EP3542868A1

EP3542868A1 - A mask-based breathing apparatus and control method

Info

Publication number: EP3542868A1
Application number: EP18162442.0A
Authority: EP
Inventors: Jun Shi; William Su; Wei Zhong Chen; Peng Zhang
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2018-03-19
Filing date: 2018-03-19
Publication date: 2019-09-25

Abstract

A breathing assistance mask has an air blower arrangement at least for drawing air from outside into the mask volume. The temperature and relative humidity is monitored inside the mask volume and from this a threshold temperature can be determined, such as the dew point temperature (or a temperature dependent on the dew point temperature). By monitoring at least the exterior temperature, the air blower arrangement may be controlled to maintain the temperature in the air chamber above the threshold temperature, in particular to prevent condensation forming inside the mask volume.

Description

FIELD OF THE INVENTION

This invention relates to a breathing apparatus in the form of a mask, for providing air to the wearer of the breathing apparatus, with the flow assisted by an air blower such as a fan.

BACKGROUND OF THE INVENTION

The World Health Organization (WHO) estimates that 4 million people die from air pollution every year. Part of this problem is the outdoor air quality in cities. The worst in class are Indian cities like Delhi that have an annual pollution level more than 10 times the recommended level. Well known is Beijing with an annual average 8.5 times the recommended safe levels. However, even in European cities like London, Paris and Berlin, the levels are higher than recommended by the WHO.
Since this problem will not improve significantly on a short time scale, the only way to deal with this problem is to wear a mask which provides cleaner air by filtration. To improve comfort and effectiveness one or two fans can be added to the mask. These fans are switched on during use and are typically used at a constant voltage. For efficiency and longevity reasons these are normally electrically commutated brushless DC fans.
The benefit to the wearer of using a powered mask is that the lungs are relieved of the slight strain caused by inhalation against the resistance of the filters in a conventional non-powered mask.
Furthermore, in a conventional non-powered mask, inhalation also causes a slight negative pressure within the mask which leads to leakage of the ambient contaminants into the mask, which leakage could prove dangerous if these are toxic substances. A powered mask delivers a steady stream of air to the face and may for example provide a slight positive pressure, which may be dependent on the resistance of an exhale valve, to ensure that any leakage is outward rather than inward.
There are several advantages if the fan operation or speed is regulated. This can be used to improve comfort by more appropriate ventilation during the inhalation and exhalation sequence or it can be used to improve the electrical efficiency. The latter translates into longer battery life or increased ventilation.
By providing powered inhale and/or exhale fans, the comfort of the mask is increased because the in-mask temperature, relative humidity and pressure difference can be regulated.
To regulate the fan speed, the pressure inside the mask can be measured and both pressure as well as pressure variation can be used to control the fan. For example, GB 2032284 discloses a respirator in which the pressure inside a mask is measured by a pressure sensor and the fan speed is varied in dependence on the sensor measurements.
The humidity level inside the mask is typically relatively high due to the breathing of the user. This presents a problem when the ambient temperature is low, e.g. in winter. The exhaled humid air in a cold mask will immediately cause the condensation of the water vapor inside the mask. This condensation may be uncomfortable or unpleasant for the user of the mask.

SUMMARY OF THE INVENTION

The invention is defined by the claims.
According to examples in accordance with an aspect of the invention, there is provided a mask comprising:

an air chamber;
an air blower arrangement at least for drawing air from outside the air chamber into the air chamber;
a first temperature sensor for sensing a first temperature, inside the air chamber;
a first relative humidity sensor for sensing a first relative humidity, inside the air chamber;
a second temperature sensor for sensing a second temperature, outside the air chamber;
a controller which is adapted to:
- determine a threshold temperature based on the first temperature and the first relative humidity; and
- control the air blower arrangement taking into account the second temperature to maintain the temperature in the air chamber above the threshold temperature.

This mask design determines a threshold temperature from the conditions inside the air chamber of the mask. The threshold temperature is for example associated with the dew point within the air chamber at which condensation will take place. The temperature of the air which is to be drawn into the air chamber is taken into account so that the air blower arrangement is controlled to avoid the temperature dropping below the threshold at all times, thereby to avoid condensation in the mask volume. The threshold temperature varies as the air blower draws air into the air chamber, and the air blower is controlled such that the resulting temperature inside the air chamber remains above the resulting threshold temperature.
The mask may further comprise a second relative humidity sensor for sensing a second relative humidity, outside the air chamber, wherein the controller is adapted to control the air blower arrangement further taking into account the second relative humidity. Thus, the temperature and relative humidity of the air outside the air chamber are taken into account in order to determine when condensation is likely to be caused.
The threshold temperature is for example an estimation of the dew point temperature inside the air chamber. By maintaining the actual temperature above the dew point temperature, condensation is prevented. The threshold temperature may be an estimation or approximation of the dew point temperature or it may be any suitable temperature threshold at a sufficient level for condensation to be avoided.
The temperature in the air chamber will generally be sufficiently high from the temperature of the exhaled air to prevent condensation within the air chamber, if no ambient air is introduced. The control of the air blower arrangement thus regulates the amount of air mixing in the air chamber with ambient air, which, when the control approach is most needed, is typically cold air (e.g. below 10 degrees Celsius, and more typically below 5 degrees Celsius)
The controller is for example adapted to determine a maximum inlet flow rate, below which a temperature inside the air chamber remains above the threshold temperature. By maintaining the inlet flow rate below this maximum level, it is ensured that the mixing of air in the air chamber with ambient air is not sufficient to cause the temperature to drop below the (recalibrated) dew point temperature.
The air blower arrangement may be further for drawing air from inside the air chamber to the outside, wherein the controller is adapted to determine a minimum exhaust flow rate for reducing the relative humidity inside the air chamber. This exhaust flow lowers the threshold temperature and hence assists in maintaining the chamber temperature above the threshold temperature.
Thus the air blower arrangement may comprise only an inlet air blower or it may comprise an inlet air blower and an exhaust air blower. Even if only in inlet air blower is used, it may be controlled to ensure that air is drawn exits the mask from a seal area even during inhaling, to provide a supply of fresh air to the user's face.
When both the inlet flow and exhaust flow are controlled, the controller is adapted to control both the inlet flow to the air chamber and the exhaust flow from the air chamber to maintain the air chamber temperature above the threshold temperature.
The mask may further comprise a pressure monitoring system, wherein the controller is adapted to determine a respiration cycle from the pressure monitoring system, and to control the air blower arrangement in dependence on the phase of the respiration cycle.
Thus, the inlet cycle and optionally also an exhaust cycle of the air blower arrangement may be controlled to provide assisted inhalation and optionally also assisted exhalation, as well as providing control of the conditions inside the air chamber to maintain comfort levels and prevent condensation.
The mask may comprise an outlet valve for controllably venting the air chamber to the outside, wherein the outlet valve comprises a passive pressure-regulated check valve or an actively driven electrically controllable valve.
The valve prevents back flow of air into the air chamber, which may for example comprise unfiltered air. When a pressure monitoring system is provided, and the controller determines a respiration cycle from the pressure monitoring system, the outlet valve may also be controlled in dependence on the phase of the respiration cycle.
The mask may comprise a filter which may comprise an outer wall of the air chamber, or a filter member in series with the air blower arrangement. The mask may be part of a respirator to provide assisted breathing or it may additional provide a filtering function
The invention also provides a method of controlling a mask which comprises an air blower arrangement at least for drawing air from outside an air chamber of the mask into the air chamber, the method comprising:

sensing a first temperature, inside the air chamber;
sensing a first relative humidity, inside the air chamber;
sensing a second temperature, outside the air chamber;
determining a threshold temperature based on the first temperature and the first relative humidity; and
controlling an air blower arrangement for drawing air into an air chamber of the mask taking into account the second temperature such as to maintain the temperature in the air chamber above the threshold temperature.

This method uses control of an air blower arrangement to regulate the temperature inside a mask air chamber, preferably to prevent condensation inside the air chamber of the mask.
The method may further comprise sensing a second relative humidity, outside the air chamber, and controlling the air blower arrangement further based on the second relative humidity. This enables the effect of the external air on the possible condensation inside the air chamber of the mask to be determined more accurately.
The threshold temperature is for example an estimation of the dew point temperature inside the air chamber.
The method may further comprise determining a maximum inlet flow rate, below which a temperature inside the air chamber remains above the threshold temperature.
A pressure may be monitored inside the air chamber so that a respiration cycle can be determined and the air blower arrangement may then be controlled in dependence on the determined respiration cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows a face mask which includes a system for preventing condensation in the mask;
Fig. 2 shows one example of the components of the mask system; and
Fig. 3 shows a mask operating method.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a breathing assistance mask which has an air blower arrangement (such as a one or more fans) at least for drawing air from outside into the mask volume. The temperature and relative humidity is monitored inside the mask volume and from this a threshold temperature can be determined, such as the dew point temperature (or a temperature dependent on the dew point temperature). By monitoring at least the exterior temperature, the air blower arrangement may be controlled to maintain the temperature in the air chamber above the threshold temperature (including as the threshold temperature evolves over time), in particular to prevent condensation forming inside the mask volume.
Fig. 1 shows a subject 10 wearing a face mask 12 which covers the nose and mouth of the subject. The purpose of the mask is to filter air before it is breathed in by the subject and to provide active control of the flow of air into an air chamber 18 (i.e. the mask volume) and optionally also out of the air chamber. An air blower arrangement 20 provides active flow control. In the example below, the air blower arrangement is a fan arrangement (of one or multiple fans) but any suitable fluid flow control device may be used such as a micropump. In one example, the mask body itself acts as an air filter 16. In an alternative arrangement, the mask body is not gas permeable, and a filter 21 is provided in series with the fan arrangement 20.
Air is drawn in to the air chamber 18 by inhalation with assistance provided by the fan arrangement. During inhalation, an outlet valve 22 such as a check valve is closed due to the low pressure in the air chamber 18.
When the subject breathes out, air is exhausted through the outlet valve 22. This valve is opened to enable easy exhalation, but is closed during inhalation.
The fan arrangement 20 is provided at least for providing a flow of air into the air chamber 18 during inhalation, thus providing assisted breathing. The fan arrangement may also provide a flow during exhalation (with the outlet valve open). During exhalation, more air may be removed from the air chamber 18 than is exhaled so that additional air is supplied to the face. This increases comfort due to lowering relative humidity and cooling. For example, a minimum exhaust flow rate may be provided for reducing the relative humidity inside the air chamber
During inhalation, by closing the valve, it is prevented that unfiltered air is drawn in. The timing of the outlet valve 22 is thus dependent on the breathing cycle of the subject. The outlet valve may be a simple passive check valve operated by the pressure difference across the mask. However, it may instead be an electronically controlled valve.
The operation of the fan arrangement is preferably also timed with the breathing cycle of the user. For this purpose, a pressure sensor 24 is provided either for measuring the pressure inside the air chamber 18 or for measuring a differential pressure between the inside and outside of the air chamber.
Various mask designs are possible, with or without assisted exhalation, and with different filter designs. There may be a single for or separate inlet and exhaust fans. The inventive concept can be applied to any such mask designs, and only requires control of a fan which regulates the flow of exterior air into the air chamber which forms the mask volume.
The mask further comprises a first temperature sensor for sensing a first temperature, inside the air chamber and a first relative humidity sensor for sensing a first relative humidity, inside the air chamber. These two sensors are together shown as a first sensor unit 26 in Fig. 1.
There is a second temperature sensor for sensing a second temperature, outside the air chamber, and optionally a second relative humidity sensor for sensing a second relative humidity, outside the air chamber. These two sensors are together shown as a second sensor unit 28 in Fig. 1.
The monitoring of temperatures and relative humidity (or relative humidities) is performed to ensure that the conditions inside the air chamber 18 do not result in the formation of condensation. In particular, the dew point temperature is monitored and the temperature is maintained above the dew point temperature.
Fig. 2 shows one example of the components of the mask. The same components as in Fig. 1 are given the same reference numbers.
In addition to the components shown in Fig. 1, Fig. 2 shows a controller 30 and a local battery 32. The fan arrangement 20 comprises a single inlet fan in this example with a fan blade 20a and a fan motor 20b.
The internal (first) temperature sensor is shown as 26a and the internal (first) relative humidity sensor is shown as 26b. The external (second) temperature sensor is shown as 28a and the external (second) relative humidity sensor is shown as 28b.
The controller 30 is adapted to determine a threshold temperature based on the first temperature T and the first relative humidity RH.
The threshold temperature is for example an estimate of the dew point temperature T_dp. The dew point temperature T_dp may for example be estimated from the temperature T and the relative humidity RH using the following relationship: $γ_{m} (T, RH) = \ln (\frac{RH}{100} \exp ((b - \frac{T}{d}) (\frac{T}{c + T})))$
$T_{dp} = \frac{{cγ}_{m} (T, RH)}{b - γ_{m} (T, RH)}$
There are different possible values for the constants a, b, c. In one example set: a =611.21 Pa, b= 18.678, c= 257.14 °C, d=234.5 °C.
These two equations define the so-called Arden Buck equation. There are alternative simpler formulae (for example the so-called Magnus formula has one less constant parameter) and there are also other alternative derivations. In general, any equation for the dew point temperature may be used which relies only on measurement of the actual (dry bulb) air temperature and relative humidity inside the air chamber.
A further approximation may be made to arrive at a simpler equation: $T_{dp} \approx T - \frac{100 - RH}{5}$
The approximation is accurate to within ±1°C when the relative humidity is above 50%.
Thus, by using one of these approximations, or indeed any alternative derivation of the dew point temperature, a threshold temperature is obtained. The determined dew point temperature may function directly as the threshold or else a margin may be added to ensure inaccuracies in the estimated may be tolerated.
When the external ambient air temperature is low, when the inhaling fan is tuned on the ambient air will mix with the air inside the air chamber. Based on knowledge of the temperature and relative humidity inside the air chamber and the temperature (and preferably also relative humidity) of the air entering the air chamber through the inlet, the dew point temperature after mixing at different fan speeds can be determined.
The controller 30 has a module for determining the combined temperature and relative humidity based on different possible fan speeds, and then determining the new dew point temperature so that it can be determined if the temperature remains above the new dew point temperature. Thus, the way inlet air will influence the temperature and relative humidity as well as the way this changes the dew point temperature is determined.
To determine the new temperature and relative humidity, the controller for example takes account of one or more of:

the air chamber volume;
the flow rate into the air chamber caused by the fan, which is a flow of air with the ambient temperature T2 and relative humidity RH2;
the flow rate breathed in by the subject during inhalation (which may be approximated as a flow of air with the resulting combined ambient temperature T_C and combined relative humidity RH_C);
the flow rate breathed out by the subject during exhalation (which may be approximated as a flow of air at body temperature and a relative humidity of 100%);
any flow rate out of the air chamber during inhalation (for example a flow out of the air chamber even during inhalation which provides cooling and increases comfort). This flow may also be approximated as a flow of air with the combined temperature T_C and combined relative humidity RH_C;
any forced flow rate out of the air chamber caused by an exhaust fan.

By way of example, one simplified approach assumes that mixing of ambient air using the inlet fan results in a final temperature given by: $T_{C} = (Q_{B} T_{B} + Q_{2} T_{2}) / (Q_{B} + Q_{2})$
In this equation Q_B and Q₂ denote the flow rate of breathing out and the flow rate of the fan respectively, and T_B and T₂ denote the temperature of the breathed out air and the ambient air respectively.
The relatively humidity may be obtained using a similar equation, but in this case the absolute water ratio is first determined, or the absolute humidity of breathed out air and ambient air are obtained. This could be calculated according to the temperature and relative humidity.
After obtaining the absolute humidity (AH) values they replace the values T_B and T₂ in the above equation, giving: ${AH}_{C} = (Q_{B} {AH}_{B} + Q_{2} {AH}_{2}) / (Q_{B} + Q_{2})$
The absolute humidity AH_C of the mixed air in the cavity can thus be obtained.
Using the absolute humidity and the combined temperature Tc, the relative humidity RH_C can then be calculated accordingly.
The simplified approach above only takes account of the breathed out air mixing with the ambient air and assumes the mask volume (dead volume) is small. For example, this simplification may be suitable for a mask volume less than 50ml.
The air characteristics inside the mask volume may instead be taken into account. The parameters which are required to be used depend on the design or structure of the mask.
If the mask volume is to be taken into account, the flow rate Q may be replaced by volume V so that there are three volume contributions to the mixing (V_B, V₂, V_mask). The same type of weighted sum of temperatures and absolute humidity values can then be obtained as above.
The mixing is a dynamic process, for example the human breathing flow rate is normally a sinusoidal function rather than a constant flow. When considering the dynamic mixing effects, the temperature and humidity in the mask volume is changing gradually over the course of a breathing cycle because of the changing flow rate of breathing. However, in different breathing cycles, this change pattern will repeat.
For volume calculations, a sample step size may be selected, such as 0.1 seconds or 0.2 seconds. An average of the dew point temperature may then be calculated in one breathing cycle to estimate whether or not condensation will take place.
No matter how the calculations are performed, a new combined temperature T_C and relative humidity RH_C is used to determine how the dew point temperature has evolved, giving a new combined dew point temperature T_dpC. The flow rate is then controlled by modulating the fan speed at least during inhalation to ensure that the new temperature T_C remains above the new dew point temperature T_dpC.
The threshold temperature evolves with time, and is the instantaneous dew point temperature determined from the instantaneous temperature and relative humidity inside the air chamber.
In this way, the threshold temperature is converted by the controller into a maximum inlet flow rate, below which a temperature inside the air chamber remains above the threshold temperature. This flow rate requirement translates to a required fan speed. Thus, by control the inhaling fan speed when in a cold ambient environment, the vapor condensation inside the mask can be avoided.
The dew point temperature monitoring may take place at all times. However, to save power it may only be triggered when the ambient temperature drops below a threshold temperature, such as 10 degrees Celsius or 5 degrees Celsius.
In a most simple implementation, only the external temperature may be monitored. The relative humidity may be assumed to have a particular value. However, there is feedback control in any case, since the way the temperature and relative humidity vary in response to different fan speeds (or duty cycles) can be monitored. Thus, the mask may estimate the relative humidity of the external air from previous monitoring of the relative humidity inside the air chamber and the temperatures inside and out.
For example, the external relative humidity may be obtained by measuring the temperature and relative humidity of the mixed air in the cavity. The assumption is that the breathing temperature, relative humidity and flow rate are known. The temperature and relative humidity are normally 35°C and 100%, a fixed pre-set inlet flow rate is used to mix the breathing air and the ambient air. The mixed air temperature and relative humidity is measured by the sensor. The absolute humidity of the mixed air can thus be determined, and the absolute humidity of the ambient air can then be calculated using the mixing equation above. Once the temperature and the absolute humidity of the ambient air is known, the relative humidity can be derived. Thus, is not essential to measure the external relative humidity.
The fan arrangement may be controlled in synchronism with the breathing cycles of the user. The temperature and relative humidity monitoring may be used to derive the timing of the breathing cycles. However, the mask may instead further comprise a pressure monitoring system, wherein the controller is adapted to determine a respiration cycle from the pressure monitoring system, and to control the fan arrangement in dependence on the phase of the respiration cycle.
Thus, the inlet cycle and optionally also the exhaust cycle of the fan arrangement may be controlled to provide assisted inhalation and optionally also assisted exhalation, as well as providing control of the conditions inside the air chamber to maintain comfort levels and prevent condensation.
Fig. 3 shows a method for controlling a mask.
In step 40 a first temperature T inside the air chamber of the mask is sensed.
In step 42 a first relative humidity RH inside the air chamber is sensed.
In step 44 a dew point temperature T_dp is determined inside the air chamber. This functions as a threshold temperature.
In parallel, in step 46 a second temperature T2 outside the air chamber is sensed. Optionally, a second relative humidity RH2 outside the air chamber is sensed in step 48.
There is then an analysis of the effect of different fan speeds, for drawing ambient air into the air chamber, on the resulting combined temperature T_C and relative humidity RH_C. A fan speed is modeled in step 50, for example starting with the highest fan speed. The resulting combined temperature T_C and relative humidity RHc are derived in step 52 and the corresponding dew point temperature T_dpC is derived in step 54.
If an exhaust fan is also used, the effect of different exhaust fan speeds may also be modeled into the analysis. For simplicity, selection only of the inlet fan speed is discussed below.
In step 56 it is determined if the combined temperature exceeds the (new) dew point temperature. If it does not (i.e. the fan speed is too high, so that the temperature is too low) then the fan speed to be modeled is reduced in step 57.
New temperature, relative humidity and dew point temperatures are calculated in steps 52 and 54. This cycle repeats until a fan speed is found for which T_C > T_dpC. This fan speed may be taken as the maximum fan speed. This corresponds to the temperature being just kept above the dew point temperature.
The fan arrangement for drawing gas into the air chamber of the mask is controlled in step 58 thereby to maintain the temperature in the air chamber above the threshold. temperature. The fan control may take into account a separate control loop, shown as input 60, which for example provides cyclic control of the fan arrangement in synchronism with the breathing cycle of the user.
There may be other fan control aspects, for example for power saving or relating to battery management. When no special control is needed to maintain the temperature above the dew point temperature, there may still be control based on temperature and relatively humidity in order to control the atmosphere in the air chamber to improve the user comfort.
Note that the iterative calculations explained above are only one example. As an alternative, the maximum fan speed may be calculated directly using suitable equations taking into account the parameters mentioned above.
For the exhaust fan case, one simple embodiment is to maintain the flow rate of the exhaust fan greater the exhalation flow rate. In this way all the humid air inside the chamber is discharged immediately when it is exhaled. This is however not an energy efficient control strategy.
Another embodiment is to decrease the threshold temperature to make it below the chamber temperature during the exhalation phase. According to the dew point temperature equation, such as: $T_{dp} \approx T - \frac{100 - RH}{5}$
the chamber temperature T and relative humidity RH can be monitored in a first or first several exhalation cycles, and the exhaust fan speed can then be increased gradually to make the threshold temperature T_dp lower than the chamber temperature during the exhalation phase.
Oppositely, the maximum fan speed may be set as an initial value and the fan speed may be decreased gradually to find the critical threshold temperature T_dp and maintain the exhaust fan speed a little bit higher than the corresponding threshold fan speed. In this way, the water condensation is avoided.
Thus, it can be seen that control of an inlet fan and/or an exhaust fan may be used to regulate the temperature and/or relative humidity inside the air chamber to prevent condensation.
As discussed above, embodiments make use of a controller, which can be implemented in numerous ways, with software and/or hardware, to perform the various functions required. A processor is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform the required functions. A controller may however be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.
Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).
In various implementations, a processor or controller may be associated with one or more storage media such as volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform the required functions. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller.
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

A mask comprising:
an air chamber (18);

an air blower arrangement (20) at least for drawing air from outside the air chamber (18) into the air chamber;

a first temperature sensor (26a) for sensing a first temperature (T), inside the air chamber;

a first relative humidity sensor (26b) for sensing a first relative humidity (RH), inside the air chamber;

a second temperature sensor (28a) for sensing a second temperature (T2), outside the air chamber;

a controller (30) which is adapted to:
determine a threshold temperature (T_dp) based on the first temperature and the first relative humidity; and

control the air blower arrangement (20) taking into account the second temperature such as to maintain the temperature in the air chamber above the threshold temperature.
A mask as claimed in claim 1, further comprising a second relative humidity sensor (28b) for sensing a second relative humidity (RH2), outside the air chamber, wherein the controller is adapted to control the air blower arrangement further taking into account the second relative humidity.
A mask as claimed in claim 1 or 2, wherein the threshold temperature is an estimation of the dew point temperature inside the air chamber.
A mask as claimed in any preceding claim, wherein the controller is adapted to determine a maximum inlet flow rate, below which a temperature inside the air chamber remains above the threshold temperature.
A mask as claimed in any preceding claim, wherein the air blower arrangement is further for drawing air from inside the air chamber to the outside, wherein the controller is adapted to determine a minimum exhaust flow rate for reducing the relative humidity inside the air chamber
A mask as claimed in claim 5, wherein the controller is adapted to control both an inlet flow to the air chamber and an exhaust flow from the air chamber to maintain the air chamber temperature above the threshold temperature.
A mask as claimed in any preceding claim, comprising a pressure monitoring system (24), wherein the controller (30) is adapted to determine a respiration cycle from the pressure monitoring system, and to control the air blower arrangement in dependence on the phase of the respiration cycle.
A mask as claimed in any preceding claim, further comprising an outlet valve (22) for controllably venting the air chamber (18) to the outside, wherein the outlet valve (22) comprises a passive pressure-regulated check valve or an actively driven electrically controllable valve.
A mask as claimed in claim 8, comprising a pressure monitoring system (24), wherein the controller (30) is adapted to determine a respiration cycle from the pressure monitoring system, and to control the outlet valve (22) in dependence on the phase of the respiration cycle.
A mask as claimed in any preceding claim, wherein the filter comprises an outer wall (16) of the air chamber, or a filter member (21) in series with the air blower arrangement (20).
A method of controlling a mask which comprises an air blower arrangement at least for drawing air from outside an air chamber into the air chamber, the method comprising:
(40) sensing a first temperature inside the air chamber;

(42) sensing a first relative humidity, inside the air chamber;

(46) sensing a second temperature, outside the air chamber;

(44) determining a threshold temperature based on the first temperature and the first relative humidity; and

(58) controlling an air blower arrangement for drawing air into an air chamber of the mask taking into account the second temperature such as to maintain the temperature in the air chamber above the threshold temperature.
A method as claimed in claim 11, further comprising (48) sensing a second relative humidity, outside the air chamber, and controlling the air blower arrangement further based on the second relative humidity.
A method as claimed in claim 11 or 12, wherein the threshold temperature is an estimation of the dew point temperature inside the air chamber.
A method as claimed in any one of claims 11 to 13, comprising determining a maximum inlet flow rate, below which a temperature inside the air chamber remains above the threshold temperature.
A method as claimed in any one of claims 11 to 14, comprising monitoring a pressure inside the air chamber, determining a respiration cycle from the pressure monitoring, and controlling the air blower arrangement in dependence on the determined respiration cycle.