GB2195777A - Plate-cone viscometer with viscosity-dependent regulation of shear rate - Google Patents

Description

1 GB2195777A 1 SPECIFICATION sition the bench having a plate connected to

it, characterised in that the plate together with Plate-cone viscorneter with viscosity-de- the bench are brought into the optimum posi pendent regulation of shear rate tion relative to the measuring cone by means 70 of a motor-driven micrometer spindle and a In conventional plate-cone viscometers, the process control apparatus for each measure dynamic viscosity is determined whilst taking ment, the measuring position being signalled into account various instrument constants or by means of electrical contacts and, during calibrating factors. This can be done with vari- measurement, the drive motor for the measur- ous cone angles or diameters and speeds. 75 ing cone being controlled by the output signal Thus, when the flow characteristics of the from an analog computer inserted in a regulat samples are not (strictly) Newtonian, different ing circuit, this analog computer linking the measurements may be obtained on the same electrical signals for the metal torque (um) and sample. Instruments of this kind are suitable speed (u,) continuously and in weighted man for difficult measuring procedures but not for 80 ner according to the equation continuous routine measurements in a pro duction run in which samples of very different UN - kN + um - km - uR - k, = 0, viscosity have to be measured.

Plate-cone viscometers typically have a mea- wherein um is a voltage proportional to the suring dynamic range of 1:3, with adjustment 85 torque, UN is a voltage proportional to the of the speed and without changing the cone speed and UR is a reference voltage and km, k, geometry, if the error in measurement is to and kv are coefficients which determine the remain below 1 %. weighting.

However, in production monitoring such as Preferably the voltages UN, u, and uR are is carried out in the manufacture of alkyd or 90 between - 15V and + 15V and the coeffici polyester resins, viscosity ranges of at least ents k,, km and kv are in the range from 0.01 1:10 to 1:20 must be measured rapidly and to 20.

reproducibly. Multiple measurements made Compared with the conventional method of necessary by manual switching of the range determining viscosity by measuring the ("trial and error") would be liable to error: the 95 outflow time using an outflow cup (e.g. ac evaporation of volatile solvent components cording to DIN 53211), this significantly re makes it essential to have short measuring duces the time for reaching the correct tem times and the smallest possible surface areas perature and making the measurement gives exposed to the air. In conventional viscomet- increased accuracy, reduces the quantity of ers there is also the risk of errors if a number 100 sample required to 1/100 and expands the of conversion factors have to be included in measuring range.

calculations. I An embodiment of the invention will now Therefore, the outflow cups according to be described by way of example.

DIN 53211, which are simple to handle, are in Figures 1 to 5 show the following:

widespread use; they do have a useful visco- 105 Fig. 1 is a schematic view of the basic con- sity measuring range of 1:10, but owing to struction of the apparatus claimed. The refer the large sample quantity (100 ml) they re- ence numerals denote the following:

quire long times to be brought to the correct (1) Measuring plate temperature (about 20 minutes) and suffer fur- (2) Measuring head ther inaccuracies of measurement caused, inter 110 (3) Measuring cone alia, by air bubbles stirred in. (4) Drive motor with tachogenerator There is therefore a tendency to switch pro- (5) Test bed with plate (1), capable of being duction control to,other methods. For reasons brought to a desired temperature and swung of comparability the same viscosity-dependent out laterally shear rates have to he maintained when such 115 (6) Position and process control a changeover is made. (7) Analog computer for continuous regula- It has now been found that by suitable ad- tion of the shear rate aptation of standard commercial plate-cone (8) Indicator instruments for the bench tem- viscometers, sequential measurement of dy- perature ('C) and the measurement obtained namic viscosity can be carried out in a pro- 120 for the dynamic viscosity duction run without the usual complicated op- (9) Supply device for the drive assembly eration and evaluation. (10) Motor-driven micrometer spindle for the The invention therefore relates to a plate- plate (1) cone viscometer with viscosity-dependent re- (11) Mains supply gulation of the shear rate for simple sequential 125 Fig. 2 schematically shows the analog com- viscosity measurement, consisting of a mea- puter which links the signals from the measur suring head which contains a replaceable mea- ing head for the torque and speed to one suring cone and a drive motor with tachogen- another continuously and in weighted manner erator and a bench capable of being brought and controls the motor so as to obtain a to a desired temperature and adjustable in po- 130 transmission function as shown in Fig. 3. The 2 GB2195777A 2 linking is in accordance with the equation of a few microns may lead to errors of mea surement of several percent.

UN - kN + um - km - UR - kv = 0, In the proposed apparatus, therefore, a mo- tor-driven micrometer spindle (10) is used for wherein u, is the voltage proportional to the 70 movement of the bench and this micrometer speed of the cone and um is the voltage pro- spindle, cooperating with the process control portional to the torque and UR is a reference means (6), seeks the optimum measuring po voltage, k,4 is the coefficient for the torque, int afresh for each sample.

which determines the weighting, and kN is the The signal indicating whether the plate is coefficient for the speed and k, is the coefficitouching the cone is produced by alternating ent for the amplification. current voltage having a frequency in the By a suitable choice of dimensions for the range 10 Hz to 100 KHz and preferably in the components and by a suitable choice of coe- audio frequency range in order to prevent po- F fficients for this analog computer it is pos- larisation or decomposition effects in the sam sible, for example, to simulate the dependency 80 ple; the time sequence of this signal is shown of the average shear rate on the viscosity as in Fig. 5.

in the measurement of the outflow time ac- The test bench (5) has thermostatically con- cording to DIN 53211 (see Fig. 4), thereby trolled fluid flowing through it for bringing it to obtaining directly comparable values. a desired temperature and is set precisely at In a practical embodiment the voltages UN, 85 right angles to the cone axis by means of u, and u, are between -15V and +15V and adjustment screws. Moreover, the entire the coefficients k, k, and kv are in the range bench can be pivoted sideways out of the from 0.01 to 20 and preferably the coeffici- axis of the sample to facilitate the cleaning of ents k,, kN and kv have the following values: the plate and cone.

k, = 0.5 to 1.2, kN 0.1 to 0.3, k, = 0.7 90 The position and process control means (6) to 1. 5. control and monitor all the processes in the The reference numerals and symbols used in positioning of the test bench and in the mea- Fig. 2 have the following meanings: surement and calibration of the apparatus. As shown in Fig. 5, the flow of highly viscous 4 Drive motor with gears and tachogenerator 95 samples as a function of time as the plate 12 Measuring spring approaches the cone is taken into account 13 Non-contact distance measurement when positioning the test bench.

k, Coefficient for the torque The control means used consist of freely k, Coefficient for the speed programmable control means based on a mi kv Coefficient for the amplification 100 croprocessor fitted with slow, interference-free P Potentiometer for setting the minimum inputs and relay outputs.

speed For monitoring production in the manufac- Z Zener diode for limiting the maximum speed ture of alkyd, polyester or copolymers, used u, Voltage proportional to the torque for example as lacquer binders, an apparatus u, Voltage proportional to the speed 105 having the following technical data may be u, Reference voltage used, for example:

Fig. 3 shows, by way of example, a transfer Cone used: Diameter 50 mm function such as is obtained by using the an- Angle (relative to the plane of the bench):

alog computer (7) for the continuous regula- 110 1 degree tion of shear rates, the useful measuring range Maximum shear rate: about 600 s-1 extending from about 50 to 10,000 mPas. Minimum shear rate: about 25 s-1 Fig. 4 shows, by way of a comparison, the Viscosity measuring range Jwith the cone spe- good comparability of the apparatus to the cified above):

shear rates which occur with the DIN cup. 115 for maximum accuracy: 100-2, 000 mPa.s Fig. 5 shows the sequence of events in the for reduced accuracy: 50-10, 000 mPa.s automatic positioning of the bench as a func- (For comparison, an outflow time of 25 to tion of the signal which indicates contact be- 200 seconds using the DIN outflow cup tween the plate and cone ("ON", "OFF"). 4/20C (DIN 53211) corresponds to values of Since plate-cone viscometers theoretically 120 from 100 to 1,000 mPa.s.) function correctly only when the cone is just Resolution for viscosity: 1 mPa.s touching the plate, the greatest care must be Reproducibility: 2% taken in adjusting the bench (= plate). When Admissible zero-point error: 0.15% of the the tip of the cone presses onto the bench, maximum torque an additional frictional moment is produced 125 Resolution for temperature: 0.10C which falsifies the measurement and makes Electrical contact between plate and cone:

the cone unusable through wear. If there is a measuring voltage: max. 1.5 VPP (about 2 kHz) gap between the tip of the cone and the Typical measuring time: 50 seconds (including bench, the viscometer indicates too little, and reaching desired temperature, no cleaning) depending on the geometry of the cone gaps 130 3 GB2195777A 3

Claims (8)

1. Plate-cone viscometer with viscosity-de- pendent regulation of the shear rate for simple sequential viscosity measurement, consisting of a measuring head which contains a repla ceable measuring cone and a drive motor with tachogenerator and a bench capable of being brought to a desired temperature and adjust able in position the bench having a plate Con 0 10 nected to it, characterised in that the plate together with the bench are brought into the optimum position relative to the measuring cone by means of a motor-driven micrometer spindle and a process control apparatus for each measurement, the measuring position be ing signalled by means of electrical contacts and, during measurement, the drive motor for the measuring cone being controlled by the output signal from an analog computer in serted in a regulating circuit, this analog com puter linking the electrical signals for the metal torque (um) and speed (UN) continuously and in weighted manner according to the equation u, - k, + u. - k,, - u, - k, = 0, wherein um is a voltage proportional to the torque, UN is a voltage proportional to the speed and u, is a reference voltage and k., k, and kv are coefficients which determine the weighting.

2. A plate-cone viscometer as claimed in claim wherein the bench is laterally pivotable.

3. A plate-cone viscometer as claimed in claim or 2 wherein the voltages UNI um and u.

are between - 15V and + 15V.

4. A plate-cone viscometer as claimed in claim 1, 2 or 3 wherein the coefficients k,, km and kv are in the range from 0.01 to 20.

5. A plate-cone viscometer as claimed in any preceding claim wherein the coefficients km, k, and kv have the following values: k, 0.5 to 1.2, k, = 0.1. to 0.3, kv = 0.7 to 1.5.

6. A plate-cone viscometer as claimed in any preceding claim, characterised in that the signal indicating contact between the plate and the measuring cone is produced by alter nating-current voltage having a frequency in the range from 10 Hz to 100 kHz.

7. A plate-cone viscometer as claimed in claim 6 wherein said frequency is in the audio frequency range.

8. A plate-cone viscometer substantially as hereinbefore described with reference to the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD.

Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.