WO2019079878A1

WO2019079878A1 - Air quality and composition sensor

Info

Publication number: WO2019079878A1
Application number: PCT/CA2018/000209
Authority: WO
Inventors: James Garry
Original assignee: Rostrum Medical Innovations Inc.
Priority date: 2017-10-27
Filing date: 2018-10-29
Publication date: 2019-05-02

Abstract

A sensor apparatus, system and method for the determination of water vapour content in a stream of air is described, which may more generally also be used to determine the concentration of one fluid in a second fluid. The method comprises: (1) applying a known quantity of heat to a stream of combined fluids within a chamber; (2) measuring the timing of the resultant temperature rise of the combined fluid stream at successive points within the chamber to determine the time-of-flight of the combined fluid stream through the chamber, and thereby infer the volume of the combined fluids subjected to heating; and (3) measuring the amount of the resultant temperature rise of the combined fluid stream to determine the heat capacity of the combined fluid stream, and thereby infer the composition of the combined fluid stream by calculation based upon the different known heat capacities of each of the first and second fluids. The sensor comprises: (1) a sealable housing defining an inner chamber in fluid communication with the fluid stream; (2) an electrical heater situated within the chamber of the sealable housing; and (3) a plurality of temperature sensors or thermocouples situated within the chamber of the sealable housing downstream of the electrical heater.

Description

AIR QUALITY AND COMPOSITION SENSOR

Technical Field:

This invention relates to sensors and systems for determining the mixing fraction of fluids or gasses, and in particular to the measurement of water vapour content in a stream of air.

Background:

Conventional sensors for the measurement of water vapour content in a stream of air typically rely on the adsorption of water to a surface to trigger some form of electrical signal. Such sensors can become damaged through the presence of contaminating adsorbed materials. In addition, adsorption and desorption of any gas such as water vapour is area-dependent, so small adsorption- based sensors generally yield correspondingly small signals that may be difficult to measure with precision. The transport processes of such sensors are also influenced by absolute temperature, so some form of electronic compensation is usually required in order to compensate for the temperature dependence.

A particular application of water vapor sensors is in the field of therapeutic ventilation for hospitalized patients. Devices that seek to measure the qualities of respired air must, however, work within tight constraints: breathing patterns may, for example, be irregular and rapid, and such features of patient respiration may confound conventional sensors, particularly those that seek to measure the humidity of an airstream. Conventional sensors using either capacitive or resistive methods to transduce water uptake by hygroscopic films respond to water vapour relatively slowly (typically on timescales of about 5 - 10 seconds) and thus may be unable to respond to changes within the timescale of a single breath. Hygrometric sensors also often display poor absolute accuracy values with typical uncertainties in the region of 3% or worse, and may display poor recovery behavior after they have become saturated with water, which may lead to "blind spots" in the measurement process.

Patients who are unable to breathe spontaneously may be fitted with an endotracheal tube which bypasses parts of the upper respiratory tract which would normally provide humidity to the inhaled airstream. Such patients must be ventilated with deliberately humidified air, and it has been hypothesized (Fleming et al, 2014) that the thermodynamic qualities of their respired airstream may provide insight into the cardio-pulmonary state of the patient.

The addition of water vapour to an air stream alters a number of qualities of the air to which the vapour has been added. This principle holds true not just for water vapour in an airstream, but also for a great many other cases in which a buffer gas is contaminated to some degree by another gas, and changes in these altered parameters may be measured and used to deduce the concentration of the contaminant second gas. Many modalities have been proposed to achieve this for the particular case of water vapour in air, for example by way of: changes wrought in the speed of sound (Van Schaik et al. 2010); the relative electrical permittivity of the air (Choi and Kim, 2013); and other parameters. The relative successes of these approaches vary according to the exact choice of property used to infer the contaminant gas concentration, and the magnitude of the change created in the measured property by changes in the concentration.

It would accordingly be advantageous to provide a sensor apparatus, system and method for the determination of water vapour content in a stream of air that provides good resolution, does not rely on the adhesion of water molecules to a surface, and is insensitive to adsorbed gasses and saturation. Such a sensor apparatus, system and method may more generally also find use in the determination of the concentration of one fluid in a second fluid.

Summary:

This invention provides a novel means of measuring the temperature, speed, and composition of an airstream. The invention exploits the different heat capacities of a carried (i.e. second) gas in a stream of carrier (i.e. first) gas, and employs a calorimetric method combined with atime-of-flight scheme to deduce the heat capacity of respired air by applying a well-described amount of heat to a parcel of air and then recording the resulting temperature rise at a point away from the heater.

In embodiments of the presently disclosed subject matter, there is provided a sensor apparatus, system and method for the determination of water vapour content in a stream of air, which may more generally also be used to determine the concentration of one fluid in a second fluid.

The method for determining water vapour content in a stream of air comprises: (1) applying a known quantity of heat to a stream of combined fluids within a chamber; (2) measuring the timing of the resultant temperature rise of the combined fluid stream at successive points within the chamber to determine the time-of-flight of the combined fluid stream through the chamber, and thereby inferring the volume of the combined fluids subjected to heating; and (3) measuring the amount of the resultant temperature rise of the combined fluid stream to determine the heat capacity of the combined fluid stream, and thereby inferring the composition of the combined fluid stream by calculation based upon the different known heat capacities of each of a known first and a known second gas in the airstream.

The sensor comprises: (1) a sealable housing defining an inner chamber in fluid communication with the fluid stream; (2) an electrical heater situated within the chamber of the sealable housing; and (3) a plurality of temperature sensors or thermocouples situated within the chamber of the sealable housing downstream of the electrical heater.

Brief Description of the Drawings:

For a fuller understanding of the nature and advantages of the disclosed subject matter, as well as the preferred modes of use thereof, reference should be made to the following detailed description, read in conjunction with the accompanying drawings.

Figure 1 is a schematic functional diagram illustrating the main components of a sensor apparatus, system and method in accordance with one embodiment of the disclosed subject matter.

Figure 2A is an enlarged perspective, partially sectional and partially exploded view of the sensor apparatus of the embodiment of Figure 1.

Figure 2B is an enlarged perspective, exploded view of a sensor apparatus in accordance with another embodiment of the disclosed subject matter.

Figure 2C illustrates enlarged side and front elevations of the internal assembly of the sensor apparatus of Figure 2B.

Figure 3 is a schematic circuit diagram of the sensor apparatus and system of the embodiment of Figure 1. Figure 4 is a flow chart of the processing steps of the sensor apparatus, system and method of Figure 1.

Figure 5 is a graphical representation of temperature versus time as measured by two successive thermocouples in the sensor apparatus of the embodiment of Figure 1 ; the separation between the received signals is clear and allows the airspeed to be calculated.

Figure 6 is a graphical representation of temperature rise versus relative humidity as measured by a thermocouple of the sensor apparatus of Figure 1, and illustrates the influence that water vapour at various concentrations in air has on the overall heat capacity of the airstrearn.

Description of Specific Embodiments:

The following description of specific embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Referring to Figure 1, one embodiment of a sensor apparatus and system 10 for measuring the temperature, speed and composition of an airstrearn is shown within the context of a medical diagnostic implementation of the invention.

In Figure 1, a patient 12 is intubated with an endotracheal tube 14, and is mechanically ventilated with humidified air provided by a mechanical ventilator 16 through a conventional paired artificial airway 18a, 18b. Humidified air generated by ventilator 16 passes to the patient 12 through intake airway 18a, and respired air passes back from the patient to the ventilator through return airway 18b. A sensor apparatus 20 is inserted into return airway 18b, and comprises a housing that defines an inner chamber or passage in fluid communication with airway 18b, such that respired air from the patient 12 passes through sensor apparatus 20 (and through return airway 18b) without any significant hindrance of the respiratory process.

Sensor apparatus 20 further comprises an electrical heater 28 and a plurality of thermocouples 30 arranged sequentially downstream of the heater 28 within its inner chamber, and is connected via electrical harness 22 to system monitor 24, which is equipped with a display 26. The monitor 24 provides electrical power to sensor apparatus 20, and receives and processes signals acquired by sensor apparatus 20 to yield data characterizing the airstrearn exhaled by patient 12, as is described in further detail below. With reference to Figure 2A, the housing of sensor apparatus 20 is preferably constructed of two separate castings or moldings 32a, 32b that are hermetically sealed together during use, but that may be separated to facilitate access to the inner chamber of the housing during cleaning or maintenance. Upstream molding 32a and downstream molding 32b of the sensor apparatus 20 housing (the terms "upstream" and "downstream" being defined herein with reference to the direction of the flow of respired air through the sensor apparatus 20) may be constructed of a suitable resilient plastic material, and in preferred embodiments the thermal conductivity of at least downstream molding 32b (and in some embodiments also that of upstream molding 32a) may be both particularly well-known and lower than that of common rigid thermoplastics. The reduction in thermal conductivity may, for example, be achieved through the addition of hollow glass microspheres (known as "ballotini") to the molten plastic before it is injected into a suitable mold, and reduces heat loss to the ambient environment from the warmed airstream within sensor apparatus 20.

Suitable fasteners such as flexible clips 34 and corresponding grooves or notches 36 are formed on upstream molding 32a and downstream molding 32b, respectively, to locate and secure the two portions of the sensor apparatus housing together during use. An O-ring 38 formed of rubber or other suitable material is adhered, such as by gluing or fitment within a corresponding groove in one of the upstream and downstream moldings 32a, 32b, and provides an airtight, hermetic seal therebetween when the sensor apparatus 20 housing is secured together for use. Tube-like fittings 40 on each of the upstream and downstream moldings 32a, 32b may be formed of tough plastic material, and are sized to provide an airtight, hermetic seal between each of the upstream and downstream ends of sensor apparatus 20 and the return airway 18b.

As outlined above, sensor apparatus 20 comprises an electrical heater 28 situated upstream of a plurality of miniature thermometers or thermocouples 30. Heater 28 comprises a resistive wire 42 wound as a mesh onto a refractory frame 44, and may be warmed as a single mass by application of an electrical current supplied by monitor 24. The mesh area of heater 28 may in some embodiments present an area to the airstream that is comparable to that of the internal cross section of the sensor apparatus 20 housing, and/or to that of return airway 18b. Resistive wire 42 may comprise one or more strands, and is constructed from an alloy that allows it to sustain rapid rises in temperature without deformation, thereby enabling the heater 28 to warm air that passes through it in a controllable manner. In the illustrated embodiment, wire 42 is a continuous metal wire made formed from an alloy with a high resistivity (~10^"6 Qm) and a narrow diameter (~0.3mm), and has a total length of roughly 30cm. In some preferred embodiments, wire 42 is a nickel-chromium alloy with an 80:20 ratio of those metals, and has a diameter of roughly 0.25mm.

Refractory frame 44 may be constructed from ceramic or other heat-resistant materials, and includes a plurality of grooves machined into its two larger plane faces and/or corresponding side walls to locate and secure windings of resistive wire 42. In preferred embodiments, the grooves of frame 44 are no shallower than about 1mm, and have a groove density along the edge of no fewer than about 3 per cm.

To facilitate an easy exchange of heaters 28 when servicing the sensor apparatus 20, suitable electrical contacts on refractory frame 44 may comprise a pair of sprung electrodes 46, and downstream molding 32b may further comprise a retaining clip 48 and opposite deformable element 50 to releasably secure heater 28 in firm contact within the sensor apparatus housing. The electrical contacts of the refractory frame 44 connect resistive wire 42 to the power supply of monitor 24 via a suitable socket provided on downstream molding 32b, and via electrical harness 22.

Fine gauge thermocouples 30 are employed to sense the temperature of the airstream within sensor apparatus 20, and are dimensioned and arranged sequentially within the inner chamber of the sensor apparatus housing such that the airstream, warmed by the heater 28, flows over the thermocouples 30 with a minimum of interference. In preferred embodiments, each of the thermocouples 30 is a 40 AWG (0.2mm diameter) thermocouple. In the embodiment of Figure 1, at least three thermocouples 30 are employed.

As will be apparent to those of skill in the art, sensor apparatus 20 may readily be modified to accommodate the sensing of airflow qualities in both directions, such as may be required if, for example, a patient was breathing spontaneously or being ventilated by a mechanical ventilator through a single artificial airway (rather than through a paired airway as illustrated in Figure 1). In embodiments of this sort, since the airflow through the sensor apparatus is bi-directional, the temperature sensing elements or thermocouples will need to be present on both sides of the heating element. One such embodiment is illustrated in Figures 2B and 2C, wherein electrical heater 128 comprises resistive wires 142 situated within refractory frame 144, and two thermocouples 130 are arranged sequentially within sensor apparatus 120 on both sides of heater 128.

Figure 3 is a schematic circuit diagram of sensor apparatus and system 10, and conceptually illustrates the connections between the following elements thereof: power supply 52 capable of supplying a constant current and a high-power current; resistive wire 42 of heater 28; small value, high power resistor 54 (comparable in specification to a 0.1Ω, 10W device); microcontroller 56; memory 58 holding a database of material thermal properties; interactive display 60 configured to enable a user to select and view aspects of the operation of sensor apparatus and system 10; clock 62 to organize the thermal stimuli by heater 28, and to provide an accurate temporal record of acquired data; amplifier 64 sensing array 66 of thermocouples 30; reference junction 68 that senses the ambient temperature of the exterior of sensor apparatus 20; and electrical harness 22 that conveys power to and signals from sensor apparatus 20. Elements 42, 64, 66 and 68 of system 10 form part of the sensor apparatus 20 of the system 10, and elements 52, 54, 56, 58, 60 and 62 of system 10 form part of the monitor 24.

Figure 4 is a flow chart of the processing steps undertaken by the sensor apparatus and system 10 in preforming the method of the invention. The general ability of the invention to sense the mixing fraction of two gases is described for the particular example that addresses the ventilation of a hospitalized patient. Let the sensor apparatus 20 be placed in the ventilation circuit of a patient 12 who is receiving mechanical ventilation. The sensor apparatus 20 is then connected to its monitor 24 via electrical harness 22, and the monitor 24 is turned on.

There are three stages embodied in the operation and subsequent data collection of the system 10:

1) After verifying that the sensor apparatus 20 is powered correctly, micro-controller 56 connects a small constant current from supply 52 to the wire 42 of heater 28, and monitors the voltage developed across the resistive heating wire 42. The resistance of wire 42 depends on its temperature, and in this way the monitor 24 establishes that the wire 42 is intact, free of short- circuits, and is at an appropriate temperature. 2) The microcontroller 56 then disconnects the constant current between supply 52 and wire 42, and connects wire 42 briefly to a high-power current provided by supply 52. A sensing resistor 54 in series with that supply allows microcontroller 56 to record the current that is applied to the wire mesh 42 of heater 28 during this heating phase.

3) Following the excitation of the heater wire 42, micro-controller 56 switches wire 42 of heater 28 back to the constant current source, and monitors the voltage developed by the resistance of the wire 42.

Throughout all of these stages the voltages raised by the thermocouple array 30 are logged. In preferred embodiments, system 10 uses a sampling speed of lkHz and registers the analogue voltages with an analogue-to-digital converter having a least-significant bit value of around 15 μν.

After each heating episode and subsequent data capture, microcontroller 56 computes a number of properties of the airflow that has passed through the sensor apparatus 20. These qualities, and the steps by which they are calculated, are outlined below. a) Air flow speed

The time elapsed between the start of the mesh 42 's heating pulse and the subsequent detection of warmed air by the thermocouple elements 30 is indicative of the flow speed. A number of strategies may be employed to characterize the instants at which detection is confirmed: examples include the points in time of peak value, or the mid-point value, or the points of maximum rate of change. In Figure 5, data are shown that demonstrate how the timing between the peak temperatures as recorded by evenly spaced thermocouples might elucidate the airstream speed. Figure 6 shows the effect of varying the relative humidity, and hence the airstream heat capacity, on the TBD of the measured pulses. b) Air heat capacity

The magnitude of the temperature rise experienced by each thermocouple 30, and history of that temperature rise, is indicative of the quantity of heat that was delivered to the air stream. The relationship between the heat capacity of humid air and the amount of water vapour held in it is well understood, so the relative humidity can be recovered if the heat capacity and temperature of the air are both known. From Gatley (2013), we know that the heat capacity (J kg^"1 K^"1) of humid air at constant pressure, c_p, is: c_p = 1005 + 1820* Eq. 1

Where x is the mass fraction of water vapour per unit mass of dry air. From knowledge of the temperature of the airstream prior to the addition of any heat, and the inferred heat capacity, it is possible to constrain the amount of water vapour that must be present in the air.

Additionally, the cooling history of the heated mesh 42, as deduced from the rate at which it cools after the heating pulse had been applied, further refines both the air flow speed measure and the estimate of the air's heat capacity.

Let the resistance of the heating mesh 42 be R (W), and for a given electrical current I (A), the instantaneous power P (Watts) that is developed in the mesh for a constant resistance and current is simply;

P = I²R

For a given pulse duration, t (sec), and with a non-constant resistance the total heat that is dissipated by the heater is knowable;

The wire 42 has a constant length and geometry, and a known variation of resistivity with temperature. The temperature field at some distance, in stationary air, at some distance from a heater experiencing a square pulse of heating power follows a first-order response, with the temperature history after the peak varying with time, t, as:

-t

T(t oa l - exp —

L τ .

Where the time constant, τ, is a compound term that involves the thermal diffusivity, the airspeed, and the geometry of the heater. Information from the thermocouples 30 about the arrival time statistics of the pulse of warmed air allows the airspeed to be calculated. Given the amount of heat that is known to be dissipated into the airstream, the peak temperature detected by the thermocouples 30 downstream of the heater 28 should be inversely related to the specific heat capacity, and thus the relative humidity of the inflowing air.

Figure 5 illustrates the ability of any given pair of thermocouples 30 to be used to deduce the speed of the air flowing through the sensor apparatus 20. If the mid-points of the rising edges of each curve are used as datum points, the calculated airflow speed differs from the speed as measured by a calibrated anemometer by no more than about 3%.

Figure 6 demonstrates this relationship for a test condition involving airflows of 15 cm s ^_1, with 100ms pulses having magnitudes of 2A and 2.5A delivered to a heater mesh of resistance 25Ω using 22 wires spaced in two near-coincident planes with a 2mm wire spacing pitch. The plotted data arose from a 0.001" gauge T-type thermocouple located 32mm from the heater wires, in a tube of diameter 19 mm.

The present description is of the best presently contemplated mode of carrying out the subject matter disclosed herein. The description is made for the purpose of illustrating the general principles of the subject matter and not to be taken in a limiting sense; the described subject matter can find utility in a variety of implementations without departing from the scope of the invention made, as will be apparent to those of skill in the art from an understanding of the principles that underlie the invention.

References:

Van Schaik W, Grooten M, Wernaart T, & van der Geld C W M, 2010. High accuracy acoustic relative humidity measurement in duct flow with air, Sensors 10(8):7421-33^■ August 2010, DOI: 10.3390/sl00807421.

Choi J M and Kim T W, 2013. Humidity sensor using an air capacitor, Trans. On Electrical and Electronic Materials, 14(4), 182-186.

Fleming N, Rose D, Brodkin I, 2014. Respiratory heat loss as a potential monitor of ventilation perfusion matching, {abstracts issue} Eu. J. of Anaesthesiology, 31, 77.

Liu X, Cheng S, Liu H, Hu S, Zhang D, Ning H, 2012. A Survey on Gas Sensing Technology, Sensors 12(7), 9635-9665.

Gatley D.P, 2013. Understanding Psychrometrics, American Society of Heating, 978-1936504312

Claims

Claims:

1. A sensor for use in a system that employs a calorimetric approach combined with a time- of-flight scheme to deduce the mixing fraction of gases in an airstream, the sensor comprising: a sealable housing defining an inner chamber in fluid communication with the airstream; an electrical heater situated within the inner chamber of the sealable housing; and a plurality of thermocouples situated within the inner chamber of the sealable housing downstream of the electrical heater.

2. A system for deducing the mixing fraction of gases in an airstream, the system comprising: the sensor of claim 1 ; and, a monitor that comprises a rapidly switchable power supply for providing electrical power to the heater of the sensor; a microcircuit and memory for receiving and storing signals generated by the heater and the thermocouples of the sensor; and a processor for interrogating a database of material properties in the memory, and for applying an algorithm thereto to recover the mixing fractions of a binary gas.

3. A method for deducing the mixing fraction of gases in an airstream, the method comprising the performance, by the system of claim 2, of the following steps:

1) applying a known quantity of heat to a stream of combined gases within the chamber of the sealable housing;

(2) measuring the timing of the resultant temperature rise of the combined gas stream at successive points within the chamber to determine the time-of-flight of the combined gas stream through the chamber, and thereby inferring the volume of the combined fluids subjected to heating; and (3) measuring the amount of the resultant temperature rise of the combined gas stream to determine the heat capacity of the combined gas stream, and thereby inferring the composition of the combined gas stream by calculation based upon the different known heat capacities of each of a first and a second known gas in the airstream.