Gas flow sensing apparatus

Claim 2

Gas flow sensing apparatus, characterized in that filter means (102) are provided upstream the location of the flow sensor (108) and the cross-sectional area of the flow channel (104,106) changes such that the flow velocity is enhanced between the filter means (102) and the location of the flow sensor (108) .

Claim 3

Apparatus according to claim 1, characterized in that the sensor (108) is located in the flow channel in a sensing portion (106) of constant cross-sectional area and that the cross-sectional area of the channel (104) between the filter (102) and the inlet of said sensing portion changes smoothly from a large cross-sectional area at the position of the filter (102) to a small cross-sectional area at the area at the inlet of said sensing portion (106)

Claim 4

Apparatus according to claim 2, characterized in that the channel (104,106) is of circular cross-section and the transition portion (104) of the channel tapers from said filter (102) to the inlet of said sensing portion (106).

Claim 5

Apparatus according to one of claims 1 to 3, characterized in that the ratio of the cross-sectional areas at the position of the filter (102) and in the sensing portion (106) is greater than 100:1.

Claim 6

Apparatus according to claim 4, characterized in that the ratio is greater than 1000:1.

Claim 7

Apparatus according to one of the preceding claims, characterized in that filter means (102) are provided at both ends of the flow channel with the sensing portion (106) being located between them, whereat the cross-sectional area of the flow channel increases at both sides of the sensing portion (106) to a larger cross-sectional area at the position of the filter means (102).

Claim 8

Apparatus according to claim 6, characterized in that the filter means (102) at both ends of the flow channel (104,106) have the same cross-sectional areas.

Claim 9

Apparatus according to one of the preceding claims, characterized by a flow sensor comprising: a semiconductor body (10) having a depression (20) formed into a first surface (14) of the body; member means (32,34) comprising resistive element means (16) the member means (32,34) having a predetermined configuration suspended over.the depression (20), the member means (32,34) being connected to the first surface (14) at least at one location, the depression (20) opening to the first surface (14) around at least a portion of the predetermined configuration, the depression (20) providing substantial physical and thermal isolation between the resistive element means (16) and the semiconductor body (10); the resistive element means (16) when provided with current becoming heated, there being a predetermined relationship between the resistance and the temperature of the resistive element means (16), the resistive element means (16) being adapted to provide a signal related to the rate of air flowing over the member means (32,34).