DE102007011985B4 - Mortar measuring cell for rotation viscometer - Google Patents

Abstract

Mortar measuring cell for rotational viscometers for the rheological investigation of fluids in the form of different solid and liquid phases, designed as a double-gap measuring cell consisting of an annular gap vessel (2) and a measuring body (1), here called the basket sensor, characterized in that the effective measuring surface (3) of the basket sensor (1) is formed by a skeleton with the material embedded therein, the skeleton being designed as a mesh and the size of the meshes (4) being chosen in suitable relation to the grain size of the solid phase of the fluid.

Description

The subject matter of the invention is a measuring cell for equipping rotational viscometers for determining the flow properties of fluids, in particular those that consist of a liquid and a solid phase, such as grains, fibers or the like.

State of the art

Rotational viscometers have been used to measure the flow properties of fluids for decades. These are equipped with various measuring cells suitable for the object of investigation. For example plate/plate (DIN 53018), cylinder measuring systems (ISO 3219) or double gap measuring systems (DIN 54453, DE 66716 , DE 844 362 B U.S. 4,214,475 A DE 36 30 565 A While the flow properties of homogeneous fluids, such as oils, can be measured well with this, fluids that consist of a liquid and a solid phase, such as cement suspensions, ceramic slip, mortar or fresh concrete, cannot be evaluated with the known measuring cells because at the boundary surfaces between the measuring cell and the fluid, separation of the phases under the shear stress leads to wall sliding effects and to settling or sedimentation of the denser particles. In U.S. 50 56 358 A a double-gap measuring cell is designed on the upper side for the denser particles to be permeable through a sieve, so that sedimentation is not hindered. The measuring cell is then arranged in an area of the measuring vessel in which there is an average particle concentration. A region of the liquid phase forms above it, and below it a dense region of sedimented particles. In addition, (Metzger, Thomas: Das Rheologie Handbuch - Hanover: Vincentz, 2000, page 211)) the distance between the shearing surfaces must be at least five times (better ten times) as large as the largest dimension of the particles in the fluid. As a result, a high shear rate must be used to ensure that the entire measuring gap is actually sheared through. The high shear rate promotes the separation of solid and liquid phases through centrifugal forces. If you want to exploit the centrifugal forces in a targeted manner, you can make the outer surface of the double-gap cell permeable by perforating ( DD2 52 246 A1 ).

It was also proposed to structure or roughen the surfaces of the measuring cells ( AT 1 22 893 B ) or lamellar ( DD 2 83 058 A7 ) to prevent wall slip. Technical practice shows that it is difficult to determine a precise shearing surface for this and that the existing indentations or cavities become clogged during the measurement and thus falsify the measurement result. Another way of avoiding the formation of sliding layers and segregation is to dispense with the shearing of the fluid between defined surfaces and to use a sinker ( DE195 03 028 A1 ) or stirrer ( U.S. 4,332,158 A WO 93/05382 A1 to move through the material with a suitable device. Similar to a rotational viscometer, the force on the displacement body is measured as a function of the speed. The disadvantage of such a device is that the size of the sheared area of material cannot be precisely determined. Since the material volume actually sheared is always included in the physical description of the material flow, one can at best speak of relative comparative measurements. It is not permissible to specify a viscosity or yield point here.

In many technical areas, heuristic methods have been used for more than 100 years to assess the flow behavior of pasty fluids. Here, for example, run-out tests (EN 12706) for slip, filler, or self-compacting concrete ( JP H09- 250 980 A ) used (EN 12706). For stiffer materials, the flow table (DIN EN 12350) or the shock table (DIN EN 1215-3) are common.

Since these methods fix the rheological parameters of the material, such as viscosity and yield point, only for a specific one, only as a whole and also device-specifically through a geometric dimension or time, they can only be used for orientation.

problem

The invention specified in claim 1 is based on the problem of excluding the wall sliding effects in a measuring cell for rotational viscometers with defined areas, namely the double gap cell, and thus measuring the yield point and viscosity of mixtures of substances, in particular mortars, exactly.

solution

This problem is solved by designing the measuring body of a known double-slit measuring cell in such a way that it is mainly formed by the material to be examined. To achieve this. the measuring body consists only of a mesh-like framework or network with negligibly small wall surfaces, in which fluid particles are embedded and thus form the shearing surface.

Advantageous Effect of the Invention

The advantages achieved with the invention are, in particular, that the fluid is connected through the openings of the mesh between the material inside and outside of the measuring body and thus cohesive forces prevent the fluid from sliding along the wall. This is achieved in particular by the fact that, depending on the material, the size of the braided mesh is adapted to the fluid, above all to the maximum grain size.

Since the inner and outer surface of the measuring body contribute to the measuring signal (torque) in a double-gap measuring cell, and this is therefore very sensitive, the yield points of very free-flowing suspensions and mortar can also be determined.

If the annular gap vessel or the measuring body is rotated while the measuring body is being immersed, sloping guide profiles at the lower end ensure that the body can screw itself into the test material, even if the material is stiff.

These guide profiles also prevent sedimentation of the solid phase of the fluid during the measurement.

Leveling slots that are worked into the mesh at the upper end of the measuring body and an overflow cup in the middle of the annular gap vessel advantageously ensure that the correct level of the fluid is set automatically. The bottom of the annular gap vessel can be rounded off without affecting the measurement result, in order to enable the vessel to be cleaned easily.

The annular gap vessel can advantageously be made up of two parts, so that the inner cylinder with the overflow cup can be removed for cleaning.

An exchangeable adapter connects the measuring body with the torque sensor of the viscometer, so that the measuring cell can easily be adapted to different viscometers.

The measuring cell can be used without fundamental changes, both with viscometers with a rotating annular vessel and a fixed measuring body (Couette type) and viscometers with a fixed annular vessel and rotating measuring body (Searle type).

Presentation of the invention and its possible variants: An exemplary embodiment of the invention is shown in the drawing and is described below as a basket measuring cell. Show it 1 the basket measuring cell in section, 2 the basket measuring cell in section as a spatial representation. The basket measuring cell consists of the measuring body or basket sensor (1) and the annular vessel (2). The effective measuring surface (3) is designed as a net or mesh-like network. The size of the meshes (4) is chosen in a suitable relation to the maximum grain size of the liquid to be tested. The mesh is connected to the bracket (5) at the top. The adapter (6) with the holder for the viscometer, which is not shown here, is located in the center of the axis of rotation. The lower edge of the mesh is stabilized by a ring (7) to which slightly inclined guide profiles (8) are attached. These allow the basket sensor to dig into the fluid (9) to be tested when entering the annular gap vessel (2), even if this has a stiffer consistency. To do this, either the basket sensor (1) or the annular gap vessel (2) must rotate when retracting. The material (9), which is displaced when the basket sensor (1) is retracted, can flow through the leveling slots (10) in the mesh from the outer to the inner area of the annular gap vessel (2). Material can get into the overflow cup (11) from the inner area, so that an exact fluid level (12) is always set. The overflow cup (11) closes off the inner cylinder (13) at the top. During the measurement, the guide profiles (8) prevent any sedimentation of the test material (9). If the annular gap vessel (2) is designed in such a way that the inner cylinder (13) can be removed, this makes it easier to clean the annular gap vessel (2).

• Das zu vermessende Fluid (9) wird bis zum Soll-Niveau (12) in das Ringspaltgefäß (2) eingefüllt. Der Korbsensor (1) wird mit dem Adapter (6) am Drehmomentaufnehmer des Viskosimeters befestigt. Das Ringspaltgefäß (2) wird auf die Gefäßaufnahme des Viskosimeters gesetzt. Die Messung wird gestartet und der Korbsensor (1) fährt in das Ringspaltgefäß (2) ein, wobei sich das Ringspaltgefäß (2) oder der Korbsensor (1) bereits langsam drehen. Während der Korbsensor (1) in das Fluid (9) eintaucht, fließt vom Korbsensor (1) und durch die Leitprofile (8) verdrängtes Fluid (9) durch die Nivellierschlitze (10) in die Überlauftasse (11), so dass sich ein festes Niveau (12) einstellt. Während der eigentlichen Messung wird das Drehmoment an Achse des Adapters (6) in Abhängigkeit von der Geschwindigkeit des Korbsensors (1) oder des Ringspaltgefäßes (2) gemessen. Aus der eingetauchten Fläche des Korbsensors (1), der Spaltweite im Ringspalt sowie dem Durchmesser des Korbsensors (1) können aus dem gemessenen Drehmoment die Scherspannung und aus der Drehgeschwindigkeit das Schergefälle errechnet, und so eine Fließkurve aufgezeichnet werden, die die Fließeigenschaften des Fluids (9) beschreibt.

A measurement with the basket measuring cell according to the invention is carried out as follows:

• The fluid (9) to be measured is filled into the annular gap vessel (2) up to the target level (12). The basket sensor (1) is attached to the torque sensor of the viscometer using the adapter (6). The annular gap vessel (2) is placed on the vessel mount of the viscometer. The measurement is started and the basket sensor (1) moves into the annular vessel (2), with the annular vessel (2) or the basket sensor (1) already rotating slowly. While the basket sensor (1) is immersed in the fluid (9), the fluid (9) displaced by the basket sensor (1) and the guide profiles (8) flows through the leveling slots (10) into the overflow cup (11), so that a solid Level (12) set. During the actual measurement, the torque on the axis of the adapter (6) is measured as a function of the speed of the basket sensor (1) or the annular gap vessel (2). From the immersed area of the cage sensor (1), the gap width in the annular gap and the diameter of the cage sensor (1), the measured The shear stress is calculated from the torque and the shear gradient is calculated from the rotational speed, and a flow curve is thus recorded which describes the flow properties of the fluid (9).

After the measurement, the basket sensor (1) moves back out of the annular gap vessel (2). The annular gap vessel (2) can be easily cleaned by removing the inner cylinder (13) with the overflow cup (11).

Claims (1)

Mortar measuring cell for rotational viscometers for the rheological investigation of fluids in the form of different solid and liquid phases, designed as a double-gap measuring cell consisting of an annular gap vessel (2) and a measuring body (1), here called the basket sensor, characterized in that the effective measuring surface (3) of the basket sensor (1) is formed by a skeleton with the material embedded therein, the skeleton being designed as a mesh and the size of the meshes (4) being chosen in suitable relation to the grain size of the solid phase of the fluid.

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