Differential viscometer with solvent compressibility correction
An improved version of the capillary bridge viscometer that compensates for the effect of solvent compressibility is disclosed. A novel, yet simple and inexpensive modification to a conventional capillary bridge viscometer design can improve its ability to reject pump pulses by more than order of magnitude. This improves the data quality and allows for the use of less expensive pumps. A pulse compensation volume is added such that it transmits pressure to the differential pressure transducer without sample flowing there through. The pressure compensation volume enables the cancellation of the confounding effects of pump pulses in a capillary bridge viscometer.
1. A capillary bridge viscometer comprising:
a bridge to receive a solvent delivered to the bridge by a pump system comprising;
a first arm comprising a first pair of series connected sensing capillaries,
a second arm comprising
a second pair of series connected sensing capillaries, and
a delay volume connected in series between the second pair of series connected sensing capillaries,
wherein the second arm is connected in parallel with the first arm,
a first differential pressure transducer coupled across the first arm and the second arm,
such that the first differential pressure provides a measurement of first amplitudes of pressure pulses generated in the solvent by the pump system wherein the first differential pressure transducer is coupled to the first arm at a first junction between each of the first pair of series connected sensing capillaries, and
such that the first differential pressure transducer is coupled to the second arm at a second junction between one of the second pair of series connected sensing capillaries and the delay volume, and
a pressure compensation volume between the first junction and to the first differential pressure transducer to transmit pressure to the first differential pressure transducer,
wherein the pressure compensation volume comprises capillary tubing;
a second differential pressure transducer providing a measurement of second amplitudes of pressure pulses generated in the solvent by the pump system where the second differential pressure transducer is connected in parallel with the first arm and the second arm; and
wherein a length of the capillary tubing of the pressure compensation volume is sized such that a fluid volume of the pressure compensation volume matches a fluid volume of the delay volume to balance compressibility of fluid in the delay volume by maximizing a suppression ratio where the suppression ratio is defined by peak amplitudes among the second measured pressure pulse amplitudes divided by peak amplitudes among the first measured pressure pulse amplitudes, resulting in a minimizing of the amplitudes of pressure pulses generated in the solvent by the pump system.
2. The capillary bridge viscometer of claim 1 wherein the tubing comprises stainless steel.
3. The capillary bridge viscometer of claim 1 further comprising a valve connected to the pressure compensation volume to permit the volume to be flushed by fluid passing through the first arm.