Abstract:
An apparatus for measuring viscosity of liquids includes an external chamber (10) which encloses an internal chamber/duct (20) having an inlet (21) and an outlet (22). Internal chamber (20) houses a fan (30) for forcing air through the duct, a heater (40) for heating air to a desired temperature, a thermal ballast (50) for maintaining the temperature of the air in the duct constant, and a viscometer (72) for measuring viscosity of liquids.

Description:
TECHNICAL FIELD 
     The present invention relates to methods and apparatuses for measuring the viscosity of liquids. 
     BACKGROUND OF THE INVENTION 
     Viscosity measurement of liquids is an essential tool used in the characterization of many products. For instance, the suitability of lubricating oils for a particular purpose is closely related to viscosity and to the variation of viscosity with temperature. Most polymers, including nylon, polyester, rubber, polyethylene, cellulose, polyvinylchloride, and polystyrene are routinely characterized during production by dissolving them in a suitable solvent and then measuring the viscosity of the resulting solution. 
     The viscosity of liquids is strongly dependent on temperature. As the temperature of a liquid changes, the viscosity also changes. An accurate measurement of viscosity therefore requires accurate and stable temperature control. If viscosity is to be measured to within an accuracy of 1%, the temperature must be highly controlled. 
     Measurement of viscosity is often performed in thermostatic baths using a liquid as the thermostatic media. Such baths often have an accuracy of +/-0.01° Celsius. This method relies on the high heat capacity of a liquid to provide a stable temperature environment during viscosity measurement. In the typical viscosity measurement apparatus, a vessel of water or oil is stirred and heated using a highly accurate thermostatic controller. In such a vessel the rate of change of the temperature is low and the viscosity measurement can be performed with relative ease. Liquid baths, however, have several disadvantages. For instance, a liquid bath requires constant maintenance. If water is used as the bath medium, evaporation can pose a problem. Bacterial and algae growth in water necessitates frequent changes of the water media. 
     When higher temperatures are used, oil is normally chosen as the liquid medium. As the oil becomes hot and starts to oxidize, an objectionable odor is created in the laboratory. Another disadvantage of oil baths is the risk of dripping the oil media onto laboratory surfaces once the viscosity measurement apparatus is removed from the bath. Also, liquid baths are subject to leaks. In the case of hot oil baths, operators are exposed to the danger of burns in the event that the bath container, which often is glass, is broken. 
     An additional disadvantage with using a liquid medium as a thermostatic bath arises when viscosity must be measured at several temperatures. A separate thermostatic bath must then be maintained at each temperature. Due to the high heat capacity of liquid, the time required for changing temperature in a bath is between six to eight hours. Typically, a lubricant laboratory would have three to four liquid baths operating continuously. 
     It thus is seen that the liquid baths utilized in the prior art as part of viscosity measurement apparatuses require high maintenance and care to operate. Additionally, numerous baths are required to measure a wide range of temperatures. 
     Accordingly, a need remains for a viscosity measuring apparatus and method which can accommodate a wide range of temperature measurements and yet which is not susceptible to the problems heretofore associated with liquid bath type apparatuses. 
     SUMMARY OF THE INVENTION 
     In a preferred form of the invention, an apparatus for measuring the viscosity of liquids in an ultra stable temperature environment has a thermally insulated test chamber. A support tray for supporting open top containers of liquids to be tested within the chamber is mounted in the chamber. A thermally insulated duct having an inlet and outlet is mounted within the chamber, as is a fan for circulating air through the duct and chamber. A heater for heating air flowing through the duct is mounted adjacent to the fan. A viscometer is mounted at least partially within the duct downstream of the heater for measuring the viscosity of liquids in containers supported upon the tray. A thermal ballast of high heat conductivity and high surface area is also preferably mounted in the duct between the heater and viscometer. 
     As air from the chamber is circulated through the duct, it flows over the heater and the ballast so that the air stream is thermally stable as it passes over the viscometer and test samples. After passing over the test samples, the air is recirculated outside the duct and drawn back into the duct at the fan intake. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an apparatus for measuring viscosity that embodies principles of the invention in a preferred form with a portion shown broken away to reveal internal components. 
     FIG. 2 is a block diagram illustrating the relationship between the temperature sensor, the heating element, and a computer control system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, apparatus for measuring viscosity is illustrated in FIG. 1 which has a thermally insulated external chamber 10 having a side wall 11. External chamber 10 is preferably formed of aluminum with an outer polyurethane layer of foam insulation having a thickness of 25 mm. An insulated access door 17 for allowing entry to the interior of external chamber 10 is attached to side wall 11. An internal chamber or duct 20 having an inlet 21 and an outlet 22 is mounted within the external chamber 10 suspended from the chamber top. Internal chamber 20 is formed of aluminum sheeting approximately 3 mm thick with epoxy fiberglass composite insulation. Internal chamber 20 has an access door 26 adjacent to external chamber access door 17. 
     A fan 30 is mounted in the external chamber 10 to the inlet 21 of duct 20. An electric heater 40 is mounted in duct 20 downstream from the fan 30, and is connected through a solid state relay to a computer as shown in FIG. 2. A thermal ballast 50 is mounted in the duct between the heater 40 and the duct outlet 22. Thermal ballast 50 is made of a series of thin copper plates 0.5 millimeters thick spaced 3 millimeters apart. 
     A viscometer 72 and a temperature sensor 60 are mounted in mutual proximity within the duct between the thermal ballast 50 and the duct outlet 22. Viscometer 72 is connected by a threaded drive shaft to a motor M3 for movement of viscometer 72 in a vertical direction. A sample tray 80 is mounted within the duct 20 for movement in a horizontal plane in both X and Y directions below the viscometer 72. The sample tray 80 is coupled by unshown means with threaded drive shafts of motors M1 and M2, respectively, which respectively drive the tray in X and Y axes directions. Finally, the duct section that houses the temperature sensor, viscometer and sample tray has a door 26 aligned with the external chamber access door 17. 
     With reference to FIG. 2, it is seen that the temperature sensor 60 is coupled with the computer through an electronic amplifier and an analog to digital converter. The viscometer 72 is also coupled to the computer. The heater 40 is also controlled by the computer. Although any number of computers may be used, an IBM-PC compatible computer with a 80486 microprocessor is preferred. 
     The internal chamber is preferably aluminum in order to minimize the time required to heat up the chamber to its operating temperature. The fan 30 is a centrifugal type fan with 5 cubic meters of air per minute capacity. The heater 40 comprises a gang of nichrome heater wires having low mass in the shape of a series of wire turns. The nichrome heater provides for a temperature range between 35 to 150 degrees Celsius. If an external cooler is added to the apparatus, the temperature range can be between -10 and +150 degrees Celsius. That portion of the duct that houses the heater is preferably coated with an epoxy fiberglass composite to achieve insulative effect as well as to secure the wiring connections. The thermal ballast 50 is made of a material that has high heat conductivity such as silver, iron, aluminum, or copper. The ballast is shaped to have a high surface area to mass ratio allowing air to access all of the surface area generally uniformly. Given this factor, the shape selected may be a series of adjacent parallel plates as seen in FIG. 1, or as a gang of spirals, or in filamentous wool forms. 
     The viscometer 72 may be of the conventional single capillary bulb type. In the preferred embodiment though, the viscometer has two viscosity measuring bulbs, namely a lower capillary and measuring bulb 71, and an upper, double capillary and measuring bulb 70. 
     A viscometer of the double bulb type provides for a more extended viscosity measurement range. Once its lower bulb is filled and raised, the viscosity of the sample in the bulb is measured as it gravitates back into a container. Sample detection is made with fiber optic cables using an infrared light source. The computer monitors the data generated by the viscometer and calculates viscosity utilizing conventional calculation software. In the dual bulb type viscometer, as a sample is drawn into its lower bulb, pressure and time are recorded by the computer. If the time-pressure product is low, the sample is drawn further into the upper capillary. Conversely, if the time-pressure product is high, the sample is released through the lower capillary back into the container. The dual type viscometer allows for the measurement of viscosity over a range of 1 to 100 centistokes. By changing viscometers the entire range of viscosity from 0.3 to 30,000 centistokes can be analyzed. 
     In operation, access doors 17 and 26 are opened. The samples are placed in disposable vials which are loaded onto tray 80 in individual checkerboard arrayed receptacles or indentations along the tray top. For the testing of more volatile liquids the vials are covered with aluminum foil to inhibit evaporation once the access doors are closed and the chamber is heated. Access doors 17 and 26 are then closed and the computer activated which energizes the heater and fan. Once the desired chamber temperature is achieved, the heater is activated intermittently by the computer in order to maintain the preselected temperature level. The air temperature within the apparatus can be changed from 40° Celsius to 100° Celsius in approximately 16 to 18 minutes. 
     Downstream of the ballast the air temperature is continuously measured by the temperature sensor 60 and monitored by the computer. After the air stream in the duct passes over the viscometer and temperature sensor, it is returned outside of the duct to the intake of the fan 30 to complete a cycle. With this unique construction and operation of the apparatus, temperature fluctuations about the viscometer and sampler have been found to be limited to within 0.01 degrees Celsius. 
     Movement of the viscometer is controlled by the computer in synchronous with movements of the tray. The viscometer is lowered by motor M3 causing it to puncture the foil cover and enter into the vial directly beneath it and its sample liquid. The sample is then drawn into the viscometer and its viscosity measured. Each time this is done the viscometer is cleansed by an unshown cleansing device and solvent. The tray 80 is then indexed so as to bring another vial into position directly beneath the viscometer. The viscometer is then driven down into that vial, again puncturing the aluminum foil, and the process repeated until all of the vial samples have been tested and the viscosity measurement for each vial recorded. 
     EXAMPLE 
     The apparatus of FIGS. 1 and 2 was tested with a thermal ballast 50 consisting of 32 sheets of copper each having a dimension of 8 cm×23 cm by 0.5 cm. The total mass of the copper used was about 2.5 kg and the total surface area exposed to the air stream about 11,776 sq. cm. Twelve samples were tested. During the period of time in which the samples were tested, the temperature was found not to vary more than 0.01 degrees Celsius. As shown in Table A, viscosity percent deviation from average was maintained at a low level. Notwithstanding movement of the tray and samples, and the puncturing of foil under these conditions, this did not distract from the effectiveness of the apparatus, in maintaining a highly thermally stable environment. 
     
                       TABLE A______________________________________Sample    Viscosity(Position)     (Centistokes)                % Deviation From Average______________________________________A         37.23      +0.05B         37.17      -0.11C         37.23      +0.05D         37.09      -0.34E         37.15      -0.16F         37.33      +0.32G         37.22      +0.03H         37.16      -0.13I         37.28      +0.19J         37.23      +0.05K         37.30      +0.24L         37.11      -0.27______________________________________ Average 37.21 Max % Deviation: +0.32, -0.27 
    
     It thus is seen that a viscosity measuring apparatus and method are now provided which allow for the measurement of viscosity in an extremely thermally stable environment and yet which can accommodate a wide range of temperatures measured. Not being a liquid bath type, it avoids problems previously associated with such viscosity measuring apparatus. Though the apparatus and method has been shown and described in its preferred form, many modifications, additions, and deletions may be made thereto without departing from the spirit and scope of the invention as set forth in the following claims.