Sensing apparatus and method

• US 8,224,621 B2
• Filed: 05/14/2004
• Issued: 07/17/2012
• Est. Priority Date: 05/14/2003
• Status: Active Grant

• Current Assignee: Garrett Thermal Systems Ltd. (Honeywell International Inc.)
• Original Assignee: Xtralis Pty Ltd. (Honeywell International Inc.)
• Inventors: Ajay, Kemal, Rezny, Michael, Caprihan, Arjun Vinoo
• Primary Examiner(s): Charioui, Mohamed
• Assistant Examiner(s): Suarez, Felix E
• Application Number:
  US10/556,482
• Publication Number:
  US 20070084286A1
• Time in Patent Office: 2,986 Days
• Field of Search: 702/14, 702/36, 702/59, 702/183, 702/186, 702/189, 702/182, 73/40, 381/159, 340/630
• US Class Current: 702/176
• CPC Class Codes: G01F 1/66 by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters
• CPC Class Codes: G01F 1/667 Arrangements of transducers for ultrasonic flowmeters; Circuits for operating ultrasonic flowmeters
• CPC Class Codes: G01F 1/74 Devices for measuring flow of a fluid or flow of a fluent solid material in suspension in another fluid
• CPC Class Codes: G01N 1/2205 with filters
• CPC Class Codes: G01N 1/2273 Atmospheric sampling
• CPC Class Codes: G01N 1/26 with provision for intake from several spaces
• CPC Class Codes: G01N 2015/0046 in gas, e.g. smoke
• CPC Class Codes: G01N 2015/1486 Counting the particles
• CPC Class Codes: G01N 2291/0217 Smoke, combustion gases
• CPC Class Codes: G01N 2291/02408 Solids in gases, e.g. particle suspensions
• CPC Class Codes: G01N 2291/02836 Flow rate, liquid level
• CPC Class Codes: G01N 29/024 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G08B 17/10 Actuation by presence of smoke or gases , e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means
• CPC Class Codes: G08B 17/113 Constructional details

Abstract

A method for the determination of the time of flight of a signal transmitted between a transmitter (42, 44) and a receiver (44, 42). In one form, the method involves transmitting a first signal and a second signal having a waveform modification introduced at a predetermined point in time of the duration of the second signal; receiving said first and second transmitted signals; determining a point of diversion between the first and second received signals to determine an arrival time of the introduced waveform feature modification at the receiver. In addition, the invention provides an accurate time of flight determination of ultrasonic signals in a flow sensor (24) adapted for a smoke detector system (10).

Claims

1. A method of determining a time of flight of a signal transmitted between a transmitter and a receiver, said method comprising the steps of:

at a signal transducer of the transmitter, generating a first ultrasonic signal comprising a plurality of cycles of a characteristic waveform feature;

at the signal transducer of the transmitter, generating a second ultrasonic signal comprising a plurality of cycles of the characteristic waveform feature and further comprising a waveform modification being a phase shift in the cycle of a characteristic waveform feature introduced at a predetermined point in time of a duration of the second ultrasonic signal;

receiving said first and second generated signals at the receiver;

determining a time of reception of the introduced phase shift in the second ultrasonic signal by comparing the waveform of the first received signal to the waveform of the second received signal and determining a point of diversion between corresponding characteristic waveform features of the first and second received signals;

determining a time of flight of the second ultrasonic signal based on the determined time of reception of the introduced phase shift and its time of generation.

2. The method of claim 1 wherein the step of determining a point of diversion further comprises:

calculating a difference between a value of the first received signal and a corresponding value of the second received signal at each point of occurrence of a characteristic waveform feature within the first received signal;

designating the point of diversion as the first point of occurrence at which the calculated difference is greater than the value of the second received signal.

3. The method of claim 2 further comprising the step of:

calculating the difference between the time of the point of diversion and the time of generation of the introduced phase shift.

4. The method of claim 2 further comprising the steps of:

measuring a time relationship between the introduced phase shift and the point of diversion and;

calculating the difference between the time of reception, based on the measured time relationship, and the time of generation of the introduced phase shift.

5. The method of claim 1 further comprising the step of:

repeating the steps of generating and receiving such that successive first and second ultrasonic signals are super positioned at the step of determining.

6. The method of claim 1, wherein the characteristic waveform feature of a signal is one of:

a) a peak;

b) a combination of peaks;

c) a zero-crossing;

d) a combination of zero-crossings.

7. The method of claim 1, wherein the phase shift is introduced near the start of the second ultrasonic signal.

8. The method of claim 7 wherein the phase shift is introduced at one of a third, fourth or fifth waveform peak after the start of the second ultrasonic signal.

9. The method of claim 1, wherein the phase shift comprises a phase inversion.

10. The method of claim 1, wherein the signal transducer of the transmitter is driven at a resonant frequency in a frequency range of about 60 kHz to about 90 kHz.

11. The method of claim 1 further comprising the steps of:

selecting a characteristic waveform feature of the first ultrasonic signal in accordance with predetermined selection criteria based on the point of diversion;

generating and receiving a plurality of first ultrasonic signals;

detecting zero-crossings of the received plurality of first ultrasonic signals which indicate the presence of the selected characteristic waveform feature in each of the received plurality of first ultrasonic signals;

estimating a position of the selected characteristic waveform feature of the received plurality of first ultrasonic signals in accordance with predetermined estimation criteria based on the detected zero crossings to provide a position estimation value;

processing the position estimation value to determine a corresponding estimation time;

calculating the time of arrival of the selected characteristic waveform feature of at least one of the received plurality of first ultrasonic signals by adding a predetermined delay time to the estimation time.

12. The method of claim 11 wherein the predetermined selection criteria comprise one of:

a) adding a predefined delay to the time of the point of diversion;

b) subtracting a predefined delay from the time of the point of diversion.

13. The method of claim 11 wherein the predetermined estimation criteria comprise:

a) measuring the time of zero-crossings adjacent the selected characteristic waveform feature and;

b) averaging the measured time of zero-crossings.

14. Apparatus adapted to determine the time of flight of a signal transmitted between a transmitter and a receiver, said apparatus comprising:

processor means adapted to operate in accordance with a predetermined instruction set,

said apparatus, in conjunction with said instruction set, being adapted cause a transducer of a transmitter to transmit, to a receiver, a first ultrasonic signal comprising a plurality of cycles of a characteristic waveform feature; and

to transmit, to the receiver, a second ultrasonic signal comprising a plurality of cycles of the characteristic waveform feature and further comprising a waveform modification being a phase shift in the cycle of a characteristic waveform feature said waveform modification being introduced at a predetermined point in time of a duration of the second ultrasonic signal;

the processor means being further adapted to;

determine a time of reception of the introduced phase shift in the second ultrasonic signal at the receiver by comparing the waveform of a first received signal to the waveform of a second received signal and determining a point of diversion between corresponding characteristic waveform features of the first and second received signals; and

determine a time of flight of the second ultrasonic signal based on the determined time of reception of the introduced phase shift and its time of generation.

15. The apparatus of claim 14, wherein the transducer of the transmitter is controlled to generate a plurality of first ultrasonic signals, said plurality of first ultrasonic signals being received at the receiver;

said apparatus further comprising;

waveform feature selection means operatively connected to the receiver and the processor means for selecting a characteristic waveform feature of the first ultrasonic signal in accordance with predetermined selection criteria based on the point of diversion;

zero-crossing detection means operatively connected to the receiver and the processor means for detecting zero-crossings of the received plurality of first ultrasonic signals which indicate the presence of the selected characteristic waveform feature in each of the received plurality of first ultrasonic signals;

signal position estimation means operatively connected to the zero-crossing detection means and the processor means for estimating a position of the selected characteristic waveform feature of the plurality of received first ultrasonic signals in accordance with predetermined estimation criteria based on the detected zero-crossings to provide a position estimation value;

wherein the processor means;

processes the position estimation value to determine a corresponding estimation time; and

calculates the time of arrival of the selected characteristic waveform feature of at least one of the plurality of received first ultrasonic signals by adding a predetermined delay time to the estimation time.

16. The apparatus of claim 15 wherein said signal position estimation means comprises a dual slope integrator.

17. The apparatus of claim 15 wherein said plurality of received first ultrasonic signals are digitised and said processor means comprises digital data processing means comprising said zero-crossing detection means and said signal position estimation means.

18. A method of determining a time of flight of a signal transmitted between a transmitter and a receiver, said method comprising the steps of:

generating at a transducer of the transmitter a first and a second ultrasonic signal, where both signals comprise plurality of cycles of a characteristic waveform feature, and the second ultrasonic signal further comprises a waveform modification introduced at a predetermined point in time of the duration of the second ultrasonic signal, and said waveform modification comprises a phase shift in a cycle of the characteristic waveform feature;

receiving said first and second generated signals at the receiver;

scanning through said the first received signal and the second received signal in time to determine a point of diversion between the characteristic waveform features of the first received signal and the corresponding characteristic waveform feature of the second received signal, wherein said point of diversion corresponds to a time of reception of the introduced waveform modification at the receiver;

determining the time of flight of the second ultrasonic signal on the basis of the time of reception of the introduced waveform modification and its time of introduction into the second ultrasonic signal.

19. The method of claim 18 further comprising the steps of:

for each characteristic waveform feature received in the first received signal, calculating a difference between a value of the first received signal and a temporally corresponding value of the second received signal;

designating the first point at which the calculated difference is greater than the value of the second received signal as a point of diversion.

20. The method of claim 19 further comprising the step of:

calculating the difference between the time of the point of diversion and the time of generation of the introduced phase shift.

21. The method of claim 19 further comprising the step of:

measuring a time relationship between an introduced phase shift and the point of diversion and calculating the difference between the time of reception, based on the measured time relationship, and the time of generation of the introduced phase shift.

22. Apparatus adapted to determine the time of flight of a signal transmitted between a transmitter and a receiver, said apparatus comprising:

processor means adapted to operate in accordance with a predetermined instruction set, said apparatus, in conjunction with said instruction set, being adapted to;

cause the transducer of the transmitter to generate a first and a second ultrasonic signal, where both signals comprise a plurality of cycles of a characteristic waveform feature, and the second ultrasonic signal further comprises a waveform modification introduced at a predetermined point in time of the duration of the second ultrasonic signal, said waveform modification comprising a phase shift in a cycle of the characteristic waveform feature;

receive said first and second signals from the receiver;

scan through said the first received signal and the second received signal in time to determine a point of diversion between the characteristic waveform features of the first received signal and the corresponding characteristic waveform feature of the and second received signals, wherein said point of diversion corresponds to a time of reception of the introduced waveform modification at the receiver;

determine the time of flight of the second ultrasonic signal on the basis of the time of reception of the introduced waveform modification and its time of introduction into the second ultrasonic signal.

23. A method of monitoring flow through a particle detector of an aspirated smoke detector system, the aspirated smoke detector comprising a flow path along which air is drawn, and a flow sensor including a first transducer and a second transducer arranged to detect flow in the flow path, the method comprising the steps of:

transmitting, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmitting, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmitting, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmitting, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

determining a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determining a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determining a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determining a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determining a volumetric flow rate, f, in the flow path using the general flow calculation;

f=S×

A

where

A=cross sectional area of an air flow path through the detector system;

s=speed of air through the detector system such that s is given by;

24. Apparatus adapted to monitor air flow through a flow path of a particle detector of an aspirated smoke detector system, said apparatus comprising:

first and second transducers arranged to transmit ultrasonic signals through the air flow in the flow path,

processor means adapted to operate in accordance with a predetermined instruction set, said apparatus, in conjunction with said instruction set, being adapted to;

transmit, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmit, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmit, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmit, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

the processor means being configured to;

determine a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determine a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determine a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determine a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determine a volumetric flow rate, f, in the flow path using the general flow calculation;

f=S×

A

where,

A=cross sectional area of the air flow path;

s=speed of air in the flow path given by;

25. A method of detecting one or more blocked sampling holes in a pipe of an aspirated smoke detector system, said aspirated smoke detector system comprising a sampling pipe network including one or more sampling holes, and aspirator for drawing air through the sampling pipe network to the detector;

and a flow sensor arranged to detect air flow in an airflow path of the aspirated smoke detector system;

said method comprising;

monitoring the base flow of fluid through the airflow path by;

transmitting, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmitting, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmitting, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of air flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmitting, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

determining a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determining a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determining a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determining a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determining the base volumetric flow rate, f₁, in the flow path using the general flow calculation;

f₁=S₁×

A,

where

A=cross sectional area of an air flow path through the detector system;

S₁=speed of air through the detector system such that s is given by;

26. The method of claim 25, wherein the difference between base volumetric flow rate f₁ and subsequent volumetric flow rate, f₂ is compared over a length of time.

27. An aspirated smoke detector comprising:

a particle detector,

a sampling network including one or more sampling points; and

an aspirator for drawing air through the sampling network, through the detector along a flow path and

an ultrasonic flow sensor in fluid communication with the particle detector and including first and second transducers arranged to monitor a flow of air in the flow path, wherein the ultrasonic flow sensor is configured to;

transmit, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmit, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmit, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmit, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

a processing system configured to;

determine a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determine a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determine a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determine a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determine a volumetric flow rate, f, in the flow path using the general flow calculation;

f=S×

A

where,

A=cross sectional area of the air flow path;

s=speed of air in the flow path given by;

28. The detector of claim 27 wherein the ultrasonic flow sensor is in fluid communication with the sampling network and is operationally arranged to measure the partial flow of fluid through the sampling network.

29. The smoke detector of claim 27, wherein the particle detector detects particles in a portion of the air flow flowing through the sampling network.

30. The smoke detector of claim 27 wherein the ultrasonic flow sensor is located in the sampling network.

31. The smoke detector of claim 27, wherein the ultrasonic flow sensor is located in a housing of the particle detector.

32. The smoke detector of claim 27, having a branch in flow path allowing air to bypass the particle detector.

33. A non-transitory computer readable recording medium having embodied thereon a computer program for executing a method of determining a time of flight of a signal transmitted between a transmitter and a receiver, said method comprising the steps of:

at a signal transducer of the transmitter, generating a first ultrasonic signal comprising a plurality of cycles of a characteristic waveform feature;

at the signal transducer of the transmitter, generating a second ultrasonic signal comprising a plurality of cycles of the characteristic waveform feature and further comprising a waveform modification being a phase shift in the cycle of a characteristic waveform feature introduced at a predetermined point in time of a duration of the second ultrasonic signal;

receiving said first and second generated signals at the receiver;

determining a time of reception of the introduced phase shift in the second ultrasonic signal by comparing the waveform of the first received signal to the waveform of the second received signals and determining a point of diversion between corresponding characteristic waveform features of the first and second received signals;

determining a time of flight of the second ultrasonic signal based on the determined time of reception of the introduced phase shift and its time of generation.

34. A non-transitory computer readable medium having embodied thereon a computer program for executing a method of monitoring flow through a particle detector of an aspirated smoke detector system, the aspirated smoke detector comprising a flow path along which air is drawn, and a flow sensor including a first transducer and a second transducer arranged to detect flow in the flow path, the method comprising the steps of:

transmitting, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmitting, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmitting, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmitting, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

determining a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determining a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determining a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determining a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determining a volumetric flow rate, f, in the flow path using the general flow calculation;

f=S×

A

where

A=cross sectional area of an air flow path through the detector system;

s=speed of air through the detector system such that s is given by;

35. A non-transitory computer readable medium having embodied thereon a computer program for executing a method of detecting one or more blocked sampling holes in a pipe of an aspirated smoke detector system, said aspirated smoke detector system comprising a sampling pipe network including one or more sampling holes, and aspirator for drawing air through the sampling pipe network to the detector;

and a flow sensor arranged to detect air flow in an airflow path of the aspirated smoke detector system;

said method comprising;

monitoring a the base flow of fluid through the airflow path by;

transmitting, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmitting, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmitting, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of air flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmitting, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

determining a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determining a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determining a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determining a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determining the base volumetric flow rate, f₁, in the flow path using the general flow calculation;

f₁=S₁×

A,

A=cross sectional area of an air flow path through the detector system;

s=speed of air through the detector system such that S₁ is given by;

36. A method of monitoring flow through a particle detector of an aspirated smoke detector system, the aspirated smoke detector system including a flow path along which air is drawn, and a flow sensor, including a first and second transducers, arranged to detect flow in the flow path, the method comprising the steps of:

ascertaining a base volumetric fluid flow f₁ through a particle detector by;

transmitting, from the first transducer to the second transducer and in a forward direction, being generally in a direction of flow along the flow path, a first ultrasonic signal comprising a plurality of cycles of a first characteristic waveform feature;

transmitting, from the first transducer to the second transducer and in said forward direction, a second ultrasonic signal comprising a plurality of cycles of the first characteristic waveform feature and further comprising a waveform modification being a first phase shift in a cycle of the first characteristic waveform feature that is introduced at a predetermined point in time of a duration of the second ultrasonic signal;

transmitting, from the second transducer to the first transducer and in a return direction, being generally opposite the direction of air flow along the flow path, a third ultrasonic signal comprising a plurality of cycles of a second characteristic waveform feature; and

transmitting, from the second transducer to the first transducer and in said return direction, a fourth ultrasonic signal comprising a plurality of cycles of the second characteristic waveform feature and further comprising a second waveform modification being a second phase shift in a cycle of the second characteristic waveform feature that is introduced at a predetermined point in time of a duration of the fourth ultrasonic signal; and

determining a time of reception of the introduced first phase shift in the second ultrasonic signal at the second transducer, by comparing the waveform of the first received ultrasonic signal to the waveform of the second received ultrasonic signal and determining a point of diversion between corresponding first characteristic waveform features of the first and second received signals;

determining a time of reception of the introduced second phase shift in the fourth ultrasonic signal at the first transducer by comparing the waveform of the third received ultrasonic signal to the waveform of the fourth received ultrasonic signal and determining a point of diversion between corresponding second characteristic waveform features of the third and fourth received signals;

determining a time of flight (t₁) of the second ultrasonic signal based on the determined time of reception of the introduced first phase shift and its time of generation;

determining a time of flight (t₂) of the fourth ultrasonic signal based on the determined time of reception of the second introduced phase shift and its time of generation; and

determining the base a volumetric flow rate, f₁, in the flow path using the general flow calculation;

f₁=S₁×

A

A=cross sectional area of an air flow path through the detector system;

S₁=speed of air through the detector system such that s is given by;