Thermal mass flow meter and method of making same

Claim 2

In a method for thermal measurement of fluid flow the steps of: partitioning the fluid flow to be measured into a plurality of generally parallel elongated ribbon-shaped partitioned flow stream portions, each ribbon-shaped stream portion having in cross-section a broad dimension and a narrow dimension for providing rectified flow conditions over a relatively wide range of fluid flow rates; and disposing a thermal flow sensor across the said narrow dimensions of a plurality of said partitioned ribbon-shaped flow portions and in heat-exchanging relation predominantly with the narrow dimensions of a plurality of said ribbon-shaped flow stream portions for measuring the fluid flow.

Claim 3

The method of claim 1 wherein the step of partitioning the flow into a plurality of ribbon-shaped flow stream portions includes the step of: anisotropically etching a member to define a comb-shaped structure having a plurality of vane-shaped septa each having major faces and carried from a common spine member, and disposing the comb-shaped structure in the flow stream for partitioning the flow stream into the plurality of ribbon-shaped flow stream portions flowing inbetween adjacent speta.

Claim 4

The method of claim 2 wherien the anisotropically etched member comprises a wafer of diamond cubic material having (110) major faces, with the major faces of the vane-shaped septa being defined by (111) planes of the diamond cubic wafer material.

Claim 5

The method of claim 2 wherein the step of disposing the thermal fluid flow sensor in heat-exchanging relation predominantly with the narrow face of at least one of the ribbon-shaped flow stream portions includes the step of supporting the thermal fluid flow sensor upon a web extending across the fluid flow stream portions flowing inbetween adjacent septa.

Claim 6

The method of claim 1 wherein the step of disposing the thermal fluid flow sensor in heat-exchanging relation predominantly with the narrow dimensions of a plurality of said ribbon-shaped flow stream portions includes the steps of: disposing first and second sensing resistors serially to the narrow dimensions of the partitioned flow stream portions with the first resistor being upstream of the second resistor; deriving an electrical output signal from said first and second sensing resistors which is a function of the temperature difference between said sensing resistors produced by the flow of fluid in the fluid stream; said derived output signal being characterized by a critical flow rate defined as the flow rate at which the amplitude of the temperature difference between said first and second sensing resistors changes from a direct function of flow rate to an inverse function of flow rate with increasing flow rate; and thermally loading said first and second sensing resistors as arranged relative to said partitioned flow stream by increasing the loss of heat from said resistors to their surrounds exclusive of said flow stream portions so as to substantially increase the fluid flow rate corresponding to the critical flow rate, whereby the flow regime wherein the temperature difference between said first and second resistors is a direct function of flow rate is extended substantially to higher flow rates.

Claim 7

The method of claim 5 wherein the step of thermally loading said first and second sensing resistors includes the step of coupling a thermally conductive element in heat-exchanging relation with said first and second resistors.

Claim 8

In a method for making a fluid flow sensor, the steps of: forming a layer of electrically and thermally insulative material on the major face of a substrate wafer; forming a temperature dependent resistor on said layer of thermally insulative material; selectively etching said substrate material from underneath said insulative layer so as to provide a web of said layer of thermally insulative material supported from an unetched portion of said substrate material; partitioning the fluid flow to be measured into a plurality of generally parallel, elongated ribbon-shaped flow stream portions, each stream portion having in cross-section a broad dimension and a narrow dimension for providing rectified flow conditions over a relatively wide range of fluid flow rates; and disposing said temperature dependent resistor as supported upon said web of thermally insulative material in heat-exchanging relation predominantly with the narrow dimensions of a plurality of said ribbon-shaped flow stream portions for measuring the fluid flow.

Claim 9

In a method for making a fluid flow sensor, the steps of: forming a layer of electrically and thermally insulative material on the major face of a substrate wafer; forming a temperature dependent resistor on said layer of thermally insulative material; selectively etching said substrate material from underneath said insulative layer so as to provide a web of said layer of thermally insulative material supported from an unetched portion of said substrate material; partitioning the fluid flow to be measured into a plurality of generally parallel, elongated ribbon-shaped flow stream portions, each stream portion having in cross-section a broad dimension and a narrow dimension for providing rectified flow conditions over a relatively wide range of fluid flow rates; and disposing said temperature dependent resistor as supported upon said web of thermally insulative material in heat-exchanging relation predominantly with the narrow dimensions of a plurality of said ribbon-shaped flow stream portions for measuring the fluid flow.

Claim 10

The method of claim 7 wherein said temperature dependent resistor includes first and second resistor portions serially disposed relative to the narrow dimensions of the partitioned flow stream portions, with the first resistor portion being upstream of the second resistor portion.

Claim 11

The method of claim 8 including the steps of: deriving an electrical output signal from said first and second resistor portions which is a function of the temperature difference between said resistor portions produced by the partitioned flow of fluid; said derived output signal being characterized by a critical flow rate at which the amplitude of the temperature difference between said first and second sensing resistor portions changes from a direct function of flow rate to an inverse function of flow rate with increasing flow rate; and thermally loading said first and second resistor portions as arranged relative to said partitioned flow stream by increasing the loss of heat from said resistor portions to their surrounds exclusive of said flow stream portions so as to substantially increase the critical flow rate, whereby the flow regime wherein the temperature difference between said first and second resistor portions is a direct function of flow rate is extended substantially to a higher flow rate.

Claim 12

The method of claim 9 wherien the step of thermally loading said first and second resistor portions includes the step of coupling a thermally conductive element in heat-exchanging relation with said first and second resistor portions.

Claim 13

In a thermal fluid flow apparatus: partitioning means for partitioning the fluid flow to be measured into a plurality of generally parallel, elongated narrow ribbon-shaped partitioned flow stream portions, each stream portion having in cross-section a broad dimension and a narrow dimension for providing rectified flow conditions over a relatively wide range of fluid flow rates; and thermal fluid flow sensor means disposed in heat-exchanging relation predominantly with the narrow dimensions of a plurality of said partitioned ribbon-shaped flow stream portions for measuring the fluid flow.

Claim 14

In a thermal fluid flow apparatus: partitioning means for partitioning the fluid flow to be measured into a plurality of generally parallel, elongated narrow ribbon-shaped partitioned flow stream portions, each stream portion having in cross-section a broad dimension and a narrow dimension for providing rectified flow conditions over a relatively wide range of fluid flow rates; and thermal fluid flow sensor means disposed in heat-exchanging relation predominantly with the narrow dimensions of a plurality of said partitioned ribbon-shaped flow stream portions for measuring the fluid flow.

Claim 15

The apparatus of claim 11 wherein said partitioning means comprises a comb-shaped structure having a plurality of vane-shaped septa each having major faces and carried from a common spine member for partitioning the fluid flow stream into a plurality of ribbon-shaped flow stream portions flowing inbetween adjacent septa.

Claim 16

The apparatus of claim 12 wherein said comb-shaped structure is made of diamond cubic material the major faces of said septa being defined by (111) planes of the diamond cubic material.

Claim 17

The apparatus of claim 12 wherein said thermal fluid flow sensor means includes a web extending across the flow stream portions flowing inbetween adjacent septa; and temperature dependent resistor means carried from said web in heat exchanging relation with the flow of fluid between said septa.

Claim 18

The apparatus of claim 11 wherein said fluid flow sensor means includes first and second sensing resistors serially disposed to the narrow dimensions of the partitioned flow stream portions with the first resistor being upstream of the second resistor; electrical circuit means for deriving an electrical output signal from said first and second sensing resistors which is a function of the temperature difference between said sensing resistors produced by the flow of fluid in the fluid stream; said derived output signal being characterized by a critical flow rate defined by the flow rate at which the amplitude of the temperature difference between said first and second sensing resistors changes from a direct function of flow rate to an inverse function of flow rate with increasing flow rate; and thermal loading means for thermally loading said first and second sensing resistors as arranged relative to said partitioned flow stream and for increasing the loss of heat from said sensing resistors to their surrounds exclusive of said flow stream portions so as to substantially increase the fluid flow rate corresponding to the critical flow rate, whereby the flow regime wherein the temperature difference between said first and second resistors is a direct function of flow rate is extneded substantially to higher flow rates.

Claim 19

The apparatus of claim 15 wherein said thermal loading means comprises a thermally conductive element disposed in heat-exchanging relation with said first and second resistors.