GB2058341A - Viscometer - Google Patents

Abstract

A viscometer comprises a bob (3) which is positioned in the liquid whose viscosity is to be measured and is rotated at constant speed by a motor (1) and drive assembly (2, 26). The drive assembly includes a resiliently twistable section (2) and the angle of twist over its length is proportional to the viscosity of the liquid. Respective arms (4, 5) are secured to the drive assembly at both ends of the section (2). An electronic processor (27) counts the time between a first optical detector (6), and then a second optical detector (7), detecting the passage of an associated arm (4, 5) past that detector and then produces an output indication, related by a predetermined function to the counted time interval, of the viscosity of the liquid. <IMAGE>

Description

SPECIFICATION Viscometer This invention relates to a viscometer.

Viscometers are known which comprise a cup containing a liquid, whose viscosity is to be measured, an upright shaft having mounted on the lower end thereof a bob (paddle) positioned in the liquid in the cup, a drive motor above the bob, and a flat coil spring, one point on which is connected to the motor shaft and another point to the upright shaft. Usually a pointer and dial are attached to the spring respectively at the aforementioned two connection points. The motor is driven at constant speed and the deflection angle through which the spring is wound due to the viscous drag forces acting on the bob is measured. This measurement is made for example using a trigger device to clamp the pointer to the dial while simultaneously de-clutching the motor drive, or using a stroboscope, so as to enable the spring deflection angle as between the two connection points to be read.By applying suitable conversion factors to the measured spring deflection angle and motor speed, the viscosity of the liquid in the cup can be calculated.

Such known viscometers are disadvantageous in that a calculation is necessary from the measured deflection angle to determine the viscosity. Thus, they do not provide a direct indication of the viscosity. Also reading the deflection angle from the pointer and dial using either of the above-mentioned two techniques is not as convenient as it would be if it were possible to display readings on a display device, e.g. an electronic digital display.

According to the invention there is provided a viscometer which comprises a drive motor, a bob for positioning in a cup of liquid whose viscosity is to be measured, a rotary drive assembly arranged to transmit drive from the motor to the bob, the drive assembly including a resiliently twistable section extending between first and second locations on said drive assembly so that, in use, said first and second locations undergo relative angular displacements, from their relative rest positions, about the axis of said drive assembly dependent upon the viscosity of the liquid, first and second detector means each operative to respond each time said first or said second location, respectively, passes through a particular angular position about the axis of said drive assembly, and electronic timing means arranged to produce an output indication, related by a predetermined function to the time difference between successive corresponding responses of said first and second detector means, of the viscosity of the liquid.

By suitable scaling of the response of the timing means, the output indication can provide a direct indication of the viscosity. In addition, the output indication can conveniently be used to operate a display unit, such as an electronic digital display.

Generally, the output indication from the electronic timing means is linearly directly proportional to the measured time difference.

Conveniently, the electronic timing means comprises a pulse generator, a counter arranged to count the pulses generated in use by the pulse generator and a start/stop measurement circuit arranged to control initiation and termination of counting in dependence upon the output conditions of the first and second detector means.

Preferably a range control is provided for adjusting the generation rate of the pulse generator.

In one arrangement, a datum indicating arm and a displacement angle detection arm are respectively secured to the drive assembly at the said two locations and each of the two associated detector means which are optical detectors is operative to respond to the movement of the associated arm into and out of the detection region of the optical detector, preferably with the two arms occupying the same angular position about the axis of the drive assembly, a logic element is arranged to detect coincidence of response by the first and second optical detectors, and means are provided to discriminate whether coincidence of response is due to an off-limit condition of the resiliently twistable section as opposed to a zero reading and, if so, to enable the logic element so as to give an indication of the offlimit condition.The discrimination can be effected by the start/stop measurement circuit, which is arranged to check whether detection of one of the arms is followed by detection of the other arm before the said arm is again detected and, if not, to enable the logic element. The logic element can be arranged to modify, upon detection of an off-limit condition, the setting of the range control so as to restore the viscometer to operation within the normal operating range of the resiliently twistable section. Alternatively, or in addition, the logic element can be arranged so as, upon detection of an off-limit condition, to reduce the operating speed of the drive motor.

In a further arrangement, a slot is formed in one of the said arms in such a position that it will pass through the detection region of the associated optical detector during each revolution of the drive assembly, and means are provided which are arranged, upon detection of the slot by the said associated optical detector, to reset the counter to zero. A control circuit may be provided for producing a command signal to override the resetting means so as to hold the last output indication produced by the electronic timing means.

Means may be provided to provide an indication in the event of the counter overrunning its operating range.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a side eievation one form of viscometer in accordance with the invention.

Figure 2 shows, in plan view, datum indicating and spring displacement angle detection eiements of the viscometer, and Figure 3 is circuit block diagram of a processor forming part of the viscometer.

Referring to Figures 1 and 2, a viscometer is shown comprising a driving motor 1 , with or without a gearbox, a flat coil spring 2, one point on which is connected to the motor shaft via a datum indicating arm 4, a bob or paddle 3 on the lower end of an upright shaft 26 which is connected via a spring displacement angle detection arm 5 to a further point on the spring, optical angular position detectors 6 and 7, respectively associated with the arms 4 and 5, and an electronic processor 27 which processes the output signals from the detectors. The datum indicating arm, together with detector 6, responds each time the location on the spring at the associated point passes through a particular angular position about the vertical axis of the rotating drive shaft assembly.

This is achieved by arranging for the detector to respond when a trailing or datum edge 9 on the arm 4 passes beneath the detector 6. The arm 4 is also provided with an off-limit detection sector 1 8 and a slot 8, the purpose of which will be explained hereinbelow.

Referring to Figure 3, the outputs from detectors 6 and 7 are processed electronically in the following manner. Detector 6 signals each time the slot 8 is detected that a measurement is about to start and causes a start/stop logic circuit 10 to return a counter 11 to zero. The counter has a counting input connected to the output of a pulse generator 13 which continuously feeds pulses to the counter 11. On being reset to zero by the logic circuit 10 and once the detector 6 detects the datum edge 9, the counter 11 initiates a new count, providing that a sample/hold control circuit 12 is in the sample mode and no off-limit condition, to be described below, is detected.The control circuit 12 is provided in order that the viscometer can be commanded, either locally or remotely 5, to continue sampling or to hold and display its last measurement. Every time the angle detection arm 5 passes detector 7, the start/stop logic circuit 10 stops the counter 11 and a number is displayed on a display unit 14 connected to the counter output or is transmitted via an external output 1 5.

When the motor and paddle are at rest or are rotating in a medium (air for example) having a viscosity below the measuring range of the instrument, the trailing edge 9 and angle detection arm 5 are conveniently (though not necessarily) so arranged that the detectors respond simultaneously. Whatever arrangement is employed, under rest conditions or for a medium of negligible viscosity, a zero output reading will always be given. When, however, the paddle is positioned in a stationary cup of liquid, the viscous drag forces acting on the paddle while it is rotated at constant speed cause the spring to wind up to bring about a change in relative angular positions between the trailing edge 9 and the angle detection arm 5.It will accordingly be appreciated that the number of pulses counted and displayed (possibly with the addition or subtraction of a number of pulses in cases where the arms 4, 5 are not in phase in the rest position) after the detectors 6 and 7 have started and stopped the measurement will be a function of the angle subtended between the trailing edge 9 and arm 5, which is proportionally related to the viscous drag acting on paddle 3, the speed of rotation of the paddle 3, the paddle shape and size, and the rate at which pulses are produced by pulse generator 13. The pulse generator has its pulse rate accurately set to take these factors into account and to produce an output from counter 11 to display 14 in units of viscosity (e.g. centipoise).It will be realised that the viscosity measurement is related to the above factors.- Of course, for each successive revolution of the paddle, a new measurement of viscosity is made. Provision is made for the pulse generator 13 to have its pulse generation rate changed by a range control 16, either locally or remotely 29, to take account of paddle size, and other variables, such as temperature, in order that a direct reading of centipoise (or other units of viscosity measurement) is displayed at 14, or transmitted externally 1 5. The range control 1 6 also allows tolerances in the spring rate of the coil spring 2 to be compensated for during calibration.

An off-limit condition (referred to above) can be found where the angle detection arm 5 is dragged by the paddle 3 to an overlapping situation with the datum indicating arm 4, i.e. the viscometer is at the end of its operating range. This condition is identified by a measurement off-limit logic element 17, which detects coincidence of detection by the detectors 6 and 7, of the off-limit detection sector 1 8 and the angle detection arm 5, respectively.To ensure that the logic element 17 does not confuse a zero reading with an offlimit condition (coincidence of detection by the detectors 6 and 7 will exist in both cases), the logic circuit 10 monitors the outputs from the detectors 6, 7 to check that detection of the datum edge 9 is followed by detection of the arm 5 before the detection sector 1 8 is next detected and, if not, it generates an enabling signal at an output 30. As shown, this output is connected to a control input of the measurement off-limit logic element 1 7 and this element is so arranged that when it identifies an off-limit condition, and simultaneously receives an enabling signal from the logic circuit 10, it produces an alarm which can drive a visual or audio indicator 20, or produce an external alarm signal 21. The alarm signal is also available for changing the motor speed 1 9 and range control 1 6 automatically so as to produce a measurement within the operating limits of the spring.

A further off-limit condition can occur whereby the counter 11 overruns its operating range. A visual or audio alarm 22 can be connected to the counter output to provide an alarm indication of the overrun condition, and an external warning signal 23 for speed and range control can also be provided. An off-limit cancel control 24 is provided on the viscometer and externally at 28.

It will be appreciated the viscometer disclosed herein provides facility for remote control and data collection as well as the inputting of correction factors, so as to allow for changes such as in temperature.

Claims (14)

1. A viscometer which comprises a drive motor, a bob for positioning in a cup of liquid whose viscosity is to be measured, a rotary drive assembly arranged to transmit drive from the motor to the bob, the drive assembly including a resiliently twistable section extending between first and second locations on said drive assembly so that, in use, said first and second locations undergo relative angular displacements, from their relative rest positions, about the axis of said drive assembly dependent upon the viscosity of the liquid, first and second detector means each operative to respond each time said first or second location, respectively, passes through a particular angular position about the axis of said drive assembly, and electronic timing means arranged to produce an output indication, related by a predetermined function to the time difference between successive corresponding responses of said first and second detector means, of the viscosity of the liquid.

2. A viscometer according to Claim 1, wherein the electronic timing means comprises a pulse generator, a counter arranged to count the pulses generated in use by the pulse generator and a start/stop measurement circuit arranged to control initiation and termination of counting in dependence upon the output conditions of the first and second detector means,

3. A viscometer according to Claim 2, wherein a range control is provided for adjusting the generation rate of the pulse generator.

4. A viscometer according to any preceding claim, wherein a datum indicating arm and a displacement angle detection arm are respectively secured to the drive assembly at the said two locations and each of the two associated detector means which are optical detectors is operative to respond to the movement of the associated arm into and out of the detection region of the optical detector.

5. A viscometer according to Claim 4, wherein with the two arms occupying the same angular position about the axis of the drive assembly, a logic element is arranged to detect coincidence of response by the first and second optical detectors, and means are provided to discriminate whether coincidence of response is due to an off-limit condition of the resiliently twistable section as opposed to a zero reading and, if so, to enable the logic element so as to give an indication of the off-limit condition.

6. A viscometer according to Claim 5, as appended to Claim 2, wherein the discrimination is effected by the start/stop measurement circuit which is arranged to check whether detection of one of the arms is followed by detection of the other arm before the said one arm is again detected and, if not, to enable the logic element.

7. A viscometer according to Claim 5 or to Claim 2 to 6, wherein the logic element is arranged to modify, upon detection of an off-limit condition, the setting of the range control so as to restore the viscometer to operation within the normal operating range of the resiliently twistabie section.

8. A viscometer according to Claim 5, 6 or 7, wherein the logic element is arranged so as, upon detection of off-limit condition, to reduce the operating speed of the drive motor.

9. A viscometer according to Claim 4 as appended to Claim 2 or 3, or according to any one of Claims 5 to 8 as appended directly or indirectly to Claims 2 and 4 or Claims 2, 3 and 4, wherein a slot is formed in one of the said arms in such a position that it will pass through the detection region of the associated optical detector during each revolution of the drive assembly, and means are provided which are arranged, upon detection of the slot by the said associated optical detector, to reset the counter to zero.

10. A viscometer according to Claim 9, wherein a control circuit is provided for producing a command signal to override the resetting means so as to hold the last output indication produced by the electronic timing means.

11. A viscometer according to Claim 2 or any one of the Claims 3 to 10 as appended to Claim 2, wherein means are provided to provide an indication in the event of the counter overrunning its operating range.

12. A viscometer according to any preceding claim, further comprising an electronic display unit to provide a digital display of said output indication.

13. A viscometer according to any preceding claim, wherein said output indication is a direct indication of the viscosity of the liquid under investigation.

14. A viscometer according to any preceding claim, wherein said output indication is linearly directly proportional to the measured time difference.

1 5. A viscometer substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.