Measuring low flowrates

Apparatus for making a measurement of the ultrafiltration flowrate in a fluid flow transfer device having a first liquid flowing on one side of a semipermeable membrane at a first pressure and a second liquid flowing on the othe side of said membrane at a lower pressue, said ultrafiltration being subject to short-term variations in flowrate, said apparatus comprising,

• a vertically-positioned collection tube with a known volume-versus-height relationship,
• means for directing a flow of liquid equal in amount to that resulting from ultrafiltration into said collection tube,
• means for periodically generating electrical signals representing the height of the surface of liquid in said tube at different times while said height is increasing during a collection interval in which there are short-term variations in flowrate, and
• means for receiving said signals and determining the time-averaged flowrate value at the end of said collection interval based on a succession of height-versus-time data taken during said collection interval to avoid distortion of flowrate value owing to said short-term variations.

A method of measuring the flowrate of a flowing liquid subject to short-term variations in flowrate comprising

• providing a vertically-positioned collection tube with a known volume-versus-height relationship,
• directing said flowing liquid into said collection tube,
• periodically generating electrical signals representing the height of the surface of said liquid in said tube at different times while said height is increasing during a collection interval in which there are short-term variations in flowrate, and
• receiving said signals and determining the time-averaged flowrate value at the end of said collection interval based on a succession of height-versus-time data taken during said collection interval to avoid distortion of the flowrate value owing to said short-term variations.