Method and device for measuring the viscosity of a fluid

A method for monitoring the viscosity of a fluid, which method comprises the steps of:

• a. passing the fluid through a device comprising two different characteristic flow restrictors connected in series, in which device the variation in the flow of fluid through one of the restrictors under the conditions of operation of the device with respect to a change in viscosity of the fluid differs by a detectable amount from the variation of flow through the other restrictor for the same change in viscosity of the fluid flowing therethrough;
• b. monitoring the pressure in the fluid between the restrictors relative to the pressure drop across the device to give an indication of a change in the viscosity of the fluid flowing through the device;
• and
• wherein the method further comprises feeding fluid under pressure to the inlet of a first restrictor which is connected by a flow conduit chamber to the inlet of a second flow restrictor, the flow restrictors serving as the inlet and outlet to the chamber;
• measuring the pressure of the fluid within the chamber by means of a first pressure sensor;
• monitoring at least one of the inlet and outlet pressure of the fluid fed to and fed from the chamber by a second pressure sensor to provide an indication of the pressure drop across the chamber;
• by relating the pressure within the chamber to the pressure drop across the chamber, obtaining an indication of any changes in viscosity in the fluid flowing through the chamber;
• and utilising that indication to modify the viscosity of the fluid so as to maintain the viscosity within a desired range.

A device for monitoring the viscosity of a fluid, comprising:

• a. two different character flow restrictors connected in series, the flow restrictors being selected such that under the intended conditions of use the flow rate of fluid through a first one of the said restrictors is affected more by a given change in the viscosity of the fluid than is the flow rate through the other restrictor for the same change in viscosity;
• b. means provided for monitoring the pressure in the fluid between the restrictors;
• and
• c. means provided for relating the pressure between the restrictors to the pressure drop across the device and for providing an indication of a change in the viscosity of fluid flowing through the duct.

A device for monitoring the viscosity of a fluid, comprising:

• a. two different character flow restrictors connected in series, the flow restrictors being selected such that under the intended conditions of use the flow rate of fluid through a first one of the said restrictors is affected more by a given change in the viscosity of the fluid than is the flow rate through the other restrictor for the same change in viscosity;
• b. means provided for monitoring the pressure in the fluid between the restrictors;
• c. means provided for relating the pressure between the restrictors to the pressure drop across the device and for providing an indication of a change in the viscosity of fluid flowing through the duct;
• and
• wherein the device comprises a hollow chamber having one of said restrictors as the fluid inlet thereto and the other of said restrictor as the fluid outlet thereto.

An ink jet printer comprising:

• a. a reservoir for ink, a pump for delivering ink under pressure via a pressurised ink line to at least one ink jet nozzle for printing or for collection in a gutter for recycling to the reservoir;
• b. a device in a duct connected between the pressurized ink line and the reservoir, which device comprises a chamber through which the ink is to flow, said chamber having as an ink inlet thereto a first flow restrictor and as an ink outlet thereto a second flow restrictor, and the flow rate through the said first restrictor being affected to a different extent than the flow rate through the said second flow restrictor with respect to a change in viscosity in the ink;
• c. means comprising a pressure sensor to measure the pressure within the chamber, and optionally, at least one other pressure sensor to monitor at least one of the inlet and outlet pressures to the chamber to provide an indication of the pressure drop across the said device, said sensors producing output signals in accordance with the pressure observed by the sensor, and means for relating the signal from the said first sensor to the pressure drop across the device so as to produce an output signal from the device indicating a change in the viscosity of the fluid flowing through the chamber;
• and
• d. means responsive to the output signal from the device for feeding a solvent or carrier medium to the reservoir to maintain the viscosity of the ink within a desired range of values.