GB2578642A

GB2578642A - Fluid temperature measurement method, apparatus and computer program

Info

Publication number: GB2578642A
Application number: GB1817969.7A
Authority: GB
Inventors: Cary Barnard Andrew; Cary Barnard Simon
Original assignee: Scb Ass Ltd
Current assignee: Scb Ass Ltd
Priority date: 2018-11-02
Filing date: 2018-11-02
Publication date: 2020-05-20
Anticipated expiration: 2038-11-02
Also published as: GB201817969D0; GB2578642B

Abstract

Various methods are disclosed for providing software-implemented acoustic temperature measurement in mobile electronic devices, such as smartphones, and domestic smart devices. In a hand-held computing device, comprising a speaker and a microphone, a computer program generates an output signal having a frequency; the speaker is used to generate a sound from the output signal; the sound is detected using the microphone; and a measured phase difference between the output signal and the detected sound is used to calculate a fluid temperature between the speaker and microphone. The phase difference may be determined using a phase comparator function which is part of a phase locked loop. In order to address the issues of clock difference and clock drift where the microphone and speaker are controlled by difference processors on different clocks, a reference output signal may be used to generate a crosstalk signal in a signal path between the microphone and the hand-held computing device. The crosstalk signal may be detected and a reference phase difference determined between the reference output signal and the crosstalk signal. The reference phase difference may be used to correct the measured phase difference. A trend in the reference phase difference and/or measured phase difference may be determined and at least one of the phase differences extrapolated to a common time point, and the extrapolated phase difference(s) used to calculate the corrected measured phase difference. Embodiments are also claimed comprising a second speaker and a second microphone. The domestic smart device could be a thermostat or a smart doorbell. The sound may be reflected from a fixed surface.

Description

Fluid temperature measurement method, apparatus and computer program The present invention is concerned with a method of measuring a fluid temperature, and apparatus for making such a measurement and a computer program comprising instructions for carrying out such a method. More specifically, the present invention is concerned with a method, apparatus and computer program for carrying out acoustic measurement of air temperature using a smartphone or similar device.

Most smartphones do not contain means for measuring ambient air temperature. Implementing such functionality by inclusion of e.g. a thermocouple (or other conductive temperature measurement means) would increase the cost of the smartphone and may necessitate increasing the size of the device due to the additional packaging space required. It would also be desirable to position the thermocouple away from parts of the phone which increase in temperature with use (such as the battery and processor(s)). Therefore, it is generally seen as commercially and technically challenging to incorporate a temperature measurement function on such a device. Instead, most smartphones use weather data and location to predict the outside temperature at any given point in time.

A further disadvantage with known temperature measurement means is that in order to function, part of the means needs to reach thermal equilibrium with the air (or other surrounding fluid whose temperature is to be measured). The transfer of heat between a thermocouple and air is a slow process, and as such many thermocouples exhibit "lag". This means they cannot respond quickly to dynamic changes in the temperature of the surrounding air. Further, thermocouples necessarily need to be attached to the smartphone, which means that as well as conduction from the surrounding air, there will always be conduction from the device itself, which may be at a different temperature to the ambient air. The mass of a smartphone is generally significantly greater than any thermocouple which would be installed therein, and as such the thermocouple would need to be thermally isolated from this "thermal mass" if true air temperature is to be measured. If, for example, a smartphone with a thermocouple is moved from room temperature (20C) into cold air (e.g. OC), then it will take time for the device to cool. This slows down the response of the thermocouple.

What is required is a method of measuring air temperature which does not exhibit these problems. As such, it is an aim of the present invention to overcome, or at least mitigate the above problems.

According to a first aspect of the invention there is provided a hand-held computing device comprising: a speaker; a microphone; and, a computer program comprising instructions which, when the program is executed by the hand-held computing device cause the hand-held computing device to: generate an output signal having a frequency; use the speaker to generate a sound from the output signal; detect the sound using the microphone; determine a measured phase difference between the output signal and the detected sound; and, calculate a fluid temperature between the speaker and microphone using the measured phase difference.

This allows fluid temperature to be directly (acoustically) measured without any specialist equipment on the device.

According to a second aspect, there is provided a computer-implemented method of detecting a fluid temperature comprising the steps of: providing a reference signal; using the reference signal to generate a crosstalk signal; detecting the crosstalk signal; determining a reference phase difference between the reference signal and the crosstalk signal; providing an output signal; providing a received audio signal from a microphone, the received audio signal representative of the output signal having passed through a fluid; determining a measured phase difference between the received audio signal and the output signal; calculating a corrected phase difference from the measured phase difference and the reference phase difference; and, calculate a fluid temperature between the speaker and microphone using the corrected phase difference.

Advantageously, this compensates for any inherent (static) difference in clock time between the transmitted and detected signals.

Preferably the method also comprises the steps of: monitoring the change in at least one of the reference phase difference and the measured phase difference; determining a trend in the at least one of the reference phase difference and the measured phase difference; extrapolating at least one of the reference phase difference and the measured phase difference to a common time point; using the extrapolated phase difference or phase differences to calculate the corrected measured phase difference.

This compensates for clock "drift".

According to a third aspect, there is provided a domestic smart device comprising a first speaker; a first microphone; and, a computer program comprising instructions which, when the program is executed by the domestic smart device cause the domestic smart device to: generate an output signal having a frequency; use the first speaker to generate a sound from the output signal; detect the sound using the first microphone; determine a first measured phase difference between the output signal and the detected sound; and, calculate a fluid temperature between the speaker and microphone using the measured phase difference.

Example methods, apparatuses and computer programs according to the invention will now be described with reference to the accompanying Figures, in which: FIGURE 1 is a front view of a smartphone having a program according to the invention installed thereon; FIGURE 2 is a schematic of how the smartphone of Figure 1 is operated according to a first embodiment of the invention; FIGURE 3 is a schematic of how a smartphone is operated according to a second embodiment of the invention; FIGURE 4 is a schematic of how a smartphone is operated according to a third embodiment of the invention; FIGURE 5 is a schematic of how a smartphone is operated according to a fourth embodiment of the invention; FIGURE 6 is a front view of a smartphone having a program according to the invention installed 10 thereon; FIGURE 7 is a schematic of how the smartphone of Figure 6 is operated according to a fifth embodiment of the invention; FIGURE 8 is a flow diagram of the process undertaken by the fifth embodiment; FIGURE 9 is a flow diagram of the process undertaken by an improved fifth embodiment; and FIGURE 10 is a schematic of a sixth embodiment.

The first embodiment Referring to Figure 1, a smartphone 10 comprising a case 12 having a touchscreen 14. The smartphone 10 comprises a memory 16, processor 18, speaker 20 and microphone 22. The processor 18 is configured to generate an output signal to the speaker 20 which creates sound. The processor 18 is configured to control the frequency and amplitude of the sound generated by the speaker 20. The microphone 22 is configured to detect sound and convert it to an electrical output which can be processed by the processor 18. As such, the smartphone 10 is conventional in nature, and of known construction. The speaker 20 and microphone 22 are a distance dl apart.

The memory 16 stores a computer program 24, specifically in the form of an "app" (Figure 2), comprising instructions configured to carry out the method of the present invention using the processor 18.

Referring to Figure 2, the functional elements of the computer program 24 are shown in schematic form. A sound generation function 26 is provided which instructs the processor to control the speaker 20 to generate a sound 28 via a digital transmitted signal 30. In this embodiment, the sound 28 has a predetermined constant frequency f. The frequency f is "high acoustic"-i.e. just below ultrasound (20kHz). In this embodiment, the frequency is 18.75kHz (although it will be understood that this can be varied). Such a frequency provides a balance between audibility (ideally the sound should be minimally audible, or inaudible) and the capabilities of the equipment (limited by (i) the Nyquist limit, or 50% of the sampling frequency-typically 48kHz and (ii) the frequency response of the microphone and speaker). The microphone 22 is able to detect the sound 28 and convert it to a digital received signal 32.

The transmitted signal 30 and received signal 32 are both received by a digital phase comparator function 34. Phase comparators are known in the field of signal processing, and a detailed description is not provided here. Essentially the phase comparator 34 compares the oscillatory received signal 32 and oscillatory transmitted signal 30 and determines a phase difference 0 (theta). The phase difference El is transmitted to a temperature calculation function 38 in the form of a phase difference signal 36.

In one embodiment, the phase comparator may be part of a phase locked loop (PLL) in which a reference signal is locked to the received signal. The phase offset between the reference signal and the transmitted signal will then provide the phase difference 8 (theta) between the transmitted signal and the received signal.

The temperature calculation function 38 is then configured to calculate the average temperature of the fluid between the speaker 20 and microphone 22 as follows.

The present invention relies on the principle that as the fluid (e.g. air) temperature varies, its density also varies. The change in air density causes the speed of sound to vary according to: v = 331.341 + Where v is the speed of sound in m.s-1; and, t is the temperature in C. The phase shift in a sound travelling along a path in a fluid is: 27rf 0= Gil.-v Where 0 is the phase shift in rad; f is the transmitted sound frequency in Hz; cla is the distance travelled in m; and t v is the speed of sound in m.s-1.

Therefore, from the phase difference provided by the signal 36, and the frequency f of the transmitted signal 30, the velocity can be calculated by: v = dl 27rf And this can be used to calculate the temperature in C: The second embodiment t = 273 (72)2) 331.3) 1) Turning to Figure 3, the functional elements of a computer program 124 according to a second embodiment are shown in schematic form. The program 124 is configured to run on a smartphone with a first speaker 120 at a distance dl from a microphone 122 and a second speaker 121 at a distance d2 from the microphone 122. dl and d2 are different, i.e. dl # d2. It will be noted that dl and d2 are shown as being parallel in Figure 3 for clarity, but it should be noted that in reality they represent the shortest distance between the various components.

The smartphone has a processor 118 configured to carry out the instructions of the program 124.

A sound generation function 126 is provided which instructs the processor 118 to control the first speaker 120 to generate a first sound 128 via a first transmitted signal 130, and to control the second speaker 121 to generate a second sound 129 via a second transmitted signal 131. The first and second transmitted signals 130, 131 are identical in terms of frequency and phase in this embodiment.

The sounds 128, 129 have the same predetermined constant frequency f. The microphone 122 is able to detect both sounds 128, 129 which are comprised in a received signal 132. The sounds 128, 129 will be out of phase due to the difference between dl and d2.

The common transmitted signal and received signal 132 are both received by a phase comparator function 134. The phase comparator 134 is configured to determine: (i) a phase difference el between the common transmitted signal and the corresponding part of the received signal 132 representing the first sound 128; and, a phase difference 02 between the common transmitted signal and the corresponding part of the received signal 132 representing the second sound 129.

AO is the phase offset and represents the phase difference between dl and d2. The use of a different value for dl and d2 can be used to mitigate any latency or delay in the generation, playing or recording of the sounds. By way of example, if an inherent delay causes a delay phase offset ()delay, this will be present in both phase differences: 2n-f 01 = d1. -+ ()delay

V 2n1

02 = c12. -± °delay

V

A phase offset be determined by: AO = 62 -01 Which eliminates the delay: 2n-f 214' 2n-f AO = (d2 + °delay) -(d1 + @delay) = (d2 -dn.

v = (d2 -d1).

AO

And this can be used to calculate the temperature in C: t = 273 12)2) k..331.3) 1) It will also be noted that if dl and d2 are the same (d1=d2), the above technique will not work, but instead the mean of two separately calculated temperatures according to the first embodiment can be determined to increase accuracy.

The third embodiment Turning to Figure 4, the functional elements of a computer program 224 according to a third embodiment are shown in schematic form. The program 224 is configured to run on a smartphone with a speaker 220 at a distance dl from a first microphone 222 a distance d2 from a second microphone 223. dl and d2 are different, i.e. dl # d2. It will be noted that dl and d2 are shown as being parallel in Figure 4 for clarity, but it should be noted that in reality they represent the shortest distance between the various components.

The smartphone has a processor 218 configured to carry out the instructions of the program 224.

A sound generation function 226 is provided which instructs the processor 218 to control the speaker 220 to generate a sound 228 via a transmitted signal 230. i.e. 2rtf

The sound 228 has a predetermined constant frequency f. The first microphone 222 and second microphone 223 are able to detect the sound 228 and generate a respective first received signal 232 and a second received signal 233. The sound received at each microphone will be out of phase with the other due to the difference between dl and d2.

The transmitted signal 230 and received signals 232, 233 are received by a phase comparator function 234. The phase comparator 234 is configured to determine: (I) a phase difference 01 between the transmitted signal and the first received signal 232; and, a phase difference 02 between the transmitted signal and the second received signal 233.

The temperature can then be calculated in the same way as per the second embodiment, and has the same advantages.

The fourth embodiment Turning to Figure 5, the functional elements of a computer program 324 according to a third embodiment are shown in schematic form. The program 324 is configured to run on a smartphone with a first speaker 320, a second speaker 321, a first microphone 322 and a second microphone 323.

The following distances are defined: Distance From To dl First speaker 320 First microphone 322 d2 Second speaker 321 Second microphone 323 d3 First speaker 320 Second microphone 323 d4 Second speaker 321 First microphone 322 It will be noted that dl to d4 are shown as being parallel in Figure 5 for clarity, but it should be noted that in reality they represent the shortest distance between the various components.

In this embodiment dl to d4 are all different-no two values are the same.

The smartphone has a processor 318 configured to carry out the instructions of the program 324.

A sound generation function 326 is provided which instructs the processor 318 to control the speaker 320 to generate a sound 328 via a transmitted signals 330, 331.

The sound 328 has a predetermined constant frequency f. The first microphone 322 and second microphone 323 are able to detect the sound 328 and generate a respective first received signal 332 and a second received signal 333. Note that each signal will comprise two phases generated by the two speakers. The sound received at each microphone will be out of phase with the other due to the difference between dl, d2, d3 and d4.

The transmitted signal 330 and received signals 332, 333 are received by a phase comparator function 334. The phase comparator 334 is configured to determine four phase differences 01, 02, 03, 04.

The temperature can then be calculated in the same way as per the second embodiment and has the same advantages.

The fifth embodiment The above embodiments require each of the components (the speaker(s) and microphone(s)) to be synchronised. In other words, they need to be controlled by the same processor, or at least two processors which are accurately synchronised to the same clocks. In many smartphones, the speakers and microphones are controlled by different processors on different clocks. This presents a problem-upon startup there may be a timing offset between the clocks. This timing offset is not constant, and changes upon each power-up of the device (and across devices).

Furthermore, inasmuch as the smartphone needs to be controlled by contact with the touchscreen 14 during the sensing operation, this can interfere with the passage of sound from the microphone to the speaker.

Referring to Figures 6 to 8, the fifth embodiment comprises the smartphone 10 as described with respect to the first embodiment. The smartphone 10 is equipped with a set of headphones 24. These are of convention design and comprise a plug 26 for interface with the smartphone, 10, a first wire 28 to a first headphone speaker 30 and a second wire 32 to a second headphone speaker 34. The first and second wires 28, 32 are joined together along a first portion 36 between the plug 26 and a split point 38. A microphone 40 is provided in the second wire 32, which is typically used to pick up voice calls when listening to the headphones 30, 34.

A template or jig 42 is provided to which the second speaker 34 and microphone 40 are attached. The jig 42 is configured to provide a predetermined distance dl. The jig 42 has receiving formations to facilitate location of both the second speaker 34 and microphone 40. The jig 42 may also be configured to hold the first headphone speaker 30 at a far distance from the microphone 40, or indeed at least partially block the passage of sound between the first speaker 30 and microphone 40. This is discussed further below.

Referring to Figure 7, the functional elements of a computer program 424 according to a fifth embodiment are shown in schematic form. The program 424 is configured to run on the processor 18 smartphone 10 of Figure 6. Referring to Figure 8, a flow diagram for the sequence of operation is shown.

At step 500 (Figure 8), a sound generation function 426 is provided which instructs the processor 18 to control the first speaker 30 to generate a first sound 428 via a first transmitted signal 430. At this point, the second speaker 32 is silent.

At step 502, the microphone 40 is used to detect a received signal 432. According to the invention, the first speaker 30 is positioned as far away from the microphone 40 as possible, and preferably so far that little or no audio can pass from the first speaker 30 to the microphone 40 through the fluid.

In some embodiments, the speaker 30 may be acoustically insulated with a sound-attenuating material.

The detected signal 432 predominantly comprises "crosstalk" from the first speaker 30. Most audio equipment experiences "crosstalk". This is usually a result of some kind of capacitive or inductive coupling between adjacent channels. In this case, it is a coupling between the first speaker 30 (first transmitted signal 430) and the microphone 40 (received signal 432).

At step 504, the sound generation function 426 is provided which instructs the processor 18 to control the second speaker 32 to generate a second sound 429 via a second transmitted signal 431.

The first and second transmitted signals 430, 431 are identical in terms of frequency and phase in this 20 embodiment.

At step 506, the microphone 40 is used to detect the received signal 432. Unlike the first speaker 30, the second speaker 32 is positioned such that the microphone 40 can detect the sound 429 passing across the distance cll.

Steps 502, 506 produce two waveforms-a reference waveform 550 and a measurement waveform 552.

At step 508, the phase difference 0,0 between the first transmitted signal 430 and the reference waveform is determined using the phase comparator 434.

At step 510, as with the first embodiment, a phase comparator 434 is used to determine the phase difference e between the second transmitted signal 432 and the measurement waveform 552. As with the first embodiment, the phase difference emeasured can be used to calculate the speed of sound in air across dl at function 438. As discussed above, this is problematic as the clock on which the second transmitted signal 432 is based may differ from the clock on which the measurement waveform 552 is based.

In order to overcome the set difference in the two clocks, a corrected phase difference at step 512 can be determined by: °corrected = °measured -°ref Improvement to the fifth embodiment A further problem with clock timing is clock "drift". This may result in the time difference between the clocks used in the present invention changing.

This results in an unknown and constantly changing offset between the signals. The fifth embodiment cannot account for this because the two measurements of reference and measured phase difference are necessarily taken at different times.

The "drift" experienced is usually of a relatively low frequency compared to the signal / ultrasonic waveform. For example, the drift may be 0.5Hz as compared to e.g. 18.75kHz.

Referring to Figure 9, a process identical to the process of Figure 8 is shown, up to step 506.

At step 508', instead of a single phase offset erg being calculated, data representing the changing phase offset with respect to time is calculated. The same is done at new step 510' for Bmeasured. The result is two traces of phase difference over time (one reference, from the crosstalk channel, and one measured).

At step 512', regression is used to model the variation in phase difference over time. This results in: ere/ = f (t) °measured = 9(0 Due to the fact that the drift frequency is so much lower than the signal frequency, in this embodiment straight line regression is used. Such that: e"f = f (t) = A. t Ome,""ed = g(t) = B. t Where A and B are constants.

At step 514' Given that the difference in time between steps 502 and 506 are known (At), Oref is extrapolated forward by At/2 and Omeasured is extrapolated backward by At/2 to estimate the phase differences of Bref and °measured at the same time.

At step 516', the interpolated / extrapolated phase values are used to determine: °corrected = °measured -°ref This method takes account of both the absolute difference in clock times, and the dynamic drift. Variations Variations to the above embodiments fall within the scope of the present invention.

Although the above invention has been described in terms of smartphone use, there are other applications. All that is required is a memory, processor, at least one speaker and at least one microphone.

A non-exhaustive list of examples is provided below: Tablet computers These are very similar to smartphones, and the skilled addressee would understand that the aforementioned technology is easily adapted to such devices. It is also noted that these devices also comprise multiple speakers which facilitate the embodiments which use them. Such devices typically have headphone jacks for implementation of the fifth embodiment.

Laptop / desktop computers These devices typically have internal microphones and speakers, and furthermore typically have headphone jacks for implementation of the fifth embodiment.

Smart speakers So-called "smart speakers" such as the Amazon (TM) Echo (TM) range, Apple (TM) HomePod (TM) etc typically have several microphones and speakers which can be used to implement the present invention. In such circumstances, the devices may be configured to activate the method according to the invention in response to a voice command requesting the local temperature.

In one embodiment, two smart speakers spaced apart across a room could be used to transmit and receive sounds per the present invention, although their distance apart would need to be measured either physically or e.g. by RF / Bluetooth (TM).

Door security systems Many smart door security systems such as the Ring (TM) video doorbell and the Nest (TM) Hello (TM) video doorbell have both speakers and microphones. As such, external temperature can be measured and reported to a user e.g. on a smartphone app.

Smart thermostats Smart thermostats such as the Nest (TM) thermostat can be provided with microphones and speakers to provide local acoustic temperature measurement. This may be in addition to, or instead of, traditional conductive sensors. In one embodiment, a speaker may be positioned across the room or area being controlled by the thermostat, as such providing an average air temperature across the room (rather than in the local vicinity of the thermostat). This is shown e.g. in Figure 10. A smart thermostat 600 comprises a memory, processor and microphone. A speaker unit 602 is configured to emit a sound 604 which travels across a room 606, distance dl. The smart thermostat 600 can then generate a room average temperature according to the present invention. It will be noted that the clocks of the smart thermostat 600 and speaker unit 602 need to be synchronised, but this can be achieved using e.g. RF signals between the two.

In a different embodiment, the speaker 602 is incorporated into the smart thermostat 602 and is used to transmit a sound which is reflected off a surface a known distance from the thermostat 602 (for example the opposite wall). The total travel path of the sound (2 x dl) is then known, and can be used to calculate average acoustic temperature across the flight path. The distance from the smart thermostat 602 to the wall can be physically measured, or e.g. measured by electromagnetic (e.g. light, or RF) signals emitted from the unit.

Claims

Claims 1. A hand-held computing device comprising: a speaker; a microphone; and, a computer program comprising instructions which, when the program is executed by the hand-held computing device cause the hand-held computing device to: generate an output signal having a frequency; use the speaker to generate a sound from the output signal; detect the sound using the microphone; determine a measured phase difference between the output signal and the detected sound; and, calculate a fluid temperature between the speaker and microphone using the measured phase difference.
2. A hand-held computing device according to claim 1, wherein the step of determining a measured phase difference between the output signal and the detected sound is carried out using a phase comparator function.
3. A hand-held computing device according to claim 2, wherein the phase comparator function is part of a phase locked loop (PLL).
4. A hand-held computing device according to any preceding claim, wherein the computer program is configured to: transmit a reference output signal to create a crosstalk signal in a signal path between the microphone and the hand-held computing device; detect the crosstalk signal; determine a reference phase difference between the reference output signal and the crosstalk signal; use the reference phase difference to correct the measured phase difference; and, calculate the fluid temperature between the speaker and microphone using the corrected measured phase difference.
5. A hand-held computing device according to claim 4, wherein the corrected phase difference is the measured phase difference minus the reference phase difference.
6. A hand-held computing device according to any preceding claim, wherein the speaker and the microphone are part of a headphone set.
7. A hand-held computing device according to claim 6, dependent on claim 4 or 5, wherein: the headphone set comprises a first and second speaker; the first speaker is used to generate a sound from the reference output signal; and, the second speaker is used to generate a sound from the output signal.
8. A hand-held computing device according to claim 7, wherein the first speaker is positioned at a larger distance from the microphone than the second speaker, and / or sound attenuating means is provided between the first speaker and the microphone.
9. A hand-held computing device according to claim 4, or any of claims 5 to 8 dependent thereon, wherein the computer program is configured to: monitor the change in at least one of the reference phase difference and the measured phase difference; determine a trend in the at least one of the reference phase difference and the measured phase difference; extrapolate at least one of the reference phase difference and the measured phase difference to a common time point; use the extrapolated phase difference or phase differences to calculate the corrected measured phase difference.
10. A hand-held computing device according to claim 9, wherein the computer program is configured to: monitor the change in the reference phase difference over a first time period; determine a trend in the reference phase difference; monitor the change in the measured phase difference over a second time period; determine a trend in the measured phase difference; extrapolate both the reference phase difference and the measured phase difference to a common time point; use the extrapolated phase difference or phase differences to calculate the corrected measured phase difference.
11. A hand-held computing device according to claim 10, wherein the computer program is configured to: extrapolate both the reference phase difference and the measured phase difference by the same amount of time.
12. A computer-implemented method of detecting a fluid temperature comprising the steps of: providing a reference signal; using the reference signal to generate a crosstalk signal; detecting the crosstalk signal; determining a reference phase difference between the reference signal and the crosstalk signal; providing an output signal; providing a received audio signal from a microphone, the received audio signal representative of the output signal having passed through a fluid; determining a measured phase difference between the received audio signal and the output signal; calculating a corrected phase difference from the measured phase difference and the reference phase difference; and, calculate a fluid temperature between the speaker and microphone using the corrected phase difference.
13. A computer-implemented method according to claim 12, wherein the corrected phase difference is the measured phase difference minus the reference phase difference.
14. A computer-implemented method according to claim 12 or 13, comprising the steps of: monitoring the change in at least one of the reference phase difference and the measured phase difference; determining a trend in the at least one of the reference phase difference and the measured phase difference; extrapolating at least one of the reference phase difference and the measured phase difference to a common time point; using the extrapolated phase difference or phase differences to calculate the corrected measured phase difference.
15. A computer-implemented method according to claim 14, comprising the steps of: monitoring the change in the reference phase difference over a first time period; determining a trend in the reference phase difference; monitoring the change in the measured phase difference over a second time period; determining a trend in the measured phase difference; extrapolating both the reference phase difference and the measured phase difference to a common time point; using the extrapolated phase difference or phase differences to calculate the corrected measured phase difference.
16. A computer-implemented method according to claim 15, comprising the steps of: extrapolating both the reference phase difference and the measured phase difference by the same amount of time.
17. A data processing apparatus comprising means for carrying out the method of any of claims 12 to 16.
18. A computer program comprising instructions which, when the program is executed by a data processing apparatus, cause the data processing apparatus to carry out the method of any of claims 12 to 16.
19. A computer-readable data carrier having stored thereon the computer program of claim 18.
20. A domestic smart device comprising a first speaker; a first microphone; and, a computer program comprising instructions which, when the program is executed by the domestic smart device cause the domestic smart device to: generate an output signal having a frequency; use the first speaker to generate a sound from the output signal; detect the sound using the first microphone; determine a first measured phase difference between the output signal and the detected sound; and, calculate a fluid temperature between the speaker and microphone using the measured phase difference.
21. A domestic smart device according to claim 20, comprising a second speaker, in which when the program is executed by the domestic smart device cause the domestic smart device to: generate an output signal having a frequency; use the second speaker to generate a sound from the output signal; detect the sound using the first microphone; determine a second measured phase difference between the output signal and the detected sound; calculate an adjusted phase difference from the first and second measured phase differences; and, calculate a fluid temperature between the speaker and microphone using the adjusted phase difference.
22. A domestic smart device according to claim 21, comprising a second microphone, in which when the program is executed by the domestic smart device cause the domestic smart device to: generate an output signal having a frequency; use the first and / or second speaker to generate a sound from the output signal; detect the sound using the first and / or second microphone; determine a plurality of measured phase differences between the output signal and the detected sound; calculate an adjusted phase difference from the plurality of measured phase differences; and, calculate a fluid temperature between the speaker and microphone using the adjusted phase difference.
23. A domestic smart device according to any of claims 20 to 22, configured to control a heating and / or cooling system using the fluid temperature
24. A domestic smart device according to any of claims 20 to 23, comprising a voice activation function configured to initiate temperature measurement on receipt of a predetermined voice command.
25. A domestic smart doorbell according to any of claims 20 to 22.
26. A method of calculating fluid temperature comprising the steps of: providing a domestic smart device according to claim 20; positioning the domestic smart device opposite a fixed surface; using the first speaker to generate a sound from the output signal; allowing the sound to be reflected off the fixed surface; detecting the sound after is has been reflected.