Patent Publication Number: US-2005132783-A1

Title: Rheometer

Description:
This application claims Paris Convention priority of DE 103 50 554.7 filed Oct. 29, 2003 the complete disclosure of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      The invention concerns a rheometer, in particular, a rotational or extensional rheometer, comprising a lower measuring part and an upper measuring part, between which a sample space is formed for receiving a material sample, and which can be moved relative to each other, wherein the sample space is surrounded, at a separation, by a cover, to form an inner, closed measuring chamber, and with a temperature sensor for detecting the temperature in the measuring chamber and a temperature control device for the measuring chamber.  
      A rheometer for determining rheological values or characteristics of a viscous material usually comprises a lower measuring part and an upper measuring part which can be adjusted relative to the lower measuring part. The upper measuring part of a rotational rheometer can be rotated or oscillated. A sample space for receiving a sample of the substance to be examined is formed between the measuring parts. The forces and tensions which occur during relative adjustment between the upper and lower measuring parts can be determined, from which the desired rheological characteristics can be calculated.  
      In an extensional or stretching rheometer, the upper measuring part can be axially adjusted relative to the lower measuring part. In one particular embodiment of an extensional rheometer, the measurement is based on the exact knowledge of the time-dependent diameter of the material sample which is extended between the two measuring parts. The rheological properties of most material samples are highly temperature-dependent. For this reason, in a known design of a rheometer, a hood-like cover is provided which surrounds the sample space at a separation and forms a substantially closed inner measuring chamber whose temperature is controlled. The temperature inside of the measuring chamber is detected using a temperature sensor and a temperature-control device, e.g. an electric heating device or a so-called Peltier temperature control is provided to keep the temperature within the measuring chamber at a predetermined value.  
      The rheological characteristics of certain material samples, such as food or colors, also largely depend on the surrounding conditions during the measurement. There is, in particular, the danger that the material sample dessicates during the measurement. This problem occurs, in particular, in an extensional rheometer, since extension of the material sample largely increases its surface thereby considerably increasing exposure of the material sample to the surrounding conditions. In a conventional rheometer, excessive dessication of the material sample is prevented by the cover used.  
      In practice, it has, however, turned out that variations in the measured rheological values occur despite the use of a cover and despite constant or at least approximately constant temperatures within the measuring chamber.  
      It is the underlying purpose of the invention to provide a rheometer of the above-mentioned type for determining rheological characteristics of a material sample with great accuracy and reproducibility.  
     SUMMARY OF THE INVENTION  
      This object is achieved in accordance with the invention with a rheometer comprising the features characterizing the independent claim. A further sensor device is thereby provided for detecting the air moisture and/or the pressure within the measuring chamber, and gas and/or liquid can be supplied to the measuring chamber via a supply line or be discharged via a discharge line.  
      The invention is based on the fundamental idea that variations in the rheological characteristics of a conventional rheometer are based mainly on the fact that the ambient conditions of a measuring chamber surrounded by a cover, cannot be controlled. In particular, the air moisture or the liquid, in particular solvent, saturation of the gaseous medium located within the measuring chamber, are unknown and subject to great variations in practice. This may have a strong influence on the accuracy of the rheological measurement. In accordance with the invention, the ambient conditions of the material sample within the measuring chamber are set, maintained or optionally changed in a controlled manner. Towards this end, the liquid or solvent saturation of the gaseous medium located within the measuring chamber is detected. Since the gaseous medium is generally air and the solvent is generally water, the term “air moisture” is used for simplification. The invention is, however, not limited to these media or substances. Additionally or alternatively, the pressure within the measuring chamber is also detected. The status data detected by the sensor device is passed to a control device in the form of actual signals. The control device checks whether the actual signals concerning the actual pressure and/or the actual air moisture within the measuring chamber correspond to predetermined desired signals, within predetermined limits. If the predetermined limit values are fallen below or exceeded, the control device may initiate supply or discharge gas, e.g. air and/or solvent, e.g. water, into or out of the measuring chamber. In this manner, the actual pressure within the measuring chamber and/or the actual air moisture within the measuring chamber can be kept at predetermined, desired and, in particular, constant values.  
      The cover should surround the sample space at a minimum separation such that the measuring chamber has a relatively small volume. This permits rapid compensation of changes or variations in the pressure and/or air moisture in the measuring chamber.  
      In a preferred embodiment, the liquid, in particular water, is supplied to the measuring chamber in the form of mist or aerosol.  
      The temperature sensor and/or the sensor device for detecting the pressure and/or the air moisture may be disposed directly in the cover. It is, however, also possible to dispose these sensors within the measuring chamber or also directly on the material sample or in its vicinity.  
      In a further development of the invention, an analysis sensor is additionally provided for detecting the composition of the gaseous medium, in particular air, located in the measuring chamber. In this manner, the composition of the gaseous medium located in the measuring chamber can be taken into consideration in the calculation of the rheological characteristics.  
      Further details and features of the invention can be extracted from the following description of embodiments with reference to the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  shows a schematic sectional view of an extensional rheometer; and  
       FIG. 2  shows a schematic sectional view of a rotational rheometer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  shows a rheometer  10  in the form of an extensional rheometer comprising a lower base part  12  which bears an upwardly projecting die which forms a lower measuring part  11 . An upper die-shaped measuring part  13  is disposed above the lower measuring part  11 , is connected to a shaft  14  on its side facing away from the lower measuring part  11 , and can be adjusted together with same in the axial direction (indicated by double arrow V). A sample space is formed between the lower measuring part  11  and the upper measuring part  13 , in which a material sample is disposed which is in contact with the lower measuring part  11  and the upper measuring part  13 . Through axial displacement of the upper measuring part  13  relative to the lower measuring part  11 , the dimensions of the sample space can be enlarged thereby extending or stretching the material sample P. The shaft  14  is connected to an upper holding part  15  at its upper end facing away from the material sample P.  
      The lower base part  12 , the lower measuring part  11 , the upper measuring part  13 , the shaft  14  and the upper holding part  15  are surrounded by a housing-like cover  16  whose inside defines a measuring chamber  17  in which the rheological characteristics of the material sample P are measured. The measuring chamber  17  is filled with a gaseous medium, in particular, with air. The wall of the cover  16  has a first opening  23   a  which houses a temperature sensor  29  for detecting the temperature within the measuring chamber  17 . The temperature in the measuring chamber  17  may also be conventionally controlled using a temperature-control device T (only schematically shown). Cooperation between the temperature sensor  29  and the temperature-control device T permits maintenance of the temperature in the measuring chamber  17  at a predetermined value.  
      The wall of the cover  16  has a further opening  23   b  close to the material sample P within which a sensor device  22  for detecting the pressure and/or the air moisture in the measuring chamber  17  is disposed. The sensor device  22  is connected to a control device  28  via a line  24 . A supply line  18  is disposed in a further opening  19  of the wall of the cover  16  which terminates in the measuring chamber  17  and through which a gas, in particular, air and/or a solvent, in particular water, can be supplied to the measuring chamber  17  (indicated by arrow A).  
      A discharge line  20  is located in a further opening  21  of the wall of the cover  16  through which air and/or the solvent can be discharged from the measuring chamber  17 , in particular suctioned (as indicated by arrow B).  
      The cover  16  moreover comprises an analysis sensor  31  for detecting the composition of the gaseous medium located in the measuring chamber  17  and transferring same to a storage or calculation unit (not shown) via a line  32 .  
      The sensor device  22  detects the actual condition in the measuring chamber with respect to actual pressure and/or actual air moisture and transmits corresponding actual signals to the control device  28  which compares the actual signals with predetermined desired values. In dependence on the actual signals of the sensor devices  22 , the supply or discharge of gas and/or solvent into or out of the measuring chamber  17  is controlled by the control device. In this manner, it is possible to ensure predetermined ambient conditions with regard to pressure and/or air moisture and/or temperature in the measuring chamber  17 , and thereby determine the rheological characteristics of the material sample P in the defined surrounding conditions.  
       FIG. 2  shows a rotational rheometer  10  which also embodies the invention. The rheometer  10  has an upper measuring part  13  which is connected to a rotatable or pivotable shaft  14  (indicated by arrow D). The upper measuring part  13  has the shape of a substantially horizontal plate which is disposed at a separation above the surface of a lower measuring part  11 . A sample space, which contains a material sample P, is formed between the lower measuring part  11  and the upper measuring part  13 .  
      The sample space and the upper measuring part  13  are covered at a separation by a hood-like cover  16  which is supported at its outer end region on the surface of the lower measuring part  11  and sealed. At its radial inner end close to the shaft  14 , the cover  16  is sealed-off from the shaft  14  via a fluid seal  25 . The fluid seal  25  comprises a supporting sleeve  26 , disposed on the shaft  14 , which has a radially outwardly projecting shoulder  27  of L-shaped cross-section, to form an annular chamber which extends around the supporting sleeve  26 . The cover  16  has a sleeve-like projection  16   a  at its radially inner end, which extends parallel to the sealing sleeve  26  and is immersed into the sealing liquid F. In this manner, a measuring chamber  17  which is formed inside the cover  16  and surrounds the sample space, the material sample P, and the upper measuring part  13 , is sealed from the surroundings by the fluid seal  25 .  
      The hood-like cover  16  has a first opening  23   a  which houses a temperature sensor  29  for detecting the temperature within the measuring chamber  17 . The temperature of the lower measuring part  11  and of the material sample P may be controlled in a conventional manner via a temperature control device T (only schematically indicated). The temperature sensor  29  transmits temperature signals via a data line  30 , which form the basis for keeping the temperature within the measuring chamber  17  at a constant value using the temperature-control device T.  
      The cover  16  has a second opening  23   b  which houses a sensor device  22  for detecting the air moisture and/or the pressure within the measuring chamber  17 . The sensor device  22  transmits corresponding actual signals to a control device  28  via a line  24 .  
      A supply line  18  penetrates the cover  16  at a further opening  19 . Gas and/or solvent, in particular in the form of mist, may be introduced into the measuring chamber  17  via the supply line  18 . In correspondence thereto, gas and/or solvent may be discharged from the measuring chamber  17  via a discharge line  20  which is located in a corresponding opening  21  of the cover  16 . The supply and discharge are indicated by arrows A and B, respectively.  
      The hood-like cover  16  also comprises an analysis sensor  32  for detecting the composition of the gas located in the measuring chamber  17 . The analysis sensor  31  transmits corresponding data to a storage or calculation unit via a line  32 .  
      Cooperation between the sensor device  22  and the control device  28  and supply and discharge of gas and/or solvent into or from the measuring chamber  17  permits adjustment or maintenance of the ambient conditions of the material sample P within the measuring chamber  17  as desired.