1、INTERNATIONAL STANDARD INTERNATIONAL ORGANIZATION FOR STANDARDIZATION ORGANISATION INTERNATIONALE DE NORMALISATION MEXJJYHAPOAHAR OPTAHM3AL/MR I-IO CTAHAPTM3Al! iy(.;, g:+it : -J however, at rotational frequen- cies above 1 s-t it should be ascertained that inertia forces are negligible. 2 Reference
2、 IEC Publication 584-1, Thermocouples - Part 1: Reference tables. 3.1 Field of flow, crucible and plunger 3.1.1 field of flow: The whole gap filled by the molten glass sample and the spatial distribution of the flow velocities within it, including its boundaries. 3.12 crucible : The outer boundary o
3、f the molten glass sam- ple corresponding to the inner surface of the crucible up to the level of the melt. 3.1.3 plunger : The inner boundary of the molten glass sam- ple corresponding to the outer surface of the plunger up to the level of the melt. 3.2 Flow field coefficient and instrument constan
4、t 3.2.1 Rotation viscometers are either of the Searle type or of the Couette type, both of which allow the determination of the viscosity according to basic equation (I 1: rl=fT . . . n where q is the viscosity; (I) 1 IS0 7884-2 : 1987 (El f is the flow field coefficient; T is the torque applied to
5、the plunger; n is the rotational frequency either of the plunger (Searle type) or of the crucible (Couette type). The flow field coefficient f is a function only of the geometrical shape of the field of flow and has the dimension of reciprocal cubic length. The analytical calculation of the flow fie
6、ld coefficient is possible in a restricted number of geometries, e.g. in the following cases (see also annex A). a) Crucible and plunger are shaped like infinite concentric cylinders; methods of eliminating experimentally the effect of the plane end surfaces are known from literature, but they are d
7、ifficult to apply in the case of glass melts. b) Crucible and plunger form two confocal rotation sur- faces of the second order, e.g. two half-ellipsoids, cut perpendicularly to the axis of rotation. If the shapes of the crucible and of the plunger are made up by several rotational surfaces, the bou
8、ndaries of the perpendicular median cuts being irregular curves (e.g. cylinders with plane or cone-shaped or hemi-spherical ends), the flow field coefficient can be determined only by means of viscometric standard liquids with Newtonian behaviour.1) 3.2.2 If, for the torque, the length and/or the ro
9、tational fre- quency, units are used which do not correspond to the unit for the viscosity, and if the resulting factor is combined with the flow field coefficient, a new constant, the instrument con- stant k, can be defined by equation (2) : . . . (2) q is the viscosity measured, in decipascal seco
10、nds; k is the instrument constant (numerical value), resulting from equation (3); T is the torque, in newton millimetres; n is the rotational frequency, in reciprocal seconds. The instrument constant k is related to the flow field coefficient f by equation (31, if the flow field coefficient f is exp
11、ressed in reciprocal cubic millimetres (mm -3) : k = IOf . . . (3) NOTE - The factor 107 in equation (3) results from the relation 1 N.s/mm* = 107 dPa.s 3.2.3 In many cases the rotational frequency n and the torque Tare not read or recorded directly. For the sake of convenience all the factors neede
12、d for their calculation and the flow field coefficient can be combined into a special instrument cons- tant k*. This constant shall be determined by calibration using viscometric standard liquids with Newtonian behaviour. 1) 3.2.4 The flow field coefficientf and the instrument constants k and k” are
13、, for Newtonian liquids, independent of the rota- tional frequency, of the torque and of the viscosity. They are slightly dependent on temperature because of the thermal ex- pansion of the glass melt, of the crucible and of the plunger (see 7.3). 3.3 Torques 3.3.1 driving torque : The torque applied
14、 by the drive to the boundary surface rotating with the rotational frequency n. 3.3.2 frictional torque: Torque of the opposite sign to the driving torque, arising from the viscous flow between the two boundaries, the one rotating and the other being at rest. 3.3.3 torque qf mechanical losses : The
15、frictional resistance caused by influences other than the glass melt (e.g. friction in bearings, air friction; in the case of rotational vibra- tions, also the self-damping of the torsion spring). 3.3.4 In the case of stationary rotation, which is essential for obtaining correct results, these three
16、 torques add up to zero (action = reaction). Torques of mechanical losses, caused by the particular construction of the instrument, shall be eliminated through a correction. 4 Apparatus (see annex B) 4.1 Rotation viscometer Two types of rotation viscometer comply with this part of IS0 7334: a) Visco
17、meter of the Couette type with adjustable, revolving crucible. The crucible stands on a turntable which is to be positioned in the furnace through its lower opening; the shaft of the plunger reaches through the upper opening of the furnace up to the torque-measuring device. b) Viscometer of the Sear
18、le type with the crucible at rest and the plunger revolving, the revolving movement be- ing induced through a shaft reaching out of the upper open- ing of the furnace. The torque-measuring device is applied to the shaft of the plunger. Both types can be operated a) either at fixed rotational frequen
19、cies (e.g. by means of a combination of synchronous motor and gear transmission, the variation range of the rotational frequency n being at 1) See for example IS0 7884-l : 1987, annex B, “Examples of certified reference glasses for viscometric calibration”. 2 , IS0 7884-2 : 1987 (E) least 1:lOO) wit
20、h a torque-measuring device (torsion wires, torsion spring or torsion balance) whose possible inaccur- acy shall not exceed 2 % at a torque of about 5 N-mm; b) or at fixed torques (e.g. by means of a weight and pulley system or by means of an electric motor) with a measuring device for the rotationa
21、l frequency (frequency meter, electronic speedmeter with optical or inductive sen- sor, microscope for very low rates). To protect these parts of the apparatus, special screening against heat or, if necessary, water-cooling is recommended. 4.2 Furnace Electrically heated tube-shaped furnace, designe
22、d for a vertical working position, for temperatures up to 1 400 OC (in special cases up to I 600 OC), with covers for the upper and the lower ends of the tube which shall be made of heat-resistant ceramic material. The temperature in the flow field area or in the adjacent space in the furnace shall
23、be constant to + 2 OC with respect to time, and the temperature gradient shall not ex- ceed 1 OC/cm. NOTE - This requirement is achieved by one or more of the following devices: extra heaters at the two ends of the ceramic tube; baffles made from noble metals (e.g. platinum); a suitable cover on the
24、 cru- cible; a thick-walled crucible made of noble metal. Good thermal insu- lation of the furnace is required in any case. 4.3 Temperature measuring and indicating instruments 4.3.1 The alumina-insulated platinum-10 % rhodium/platinum (type S according to IEC 584-I), or (for extensive use above 1 2
25、00 OC) platinum-30 % rhodium/platinum-6 % rhodium (type B according to IEC 584-l) thermocouples shall exhibit low thermal inertia (the diameter of the wires should not be greater than 0,5 mm). The wires shall have a sufficient length within the furnace (with respect to heat conduction along the wire
26、s). 4.3.2 Control thermocouples should be located as near as possible to the furnace winding for fast response. The hot junc- tion of the measurement thermocouple, however, shall be placed in the immediate vicinity of the flow field (the crucible). In accordance with IS0 7884-1, the measurement ther
27、mo- couple shall be calibrated and the calibration checked regularly. NOTE - Further improvement in the accuracy of the temperature determination is achieved by dipping the (electrically isolated) measurement thermocouple into the melt and/or by observation of the temperature distribution by means o
28、f two further thermocouples placed above and below the crucible. If the construction of the viscom- eter permits the positioning of the measurement thermocouple at the centre of the plunger, the best assignment of temperature to the shear area, which is mainly responsible for the measured viscosity,
29、 is achiev- ed; in that case other special devices (see note to 4.2) may be omitted. 4.3.3 The electrical output of the thermocoup!es shall be determined at zero current by means of potentiometers or high- resistance electronic amplifiers having a sensitivity of 1 pV. Precautions shall be taken that
30、 the ice-bath for the cold junc- tion is maintained at 0 OC throughout the test. If the temperature measuring equipment is fitted with automatic cold junction compensation, the ice-bath can be omitted. 4.4 Crucible and plunger 4.4.1 Crucibles According to their chemical resistance to the glass melt
31、under test, the crucibles shall be made of ceramic material (e.g. alumina) or noble metal. If ceramic crucibles are used (normally they can be used only once), it is recommended that such a crucible be placed in a larger crucible made of thin noble sheet- metal, in order to protect the furnace in ca
32、se the ceramic .cru- cible breaks. Noble metal crucibles (preferably platinum or platinum-rhodium alloys) require cleaning, e.g. in a hydro- fluoric acid bath. The volume of the crucibles (sample volume) shall normally be between 20 and 250 ems. 4.4.2 Plungers The plungers shall be made of noble met
33、al, preferably of platinum-rhodium alloys. They shall be welded to a long thin shaft, reaching out of the furnace to the torque-measuring in- strument. The shaft may be made entirely of noble metal, or alternatively only that part of it which plunges into the melt may be of noble metal, the part abo
34、ve the melt being made of ceramic material. The lower end of a cylindrical plunger shall be cone-shaped, not plane, in order to avoid bubbles adhering to it; this would produce scatter in the results. Moreover, the plunger shall have no sharp edges, as sharp edges would be more easily attacked by th
35、e melt. Therefore, the plungers should be spheres, cylinders with hemi-spherical ends, or ellipsoids. NOTE - The contribution to the torque of flow arising from the shaft can be restricted to less than 15 %. If the flow field coefficient is to be determined by calculation, the shape of the plunger s
36、hall be chosen accordingly. By means of a set of plungers of different sizes, the ratio of the sizes being for example 1:2, the possible range of measurement can be extended by about one order of magnitude. 4.5 Apparatus for quantifying the test sample One of the following methods shall be used : a)
37、 a balance and device for determining the density of the glass at room temperature (between 18 and 28 “Cl; or b) a device for determining the distance between the sur- face of the melt and the upper edge of the crucible at a temperature above the melting range of the sample, e.g. at 1 000 OC; or c)
38、a device for the visual determination of the level of the melt in the crucible. 4.6 Devices for the adjustment of the flow field 4.6.1 Device for the adjustment of the position of the furnace and/or of the torque-meter, if necessary with a facility for swinging the furnace or the viscometer aside an
39、d precisely back in the same plane again. 3 IS0 7664-2 : 1967 (E) 4.6.2 Device for centring crucible and plunger. 4.6.3 Device for checking the concentric running of the plunger or of the crucible; deviations shall not exceed l/100 of the plunger diameter; such faults normally originate from bends i
40、n the plunger shaft. It is recommended to use a self-centring suspension device, e.g. a universal joint; its effectiveness shall be checked. When the position of the apparatus is changed or the furnace and/or the measuring device are swung aside, the adjustment of the flow field shall be maintained.
41、 4.6.4 Device to check the relative positions of crucible and plunger in the furnace, e.g. scale or marks or windows in the wall of the furnace. NOTE - If it is i.mpossible to avoid the use of windows, they should be of minimum size. Take care to minimize disturbance of the temperature homogeneity,
42、e.g. by suitable wiring of the heaters. 4.7 Other equipment The following may be necessary : - crucible pincers with platinum points; - rods of ceramic material, diameter between 3 and 10 mm, length about 400 mm; - - - - tools for crushing the glass; thermal insulating gloves; infra-red protective g
43、oggles; melting furnace. 5 Test specimen 5.1 The sample of the glass to be tested shall consist of pieces larger than 3 mm. If the pieces do not fit into the cru- cible, they shall be crushed carefully, avoiding any contami- nation (see IS0 7884-l : 1987, sub-clause 7.1). 5.2 The quantity needed sha
44、ll be determined according to the size of the flow field, known from the calibration experiment. 5.3 The glass specimen shall be melted either in the furnace of the viscometer or in a separate melting furnace; the melt shall be free from bubbles. Normally the bubbles rise quickly enough if the visco
45、sity is about 1 000 dPas. The melting temperature shall be chosen accordingly. It shal! be taken into account that, with increasing temperature (decreasing vis- cosity), the sample can change its properties through incongru- ent evaporation and/or changes in water content. For the treat- ment of mel
46、ts, see also IS0 7884-l : 1987, sub-clause 7.3. NOTE - For the usual flat glasses a melting temperature between 1 209 and 1 250 OC is sufficient. Glasses with an unusually high tendency to foam, evaporate or attack chemically the crucible and plunger materials give rise to special requirements for t
47、he preparation of the sample and for the materials which come into contact with the melt. 6 Procedure 6.1 Calibration The rotation viscometer shall be calibrated by means of viscometric standard liquids with Newtonian behaviour, e.g. certified reference glasses (see IS0 7884-l : 1987, annex B), unle
48、ss the flow field coefficient and the instrument constant can be determined by calculation. When calibrating with stan- dard liquids other than reference glasses, measurable heating by friction shall be avoided and the torque of mechanical loss shall be taken into account. Thermocouples shall be cal
49、ibrated by comparison with a calibrated standard and/or with thermometric fixed points. Every calibration shall be repeated regularly. 6.2 Preparation The crucible containing the molten glass test specimen shall be positioned in the heated furnace of the viscometer. The plunger shall be dipped slowly into the melt, down to its desired posi- tion relative to the position of the crucible, the viscosity of the melt being less than 1 000 dPas. NOTE - It is advantageous to rotate slowly the plunger - or the cup in the case of a Couette-type device - while dipping the plunger slow- ly into the
