GB2571999A - Dry wick indicator - Google Patents

Abstract

A dry wick indicator for a vaping device comprises a detecting means for detecting a decomposition product from the vaping device, and means for producing a signal indicating a dry wick condition based on an output of the detecting means. The indicator may further comprise a discriminator 36 for comparing an output of the detector with a reference, wherein the signal is output based on a result of the comparison. The decomposition product may be formed by pyrolysis, thermal degradation or combustion of an e-liquid base, may not be present in an e-liquid from which vapour is produced, and may be selected from carbon monoxide, carbon dioxide, formaldehyde, acetone, acrolein and/or acetaldehyde. The detecting means may use non-dispersive infrared spectroscopy, Fourier transform infrared spectroscopy, colorimetric analysis and/or reversible oxidation, and/or may comprise an electromechanical cell 28 having a working electrode, a reference electrode and a counter electrode in contact with an electrolyte. A testing apparatus with means 22 for puffing the vaping device to produce a vapour is also provided, wherein the apparatus may comprise an exposure chamber 26 in which the detecting means is at least partially located.

Description

The present invention relates to a dry wick indicator for indicating a dry wick condition in a vaping device. Some embodiments of the invention relate to a vaping machine comprising such a dry wick indicator. Other embodiments of the invention relate to a vaping device comprising a dry wick indicator.

Vaping devices, also known as Electronic Nicotine Delivery Systems (ENDS), electronic cigarettes or e-cigarettes, have been developed mainly as a response to the health risks associated with smoking tobacco. Vaping devices work by vaporising a liquid which is then inhaled by the user. Vaping devices typically comprise a mouthpiece, a tank, an atomizer, a control unit, and a battery. The liquid to be vaporised (the “e-liquid” or “e-juice”) is contained in the tank. The tank may be replaceable or refillable. The atomizer comprises a wicking material that draws the e-liquid onto a heating element. When the user pushes a button or activates a pressure sensor by inhaling, the heating element atomises the e-liquid to create an aerosolized vapour which is inhaled by the user.

E-liquids typically comprise a base (or carrier) of propylene glycol (PG) and/or vegetable glycerine (VG), together with nicotine and flavourings. The base holds the nicotine and flavour in suspension/solution so the vaporizer can produce clouds of smoke-like vapour.

With the number of e-cigarette users increasing, there is an increasing need to analyse the vapour produced by vaping devices for regulatory and quality control purposes. Analytical vaping machines have therefore been developed for such studies. Vaping machines typically puff or “vape” one or more e-cigarettes in order to produce a vapour stream which can then be analysed.

It is known that, at the end of life of some ENDS products, the composition of the vapour changes as the wick dries and becomes directly heated. Dry puffing produces an acrid taste and has the potential to expose the user to thermal degradation products. Some of these breakdown products can be injurious to health. For example, propylene glycol can degrade into formaldehyde as part of an incomplete combustion reaction. It is thermal (heat-related) degradation, so the temperatures involved are central to the amount of formaldehyde that comes from PG. Vegetable glycerine can degrade into the toxin acrolein, in a similar way. The temperature of the heating coil, surface area of the heating element, material of the coil, ingredients of the e-juice, material of the wick and duration of the puff all have an influence on the quantity of toxin produced during dry puffing.

Some e-cigarettes avoid these problems by having safety cut outs. For example, some disposable devices have excess e-juice so that the battery is exhausted before dry wicking occurs, or in some closed tank systems the number of puffs from a tank are counted and the device ceases to operate with that tank whilst there is still excess e-juice present. Open systems, where the e-juice is replaced by the user from a bottle, do not have such simple safety measures and this becomes more acute as the diversity of the e-cigarettes increases, particularly in the case of “mods” where the user can control the heating power of the coil which in turn impacts the quantity of vapour formed in each puff and the speed at which the reservoir is depleted.

It is generally assumed that a consumer would be deterred from this “dry wick” condition and cease puffing as the taste is poor. However, when studying ecigarettes on analytical vaping machines there is no such indication of dry wicking, and vaping may continue past the point of end of life. This may be accompanied by the formation of unwanted toxins. Not realising that a dry wicking situation has occurred and that this would be rejected by a consumer as unsatisfactory, analysis of the gases could be made and incorrect levels of toxicity that a user could be exposed to could be ascribed to e-cigarettes. This may incorrectly magnify the risks of e-cigarettes.

According to one aspect of the present invention there is provided a dry wick indicator for indicating a dry wick condition in a vaping device, the dry wick indicator comprising detecting means for detecting a decomposition product from the vaping device, and means for producing a signal indicating a dry wick condition based on an output of the detecting means.

The present invention may provide the advantage that, by detecting a decomposition product from the vaping device, it may be possible to indicate that a dry wick condition has occurred. This may be an indication that the vaping device is at end-of-life or requires replenishment of e-juice or replacement of the wicking material and coil. Appropriate action may then be taken, such as ceasing vaping or producing an alarm signal. In this way, it may be possible to avoid the production of toxins in the vapour.

The vaping device may be any type of device designed to vaporise an e-liquid, such as an e-cigarette, an electronic pipe, an electronic hookah, or any similar type of device.

Preferably the decomposition product is in the gas or vapour phase, which may facilitate detection. Preferably the detection takes place in real time.

Preferably the dry wick condition is a condition in which a wick of the vaping device (which is used to vaporise the e-liquid) is not fully saturated with e-liquid yet continues to be heated. This may result in pyrolysis, thermal degradation or combustion of the e-liquid. A dry wick condition will typically occur when the vaping device is at end-of-life (as the e-liquid is depleted). However, a dry wick condition may also occur in other circumstances such as due to a fault or blockage in the vaping device.

Preferably the decomposition product is a product formed by pyrolysis, thermal degradation, pyrolysis or combustion of an e-liquid base (such as propylene glycol and/or vegetable glycerine) or the wick material. This may allow a dry wick condition to be detected by the emergence of such a product in the vapour during dry wicking.

The decomposition product of interest is preferably a product that is that is not normally present in an e-liquid from which the vapour is produced. This may allow a dry wick condition to be detected by the emergence of a product in the vapour that is not present during normal vaping, which may be used to prevent false alarms.

Preferably the decomposition product is selected from at least one of: carbon monoxide; carbon dioxide; formaldehyde; acetone; acrolein; and acetaldehyde.

Each of these products has been found to be indicative of end-of-life of a vaping device.

If desired, the detecting means may be arranged to detect two or more decomposition products in the vapour. In this case the signal indicating a dry wick condition may be based on detection of each or both of the two or more decomposition products, for example, exceeding a pre-determined threshold.

It has been found pursuant to the present invention that production of carbon monoxide is associated with dry wicking of a vaping device. Furthermore, carbon monoxide is not generated during usual use, and is typically formed when burning, charring or thermally degrading the contents of a vaping device.

Therefore, in a preferred embodiment of the present invention, the decomposition product is selected to be carbon monoxide, this being relatively simple to detect as well as being a risk factor in COPD. However, if desired, other decomposition products could be used as well as or instead of carbon monoxide.

The detecting means may be arranged to output a signal indicating a concentration of the decomposition product in the vapour which may in turn be related to a threshold and alarm condition.

The detecting means may use any suitable detection technique for detecting the decomposition product with preferably an output proportional to the amount of the decomposition product in the vapour or gas phase. For example, the detecting means may use a detection technique selected from at least one of: nondispersive infrared spectroscopy; Fourier Transform infrared spectroscopy; colorimetric analysis; and reversible oxidation. If desired, two or more detection techniques may be used.

In a preferred embodiment, the detecting means comprises an electrochemical cell. Such devices may provide the advantage of being readily available, relatively inexpensive, have a high speed of response, have high selectivity, have high sensitivity, consume little power, have high accuracy and/or having a long lifetime. However other devices such as a semiconductor MOS could be used instead or as well.

The electrochemical cell may comprise a working electrode, a reference electrode and a counter electrode in contact with an electrolyte. The electrochemical cell may be arranged to produce a signal current that is related to the amount of carbon monoxide to which the cell is exposed. The dry wick indicator may comprise a control circuit for applying a bias potential to the electrochemical cell and/or for converting the signal current to an output voltage. This may provide a readily available and relatively inexpensive way of detecting a decomposition product such as carbon monoxide.

The detecting means may be at least partially located in an exposure chamber. For example, in the case of an electrochemical cell, at least the working electrode may be located in the exposure chamber. The exposure chamber may expose at least part of the detecting means to the vapour.

The dry wick indicator may further comprise a hydrophobic barrier between the vapour and the detecting means. The hydrophobic barrier is gas permeable, in order to allow the (gaseous) decomposition product to pass through to the detecting means. For example, the hydrophobic barrier may be a waterproof, breathable fabric membrane such as Gore-Tex (RTM). The hydrophobic barrier may help to prevent degradation of the detecting means through exposure to condensed vapours.

The dry wick indicator may further comprise means for comparing an output of the detecting means with a reference and producing a signal indicating a dry wick condition in dependence on a result of the comparison. For example, the signal indicating a dry wick condition may be produced when an output of the detecting means exceeds a threshold value. Alternatively, a number of thresholds could be set, and/or the output of the detecting means could be compared to a predetermined pattern which is indicative of a dry wick condition.

According to another aspect of the present invention there is provided a dry wick indicator arranged to indicate a dry wick condition in a vaping device, the dry wick indicator comprising a detector arranged to detect a decomposition product in vapour from the vaping device, and a discriminator arranged to compare an output of the detector with a reference and to output a signal indicating a dry wick condition in dependence on a result of the comparison.

In one embodiment of the invention, the dry wick indicator is used in a testing apparatus such as a vaping machine in order to provide an indication that the product under test is at end-of-life. Thus, according to another aspect of the invention there is provided a testing apparatus for testing a vaping device, the testing apparatus comprising means for puffing the vaping device to produce a vapour, and a dry wick indicator in any of the forms described above.

Preferably the apparatus is arranged to cease puffing the vaping device in dependence on the signal indicating a dry wick condition. Thus, in this embodiment, it may be possible for the testing apparatus to cease testing of the vaping device when the vaping device is at end-of-life. This may help to avoid the risks of the vaping device being incorrectly magnified. Alternatively or in addition, other action such as the production of an alarm signal could be taken instead or as well.

Preferably the apparatus comprises a control unit for controlling the puffing means, and the control unit is arranged to receive the signal indicating a dry wick condition, and to cease puffing of the vaping device in dependence thereon.

Preferably the testing apparatus comprises an exposure chamber in a fluid flow path between the vaping device and the puffing means, and the detecting means is at least partially located in the exposure chamber.

Optionally the testing apparatus may comprise a mechanism for flushing the exposure chamber with clean air. This may be activated after exposure to the vapour from the e-cigarette, in order to prevent over exposure of the detecting means to excessive levels of the gas to be detected.

In another embodiment of the invention, the dry wick indicator is incorporated into a vaping device. Thus, according to another aspect of the invention, there is provided a vaping device comprising a dry wick indicator in any of the forms described above.

The vaping device may be arranged to prevent vaping in dependence on the signal indicating a dry wick condition. This may help to prevent harmful products from being inhaled by the consumer.

Preferably the vaping device comprises a heating element for heating an e-liquid and a control unit for controlling the heating element, and the control unit is arranged to receive the signal indicating a dry wick condition, and to cease heating of the heating element in dependence thereon.

Preferably the vaping device comprises an exposure chamber in a fluid flow path for the vapour, and the detecting means is at least partially located in the exposure chamber. The exposure chamber may be formed, for example, by a housing and/or a mouthpiece of the vaping device.

Corresponding methods may also be provided. Thus, according to another aspect of the present invention there is provided a method of indicating a dry wick condition in a vaping device, the method comprising detecting a decomposition product in vapour from the vaping device, and outputting a signal indicating a dry wick condition in dependence thereon.

Features of one aspect of the invention may be provided with any other aspect. Apparatus features may be provided with method aspects and vice versa.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an example of a typical e-cigarette testing instrument;

Figure 2 shows the percentage change of various products in the vapour at end-of-life for a typical vaping device that exhibits a dry wicking condition;

Figure 3 shows concentrations of various vapour/gas phase components in the vapour during an example test routine where a dry wick condition is reached approximately 18 minutes into the test run;

Figure 4 shows parts of a vaping machine with a dry wick indicator in one embodiment of the invention;

Figure 5 shows parts of an exposure chamber and electrochemical cell in another embodiment;

Figure 6 is a flow chart showing steps taken by a control unit when preforming a test routine in an embodiment of the invention;

Figure 7 is a simplified diagram of an electrochemical cell and its control circuit;

Figure 8 shows parts of a vaping device in another embodiment of the invention; and

Figure 9 shows parts of the vaping device used to control operation of the heating element.

Vaping machines

Figure 1 shows an example of a typical e-cigarette testing instrument. In this example the testing instrument 10 comprises eight ports for testing between one and eight e-cigarettes. A puffing mechanism is provided for drawing air through the e-cigarettes. The instrument may offer the user the ability to program a puffing routine for establishing delivery from various e-cigarette devices. For example, the user may be able to select puff shape, volume, duration, interval and number of puffs taken, allowing capture on a filter pad or by impingers for offline chemical analysis.

In this example the testing instrument is a stand-alone apparatus that includes a touch-screen display which can be used for entry of test parameters and display of results. An in-built processor running the appropriate software controls the puffing routine, and provides an output interface to a printer, a Laboratory Information Management System (LIMS) and an internal database.

The output from each sample is passed through a CFH (Cambridge filter holder) fitted with a glass fibre filter pad which captures particulate and aerosol matter, which is removed after each test for weighing and then sent for subsequent chemical analysis. The before and after weights can be entered to establish delivery mass. The results are then stored in a database for later evaluation or printing. Alternatively the vapour or gas phase elements of the aerosol can be passed through a set of impingers (Dreschler tubes) containing a liquid medium that is used to capture volatile gas or vapour phase components of the aerosol such as formaldehyde. The liquid can be removed at the end of a run and then analysed. The software allows the operator to setup libraries and test regimes.

When studying e-cigarettes on known analytical vaping machines there is no indication of dry wicking and vaping may continue past the point of end of life. This may be accompanied by the formation of unwanted toxins. Not realising that dry wicking has occurred, analysis of the gases could be made, and higher levels of toxicity could be ascribed to e-cigarettes. This could incorrectly magnify the risks of e-cigarettes.

Figure 2 shows the percentage change of various toxic products found in the vapour at end-of-life, where dry wicking has occurred, for a typical vaping device and e-liquid. Some of these breakdown products can be injurious to health. Notable amongst these toxins is formaldehyde.

End-of-life indicator

The principle behind the present invention is to detect a change in chemical composition of the vapour that is associated with pyrolysis I burning I charring of the e-juice and potentially the wicking material. This change is best detected by the emergence of a chemical in the vapour stream at the point of dry wicking that is absent, or at very low concentrations, at other times during normal vaping. This would give good discrimination for the onset of dry wicking. If there is a suitably sensitive detection method and there is a sufficiently large change in chemical composition then the indication system would be very sensitive to the onset of dry wicking.

Another requirement would be that the change in chemical composition and detection system is immune to cross interference i.e. the system is selective for the target chemical change alone. For instance, if a slight change in nicotine or propylene glycol in the vapour is characteristic of dry wicking, this would be difficult to use as an end-of-life detector as a slight change on top of a high background is difficult to discriminate from random noise.

Alternatively, if formaldehyde is chosen as a dry wicking detection chemical, the low concentrations formed in the vapour stream makes this equally difficult to use because of the limited sensitivity of most detection techniques. Also, contamination of the e-juice could be a source of formaldehyde making the detector unreliable.

The present invention uses the detection of a decomposition product, that is, something that is not present in the e-juice and is formed in the gas or vapour phase by pyrolysis, thermal degradation or combustion. In this way it is highly specific for this dry wicking condition.

Tests carried out by the present inventors involved coupling an FTIR (Fourier Transfer Infra-Red) analyser directly to the exhaust of a vaping machine so as to analyse whole vapour. This provided concentrations of selected vapour components in real time, reporting concentrations as parts per million (ppm) which can be converted to mg or other suitable units.

The behaviour at end-of-life of a simple refillable tank ENDS device was examined by filling and emptying the e-liquid tank and then puffing beyond the normal condition that would indicate end of life. Simultaneous measurement of a number of vapour/gas phase components was made using FTIR. It was noted that one indicator of end-of-life was the production of carbon monoxide (CO) and/or carbon dioxide (CO₂).

Figure 3 is a chart showing concentrations of various vapour/gas phase components in the vapour during an example test routine. In this test, 55ml square wave puffs were taken, of three seconds duration, with 30 second intervals between puffs, and 50 puffs in total.

It can be seen from Figure 3 that significant quantities of CO are produced at the end-of-life of the vaping device (as the e-liquid tank empties). That this is the end-of-life is confirmed by the production of toxins such as formaldehyde at the same point in the puffing cycle.

It was therefore noted that the dry wicking conditions that result in the thermal decomposition of the vegetable glycerine and/or propylene glycol that form the basis of the carrier liquid in e-cigarettes are accompanied by elevated levels of carbon monoxide. Furthermore, unlike in conventional cigarettes, carbon monoxide is not generated during usual use, as it is only formed when burning, charring or thermally degrading the contents of an e-cigarette.

It was therefore concluded that the onset of carbon monoxide production in an ecigarette is an indication that the wick is dry. Furthermore, if the onset of carbon monoxide production can be measured then this becomes a predictor for dry wicking conditions and vaping can be terminated before harmful toxins are produced.

In order to use CO as an end-of-life indicator, an analyser for carbon monoxide (or detector) is needed that is preferably reversible in nature (recovers from exposure to be reused), has a fast reaction time (15 seconds maximum to match the most intensive cycles of puffing), high sensitivity and discrimination (threshold detection of carbon monoxide should be in the region of 50ppm whilst recognising there may be an inherent CO floor in ambient conditions of ~2-4ppm). The analyser or detector needs to be capable of raising an alarm that can be integrated into the puffing machine and stop further puffs. The detector or analyser should be irresponsive to the normal constituents of the vapour stream (free from or is resistant to cross interferences). The detector or analyser should also not become contaminated with condensed e-cigarette aerosol. Finally the detector or analyser should not be poisoned by the vapour stream in normal use (although a clean air flushing mechanism could alleviate this problem) and should be capable of periodic calibration. Low cost of the device is also advantageous for routine test applications.

There are a number of detectors I analysers based on the electrochemical principle that meet these requirements, as do a number of detectors based on tin oxide, FTIR, NDIR etc. Electrochemical cell devices are relatively low cost and have simple drive electronics. These detectors have T₉₀ response times in the order of 15 seconds and have sensitivities in the order of a few ppm of CO.

Cross interface from other organic vapours can be eliminated by the use of active carbon filters. Resistance to condensed vapours that include water can be gained by the use of hydrophobic barriers between vapour stream and detector.

The relative low cost, high sensitivity and resistance to normal vaping that can be found in such devices is ideal for use as a “dry wick” detector.

Therefore, in a preferred embodiment, the detection of CO is chosen to determine dry wicking. This may provide the following advantages:

• CO is gaseous and cannot be in the e-liquid.

• CO is formed by partial combustion of higher hydrocarbons such as found in e-liquid.

• CO detectors are available that meet the base criteria for the device of specificity to CO, and high sensitivity.

• CO detectors are available that can be manufactured in high quantities at low cost.

• CO detectors can be relatively fast.

• CO detectors are available that are low power.

• CO detectors can be miniaturised.

Dry wick indicator

Figure 4 shows parts of a vaping machine with a dry wick indicator in accordance with an embodiment of the invention. Referring to Figure 4, the vaping machine 10 in this embodiment comprises a vaping port 12 for holding an e-cigarette 14, a Cambridge filter pad holder 16 for holding a particulate capture filter 18, a gas transfer tube 20, a pump 22, a control unit 24 and a user interface 25. The control unit 24 is operable to control the pump 22 to draw a series of puffs through the e-cigarette 14 via the transfer tube 20, filter pad holder 16 and vaping port 12. The user interface 25 is used to program the control unit 24 to perform an appropriate puffing routine.

In the arrangement of Figure 4, an exposure chamber 26 is provided in the gas transfer tube 20. An electrochemical cell 28 is provided in the exposure chamber

26. In this embodiment, the electrochemical cell 28 is arranged to produce a signal current that is related to the amount of carbon monoxide that is present in the exposure chamber 26. Control circuit 30 is used to apply the appropriate bias potential to the electrochemical cell 28, and to convert the signal current to an output voltage.

The output voltage produced by the control circuit 30 is amplified by amplifier 32. The amplifier 32 may also filter the signal to remove noise, and/or perform other types of signal conditioning. The amplified signal is then converted to a digital signal by an analogue-to-digital converter 34. Thus, the output of the analogueto-digital converter 34 is a digital measurement signal, which is a measure of the amount of carbon monoxide that is present in the exposure chamber 26. The measurement signal is fed to discriminator 36. The discriminator 36 compares the measurement signal to a reference signal produced by a reference signal generator 38. If the measurement signal exceeds the reference signal, then the discriminator 36 outputs an alarm signal indicating that a dry wick condition has been detected. The alarm signal is fed to control unit 24.

When the control unit 24 receives the alarm signal, it controls the pump 22 in order to cease puffing of the e-cigarette 14. Since the onset of carbon monoxide production in an e-cigarette is an indication that the wick is dry, this can prevent “dry wicking” of the e-cigarette, and thus prevent or reduce the production of toxins that would not normally be inhaled by the consumer.

In the arrangement of Figure 4, the alarm signal is also fed to alarm device 40. The alarm device 40 provides a visual and/or audible indication that a dry wick condition has been detected. The digital measurement signal output from the analogue-to-digital converter 34 is also fed to a display 42. The display 42 displays a reading of the concentration of CO which is present in the exposure chamber 26. The digital measurement signal may also be stored in memory and/or sent to a processor for further processing.

In the arrangement described above, the reference signal produced by the reference signal generator 38 may be a predetermined threshold level. The predetermined threshold may be pre-programed into the system, and/or may be controllable by the operator or by the control software. Alternatively, a number of different thresholds could be set, with a different action being taken as each threshold is exceeded. For example, the control unit may be arranged to slow down puffing of the e-cigarette and/or sound an alarm when a first threshold is exceeded, and to cease puffing when a second threshold is exceeded.

Alternatively or in addition, the discriminator 36 may be arranged to detect a particular pattern of CO concentrations in the exposure chamber, which pattern is known to correspond to dry wicking conditions. These alternative arrangements may help to prevent false alarms, while ensuring that end-of-life of the e-cigarette is detected effectively.

Optionally, the machine may include a mechanism for flushing the exposure chamber with clean air after exposure to the vapour from the e-cigarette, in order to prevent over exposure of the electrochemical cell to excessive levels of CO and other gases.

Figure 5 shows parts of the exposure chamber 26 and electrochemical cell 28 in another embodiment. In this embodiment, a hydrophobic barrier 43 is provided between the vapour in the exposure chamber and the electrochemical cell. The hydrophobic barrier 43 is a waterproof, breathable fabric membrane such as Gore-Tex (RTM). The hydrophobic barrier 43 provides resistance to condensed vapours that include water, while allowing CO to pass through to the electrochemical cell 28.

Figure 6 is a flow chart showing steps taken by the control unit 24 when preforming a test routine on an e-cigarette 14 in an embodiment of the invention. Referring to Figure 6, in step 100 the test parameters are entered into the control unit 24 by the user using the user interface 25. The test parameters may include, for example, puff shape, volume, duration, interval and total number of puffs to be taken. These parameters may be entered individually by the user, or a predetermined routine may be selected by the user, or some combination of the two.

In step 102, the test routine is started. In step 104, the control unit controls the pump 22 to draw a puff through the e-cigarette 14. In step 106, it is determined whether an alarm signal has been received from the discriminator 36. If an alarm signal has been received, then in step 108 the test routine is terminated.

However, if no alarm signal has been received, then in step 110 it is determined whether the number of puffs taken is equal to or exceeds the total number of puffs to be taken in the test routine. If the number of puffs taken is less than the total number of puffs to be taken, then processing returns to step 104, and another puff is taken. However, if the number of puffs taken is equal to or exceeds the total number of puffs to be taken, then in step 108 the test routine is terminated.

Electrochemical cell

The electrochemical cell 28 is a type of fuel cell that is designed to produce a signal current that is related to the amount of target gas (in this case, carbon monoxide) that is present. Essentially the electrochemical cell consists of a container, electrodes, connection wires and an electrolyte, typically sulfuric acid. When a potential difference is applied, carbon monoxide is oxidized at one electrode to carbon dioxide while oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime.

Practical implementations of the electrochemical cell 28 may require three electrodes and usually a choke to control gas flow and a hydrophobic barrier to stop condensation on the surface of the cell and provide a support that is gas permeable for the working electrode.

A simplified diagram of the electrochemical cell and its control circuit is shown in Figure 7. Referring to Figure 7, the electrochemical cell 28 comprises an electrolyte 44 held in a container 45. A working electrode 46, a reference electrode 47 and a counter electrode 48 are in contact with the electrolyte. The working electrode 46 is also in contact with the target gas 49 via the gas permeable membrane 43.

In use, the target gas 49 reacts at the working electrode 46 producing a small current from the reaction. The electrons involved in the electrochemical reaction flow from the working electrode through the external circuit 30, producing the output signal of the sensor. In order for the reaction to take place, the thermodynamic potential of the working electrode is of importance. The reference electrode 47 provides a stable electrochemical potential in the electrolyte. The reference electrode is protected from exposure to the CO gas, so that its thermodynamic potential is always the same and remains constant. Also, no current is allowed to flow through the reference electrode (this would change the thermodynamic potential). The counter-electrode 48 is provided to complete the circuit of the electrochemical cell. The counter electrode functions solely as the second half-cell, and allows electrons to enter or leave the electrolyte. The circuit 30 that controls the potential of the working electrode and converts the signal current to a voltage is called a potentiostat.

The signal (current) from the working electrode 46 is converted to a voltage by operational amplifier U2. This circuit also maintains the voltage of the working electrode at the bias potential, Vbias. The potential of the reference electrode 47 is compared to the stable input voltage, Vbias. The op-amp U1 generates a voltage at the counter electrode 48 which is just sufficient to produce a current that is exactly equal and opposite to the working electrode current. At the same time, a constant voltage is maintained between the reference electrode and working electrode.

The reaction at the working electrode is:

CO + H₂O CO₂+ 2H⁺ + 2e

And at the counter:

(1/2) O₂ + 2H⁺+ 2e H₂O

Vaping device

Figure 8 shows parts of a vaping device in accordance with another embodiment of the invention. Referring to Figure 8, the vaping device 50 comprises a cylindrical housing 52 which accommodates a battery 54, a circuit board 56, an airflow sensor 58, a check valve 60, an atomizer 62, and an e-liquid tank 64. A mouthpiece 66 is provided at the end of the housing away from the battery. The liquid to be vaporised (e-liquid) is contained in the tank 64. The tank may be replaceable or refillable. The atomizer 62 comprises a wicking material that draws the e-liquid onto a heating element.

In operation, a user sucks on the mouthpiece 66, which causes air to flow along an airflow passage 68 between the tank 64 and the housing 52. Air inlets are provided in the housing on the side of the airflow sensor 58 away from the mouthpiece. The airflow sensor 58 detects the airflow, and outputs a signal to a control unit on the circuit board 56. When the control unit receives this signal, it turns on the heating element in the atomizer 62. The heating element then atomises the e-liquid to create an aerosolized vapour which is inhaled by the user. Alternatively or in addition, a button may be provided which can be pushed by the user to activate the heating element.

Also shown in Figure 8 is electrochemical cell 70 (or other miniaturised CO detection device) which is located in a chamber 71 between the mouth piece 66 and the airflow passage 68. The electrochemical cell 70 may be the same as or similar to that described above. The electrochemical cell 70 is connected to the circuit board 56 via wires which pass through or along the housing 52. The circuit board 56 includes a control circuit that controls the potential of the working electrode in the electrochemical cell and converts the signal current to a voltage. The control unit on the circuit board is arranged to turn off the heating element in the atomizer 62 when CO is detected, as will be described in more detail below.

Figure 9 shows parts of the vaping device used to control operation of the heating element. Referring to Figure 9, a control unit 72 is provided which controls operation of the heating element 74. The control unit 72 comprises a processor which is programmed with the appropriate control software. The control unit 72 receives a signal from a switch 76, which may be the airflow sensor 58, or a button pushed by the user. When the switch is activated, the control unit 72 turns on the heating element 74, in order to atomise the e-liquid and create an aerosolized vapour. The control unit 72 is located on the circuit board 56 of Figure 8.

Still referring to Figure 9, the electrochemical cell 70 is connected to a control circuit 78. The electrochemical cell 70 is arranged to produce a signal current that is related to the amount of carbon monoxide that is present in the chamber 71 of Figure 8. The control circuit 78 controls the potential of the working electrode and converts the signal current to a voltage. In this embodiment, control circuit 78 is also located on the circuit board 56, although it may be located elsewhere.

The signal voltage output by the control circuit 78 is amplified by amplifier 80. The amplifier 80 may also filter the signal to remove noise, and/or perform other types of signal conditioning. The amplified signal is then converted to a digital signal by an analogue-to-digital converter 82. The output of the analogue-todigital converter 82 is a digital measurement signal, which is a measure of the amount of carbon monoxide that is present in the chamber 71 of Figure 8. The measurement signal is fed to discriminator 84. The discriminator 84 compares the measurement signal to a reference signal produced by a reference signal generator 86. If the measurement signal exceeds the reference signal, then the discriminator 84 outputs an alarm signal indicating that a dry wick condition has been detected. The alarm signal is fed to control unit 72. When the control unit 72 receives the alarm signal, it functions to cease any further heating of the heating element 74.

Since the onset of carbon monoxide production is an indication that the wick is dry, this can prevent “dry wicking” of the vaping device, and thus prevent or reduce the production of toxins that might otherwise be inhaled by the user. Alternatively or in addition, some other action may be taken, such as producing an alarm.

The reference signal produced by the reference signal generator 86 may be a predetermined threshold level, which may be pre-programed into the device, and/or controllable by the user or by the control software. Alternatively, a number of different thresholds could be set, with a different action being taken as each threshold is exceeded. For example, the control unit may be arranged to reduce the heat of the heating element when a first threshold is exceeded, and to cease heating when a second threshold is exceeded. Alternatively or in addition, the discriminator 86 may be arranged to detect a particular pattern of CO concentrations in the chamber 71, which pattern is known to correspond to dry wicking conditions.

In any of the above embodiments, different detector types and/or different decomposition products could be used as well as or instead of those described, with some examples being given below.

Detector types

In alternative embodiments, other detector types could be used as well as or instead of an electrochemical cell. Methods of detection can be chemical or spectroscopic, and there are advantages and disadvantages to each approach. The following discussion is primarily restricted to the use of CO as the detection species.

1. Infrared

a. Non Dispersive Infra Red spectroscopy: - This is a technique where light of a single wavelength is passed through a sample and the light is adsorbed by a chemical when present in the gas. The light wavelength I energy is selected such that it excites vibration in a particular bond in the target molecule (does not work for diatomic such as oxygen). As the energy required to cause a vibration is specific and quantised this can be a very specific technique. In practice the vibrational frequency is influenced by other energy states in the molecule such as translational, rotational and electronic states and so the adsorption energy tends to become broadened. This is exacerbated by the practical difficulties in producing a monochromatic infra-red light beam. Two approaches can be used a broad band emitter and a band pass filter that lets light of a particular wavelength through or use of a monochromatic light source. Bandpass filters tend to be relatively broad in passing light so interferences from other gas / vapour phase chemicals is possible. Alternatively a tuneable diode laser can be used to produce a monochromatic beam, although the technology for ensuring that the wavelength is stable and located at a point of interest can be complicated. In both cases the potential for the “brightness” of the illuminating source changing with time is a real one and quantitative analysis becomes more challenging. This can be eliminated with repeated calibrations or through use of a reference filter for the same source that absorbs light away from the area of interest and ratio metric measurements are taken. In the case of a tuneable diode laser, the laser is scanned to another area of the spectrum. This can be achieved by changing the temperature of the laser a few degrees, for example, by using a Peltier heat pump and changing the current passing through the pump.

b. Fourier Transform infrared spectroscopy. In this a broad band infrared source is deployed. FTIR offers quantitative and qualitative analysis for organic and inorganic samples. Fourier Transform Infrared Spectroscopy identifies chemical bonds in a molecule by producing an infrared absorption spectrum. The spectra produce a profile of the sample, a distinctive molecular fingerprint that can be used to screen and scan samples for many different components simultaneously. In FTIR spectroscopy, a beam of light is passed through a series of mirrors that cause the beam’s individual wavelengths to hit each other in a way that allows a sample to absorb some wavelengths, while others are blocked. Light absorption is measured and a computer infers the absorption rate of each wavelength within the beam. FTIR spectroscopy measures the absorption rate of many different frequencies simultaneously and through use of libraries can deconvolute complex mixtures for qualitative and quantitative analysis. The disadvantage of this technique for a simple detection system for dry wicking is the inherent cost of such systems, the complexity of the engineering and the size of the device.

2. Colorimetric analysis

A number of chemical compounds change colour on exposure to carbon monoxide, the most notable and commonly occurring being haem in blood which on acute exposure to carbon monoxide changes irreversibly to cherry red. There are a number of chemicals that have the same properties, and these can be impregnated into a substrate and exposed to carbon monoxide. A light source is shone at the chemically treated target and the amount of light reflected can be monitored. As the substrate darkens the amount of light reflected reduces and an accurate measurement of the amount of CO present can be made. The system can be small, and inexpensive. However colour changes are irreversible and cumulative so low levels of CO can cause darkening that builds up with time and once used the colour strip needs to replaced. Commercially this has some advantages as it produces a readymade consumables market but it has considerable limitations for practical use.

3. Reversible oxidation

a. Electrochemical cell.

In this a platinum catalyst reacts with carbon monoxide in the presence of air to create carbon dioxide and electrons (current). This oxidation reaction does not consume the electrode and so in theory the electrochemical cell has theoretically infinite life. This type of detector was discussed above.

b. Semiconductor MOS: In clean air, donor electrons in tin dioxide are attracted toward oxygen which is adsorbed on the surface of the sensing material, preventing electric current flow. In the presence of reducing gases, the surface density of adsorbed oxygen decreases as it reacts with the reducing gases. Electrons are then released into the tin dioxide, allowing current to flow freely through the sensor. Disadvantages these are high power for the heater element that sits behind the tin oxide substrate that must be kept at 200°C and the potential for poisoning of the tin oxide from almost every conceivable chemical which requires a cleaning and reconstitution cycle at high temperatures.

Other decomposition products

The production of other products instead of, or as well as CO, could be used as an end-of-life indicator. Such products include:

• Carbon dioxide (CO₂). This is formed during combustion I pyrolysis I thermal degradation and as a gas would not be present in the e-juice. Disadvantages are that the simple detectors that can be made cheaply, miniature and low power are susceptible to changes in humidity. Also higher sensitivity is required for CO₂ detectors due to the requirement that higher hydrocarbons are fully combusted which is usually a high temperature process and so would only indicate dry wicking beyond the point where partial combustion I pyrolysis occurs.

• Formaldehyde. The production of this chemical is characteristic of pyrolysis of the carrier liquids of the e-juice such as Glycerol and Propylene glycol. It is therefore highly characteristic of dry wicking. Disadvantages are that formaldehyde may be present in the juice (liquid), could be formed by other chemical processes in vaping, is at a relatively low level in e-juice when dry wicking, detectors tend to be sophisticated, large and expensive if selective or less selective when cheap and compact e.g. colorimetric methods.

• Acetone. The production of this chemical is characteristic of pyrolysis of the carrier liquids of the e-juice such as Glycerol and Propylene glycol. It is therefore highly characteristic of dry wicking. Disadvantages are that acetone may be present in the e-juice (liquid), could be formed by other chemical processes in vaping, and is at a relatively low level in e-juice when dry wicking. Detectors tend to be sophisticated, large and expensive if selective or less selective when cheap and compact e.g. colorimetric methods.

• Acrolein. Production of this chemical is characteristic of pyrolysis of the carrier liquids of the e-juice such as Glycerol and Propylene glycol. It is therefore highly characteristic of dry wicking. Disadvantages are that acrolein may be present in the e-juice (liquid), could be formed by other chemical processes in vaping, and is at a relatively low level in e-juice when dry wicking. Detectors tend to be sophisticated, large and expensive if selective or less selective when cheap and compact e.g. colorimetric methods.

• Acetaldehyde. Production of this chemical is characteristic of pyrolysis of the carrier liquids of the e-juice such as Glycerol and Propylene glycol. It is therefore highly characteristic of dry wicking. Disadvantages are that acetaldehyde may be present in the e-juice (liquid), could be formed by other chemical processes in vaping, and is at a relatively low level in ejuice when dry wicking. Detectors tend to be sophisticated, large and expensive if selective or less selective when cheap and compact e.g. colorimetric methods.

• Other organic species

In the above description, preferred features of the invention have been described with reference to various embodiments. It will be appreciated that features of one embodiment may be used with any other embodiment. Furthermore, the invention is not limited to these embodiments, and variations in detail may be made within the scope of the appended claims.

Claims (26)

1. A dry wick indicator for indicating a dry wick condition in a vaping device, the dry wick indicator comprising detecting means for detecting a decomposition product from the vaping device, and means for producing a signal indicating a dry wick condition based on an output of the detecting means.

2. A dry wick indicator according to claim 1, wherein the decomposition product is a product formed by pyrolysis, thermal degradation or combustion of an e-liquid base.

3. A dry wick indicator according to claim 1 or 2, wherein the decomposition product is a product that is that is not present in an e-liquid from which vapour is produced.

4. A dry wick indicator according to any of the preceding claims, wherein the decomposition product is selected from at least one of: carbon monoxide; carbon dioxide; formaldehyde; acetone; acrolein; and acetaldehyde.

5. A dry wick indicator according to any of the preceding claims, wherein the decomposition product is carbon monoxide.

6. A dry wick indicator according to any of the preceding claims, wherein the detecting means is arranged to output a signal indicating a concentration of the decomposition product in vapour from the vaping device.

7. A dry wick indicator according to any of the preceding claims, wherein the detecting means uses a detection technique selected from at least one of: nondispersive infrared spectroscopy; Fourier Transform infrared spectroscopy; colorimetric analysis; and reversible oxidation.

8. A dry wick indicator according to any of the preceding claims, wherein the detecting means comprises an electrochemical cell.

9. A dry wick indicator according to claim 8, wherein the electrochemical cell comprises a working electrode, a reference electrode and a counter electrode in contact with an electrolyte.

10. A dry wick indicator according to claim 8 or 9, wherein the electrochemical cell is arranged to produce a signal current that is related to the amount of carbon monoxide to which the cell is exposed.

11. A dry wick indicator according to any of claims 8 to 10, further comprising a control circuit for applying a bias potential to the electrochemical cell and/or for converting the signal current to an output voltage.

12. A dry wick indicator according to any of the preceding claims, wherein the detecting means is at least partially located in an exposure chamber.

13. A dry wick indicator according to any of the preceding claims, further comprising a hydrophobic barrier between vapour from the vaping device and the detecting means.

14. A dry wick indicator according to any of the preceding claims, further comprising means for comparing an output of the detecting means with a reference and producing a signal indicating a dry wick condition in dependence on a result of the comparison.

15. A dry wick indicator according to claim 14, wherein the signal indicating a dry wick condition is produced when an output of the detecting means exceeds a threshold value.

16. A dry wick indicator arranged to indicate a dry wick condition in a vaping device, the dry wick indicator comprising a detector arranged to detect a decomposition product in vapour from the vaping device, and a discriminator arranged to compare an output of the detector with a reference, and to output a signal indicating a dry wick condition in dependence on a result of the comparison.

17. A testing apparatus for testing a vaping device, the testing apparatus comprising:

means for puffing the vaping device to produce a vapour; and a dry wick indicator according to any of the preceding claims.

18. A testing apparatus according to claim 17, wherein the apparatus is arranged to cease puffing the vaping device in dependence on the signal indicating a dry wick condition.

19. A testing apparatus according to claim 17 or 18, the apparatus comprising a control unit for controlling the puffing means, wherein the control unit is arranged to receive the signal indicating a dry wick condition, and to cease puffing of the vaping device in dependence thereon.

20. A testing apparatus according to any of claims 17 to 19, the testing apparatus comprising an exposure chamber in a fluid flow path between the vaping device and the puffing means, wherein the detecting means is at least partially located in the exposure chamber.

21. A testing apparatus according to claim 20, the apparatus comprising means for flushing the exposure chamber with clean air.

22. A vaping device comprising a dry wick indicator according to any of claims 1 to 16.

23. A vaping device according to claim 22, wherein the vaping device is arranged to prevent vaping in dependence on the signal indicating a dry wick condition.

24. A vaping device according to claim 22 or 23, the vaping device comprising a heating element for heating an e-liquid, and a control unit for controlling the heating element, wherein the control unit is arranged to receive the signal indicating a dry wick condition, and to cease heating of the heating element in dependence thereon.

25. A vaping device according to any of claims 22 to 24, the vaping device comprising an exposure chamber in a fluid flow path for the vapour, wherein the detecting means is at least partially located in the exposure chamber.

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26. A method of indicating a dry wick condition in a vaping device, the method comprising detecting a decomposition product in vapour from the vaping device, and outputting a signal indicating a dry wick condition in dependence thereon.