High accuracy turbine flowmeter with optical sensor

A method of optically detecting rotation of a turbine having multiple vanes and rotating at least partly within a substantially cylindrical bore within a body of a flowmeter, the method comprising the steps of:

• (a) providing a protective window mounted within the body through which an edge portion of each vane may be observed in sequence as each vane passes by a predetermined location within the body;
• (b) illuminating the predetermined location with an artificial light source remotely located on a first side of the window opposite the vanes;
• (c) detecting light from the artificial light source reflected off a predetermined edge portion of each vane as such portion passes through the predetermined location;
• (d) mounting the turbine within the bore such that the turbine has an axis of rotation concentric to the axis of the bore;
• and
• (e) sizing the turbine to have a clearance with respect to the bore that allows substantially minimum resistance to turbine rotation in response to extremely low rates of fluid flow through the bore,
• steps (b) and (c) being carried out by transmitting light from the artificial source at a remote location through a fiber optic cable to a location near the first side of the window and transmitting light reflected off of the edge of the vane in the predetermined location back through the fiber optic cable to a photodetector at the remote location.

An optical sensor housing in combination with a flowmeter having a body with a passage therethrough through which the fluid to be measured flows and having a turbine provided with multiple vanes located at least partially within the passage, the sensor housing comprising:

• a rigid casing having an elongated hole therethrough having first and second ends, the hole provided with first and second regions of differing nominal cross-sectional areas, such that the casing has a shoulder having an annular surface between the first and second regions;
• a window mounted in the hole near the first end thereof, the window being made at least primarily of optically transparent material and having first and second opposed polished surfaces, and an annular recess in the window adjacent to the second polished surface of the window somewhat smaller in lateral dimension than the annular surface of the shoulder;
• and
• means for cushioning the window as it is pushed in the direction of the shoulder by the high-pressure fluid in the passage, said means including flexible material that deforms under pressure and which is located at least partly in the annular recess of the window,
• the shoulder supporting the window along at least substantially all of its periphery in a direction generally perpendicular to the second polished surface, and
• the shoulder, recess in the window and means for cushioning cooperating to enable the window to withstand a static pressure difference of at least about 300 atmospheres across the first and second polished surfaces of the window produced by a high-pressure condition in the fluid in the passage.