Abstract:
A fluid level indicator for determining the level of fluid in a container includes a dipstick that is entered into the fluid in the container and withdrawn from the fluid, a billet of thermochromic material having a reference temperature, a billet length extending along a portion of the dipstick, and a fluid level range marking on the dipstick along the length of the billet.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to determining the level of fluid in a container. More particularly the invention pertains to sensing the temperature of the fluid. 
         [0003]    2. Description of the Prior Art 
         [0004]    Accurately measuring transmission oil or engine crankcase oil level in a repeatable manner has been challenging because, when the dipstick is pulled out of a fill tube, the oil level is not clearly indicated. An oil film left on the dipstick does not provide a clearly visible demarcation. In addition, the dipstick can contact the side of the fill tube causing oil film to be formed in areas above where the actual oil level is located. This results in a measurement system that does not provide a repeatable, reliable indication of the oil level in the vehicle. 
         [0005]    Additionally, transmission oil should be warmed to be about 150° F. before reading the oil level. Often the vehicle operator or service technician has no easy technique way to determine transmission oil temperature without the use of a diagnostic tool. Transmission oil level is very important in automatic transmission function since too much oil may reduce fuel economy and can result in venting fluid. Too little oil degrades function under cold conditions. These constraints often lead to a very tight oil level tolerance (about 0.5 liters), which may be difficult to verify using conventional dipsticks. 
         [0006]    There is a need in the industry for a fluid level indicator that responds to fluid temperature and provides clear, reliable, visible indication of fluid level in a confined space, such as a transmission oil sump or an engine crankcase. 
       SUMMARY OF THE INVENTION 
       [0007]    The fluid level indicator employs a heat sensitive material, preferably thermochromic plastic, which changes color at a reference temperature, sometimes called a transition temperature. Thermochromic plastics are available in many different colors, which fade to white above the transition temperature or become transparent above the transition temperature. The fluid level indicator provides a visible indication of the level of fluid in a container that is accurate, repeatable, and easy to use. 
         [0008]    A billet, secured to an end of a dipstick, is made of multiple lengths of thermochromic plastic materials, each material selected to transition in color at the reference temperature. A thermal insulator, located between each length, minimizes conductive heat transfer among the lengths along the billet&#39;s length. 
         [0009]    The billet can then be secured to the indicator end of any dipstick. The middle two sections of the billet&#39;s length can represent the mid-range of fluid level specified by the vehicle manufacturer and the outermost sections indicate underfill and overfill fluid levels, respectively. A larger number of billet sections provide increased precision of the indicated fluid level. 
         [0010]    The thermochromic plastic indicator provides the user a clear and certain indication of both the oil level and that the fluid has been heated to the proper temperature compatible with the manufacturer&#39;s recommendation for reading the oil level. 
         [0011]    A fluid level indicator for determining the level of fluid in a container includes a dipstick having a support surface that is entered into the fluid in the container and withdrawn from the fluid, a thermochromic billet secured to the support surface, the billet having a reference temperature, a length extending along a portion of the dipstick, and a width extending across the dipstick, and a fluid level range marking on the dipstick along the length of the strip. 
         [0012]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a side view of a dipstick for indicating the level of fluid in a sealed housing; 
           [0015]      FIG. 2  is side view of a tube into which a length of the dipstick of  FIG. 1  is inserted; and 
           [0016]      FIG. 3  is a side view of a portion at the tip of the dipstick of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]      FIGS. 1 and 2  illustrates a dipstick assembly  10  in the form of a thin rod that is inserted into a housing  12 , such as a transmission case or an engine block, to produce visually on the surface of the dipstick an indication of the level of fluid contained in a oil sump located in the transmission case or in a crankcase located in the engine block. The dipstick  10  is inserted into the case or block  12  through a cylindrical tube  14  secured to a vehicle and having an open lower  16  end that communicates with the case or block and an open upper end  18  formed with a surface  20  that can be engaged by a screw thread  22  formed on the dipstick. Engagement of the screw thread  22  on dipstick  10  with the tube  14  secures the dipstick to the tube such that the lower end of the stick in located in the transmission sump or engine crankcase. A cap  24  on the upper end of the dipstick seats on an end of the tube and compresses a seal  26  against the tube when the dipstick is threaded onto the tube. 
         [0018]    Referring to  FIG. 3 , billet  29  is formed with flat surfaces  28  on opposite sides of the billet, which is secured to a lower end of a cylindrical rod portion  30  of the dipstick assembly  10 . The billet  29  is formed of plastic containing at least one and preferably multiple thermochromic pigments  36  molded integrally with the plastic and extending along a length of each billet surface  28  from 32 to 34 and across its width. The thermochromic pigments  36  change color along the length of the billet  29  as the temperature of the billet changes in response to the temperature of oil contacting the billet. The thermochromic pigments  36  change color along the length of the billet  29  when the temperature of the billet passes above and below a reference temperature of the respective thermochromic pigments  36 , thereby indicating the temperature of the transmission oil. 
         [0019]    An appropriate billet  29  of molded plastic may contain thermochromic pigments available commercially from Kelly Chemical Corporation 9F, No. 155, Sec. 1, Keelung Rd, Taipei, Taiwan 
         [0020]    Thermochromic plastics are available in various colors below a reference temperature, which colors change automatically to white when their temperature becomes substantially equal to or greater than the reference temperature. A preferred reference temperature for this application is about 65° C., i.e., 150° F. 
         [0021]    In the embodiment of  FIG. 3 , surfaces  28  of billet  29  can be formed of four different thermochromic materials  36 ,  38 ,  40 ,  42 , each having a characteristic color which changes to a reference color, such as white, at temperatures above 150° F. A thermal insulator  44  extending across the width of surfaces  28  is located between each of the four materials  36 ,  38 ,  40 ,  42  to minimize the effects of conductive heat transfer among the thermochromic materials located on surfaces  28 . Any material having a low thermal conductivity (such as a ceramic) can be used as the thermal insulator  44 , thereby avoiding temperature increase up the length of the dipstick  10 . 
         [0022]    In operation, the vehicle operator runs the engine for a period during which the oil temperature increases to a test temperature defined by the vehicle manufacturer as the minimum temperature at which the fluid level should be determined using the dipstick  10 . When the billet  29  is inserted into the oil sump, the oil in the sump wets and heats the thermochromic materials  36 ,  38 ,  40 ,  42  on the billet. If the temperature of the oil in the sump is equal to or greater than the minimum defined test temperature, the thermochromic materials wetted by the sump oil change color and become white. Oil on the billet  29  is located on thermochromic materials  36 ,  38 ,  40 ,  42 , which will have a color corresponding to the oil temperature. The height of the oil in the sump is indicated by the location on the billet where the color changes. Above the oil height, the temperature of the thermochromic materials  36 ,  38 ,  40 ,  42  that are not wetted by the sump oil will be lower than the reference temperature, and those materials will have the color they had before inserting the dipstick  10  into the sump. Below the oil height, the temperature of the thermochromic materials  36 ,  38 ,  40 ,  42  that are wetted by the sump oil will be greater than the reference temperature, and those materials will have changed color from the color they had before inserting the dipstick  10  into the sump. The volume of the oil in the sump is indicated on the thermochromic materials  36 ,  38 ,  40 ,  42  by a line across the billet produced at the height of oil in the sump. 
         [0023]    Alternatively, oil on the billet  29  is located on thermochromic materials  36 ,  38 ,  40 ,  42 , which provide a clear background in contrast to the color of the oil. The volume of the oil in the sump is indicated on the thermochromic materials  36 ,  38 ,  40 ,  42  by a line across the billet produced at the height of oil in the sump. 
         [0024]    The thermochromic material  36  and  42  on the billet  29  can be molded with abbreviations such as “MIN” and “MAX” to indicate the volume of oil in the sump that corresponds to the location of the oil line on the dipstick, thereby providing a clear, visual indication of the fluid level in a transmission. 
         [0025]    The thermochromic material  38 ,  40  located between materials  36 ,  42  can be used to represent the specification level of sump oil defined by the vehicle manufacturer, and materials  36 .  42  can indicate underfill or overfill oil levels. 
         [0026]    More than four thermochromic materials  36 ,  38 ,  40 ,  42  can be used along the length of billet  29 , to increase precision. The temperature range of the sump oil can be indicated using a billet  29  containing multiple thermochromic pigments, each having a different reference temperature, at which a color change occurs. 
         [0027]    Alternatively, a single thermochromic material  36  can be used along the length of billet  29 . 
         [0028]    The entire billet  29  may be formed of metal or plastic covered with one or more layers of thermochromic material containing thermochromic pigments, each layer extending a portion of the length of the billet. 
         [0029]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.