Patent Description:
Air/Oxygen blenders are known for use in a variety of healthcare applications to offer a reliable and accurate method of delivering the gas to the patient. Fixed lines at a wall of a hospital provide gases at a high pressure typically between <NUM> and <NUM> bar, and these gases need to be mixed in desired proportions and delivered to a patient at a particular pressure and/or flow rate.

Medical gas devices that are driven from high pressure air and oxygen typically fall into two categories:.

dimensions using a fine screw thread which moves the pin in and out.

The ability to provide precisely mixed air and oxygen in a safe, easy and controlled manner is becoming increasingly important, particularly in critical care and therapeutic care of premature infants, where there are severe consequences associated with the supply of either too little or too much oxygen. Accordingly, known gas blender systems are typically developed based on a mechanical ventilator, with a regulator to moderate the gas pressure from the wall before the mixing and delivery function is performed.

The accuracy of mechanical ventilator systems makes them particularly suitable for modification for use as gas blenders. However, unnecessary complexity often remains even after modification or simplification of a mechanical ventilator, and a first stage pressure regulator is required to drop the input pressure prior to the final flow control. This leads to unnecessary cost and complexity in the final gas blender, which does not need the millisecond response times the ventilator arrangement is designed for.

A medical flow meter does not typically have a pressure regulator to reduce the initial flow pressure, but still needs to be able to give sufficient control when both low flows at high driving pressures, and high flows at low driving pressures, are required. This is achieved by the fine screw thread, meaning that many turns of the thread are required to show any movement or change in flow when operating at the high flow/low driving pressure end. This reduces the responsiveness of the valve, impeding the ability of a flow meter to respond to sudden fluctuations in supply pressure or to adjust an output flow rate rapidly as might be required, for example, in a gas blender application. Although the supply of gas from the wall in hospitals is regulated by an ISO standard, and supply is generally between <NUM> and <NUM> bar, there are situations where there are pressure fluctuations in the wall supply. A desire existed, therefore, for a simpler more cost-effective gas blender unit which retained the benefits responsive flow control over a range of flow rates.

This desire led to a complete re-assessment of the process of designing a medical gas blender. It was determined that through including an improved flow control valve, with a high turndown ratio, a system could be provided which could overcome or help to mitigate the problems identified above.

All valves have a certain 'turndown ratio', defined as the ratio of maximum to minimum flow. In most instances, valves provided in mechanical ventilators and similar generally have a turndown ratio of up to or around <NUM>:<NUM>. That is to say the valve provides accurate flow output between, for example, <NUM>lmin-<NUM> and <NUM>lmin-<NUM>. At a fixed driving pressure this can provide control at suitable accuracy over a range of, for example, <NUM>lmin-<NUM> to <NUM>lmin-<NUM>. It would not be capable of providing that control if the driving pressure were varied from <NUM> to <NUM> bar, i.e. without pre-regulation. The remaining aspects of a gas blender can be greatly simplified through the use of a valve with a considerably higher turndown ratio. The proposed design offers ratios in excess of <NUM>:<NUM>, allowing a simple gas blender to be provided which allows both the 'active' regulation and simultaneous metering of a gas with no need for a first stage regulation of flow pressure upstream of the mixing step.

The development of the invention came from a realisation that the conventional approach to the design of gas flow controllers, in particular controllers for medical applications such as gas blenders, was fundamentally flawed. Moving away from the accepted development process allowed the consideration of design options that would not have otherwise been considered.

A gas blender comprises a first inlet for connection to a first gas supply, a second inlet for connection to a second gas supply, a mixing chamber, a first flow regulation valve located between the first inlet and the mixing chamber for controlling the flow of a first gas to the mixing chamber, a second flow regulation valve located between the second inlet and the mixing chamber for controlling the flow of a second gas to the mixing chamber, an outlet, and control hardware for controlling the operation of at least one of the first and second flow regulation valves. The first and second flow regulation valves may have a turndown ratio in excess of <NUM>:<NUM>. The gas blender may further comprise an oxygen sensor.

At least one of the first and second flow regulation valves may comprise a valve member driven by a motor, and the control hardware may be configured to move the motor a number of steps based on the outcome of the comparison step.

The valve member may comprise a valve pin.

A gas flow controller is a simpler construction than the gas blender previously described, and comprises a flow regulation valve with a valve pin and a sealing element surrounding the valve pin to provide a seal between the sealing element and the valve pin at a sealing position on the valve pin. Relative movement of the valve pin and the sealing element away from the sealing position adjusts the gas flow through a range of flow rates and/or pressures.

A typical mechanical ventilator includes control valves and associated control architecture to govern the gas composition and flow rate as well as other functions allowing the ventilator to adjust, rapidly, to changes in a patient.

While the precise flow control provided by mechanical ventilators is clearly beneficial in gas blender applications, several of the more complex control functions also provided by a typical ventilator are unnecessary. It is possible for some of the redundant or unnecessary control systems or features to be removed or inhibited when producing a gas blender device, but the resulting systems still tend to be overcomplicated, and therefore costly. For example, the rapid response provided by proportional solenoid valves, or similar, is not required in typical steady flow gas blender applications.

Through the resulting development of a suitable valve pin, a gas blender can provide the necessary precision of flow rate control in a simple package and operate across a wide range of pressures, without the need for a pressure regulator to be provided upstream of or built into the device, whilst still delivering a very controlled oxygen concentration in the gas mix.

<CIT> relates to a gas mixing valve with first and second inlet chambers linked to supplies of a first and second gas, and a mixing chamber having a blended gas outlet. The inlet chambers are connected to the mixing chamber via first and second control valves, each valve having a valve seat and valve member defining a flow control orifice. Each valve seat and valve member are relatively movable between a closed position in which no flow occurs and a maximum opening position to define a series of orifices of progressively increasing area corresponding to the same geometrical progression. The position of each valve member can be controlled to provide a desired mixing ratio. Each valve seat is provided in a respective piston, and the pistons move in response to variations in pressure drop across each piston, in order to vary the orifice size to compensate for changes in flow rate without changing the mixing ratio.

<CIT> relates to a regulating valve comprising a valve needle arranged in a valve seat hole. Grooves are formed in the side walls of the valve needle to provide "micro-flow regulation".

<CIT> discloses a needle valve includes a metering stem portion or nose in which is formed an elongate fluid metering groove which extends generally lengthwise of the stem axis, said groove gradually increasing in depth in one direction. The nose may be projected predetermined distances axially through a valve seat element bore, which has such close tolerance about the nose that all fluid passed by the valve will necessarily traverse the groove, this resulting in a high velocity of fluid release which has a self-cleaning effect preventing silting. The valve seat element is self-adjusting to the stem nose, and may readily be economically replaced in the event of wear or deterioration.

In <CIT> a metering valve assembly is provided, with a valve stem having one or more tapered channels formed therein for selective engagement with a valve seat to provide incremental regulation of fluid flow through a valve housing. An adjustment knob provides one manner of reciprocal movement of the valve stem within the valve housing. Alternatively, the valve stem can be directly engaged for rotational reciprocal movement within the valve housing. Different embodiments of the valve stem are provided for alternate fluid flow regulation requirements. The metering valve assembly is comprised of a limited number of component parts for ease of manufacture as well as the replacement and exchange of component parts in the field.

<CIT> relates to flow regulating or metering valves for accurately controlling the rate of flow of a fluid between chambers maintained at different pressures, or to control the rate of flow of air to or from a pressure or vacuum chamber. The valve has a stem with a channel shaped groove extending inwardly from its outer end.

The present invention provides a valve, for use in a medical gas flow controller, as defined in the appended claim <NUM>. Further optional features are recited in the associated dependent claims.

In developing the present technology, a valve was developed that provides an increased turndown ratio, making the valve more practical to use when operating across these driving pressure ranges.

The valves used in ventilators and similar flow controllers typically have a turndown ratio which is generally around <NUM>:<NUM>. The valve of the present technology has, in some embodiments, been found to increase the turndown ratio to around <NUM>:<NUM>, essentially providing accuracy between, for example, <NUM>lmin-<NUM> and <NUM>lmin-<NUM>, by combining two flow control mechanisms, namely a tapered pin shape and an increased valve cross-section/flow area near to the closed/sealed position, into one. In other embodiments the valve of the present technology has been found to increase the turndown ratio to around <NUM>:<NUM>, essentially providing accuracy between, for example, <NUM>lmin-<NUM> and <NUM>lmin-<NUM>. Again, this is achieved by providing an increased valve cross-section/flow area near to the closed/sealed position and a tapered section of the valve pin, as well as an opening/flaring at the free end of the valve.

In all suitable applications, the improved valve pin addresses the aim of providing the largest possible turndown ratio without including a pressure regulator.

In a flow meter, the valve needs to be capable of delivering high flows at low wall pressures (ie fully open), but also manage and even out any pressure fluctuations (which is where small accurate changes in flow are necessary), particularly where the wall pressure is high but the set flow is low (ie with the valve near its closed position).

As noted above, there are situations where there are pressure fluctuations in the wall supply in hospital environments. In a conventional flow meter, the pressure setting selected by the clinician is static and cannot account for this pressure fluctuation without being manually altered. The valve arrangement and valve pin of the present invention not only allows different wall pressures to be accommodated, but also provides a rapid response that can even out the fluctuations and maintain supply at the selected flow rate.

The fine control may already be provided by certain existing systems and known valve arrangements, but the valve pin and valve of the present invention provides an alternative solution, while also allowing much more rapid reaction to pressure changes. As will be described later, conventional valves tend to provide a sudden increase in flow rate immediately once opened. To provide the necessary control in this region, a fine screw thread is typically provided to allow tiny movements of the valve pin. This, however, reduces the overall responsiveness of the valve. The improved valve pin design provides a more gradual response at low flow rates, so the movement need not be controlled so precisely to provide comparable flow control in this region. This allows faster movement of the valve, and thus faster response across its entire operating range.

In addition, to ensure accuracy and control at the lower end a conventional flow meter would require very high tolerances for the valve components. The valve pin of the present invention avoids the need for such high tolerances, and therefore provides a solution that is cheaper and easier to manufacture, less susceptible to contamination, and less prone to mechanical failure.

In the case of a gas blender, the valve extends the accurate operating range of the device without the need to include an expensive pressure regulator. The importance of delivering an accurately and precisely controlled oxygen concentration from a medical gas blender to a patient has already been discussed.

In the case of the flow meter, which may already have the required accuracy, the valve provides a more user-friendly and/or cost-effective solution to that problem.

The valve comprises a valve pin with an elongate body with an outer sealing surface and a tapered part tapering towards a free end of the body. A sealing element surrounds the valve pin to provide a seal with the outer sealing surface at a defined sealing position, and relative movement of the valve pin and the sealing element away from the sealing position provides a variable flow area between the sealing element and the valve pin. The valve pin is shaped to increase the flow area of the valve close to the sealing position.

The elongate body of the valve pin may have a first portion of substantially constant cross-section, ie an un-tapered portion, and a second separate tapered portion towards the free end. A sealing position of the valve pin may be provided on the first portion.

According to the present invention, a recess is provided in the outer sealing surface of the body close to the sealing position. The recess comprises first and second elongate slits or channels, the length of which are aligned with the length of the elongate body, for example between the first and second portions of the elongate body. The length of the first elongate slit is greater than the length of the second elongate slit.

The depth of the elongate slit, into the elongate body, may vary along the length of the slit. For example, the depth of the elongate slit may fluctuate or may reduce from the free end of the body towards the second end. The depth of the slit may flare towards the free end of the valve. The slit may extend so far that it is present in an end face of the valve pin defined by the free end of the body.

The width of the elongate slit may be constant along the length of the slit, or may vary. The width of the slit may reduce from the free end of the body towards the sealing position, and may flare towards the free end of the body. The slit may provide an open end section at the free end of the body.

The outer sealing surface may have a substantially constant cross-section, and the variation in depth of the elongate slit may provide the tapered part of the valve.

It will be understood that a flow controller such as a medical gas blender could be provided comprising a valve as described above.

The gas flow controller/blender may comprise one or more additional features as described above, and/or the gas blender may further comprise a linear stepper motor for moving the valve pin. Using a stepper motor provides certain advantages, for example regarding power consumption. However, a servo motor or, indeed any other digital or analogue motor could alternatively be used to move the pin if required.

The flow controller or gas blender may, in particular, be used in a breathing circuit, for example in a hospital environment.

The gas flow controller and gas blenders described are designed to run directly off the gas pressure at the wall in the hospital, so there is no prior pressure regulation or pressure generation required. This makes the devices highly cost effective.

Practicable embodiments of the present will now be described with reference to the accompanying drawings, of which:.

A schematic view of a type of gas flow controller, specifically a medical gas blender <NUM> is shown in <FIG>. The gas blender <NUM> briefly comprises a base housing <NUM> within which the various components of the gas blender <NUM> are housed. A pair of inlet fittings <NUM> are provided externally of the housing <NUM> for connection to gas supplies in a hospital or similar. One inlet fitting <NUM> connects, in use, to an oxygen source such as a supply of pure oxygen or <NUM>% oxygen, and the other inlet fitting <NUM> connects to pressurised air. The connections are made using separate tubes or lines connected to the supply pressure, often provided at the wall of the hospital.

Each inlet fitting <NUM> is connected to a lower end of a valve housing <NUM>. The two valve housings <NUM> are also connected to a mixing chamber <NUM> which is, in turn, connected to an outlet port <NUM> provided at the front of the housing <NUM>. It will be understood that, in use, gas flow <NUM> passes from the inlet fittings <NUM> successively through the valve housings <NUM> and mixing chamber <NUM> before exiting the gas blender <NUM> through the outlet port <NUM>. O-rings are provided at the various connection points to provide a gas-tight passageway.

The flow from the outlet port <NUM> can then be directed to a patient via standard tubes or lines. Other known components of respiratory circuits may be connected between the outlet port <NUM> and the patient if required.

The flow entering each valve housing <NUM> from its respective inlet <NUM> is regulated by a valve pin <NUM> which engages with a sealing O-ring <NUM>. A linear stepper motor <NUM> is mounted to an upper end of each valve housing <NUM>. The linear stepper motors <NUM> can be independently controlled to move the valve pins <NUM> towards or away from the sealing O-rings <NUM> to alter the cross-sectional area of each passageway, and thus control the flow between open and closed positions. The valve housings <NUM> and mixing chamber <NUM>, together with the valve pins <NUM> and their respective motors <NUM> can therefore be considered a valve assembly or flow controller.

An oxygen sensor assembly <NUM> is provided for measuring the level of oxygen concentration in the mixing chamber <NUM> and/or flowing to the outlet port <NUM>. This information can be used, possibly along with an oxygen saturation (SpO<NUM>) reading taken from a patient, to automatically calculate the required concentration of oxygen to be delivered to the patient using a suitable algorithm and feedback control if required. The gas blender <NUM> can then respond to this automatically and independently adjust the position of each motor <NUM>, and therefore its associated valve pin <NUM>, to adjust both oxygen and air flow to a desired flow rate and oxygen concentration. The required gas mixture then flows out of the outlet port <NUM> at the desired flow rate and oxygen concentration. A flow sensor assembly <NUM> is also provided downstream of the mixing chamber <NUM>.

Alternatively, or additionally, a clinician can adjust the oxygen concentration and flow manually based on the reading from the oxygen sensor <NUM> displayed on an LCD screen or similar display provided on the housing <NUM>.

Using stepper motors <NUM> to actuate the valve pins <NUM> helps to minimise power drain during use. The motors <NUM> need only be activated when the valves need adjusting, so there is no power drain during steady flow situations. This also means that the blender <NUM> can continue to function at steady state even if a power supply is interrupted. A battery may be provided to maintain power to various parts of the unit during a power interruption, and to allow some movement of the stepper motors <NUM> if required to modify the gas flow.

Additional control accuracy and precision may also be provided by modulating between steps in the stepper motors <NUM>, either by microstepping or a slow (<NUM>-<NUM> second) pulse width modulation between two steps. Microstepping involves sending instructions to switch between the fixed steps of a stepper motor faster than the motor can respond, which results in the motor 'hovering' in a position between fixed step points. A similar result can be achieved if the motor is told to repeatedly spend <NUM> seconds at one step and <NUM> seconds at the next step to produce a slow modulation and, in effect, a mid-position between the two steps.

In either case, appropriate control can effectively provide additional stop positions for the valve between the steps defined by the stepper motors <NUM>, thus improving precision. This could be beneficial, for example, if a control algorithm outputs required concentration changes of around <NUM>%-<NUM> % when the stepper motors <NUM> are only designed to achieve <NUM>% steps. Although this level of precision will rarely be required in practice, the same theory could be used to generally permit the use of less precise stepper motors than would otherwise be necessary, thus reducing costs.

<FIG> schematically shows a valve assembly from <FIG> in isolation. The valve is shown in a closed configuration with the valve pin <NUM> advanced to a sealing position where an outer surface <NUM> of the valve pin <NUM> forms a seal with the sealing O-ring <NUM>. It will be understood that the valve arrangement shown in <FIG>, ie the valve housing <NUM>, stepper motor <NUM>, valve pin <NUM> and sealing element <NUM>, could easily be incorporated into alternative flow controllers such as a flow meter if desired.

<FIG> shows a side view of a valve pin <NUM> from the gas blender <NUM> of <FIG>. The valve pin <NUM> is generally 'bullet shaped', with a generally constant diameter stem portion <NUM> and a non-linearly tapering point <NUM> at a first end. A larger diameter base <NUM> is provided at a second end of the valve pin <NUM>, opposite the tapered first end <NUM>, for attachment to a linear motor <NUM>.

In the illustrated example, the stem portion <NUM> extends over a length L1 of around <NUM> from the end of a short transition from the larger diameter base <NUM>, and has an outer diameter D1 of <NUM>. The stem portion <NUM> adjacent the base <NUM> includes a sealing position, at which the valve pin may form a complete gas-tight seal with an external sealing element such as an o-ring or similar. The tapering point <NUM> has a non-linear taper from the other end of the generally constant diameter stem portion <NUM> down to an end diameter D2 of <NUM>. The overall length L of the valve pin <NUM> from the larger diameter base <NUM> to the end of the tapered point <NUM> is around <NUM>, including the <NUM> length L2 of the transition, and the thickness T of the base <NUM> is around <NUM>.

The use of valve pins <NUM> in the gas blender results in a simpler and more cost-effective valve assembly than using proportional solenoid valves or similar, and also allows the use of stepper motors for control, which provides further benefits as outlined above. However, problems were encountered when trying to maintain the required accuracy and precision that is provided by more complex ventilator based systems, particularly when operating at relatively high inlet pressures and low flow rates.

The bullet shape of the pin <NUM> helps to linearize the valve response, but problems still arise when operating at high pressures. In particular, a small change in the motor position when the pin <NUM> is close to the O-ring <NUM> can change the cross-sectional area of the valve dramatically, leading to a loss of control.

Tapered valve pins and linear motors are most commonly used for opening and closing valves in low pressure LPG systems, and a tapered pin tends to provide adequate flow control during normal use at low pressures. However, medical gas flow controllers such as the gas blender of <FIG> are required to provide precise flow control at all operating pressures up to at least <NUM> bar (87psi, 600kPa), and testing found that the required precision/fine control was not obtainable during testing with a standard tapered valve pin. The problem could be addressed by including a regulator upstream of the control valves, but this adds complexity to the system and is not desirable.

<FIG> shows an end view of the valve pin <NUM> from the end of the tapering point <NUM>. To overcome the problems described above, a slit <NUM> or channel is provided in the side of the valve pin <NUM>. The slit <NUM> has a curved floor <NUM> within the valve pin <NUM> and generally parallel spaced sides providing a width W of <NUM>.

A cross-section of the valve pin <NUM> through the slit <NUM>, as indicated by arrows A-A, is shown in <FIG>. The slit <NUM> can be seen to vary in depth along the length of the valve pin <NUM>. At the end of the tapering point <NUM> the curved floor <NUM> of the slit <NUM>, which has a height H of around <NUM>, extends just beyond the central axis <NUM> of the valve pin <NUM>. As illustrated, the depth of the slit <NUM> decreases linearly along the length of the valve pin <NUM> towards the base <NUM>, terminating part way along the constant diameter stem portion <NUM> at a distance L3 of around <NUM> from the base <NUM>. Also shown in <FIG> is a central hole <NUM> in the base portion <NUM> for mounting the valve pin <NUM> to a motor <NUM>.

The inclusion of the slit <NUM> in the side of the valve pin <NUM>, and its varying depth along the length of the pin <NUM>, allows greater control at high operating pressures. The slit <NUM> provides a path through the valve at a near-sealed position, and provides a larger cross-sectional area for the valve than would otherwise be the case when the pin <NUM> is close to the O-ring <NUM>. At lower pressures and/or at higher flow rates, when the valve is more open, the effect of the slit <NUM> in the pin <NUM> is low, because the cross-sectional area of the valve is already so large.

<FIG> is a graph of motor position vs flow through a valve for three different valve pins. The middle plot <NUM> of the three illustrates the flow when using the slotted valve pin <NUM> as described above. To the right of the graph is a plot <NUM> illustrating the flow through a valve having a similar pin without the slit, while the leftmost plot <NUM> illustrates the performance of a shorter valve pin, specifically a pin with a shorter constant diameter stem portion <NUM>, again without a slit. The bullet-shape of the valve pins provides the linear response seen in plots <NUM> and <NUM>, and for the higher flow rates in plot <NUM>.

At higher flow rates, above around <NUM>lmin-<NUM>, it can be seen that all three valve pins provide very similar performance/response, specifically around <NUM>dl/step. Indeed, as shown the right-hand plot <NUM> has been offset from the Y axis to avoid overlap with the plot <NUM> for the slotted valve pin <NUM>. Below <NUM>lmin-<NUM>, however, the three plots <NUM>,<NUM>,<NUM> differ significantly.

Considering plot <NUM>, for the similar bullet-shaped valve pin without a slit, the horizontal part of the plot indicates that most of the length of the bullet produces zero flow. The plot then abruptly steepens as the tapered part of the bullet is reached, so that a small change in motor position immediately produces a large change in gas flow (around <NUM>dl/step). As a result, a large amount of the motor range is effectively wasted, and the abrupt change then makes it impossible to select a precise low flow.

Shortening the length of the bullet addresses the first of these problems, as can be seen in plot <NUM> where far less of the motor range provides zero flow because the narrowing of the bullet shape would occur at lower opening positions. However, the abrupt change in flow rate caused by the tapered part of the bullet still provides the same difficulties when trying to precisely control low flow rates.

This problem is addressed by the provision of the slit <NUM> in the valve pin <NUM>, as shown in plot <NUM>. The lower part <NUM> of the plot <NUM> shows where the slit <NUM> is active, while the upper, steeper part is when the bullet shape takes over. Unlike the other plots <NUM>,<NUM>, the slit <NUM> provides a relatively gradual increase of flow rate up to about <NUM>lmin-<NUM>, before the same faster response is provided above this. This allows for more precision when low flows are required (more motor movement for a small change in flow), while still allowing for large flows with less precision. The difference can be quantified by comparing the gradients for the two parts of the plot. The lower part <NUM> has a gradient of about <NUM>dl/step, in comparison with around <NUM>dl/step for the upper part. This means that the motor must move approximately seven times (<NUM>/<NUM>) as far to provide the same flow change at low flow rates (< <NUM>lmin-<NUM>) than at higher flow rates, allowing far more precise control in this region.

Another benefit provided by the slit <NUM> is that the valve pin <NUM> becomes less reliant on the physical method of sealing between the casing and the needle/bullet. This physical sealing typically makes the actual opening point very unreliable, which further reduces the precision of typical valve pins at low flow rates.

The plots <NUM>,<NUM>,<NUM> of <FIG> all assume a supply pressure of around <NUM> bar, but flow reduces approximately linearly with pressure, so a different supply pressure would not significantly change the relationships shown.

By way of example, <FIG> shows three overlaid plots showing the relationship between motor position and flow rate for the valve pin <NUM> at three different supply pressures. Plots <NUM> and <NUM> relate to supply pressures of <NUM> bar and <NUM> bar respectively, and both show predictable linear performance, similar to that seen in <FIG>, in both regions of the graph (governed by the slit <NUM> and bullet shape). The third plot <NUM> relates to a lower supply pressure of <NUM> bar. This is below the normal required operating pressure range for gas blender applications (typically <NUM>-<NUM> bar, and rated to operate at up to <NUM> bar before failure), but still shows similar performance trends over the majority of the range shown. Indeed, results in testing found improvements for supply pressures as low as <NUM> bar.

By providing a valve pin <NUM> which provides finer control across a wide range of pressures, the described gas blender no longer needs to include a complex and costly flow regulator. The resulting device is thus simpler and more cost-effective.

Although the described embodiment provides a pin <NUM> with just a single slit <NUM>, a pin may alternatively be provided with multiple slits, possibly of various depths and shapes. It should also be understood that alternative means of controlling or altering the cross-section of the pin could be provided.

As noted, the bullet shape of the valve pin <NUM> is beneficial in linearizing the valve response, but the additional control provided by the slit <NUM> would also be beneficial if applied to alternative valve pin shapes.

An alternative valve pin <NUM> according to the present invention is illustrated in <FIG>. Unlike the valve pin <NUM> already described, the alternative valve pin does not comprise a taper <NUM> on the exterior of the body portion. Instead, the stem portion <NUM> comprises an open end section <NUM> provided by first and second slits or channels 152A,152B which widen and deepen as they extend from the larger diameter base <NUM>, leaving a tapered central part <NUM> of the valve stem <NUM>. This provides the alternative valve pin <NUM> with a 'duckbill' appearance, as can be seen in <FIG>.

A front view of the alternative valve pin <NUM> is shown in <FIG>. The apparent taper at the end of the stem portion <NUM> is a result of the widening and deepening 152A,152B on either side of the valve stem <NUM>. The open end <NUM> can clearly be seen in the cross-sectional view of <FIG>, which is taken at the line B-B in <FIG>. The duckbill shape of the pin <NUM> is also clearly visible, as is a central hole <NUM> provided in the base portion <NUM> for mounting the valve pin <NUM> to a motor <NUM>.

<FIG> shows a side view of the alternative valve pin <NUM>, in which the generally straight sides of the valve stem <NUM> and the flared shape of the first channel 152A can be seen. The alternative valve pin <NUM> was developed, at least in part, to address problems associated with the physical method of sealing between the previously described bullet-shaped valve pin <NUM> and an external o-ring seal. While the bullet-shaped valve pin <NUM> achieve a desirable linear response over a range of flows/pressures, some problems still arose at the point the pin separated from or moved off the o-ring. The generally constant outer diameter of the alternative pin <NUM> helps to keep the stem <NUM> in constant contact with an o-ring or similar seal. Indeed, the majority of the outer surface of the alternative pin <NUM> remains in contact with the o-ring right up to the point that it completely disengages. The variable flow area is provided by the flaring of the slits 152A,152B rather than by an external taper. <FIG> also indicates the location of the cross sectional view shown in <FIG>.

The tapered shape of the central part <NUM> of the valve stem <NUM> can be seen in <FIG>. The curved taper in the illustrated example is provided by a flaring of the first and second channels 152A,152B as they deepen towards the end of the valve pin <NUM>. It should be noted that alternative tapers, such as a straight taper, could be provided if the first and second channels 152A,152B were not flared as shown. The positions and sizes of the first and second channels 152A,152B may be substantially identical. However, in the illustrated example, being part of the present invention, the total length L4 of the first channel 152A is greater than the total length L5 of the second channel 152B, meaning that the first channel 152B starts closer to the base portion <NUM> than the second channel 152B. As a result, when the alternative valve pin <NUM> is moved progressively away from a sealing position (located adjacent the base portion) during use, flow is permitted initially through the first channel 152A only, and then through both the first and second channels 152A,152B, and finally through the open end section <NUM> of the valve pin <NUM>. Various paths are therefore through the alternative valve pin <NUM>, including at a near-sealed position, and as with the valve pin <NUM> previously described this provides greater control and a more linear response at high operating pressures and/or low flow rates. The flaring of the first and second channels 152A,152B, and ultimately the open end section <NUM>, provides a more open profile to the valve at lower pressures and/or at higher flow rates. Although the alternative valve pin <NUM> abruptly disengages from an o-ring/seal, due to its generally constant external diameter, the flared channels 152A,152B and open end section <NUM> mean that the valve is largely open at the point of disengagement. Any effect of the abrupt disengagement is therefore minimal, much in the same way that the influence of the slit <NUM> in the bullet-shaped valve pin <NUM> is lower when the valve is more open, as previously described.

The dimensions of the alternative valve pin <NUM> will typically be similar to the valve <NUM> previously described. For example, the length from the larger diameter base <NUM> to the end of the open end <NUM> will likely be around <NUM>. However, shorter valve pins could be provided, where a comparable dimension may be around <NUM>, either by reducing the overall size or by effectively cutting the open end section <NUM> shorter than shown.

<FIG> shows three overlaid plots showing the relationship between motor position and flow rate for the alternative valve pin <NUM> at three different supply pressures. Plot <NUM> relates to a supply pressure of <NUM> bar, plot <NUM> to a supply pressure of <NUM> bar, and plot <NUM> to supply pressure of <NUM> bar. All three plots <NUM>,<NUM>,<NUM> show comparative performance, illustrating the predictability and suitability of the alternative valve pin <NUM> across the standard operating pressure range of <NUM>-<NUM> bar.

The valve pins <NUM>,<NUM> can be operated using linear stepper motors. This minimises the power drain of the system, because the motors need to be activated only when movement of the valve pin <NUM>,<NUM> is required, and means that the valves will remain in position, maintaining a desired flow rate, if power to the unit is interrupted.

The overall systems incorporating the new valve pins <NUM>,<NUM> provides the required level of precision in a dedicated system which avoids the unnecessary complexity found in many ventilators and removes the requirement for a regulator. The valve pins are described herein provide turndown ratios of <NUM>:<NUM> and <NUM>:<NUM>, while also being relatively simple to manufacture.

<FIG> and <FIG> are flow charts setting out elements of applicable control strategies. The control strategies as shown are iterative in a closed loop.

<FIG> illustrates a suitable control strategy <NUM> for a gas blender.

Control of the gas flow using, for example, a motorised valve pin <NUM>,<NUM> as previously described is done using a software-controlled stepper motor in a closed loop. The output flow is compared with the target flow <NUM> and the difference is used to modify the position of the pin <NUM>, thus changing the output flow.

In use, the gas blender controls two gasses independently to achieve a selected flow and oxygen concentration, with each gas being checked in turn against the target flow for that gas. The control method <NUM> therefore first selects a gas for measurement <NUM> before implementing analysis <NUM> and comparison <NUM> steps and controlling a motor based on the comparison <NUM>.

During development, it was found suitable for the control loop to be executed every <NUM>, to suit the speed of a flow sensor used in a particular medical gas blender. However, it will be understood that different repeat rates/frequencies could be used depending on the needs of a particular flow sensor and/or of a particular flow control device.

A similar control method can also be used to measure and control the flow of a single gas, for example in a medical flowmeter/regulator, as illustrated in <FIG>. The flowchart of <FIG> shows a simplified method <NUM> including the steps of measuring a gas flow <NUM> and comparing with a target value <NUM>. As in the control method <NUM> of <FIG>, a motor is then moved a number of steps based on the comparison <NUM>. Again, this is executed every <NUM>, or at a different frequency if a particular application requires.

Although illustrated as incorporating a Proportional-Integral-Differential (PID) controller, the control strategies <NUM>,<NUM> of <FIG> and <FIG> would still function if only the proportional element were used. The number of steps that the motor should move <NUM>,<NUM> is calculated from a mix of the flow error, multiplied by an empirically determined "Proportional Constant" (Kp). This constant effectively calibrates for the mechanical scaling from the incoming gas, through the motorised pin aperture and out of the device. The addition of an "Integral Constant" (Ki), again calculated empirically, would help to speed up motion toward the target flow.

Claim 1:
A valve (<NUM>,<NUM>,<NUM>) for use in a medical gas flow controller, the valve (<NUM>,<NUM>,<NUM>) comprising:
a valve pin (<NUM>) having an elongate body (<NUM>) with an outer sealing surface and a tapered part (<NUM>) tapering towards a free end of the body (<NUM>), and
a sealing element (<NUM>) surrounding the valve pin (<NUM>) to provide a seal with the outer sealing surface at a defined sealing position,
wherein relative movement of the valve pin (<NUM>) and the sealing element (<NUM>) away from the sealing position provides a variable flow area between the sealing element (<NUM>) and the valve pin (<NUM>), and
wherein the valve pin (<NUM>) is shaped to provide a recess in the outer sealing surface of the body (<NUM>) close to the sealing position to increase the flow area of the valve (<NUM>,<NUM>,<NUM>) close to the sealing position, characterized in that
the recess comprises first and second elongate slits (152A,152B), the length of the first and second slits (152A, 152B) being aligned with the length of the elongate body (<NUM>),
wherein the length of the first elongate slit (152A) is greater than the length of the second elongate slit (152B).