Sensing apparatus and method

1. 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 network including one or more pipes having sampling holes, an aspirator for drawing air through the sampling network to a detector; and a flow sensor, said method comprising:

ascertaining the base flow of fluid through a particle detector using the flow sensor;

monitoring subsequent flow through the particle detector using the flow sensor; and

comparing in a processor, the subsequent flow with the base flow, and determining that one or more sampling holes of the sampling network are blocked and indicating a fault, if the difference between the base flow and the subsequent flow exceeds a predetermined threshold.

2. The method of claim 1 wherein the flow sensor is an ultrasonic flow sensor.

3. The method of claim 1 , wherein the difference between base flow and subsequent flow is compared over a length of time.

4. The method of claim 1 wherein the flow is determined according to the following general flow calculation:

f=s×A

where f=volumetric flow;

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;

s

=

d

2

⁢

(

1

t

2

-

1

t

1

)

where t, is the transit time of a signal transmitted in a forward direction, being generally in the direction of flow, from a first transducer located adjacent the flow path to a second transducer located generally opposite the first transducer and adjacent the flow path;

t 2 is the transit time of a signal transmitted in a reverse direction, being generally against the direction of flow, from the second transducer to the first transducer; and

d is a distance travelled by the signal between the first and second transducer; and wherein both t 1 and t 2 are determined.

5. A non-transitory computer readable medium encoded with a computer program containing instructions stored thereon for causing a computer processor to control a particle detector to perform a method comprising the steps of:

ascertaining the base flow of fluid through a particle detector using a flow sensor;

monitoring subsequent flow through the particle detector using the flow sensor; and

comparing in a processor, the subsequent flow with the base flow, and determining that one or more sampling holes of the sampling network are blocked and indicating a fault, if the difference between the base flow and the subsequent flow exceeds a predetermined threshold.

6. The non-transitory computer readable medium as claimed in claim 5 , wherein in the method, the difference between base flow and subsequent flow is compared over a length of time.

7. The non-transitory computer readable medium as claimed in claim 5 , wherein in the method, the flow is determined according to the following general flow calculation:

f=s×A

where f=volumetric flow;

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;

s

=

d

2

⁢

(

1

t

2

-

1

t

1

)

where t, is the transit time of a signal transmitted in a forward direction, being generally in the direction of flow, from a first transducer located adjacent the flow path to a second transducer located generally opposite the first transducer and adjacent the flow path;

t 2 is the transit time of a signal transmitted in a reverse direction, being generally against the direction of flow, from the second transducer to the first transducer; and

d is a distance travelled by the signal between the first and second transducer; and wherein both t 1 and t 2 are determined.

8. A non-transitory computer readable medium encoded with a computer program containing instructions stored thereon for causing a computer processor to control a particle detector to perform a method comprising the steps of:

ascertaining the base flow of fluid through a particle detector using an ultrasonic flow sensor:

monitoring subsequent flow through the particle detector using the ultrasonic flow sensor; and

comparing in a processor, the subsequent flow with the base flow, and determining that one or more sampling holes of the sampling network are blocked and indicating a fault, if the difference between the base flow and the subsequent flow exceeds a predetermined threshold.

9. An aspirated smoke detector comprising:

a particle detector,

a sampling network including one or more pipes with one or more sampling holes through which samples enter the one or more pipes; and

an aspirator for drawing an air sample through the sampling network to the particle detector;

wherein the aspirated smoke detector includes an ultrasonic flow sensor arranged to detect the flow rate of air entering the particle detector, and

wherein the aspirated smoke detector further comprises a processor configured to:

ascertain the base flow of fluid through the particle detector using the flow sensor;

monitor subsequent flow through the particle detector using the flow sensor; and

compare the subsequent flow with the base flow, and determine that one or more sampling holes of the sampling network are blocked and indicate a fault, if the difference between the base flow and the subsequent flow exceeds a predetermined threshold.