Measurement of rates of flow of gases

Claim 3

A method as claimed in claim 1, wherein the third acoustic signal is a continuous sine wave, the amplitude or the frequency of which is modulated by a periodic signal having a period greater than the time taken for the third signal to travel from its point of propagation to its point of reception, the total phase difference of the third signal being measured with respect to the period of the modulating signal.

Claim 4

A method as claimed in claim 1, wherein the path length of the third signal is altered by moving the point of reception.

Claim 5

A method as claimed in claim 1, wherein the path length is altered by an amount greater than a whole apparent wavelength and the number of whole apparent wavelengths contained in the distance by which the path length is altered is ascertained.

Claim 6

A method as claimed in claim 4, wherein the number of whole apparent wavelengths contained in the distance by which the path length of the third signal is altered is ascertained by means of a zero-crossing device and appropriate associated circuitry arranged to count the number of times the wave form of the third signal passes through zero during alteration of the path length of the third signal.

Claim 7

A method as claimed in claim 1, wherein the third acoustic signal is propagated in a direction perpendicular to the direction of flow of the gas, is reflected by a reflector in the opposite direction and is received at a point adjacent to its point of propagation.

Claim 8

A method as claimed in claim 6, wherein the path length of the third signal is altered by adjusting the distance of the reflector from the points of propagation and reception.

Claim 9

A mEthod as claimed in claim 7, wherein the reflector is moved by means of a pneumatic or hydraulic actuator.

Claim 10

A method as claimed in claim 1, wherein the first and second signals are continuous sine waves, the difference in time taken by the first and second signals to travel over their respective paths is longer than one period of said sine waves, and the amplitude or the frequency of the sine waves is modulated by a periodic signal having a period greater than the said difference in time, the total phase difference between the first and second signals at their respective points of reception being measured with respect to the period of the modulating signal.

Claim 11

A method as claimed in claim 1, wherein the first and second signals are acoustic beams which are of sufficiently narrow angle and are arranged to be propagated and received in such a way that stray reflected waves received are substantially eliminated or their intensity considerably reduced.

Claim 12

A method as claimed in claim 1, wherein the gas is flowing in a conduit and stray waves that would otherwise have been reflected by the wall of the conduit are absorbed by material absorbent to acoustic waves.

Claim 13

A method as claimed in claim 1, wherein the first acoustic signal is propagated through the gas in an upstream direction, is reflected, and is then received and the second acoustic signal is propagated in a downstream direction, is reflected, and is then received.

Claim 14

A method as claimed in claim 1, wherein the first and second signals are propagated through the gas in axial planes at 90* to one another.

Claim 15

A method as claimed in claim 1, wherein the first, second and third signals are all derived from a common source.

Claim 16

A method of measuring the rate of flow of a gas as claimed in claim 1, wherein there are solid particles in suspension in the gas.

Claim 17

A method of measuring the mass flow rate of a gaseous suspension of solid particles which comprises measuring the rate of flow of the gas as claimed in claim 15 and measuring the concentration of the solid particles in the gas.

Claim 18

Apparatus for measuring the rate of flow of gas, which comprises means for propagating a first acoustic signal through the gas in an upstream direction, means for propagating a second acoustic signal through the gas in a downstream direction, the propagating means being such that the two acoustic signals are of the same known frequency and have a known phase relationship at their respective points of propagation, a first receiver for receiving the first signal positioned at a point upstream of and at a known distance from the point of propagation of the first signal, a second receiver for receiving the second signal at a point downstream of and at a known distance from the point of propagation of the second signal, means for obtaining a resultant signal dependent on the total phase difference between the first and second signals at their respective points of reception, means for propagating a third acoustic signal of known frequency through the gas in a direction perpendicular to the direction of flow of the gas, a third receiver for receiving the third signal after it has travelled over a certain path length, means for altering by a known amount the path length of the third signal in the gas and means for obtaining a signal dependent on the total difference in phase of the third signal at its point of reception before and after the alteration of the path length, the signal dependent on the said total phase difference of the first and second signals with the signal dependent on the said total phase difference of the third signal being usable to obtain a measure of the rate of flow of the gas.

Claim 19

Apparatus for measuring the rate of flow of gas, which comprises means for propagating a first acoustic signal through the gas in an upstream direction, means for propagating a second acoustic signal through the gas in a downstream direction, the propagating means being such that the two acoustic signals are of the same known frequency and have a known phase relationship at their respective points of propagation, a first receiver for receiving the first signal positioned at a point upstream of and at a known distance from the point of propagation of the first signal, a second receiver for receiving the second signal at a point downstream of and at a known distance from the point of propagation of the second signal, means for obtaining a resultant signal dependent on the total phase difference between the first and second signals at their respective points of reception, means for propagating a third acoustic signal of known frequency through the gas in a direction perpendicular to the direction of flow of the gas, a third receiver for receiving the third signal after it has travelled over a certain path length, means for altering by a known amount the path length of the third signal in the gas and means for obtaining a signal dependent on the total difference in phase of the third signal at its point of reception before and after the alteration of the path length, the signal dependent on the said total phase difference of the first and second signals with the signal dependent on the said total phase difference of the third signal being usable to obtain a measure of the rate of flow of the gas.

Claim 20

Apparatus as claimed in claim 17, wherein the third receiver is movable.

Claim 21

Apparatus as claimed in claim 18, which comprises a zero-crossing device and appropriate associated circuitry arranged to count the number of times the wave form of the third signal passes through zero during alteration of the patH length of the third signal.

Claim 22

Apparatus as claimed in claim 17, wherein the means for propagating the third acoustic signal is arranged to propagate the signal in a direction perpendicular to the direction of flow of the gas, a reflector is provided which is arranged to reflect the signal in the opposite direction and the third receiver is arranged to receive the third signal at a point adjacent to the means for propagating the third signal.

Claim 23

Apparatus as claimed in claim 20, wherein the reflector is movable so as to allow adjustment of the distance of the reflector from the means for propagating the third signal and the third receiver.

Claim 24

Apparatus as claimed in claim 21, which comprises a pneumatic or hydraulic actuator for moving the reflector.

Claim 25

Apparatus as claimed in claim 17, wherein the means for propagating the first and second signals are transmitters arranged to propagate acoustic beams of sufficiently narrow angle and the transmitters are so arranged relative to their respective receivers that stray reflected waves reaching the receivers are substantially eliminated or their intensity is considerably reduced.

Claim 26

Apparatus as claimed in claim 17, wherein the narrow angle of each of the beams is less than 50* and each of the transmitter-receiver axes is at an acute angle to the direction of flow of the gas of between 49* and 16*.

Claim 27

Apparatus as claimed in claim 17, which comprises a conduit through which the gas is arranged to flow and material absorbent to acoustic waves is arranged to absorb stray waves that would otherwise have been reflected by the wall of the conduit.

Claim 28

Apparatus as claimed in claim 17, wherein a reflector is provided to reflect the first acoustic signal before it is received by the first receiver and a further reflector is provided to reflect the second acoustic signal before it is received by the second receiver.

Claim 29

Apparatus as claimed in claim 26, wherein each of the first and second receivers is positioned on the same side of the conduit through which the gas is flowing as its respective transmitter.

Claim 30

Apparatus as claimed in claim 27, wherein material absorbent to acoustic waves is arranged between the transmitter and receiver of each of the first and second signals to absorb waves that would otherwise have been reflected by the wall of the conduit more than once.

Claim 31

Apparatus as claimed in claim 17, wherein the means for propagating the first and second signals are arranged to propagate the signals through the gas in axial planes at 90* to one another.