GB2066483A

GB2066483A - Viscosity measuring device

Info

Publication number: GB2066483A
Application number: GB8038562A
Authority: GB
Original assignee: Crosfield Electronics Ltd
Current assignee: Crosfield Electronics Ltd
Priority date: 1979-12-04
Filing date: 1980-12-02
Publication date: 1981-07-08

Abstract

A constantly rotating device for continuously measuring the viscosity of a liquid comprises a constant speed synchronous motor (3) which applies torque to a drag member (2) immersed in the liquid (9), by means of a driving spindle (1), so that the relative angular displacement of the upper end of the driving spindle and an attached optical measuring disk 4 with respect to a torsionally rigid member (6) also attached to the member 2 and to another optical measuring disk 5 gives an indication of the size of this torque which is arranged to balance the viscous drag. <IMAGE>

Description

SPECIFICATION Viscosity measuring device This invention relates to a constantly rotating device for continuously measuring the viscosity of a liquid. It is especially applicable to the automatic control of the solvent content or printing ink, where constant monitoring of viscosity is useful.

It is known in the art to measure viscosity using a drag member immersed in the liquid which is made to rotate by an electric motor. Changes in viscosity, and hence in the drag on the rotating member, are manifested by variations in the current drawn by the motor.

The object of this invention is to provide a simple method for measuring the drag on a submerged drag member which is easier and cheaper to put into effect.

According to the present invention, a device for measuring the viscosity of a liquid comprises a container for the liquid whose viscosity is to be measured and a support for the container; a driving spindle attached at one end to the constant speed motor and at the other end to a drag member located within the container so as to be immersed in the liquid; a torsionally rigid member coaxially surrounding the spindle and attached to the drag member; and means responsive to the relative angular displacement of spindle and rigid member remote from the drag member for providing an output signal varying with the angular displacement whereby the motor rotates the drag member at constant speed and the said angular displacement is a function of the viscosity of the liquid.

Two examples of apparatus embodying the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a sketch of a viscosity measuring device according to a first embodiment of the invention; and Figure 2 is a diagram of a viscosity measuring device according to a second embodiment of the invention.

In the embodiment shown in Figure 1, a driving spindle 1 made of steel is attached at one end to a drag member 2 and at the other end to a first hub 7 which is driven by a synchronous motor 3. A first optical measuring disc 4 is attached to the first 'hub.

A second optical measuring disc 5 is attached by means of a second hub 8 to a torsionally rigid tube 6 surrounding the spindle and attached at its other end to the drag member. The axes of the motor drive shaft, first and second hubs, first and second optical measuring discs, spindle, rigid tube and drag member are all coincident. The motor is made to rotate at a constant speed, and applies sufficient torque to the hub 7 to balance the resistive torque which is due predominantly to the viscous drag on the member 2 or liquid 9 contained in an enclosure 12. This applied torque causes the spindle 1 to twist, causing the first measuring disc 4 to be displaced angularly relative to the second measuring disc 5.This relative angular displacement may be measured continuously by electronic means not shown, using stationary optical detectors 10 and 11 cooperative with the optical measuring discs 4 and 5.

The optical measuring discs have identical patterns of holes spaced regularly in a ring near their edges, and each optical detector comprises a light source and a light sensor arranged on opposite sides of the corresponding disc so that light from the source may pass through any of the holes in that disc and be detected by the sensor.

The sensors supply signals to the electronic means whenever the holes in the disc pass by the detectors, so the electronic means is provided with information as to the speed of rotation of the discs (which should be constant) and, more importantly, to the delay between the times that holes on the first disc and corresponding holes on the second disc pass the detectors, and hence to the relative angular displacement of the discs.

The electronic means may be calibrated to convert this information directly into the viscosity of the liquid, or it may instruct further apparatus not shown to change the viscosity of the liquid should this be necessary. In this way, the viscosity of a liquid may be monitored automatically and continuously. It is an advantage in this invention that the torsion in the spindle is linear with respect to viscosity.

In the second embodiment of the invention, shown in figure 2, the drag member 2 includes a rod portion 1 5 which is attached to the point at which the driving spindle 1 and the torsionally rigid tube 6 are joined. The entire drag member is surrounded by a cylindrical wall 14 attached to the rigid frame of the apparatus, whose function is to protect the drag member and to improve the consistency of the results by regulating the motion of the liquid around the drag member.

In the embodiment shown ir figure 2, a hollow cylindircal cup 7 and a steel hub 8 have pins 17, 1 8 respectively attached to their rims, instead of optical measuring discs 4, 5. The pins 17, 18 pass through optical detectors 10, 11 respectively once in every revolution of the spindle, and the interval between the passage of pin 1 7 through detector 10 and pin 1 8 through detector 11 is indicative of the angular displacement of the discs and hence of the torsion applied to the drag member, which in turn depends linearly on the viscosity of the liquid 9.

A damping device shown at 1 6 on figure 2 is provided for the steel hub 8, to dampen oscillations of the drag member 2 and hub 8 which would otherwise reduce the accuracy of tne instrument. The device 1 6 comprises an annular magnet (cross-hatched) enclosed by two steel discs of a diameter nearly as great as the inner diameter of the cylindrical cup 7. Ferrofluid damping oil is held between the magnet, the two discs and the inner wall of the cup 7 by the magnetic field of the magnet, and a ferrofluidic seal is thus achieved while leaving the hub 8 free to move axially or radially.

The motor shown in figure 2 is a DC printed circuit motor in which the speed is kept constant by electric control.

In either embodiment of the invention, a stop mechanism (13, fig. 2) is preferably included between the discs to limit the amount of torsion applied to the spindle to safe value.

The diameter of the driving spindle and the drag member are chosen to provide sufficient sensitivity for the required range of torques, i.e.

viscosities, to be measured. Narrower spindles will be more sensitive to changes, but will not be capable of measuring higher values of viscosity.

The spindle is preferably made of steel or a copper-beryllium alloy.

The viscosity measuring device has been found to have high accuracy, and can be used over a wide range of viscosities.

Claims

1. A device for measuring the viscosity of a liquid, comprising: a container for the liquid whose viscosity is to be measured and a support for the container; a driving spindle attached at one end to a constant speed motor and at the other end to a drag member located within the container so as to be immersed in the liquid; a torsionally rigid member coaxially surrounding the spindle and attached to the drag member; and means responsive to the relative angular displacement of spindle and rigid member remote from the drag member for providing an output signal varying with the angular displacement whereby the motor rotates the drag member at constant speed and the said angular displacement is a function of the viscosity of the liquid.

2. A device according to claim 1, in which the motor is a DC printed circuit motor which is electronically controlled.

3. A device according to claim 1 or 2, in which the drag member is in the shape of a double cone.

4. A device according to claim 1, 2 or 3, in which the driving spindle is of steel.

5. A device according to claim 1, 2 or 3, wherein the driving spindle is of a copperberyllium alloy.

6. A device according to claim 4 or 5, wherein the coaxial rigid member is a steel tube surrounding the spindle.

7. A device according to any one of claims 1 to 6, in which the means responsive to the relative angular displacement of spindle and rigid member comprises: a first optical sensing disc attached to the end of the spindle nearer the driving means; a second optical sensing disc attached to the rigid member close to the first optical sensing disc,- both discs being coaxial with the driving spindle and rigid member; first and second optical detectors stationed by the first and second optical sensing discs respectively which provide output signals; and an electronic means responsive to the said output signal which provides an output signal varying with the viscosity of the liquid.

8. A device according to claim 7, in which each optical measuring disc has a regular array of holes, and in which each optical detector comprises a lamp and a photocell arranged on opposite sides of its associated disc so that light from the lamp may pass through each hole in turn and be detected by the photocell, the output signal from each detector being derived from each photocell.

9. A device according to claim 7, in which each optical sensing disc has a pin attached on or near its rim and positioned such as to pass by one of the optical detectors once in every revolution of the spindle, the said output signals indicating the passage of the pins past the optical detectors.

10. A device according to any of claims 7 to 9 in which there is a ferrofluidic damping means between the first and second optical sensing discs.

11. A device according to any preceding claim, in which the drag member includes a rod portion and the whole drag member is surrounded by a cylindrical wall open at the end further from the rod portion.

12. A viscosity measuring device substantially as described herein with reference to the accompanying drawinqs.