Liquid level sensor

Abstract

A nebuliser cup for maintaining a safe level of liquid during nebulization, the cup comprising a liquid level sensor comprising: a first element sensing liquid at or below a position corresponding to a filled liquid level in the nebuliser cup; a second element electrically isolated from the first element and located at a position corresponding to a filled liquid level in the nebuliser cup; and wherein when the cup is not filled with a liquid to the filled liquid level, the first element, the liquid and the second element do not form an electrically coupled circuit; and when the cup is filled with a liquid to the filled to liquid level, the first element, the liquid and the second element together form an electrically coupled circuit.

Claims

1. A method of maintaining a liquid level in a nebuliser cup in a sterilisation apparatus during operation, the method comprising: providing a first element sensing liquid at or below a position corresponding to a filled liquid level in the nebuliser cup; providing a second element electrically isolated from the first element to contact a liquid at a position corresponding to the filled liquid level in the nebuliser cup; providing sufficient liquid to the nebuliser cup to reach said filled liquid level thereby to complete an electrically coupled circuit between the first element, the liquid and the second element; and whereby a drop in the liquid level to below the filled liquid level triggers a break in the circuit between the first element, the liquid and the second element; and whereby the break in the circuit triggers dispensation of further liquid in the nebuliser cup, wherein the sterilisation apparatus comprises a sterilisation chamber, an inlet valve an outlet valve, an ultrasonic nebuliser in fluid communication with the chamber via the inlet valve, and a fan upstream of and in fluid communication with the ultrasonic nebuliser during operation, and wherein the nebuliser cup comprises a base, an upwardly extending wall, and an open top, whereby an aerosol can be generated directly from the liquid in the nebuliser cup wherein the open top is free of a cover, wherein the cover would prevent flow of the aerosol out of the open top; and wherein nebulization takes place on an intermittent basis and wherein sensing of the liquid levels is conducted when nebulization is off.

2. A method according to claim 1 wherein sufficient further liquid is dispensed to the nebuliser cup in response to the break in the circuit to restore the circuit and halt further dispensation of liquid from a reservoir.

3. A method according to claim 2 wherein the break in the circuit triggers dispensation of further liquid from the reservoir immediately.

4. A method according to claim 2 wherein the break in the circuit triggers dispensation of further liquid from the reservoir after a predetermined time.

5. A method according to claim 1 wherein the liquid used for sterilisation comprises a hydrogen peroxide solution or peracetic acid solution.

6. A method according to claim 1 wherein the first element at or below the filled liquid level is configured to maintain at least 5 mm depth of liquid in the nebuliser cup.

7. A method according to claim 1 wherein the first element at or below the filled liquid is configured to switch off a transducer when a volume of liquid in the nebuliser cup falls to about at least 1.5 cm.sup.3.

8. A method according to claim 1 wherein the nebuliser cup is in fluid communication with a sterilisation chamber.

9. A method according to claim 1 wherein the first element is a non-conducting cup incorporating an electrically conductive element not in direct contact with the liquid.

10. A method according to claim 1 wherein sensing takes place before nebulisation commences.

11. A method according to claim 1 wherein sensing takes place between nebulisation intervals.

12. A method according to claim 1 wherein, if insufficient levels are detected, the nebuliser cup is refilled before nebulisation recommences.

13. A method according to claim 1 wherein the nebuliser cup is in operative contact with an ultrasonic transducer.

14. A method according to claim 1 wherein the nebuliser cup has an ultrasonic transducer at the base.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of the nebuliser cup of the present invention.

(2) FIGS. 2 and 3 show a preferred nebuliser cup of the present invention.

(3) FIG. 4 is an assembly view of a preferred nebuliser cup of the present invention in its operating environment.

(4) FIGS. 5a and 5b show alternative embodiments of the invention

(5) FIG. 6 shows yet another embodiment of the invention.

DESCRIPTION OF THE INVENTION

(6) The present invention provides a nebuliser cup which allows for detection of liquid level in the cup whilst being of sufficiently simple and robust design to withstand harsh environments such as may be found within ultrasonic sterilizing apparatus.

(7) The general principle of the device is that at least one of the sensor elements is positioned at an upper fill level position and the other at or below the fill position, which may be called a lower position (although both sensors may be at the fill position, as long as isolated from each other when liquid is absent). When the cup is filled with liquid, the upper sensor element, the lower sensor element and the liquid form a circuit. When the liquid is consumed, the upper level of the liquid drops below the upper sensor, breaking the circuit, which in turn triggers the dispensation of additional liquid to refill the cup.

(8) The sensors can either the same or different, and can be either capacitive or conductive. The options are shown below in Table 1.

(9) TABLE-US-00001 TABLE 1 Option Sensor 1 (at or below fill level) Sensor 2 (at fill level) 1 Direct electrical contact Direct electrical contact with fluid with fluid 2 Capacitive contact with fluid Direct electrical contact with fluid 3 Direct electrical contact Capacitive contact with fluid with fluid 4 Capacitive contact with fluid Capacitive contact with fluid

(10) The apparatus is described with respect to the drawings, in which the sensors are both conductive, and in which one of the sensors, that at or below fill level, is the cup itself.

(11) Nebuliser cup 1 is in operative engagement with ultrasonic transducer 2. The transducer is in contact with the base 3 of the cup, 3. Operation of transducer 2 serves to ultrasonicate liquid, where present, in cup 1. The liquid in the cup, typically containing aqueous hydrogen peroxide, is advantageously filled at least to fill level 4. As transducer 2 operates and the liquid is ultrasonicated and converted to nebulant, which is dispersed, the upper level of the liquid drops. As the level of the liquid drops, the nebulization process can become sub optimal, and on approaching very low liquid levels, the liquid may be unable to disperse the ultrasonic energy, and the transducer 2, can begin to heat, with possibly destructive consequences.

(12) The present invention includes a sensor 5 which is positioned at the fill level 4. The fill level can be chosen to achieve the optimal liquid level height given the other operating parameters of the device. In this case, the positioning is achieved by way of an aperture 6 in the cup 1, into which is placed sensor 5 surrounded by an isolating sleeve 7. The isolating sleeve may be of any suitable material, for example, chemically resistant PVDF (polyvinylidene fluoride), PEEK, PTFE etc.

(13) Ideally, the sensor at or below fill level is configured to switch on the pump or open the valve so as to introduce more liquid into the nebuliser cup (or switch off the transducer, if the reservoir is empty) when the level of liquid in the nebuliser cup falls to a predetermined level. In preferred embodiments, that predetermined level is at least 5 mm from the base of the cup or surface of the transducer. That, is, the lower liquid level sensor is positioned at 5 mm or greater from the base of the cup so that the cup will always retain a minimum of 5 mm depth in the base of the cup to ensure that there is sufficient liquid present to absorb ultrasonic energy and prevent the transducer from destructive delamination.

(14) The typical diameter of the nebuliser cup, based upon the sizing of most ultrasonic transducers, is at least around 20 mm, so the fill sensors are typically positioned such that the liquid does not fall below a minimum volume of 1.5 cm.sup.3 of liquid.

(15) Whilst the main purpose of the present invention is to protect the nebuliser from running dry, the nebuliser cup of the present invention also allows the setting of upper and lower sensor levels to determine optimum performance of the nebuliser. The ultrasonic transducer focuses energy into the cup, causing a mist of droplets to arise. In some cases, the mist and droplet profile can vary depending upon whether the mist is generated from a full cup or from a cup with a lower liquid level. The upper and lower sensor levels can be determined, based on the transducer and cup design, so as to bracket the optimum liquid level. This means that in use, the nebuliser cup of the present invention can provide a mist that it consistently around the optimum achievable.

(16) The sensor is an AC excited coupled liquid level sensor. The excitation signal may be of any suitable form, for example, a 5 Vp-p 6 kHz square wave and is coupled via a capacitor to the sensor electrode. This ensures that there is no DC potential on the sensor electrode; avoiding any problems with electrolysis of the liquid.

(17) When the liquid level is at the fill level 4, it contacts the sensor 5. The sensor has an aluminium, or stainless steel, sensor pin 5a which contacts the liquid where present. There is a drop in the electrode signal level that occurs when the H.sub.2O.sub.2 level in the cup is high enough to contact the electrode. This shunts some of the signal to earth via the conductive path formed between the sensor and the earthed metallic cup.

(18) When the apparatus is in use, the liquid in the cup is consumed as a result of the nebulisation process, and the liquid level falls. At some time, the liquid level will drop to a sub-fill level 8. At this level, pin 5a will no longer contact the liquid and sensor 5 will stop sensing it, and that information will be fed via pathway 9 into controller 10.

(19) If controller 10 detects a low electrode signal below a predetermined threshold, then it infers sufficient liquid is present in the cup. If controller 10 detects a high electrode signal, above a predetermined threshold, then it infers insufficient liquid is present in the cup.

(20) In the case of insufficient liquid level detected by sensor 5, controller 10 triggers the opening of valve 11 which permits additional liquid to flow from reservoir 12 into cup 1, either under gravity or by pumping. Alternatively, a peristaltic pump could be used at 11 to close reservoir 12 when not pumping.

(21) Controller 10 can be configured to trigger the release of a liquid either immediately upon detection of a fall in liquid level, or at a predetermined time after a fall in liquid level is detected, usually a few seconds, during which time the liquid level will fall below the sub-fill level 8, but not so much as to reach the base of the cup. In this way a safe and operationally useful liquid level is maintained. Importantly, the sensor is sized and positioned such that liquid droplets such as nebulant particles or particles of condensate are unable to complete the circuit between the sensor the conducting cup, thereby generating a false signal that would result in destruction of the apparatus. Additionally, the positioning of the sensor and the triggering of additional liquid can be configured to take into account meniscus formation. One such example would be an insulating sheath around the sensor to prevent electrical contact between the cup and the sensor. The insulating sheath preferably provides around a 1 mm or greater gap between the sensor and the conducting cup.

(22) Because the switching is qualitative (on-off) rather than quantitative, it is more robust and less prone to error or the need for recalibration. Also, because of the large difference between on and off voltages, the system can readily operate over a large range of sterilization liquids without recalibration or standardization.

(23) Additionally, the cup can be used in apparatus where the nebulization and dispensation of liquid to the cup (refilling) occur either simultaneously or sequentially. For example, in larger apparatus, nebulization may take place continuously, and filling would then take place as needed while nebulization was on going, so as to maintain the predetermined liquid level. However, in other embodiments, nebulization takes place on an intermittent basis, and it may in those cases be advantageous to conduct sensing of the liquid levels whilst nebulization is off. For example, sensing can take place before nebulization commences, or between nebulization intervals. In that case, if insufficient levels are detected, the nebulizer cup is refilled before nebulization recommences.

(24) It may be desirable in some cases to overfill the nebulizer above the fill level. In that case, a further overfill sensor at a higher level can be used to detect an overfill condition. Alternatively, the overfill level could be determined by the refilling liquid volumetric flow rate and a time delay between the detection of liquid and the cessation of refilling.

(25) The lower level may also be selected so that it can accommodate a further predetermined time interval prior to switching off the transducer or introducing more liquid.

(26) The cup may be made from any type of conducting material, for example, a metal such as aluminium or alternatively, the cup maybe made from a conducting plastic. In an alternative embodiment, the cup is not made from a conducting material, but instead has an appropriate conductive portion located inside the cup at fill level.

(27) A further embodiment of the device is shown in FIGS. 2 and 3. Cup 1 can be tapered towards the bottom if desired. The reservoir in this apparatus can be seen to fill via port 13 which is at the bottom of the reservoir. FIG. 4 is an assembly view.

(28) FIGS. 5a and 5b show alternative embodiments of the invention. In this case, the cup 1 is not conductive, but rather the circuit is completed by way of a sensor element (e.g. an electrode of some type) which completes the circuit only when the liquid is at a fill level 4.

(29) A further embodiment is shown in FIG. 6. In this case the liquid level sensors 5 are located away from the cup body 1 but are in fluid communication with the liquid in the cup and are located at a level corresponding to the fill level 4 of the cup.

(30) When the apparatus is in use, the liquid in the cup is consumed as a result of the nebulisation process, and the liquid level falls. At some time, the liquid level will drop to a sub-fill level 8. At this level, liquid no longer remains in exit pipe 20, which has sensors 5 located along it at level corresponding to fill level 4. The sensors 5 fail to sense the presence of liquid, leading to an increase in signal, controller 10. The sensors then sense the presence of liquid in the exit pipe which in turn leads controller 10 to infer there is insufficient liquid is present in the cup. Controller 10 then triggers the opening of valve 11 which permits additional liquid to flow from reservoir 12 into cup 1, either under gravity or by pumping. Alternatively, a peristaltic pump could be used at 11 to close reservoir 12 when not pumping.

(31) Controller 10 can be configured to trigger the release of a liquid either immediately upon detection of a fall in liquid level, or at a predetermined time after a fall in liquid level is detected, usually a few seconds, during which time the liquid level will fall below the sub-fill level 8, but not so much as to reach the base of the cup. In this way a safe and operationally useful liquid level is maintained.

(32) Additionally, as mentioned, it is not necessary that either or both of the sensors are conductive. Either or both of the sensors may be capacitive. A capacitive sensor has plates which are not in direct conduct with the liquid, but is physically separated by way of a barrier layer but is nevertheless electrically coupled to it. Typically, a capacitive sensor would be present on the outside of the cup, or placed inside the cup wall material, which forms a barrier layer. When the liquid comes into contact with the barrier layer adjacent the capacitive sensor, an AC current is able to flow if the circuit is otherwise complete (e.g. by way of another sensor element either directly in contact with the liquid or in capacitive contact with the fluid).

(33) In the case of direct sensors, the current can be either AC or DC, although DC is not preferred as it can lead to electrolysis of the liquid. In the case of capacitive sensors, or mixed capacitive/conductive sensors, AC is required.