Mass flow measuring assembly having low pressure drop and fast response time

A mass flow measuring assembly for performing a multiplicity of measurements of mass flow of a gas entering the assembly at a preselected flow rate in a range from 0.25 to 10 sccm from a gas storage container at a pressure less than 10 Torr and exiting the assembly into a vacuum chamber, each measurement performed within not more than 4 seconds, the assembly comprising:

• a capillary tube having opposed first and second ends and a central portion equidistant between said first and second ends, the tube having a circumferential outer surface and a generally cylindrical bore of a preselected diameter, said outer surface and bore determining a preselected wall thickness, the tube second end in fluid communication with the gas storage container;
• a first diffuser section having first and second apertures, said first aperture attached to and in fluid communication with the tube first end, and further having a conical, distally diverging passage therethrough symmetric about an axis of symmetry substantially in line with the tube bore, said passage diverging distally at a preselected small total angle and terminating proximally in said first aperture and distally in said second aperture, said second aperture in fluid communication with the vacuum chamber, said passage minimizing pressure drop of the gas exiting said second aperture;
• a second diffuser section having first and second apertures, said first aperture attached to and in fluid communication with the tube second end, and further having a conical, distally diverging passage therethrough symmetric about an axis of symmetry substantially in-line with the tube bore, said passage diverging distally at said preselected total angle and terminating proximally in said first aperture and distally in said second aperture, said passage reducing pressure drop of the gas exiting said first aperture and entering said tube second end, said second aperture in fluid communication with the gas storage container;
• and
• first and second resistance thermometers, each thermometer being formed as a segment of an iron-nickel alloy wire of a predetermined diameter, the segments of equal length, each segment wound in a tight coil around the outer surface of the tube central portion, the coils of equal length and separated by a predetermined gap.

A system for controlling mass flow of a gas entering in at a preselected flow rate from a gas storage container at a pressure less than one atmosphere and exiting into a vacuum chamber, the system comprising:

• a mass flow measuring assembly comprising a capillary tube having opposed first and second ends and a central portion equidistant between said first and second ends, the tube having a circumferential outer surface and a generally cylindrical bore of a preselected diameter, said outer surface and bore determining a preselected wall thickness, the tube second end in fluid communication with the gas storage container;
• said measuring assembly further comprising a first diffuser section having first and second apertures, said first aperture attached to and in fluid communication with the tube first end, and further having a conical, distally diverging passage therethrough symmetric about an axis of symmetry substantially in-line with the tube bore, said passage diverging distally at a preselected small total angle and terminating proximally in said first aperture and distally in said second aperture, said second aperture in fluid communication with the vacuum chamber, said passage minimizing pressure drop of the gas exiling said second aperture;
• said measuring assembly further comprising a second diffuser section having first and second apertures, said first aperture attached to and in fluid communication with the tube second end, and further having a conical, distally diverging passage therethrough symmetric about an axis of symmetry substantially in-line with the tube bore, said passage diverging distally at said preselected total angle and terminating proximally in said first aperture and distally in said second aperture, said passage reducing pressure drop of the gas exiting said first aperture and entering said tube second end, said second aperture in fluid communication with the gas storage container;
• and said measuring assembly further comprising first and second resistance thermometers, each thermometer being formed as a segment of an iron-nickel alloy wire of a predetermined diameter, the segments of equal length, each segment wound in a tight coil around the outer surface of the tube central portion, the coils of equal length and separated by a predetermined gap.

A system for measuring mass flow of a gas entering in at a preselected flow rate from a gas storage container at a pressure less than one atmosphere and exiting into a vacuum chamber, the system comprising:

• a mass flow measuring assembly comprising a capillary tube having opposed first and second ends and a central portion equidistant between said first and second ends, the tube having a circumferential outer surface and a generally cylindrical bore of a preselected diameter, said outer surface and bore determining a preselected wall thickness, the tube second end in fluid communication with the gas storage container;
• said measuring assembly further comprising a first diffuser section having first and second apertures, said first aperture attached to and in fluid communication with the tube first end, and further having a conical, distally diverging passage therethrough symmetric about an axis of symmetry substantially in-line with the tube bore, said passage diverging distally at a preselected small total angle and terminating proximally in said first aperture and distally in said second aperture, said second aperture in fluid communication with the vacuum chamber, said passage minimizing pressure drop of the gas exiting said second aperture;
• said measuring assembly further comprising a second diffuser section having first and second apertures, said first aperture attached to and in fluid communication with the tube second end, and further having a conical, distally diverging passage therethrough symmetric about an axis of symmetry substantially in-line with the tube bore, said passage diverging distally at said preselected total angle and terminating proximally in said first aperture and distally in said second aperture, said passage reducing pressure drop of the gas exiting said first aperture and entering said tube end, said second aperture in fluid communication with the gas storage container;
• and said measuring assembly further comprising first and second resistance thermometers, each thermometer being formed as a segment of an iron-nickel alloy wire of a predetermined diameter, the segments of equal length, each segment wound in a tight coil around the outer surface of the tube central portion, the coils of equal length and separated by a predetermined gap.