating, a tilt switch activates a visual indicator and an audible alarm, and the vaporizer stops delivering vapour. In case of a line power failure, a 9 V battery back up supply only the indicators and alarms.

The pressure control system is independent of the pressure alarm system in order to prevent the failure of a single component to affect both systems. Two pressure transducers are used, one for pressure control and one for the pressure alarm. High and low pressure alarms are provided in order to avoid over or under delivery of anaesthetic agent. When the vaporizer is not in use the pressure transducers are checked for any drift.

The temperature control system is independent of the temperature alarm system for similar reasons. Two temperature sensors are used, one for temperature control and one for temperature alarms. An under temperature detection is provided for the Warm Up state. Several mechanisms are used to prevent excessive pressure build-up. If the normal temperature control circuit should fail, a separate over temperature detection circuit will shut the heaters down and activate an alarm. As further protection, thermal fuses are included in the sump heater circuit to cut off power if their temperature reaches 62°C to 66°C.

A solder filled plug is set to melt and release pressure if for some reason (e.g. an external fire),the sump temperature exceeds 72.

Any alarm state will de-energise the dial lock and die sump shut off valve in order to prevent (lie usage of the vaporizer. The alarm state is displayed by visual and audio means,

Vaporiser modes

The following modes are used in order to inform the user of the state of the vaporizer:-

WARM UP unit heating up, dial locked.

OPERATIONAL ready for use, dial can be turned.

NO OUTPUT operating or system fault detected, agent output prevented.

LOW AGENT warning of minimum agent level.

LOW BATTERY warning of flat alarm back up battery.

Tec 6 Vaporizer Displays

The following indicators are used to display the state of the vaporizer:-

OPERATIONAL	green LED
NO OUTPUT	red LED
LOW AGENT	amber LED
WARM UP	amber LED
BATTERY LOW	amber LED
Agent Level display	20 bars LCD

Tec 6 Vaporizer Sump Capacity

The sump has a capacity of 425 ml, an indicator is provided for the 240 ml level (one bottle) and the low agent alarm is activated at 50 ml.

Unique Filling system

Misfilling a non Tec 6 vaporizer with desf'lurane is extremely dangerous because of the agent's high volatility. The general rule is that agents should not be used in vaporizers for other agents. In order to prevent misfilling, agent specific filling systems has been developed.

Desflurane is supplied in a plastic coated glass bottle to protect against leakage and breakage. Because the sump in Tec 6 is pressurised, a special bottle and inlet system has been developed for filling. A crimped on, one way valve seals the bottle and mates only with the Tec 6 spring loaded inlet. This system permits filling at any time during operation. The vaporizer does not need to be in standby mode to be filled. The filling time is approx. 70

seconds for a full bottle. The sump's pressure drops significantly during filling but this does not affect the differential pressure across the pressure transducer and in consequence the output concentration is not affected during the filling of the vaporizer.

Hypoxic Mixture concerns

Because of the high agent concentration possible with the Tec 6, care must be taken that an adequate oxygen concentration is maintained in the inspired gas. This is particularly important when low fresh gas flows from the anaesthesia machines are used in a re-breathing system. Ohmeda highly recommends continuously monitoring of the oxygen concentration in the breathing system. A warning label in respect with hypoxic mixtures is attached on the top of each vaporizer.

Effect of the pressure transducer error on output concentration

A pressure transducer error will cause an offset between the pressure upstream the fixed restrictor and the pressure upstream the variable restrictor (for zero pressure error the two pressures are equal). The back pressure upstream of the fixed restrictor is proportional to the fresh gas flow and in consequence the effects of the pressure error on the output concentration will be greater for low fresh gas flows.


	Manchester 1994
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