Abstract:
The present invention relates to a Drop Counter for counting a succession of falling liquid drops. The Drop Counter includes a light emitting diode for providing a light beam directed to a falling liquid drop. A photo diode sensor is positioned in side-by-side relation with the light emitting diode and detects reflected light from the falling liquid drop. The photo diode sensor provides an output signal when reflected light is detected and further includes a counter for receiving the output signal and counting the number of times the output signals are received.

Description:
This application claims the benefit of provisional application Ser. No. 60/665,651 filed Mar. 25, 2005. 

   BACKGROUND OF THE INVENTION 
   This invention relates to apparatus for counting drops and this particular application for counting drops from a burette to obtain a measure of the amount of liquid reactant (titrant) added to an unknown solution. 
   Drop counters are known. These drop counters use apparatus directing a light beam across a chamber toward a light sensor on the other side of the chamber. When a drop falls through the chamber the light beam is interrupted, and thus providing an indication of the presence of a drop. See for example U.S. Pat. No. 4,181,130 to Bailey. Other known apparatus include a light source for directing a light beam toward a falling drop and include a beam detector located to detect a refracted light beam that has passed sidewardly out of a drop. See U.S. Pat. No. 5,982,289 to Kingsley, et. al. 
   There is a need for a compact drop counter which is reliable and not affected by ambient light or by conditions occurring when drops become fragmented. 
   SUMMARY OF INVENTION 
   The present invention relates to a Drop Counter for counting a succession of falling liquid drops. The Drop Counter includes a light emitting diode for providing a light beam directed to a falling liquid drop. A photo diode sensor is positioned in side-by-side relation with the light emitting diode and detects reflected light from the falling liquid drop. The photo diode sensor provides an output signal when reflected light is detected and further includes a counter for receiving the output signal and counting the number of times the output signals are received. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be clearly understood and readily carried into effect, a preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings wherein: 
       FIG. 1  is a schematic view of a drop counter in accordance with the present invention illustrating the drop counter in operational use; 
       FIG. 2  is a top plan view of a drop counter in accordance with the present invention; 
       FIG. 3  is a rear plan view of the drop counter shown in  FIG. 2 ; 
       FIG. 4  is a front elevational view of the drop counter shown in  FIG. 2 ; and 
       FIG. 5  is a schematic circuit diagram of an electrical circuit used with the drop counter shown in  FIGS. 2 and 3 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   A common analytical technique used in standard analytical chemical procedures is to determine the volume of titrant added to an unknown solution. This process is called titration. A drop counter  10 , as shown in  FIG. 1 , can provide assistance in measuring the amount of titrant added to this unknown solution. 
   The drop counter  10  includes a chemically-resistant plastic enclosure  12  for housing a drop sensor  14  and a circuit board  16  in an inset area  15  as shown in  FIG. 3 . In a preferred embodiment the sensor  14  and circuit board  16  are potted in the enclosure  12  after assembly to make a water-tight assembly. 
   An electrical schematic of the circuit board  16  is shown in  FIG. 5 . A conventional CAT-5, eight conductor cable  17  is used to connect the drop counter  10  to lab interface equipment (not shown). The lab interface equipment supplies electrical power to drop counter  10  and interfaces drop counter  10  with a computer where data transmitted via the CAT-5 cable  17  is collected and analyzed. The CAT-5 cable  17  includes a ground lead  18 , a data transmission lead  20  and a 9-volt power lead  22 . The 9-volt lead  22  is connected to a voltage regulator  24  which converts the 9-volts on lead  22  to 5 volts on lead  26 . The lead  26  is connected to the VCC terminal  28  of the sensor  14 . The data lead  20  is connected to the Vo terminal on the sensor  14 . The ground lead  18  is connected to the GND terminal  32  on the sensor  14 . 
   A green light emitting diode  34  is connected between the power lead  22  and the ground lead  20  to indicate when power is on to the circuit. A red light emitting diode  36  is connected between the 5-volt power lead  26  and the data transmission lead  20  and blinks every time a drop passes the drop counter  10  so as to provide a visual indication of a drop&#39;s passage. 
   In a preferred embodiment, the infrared reflective sensor  14  includes a Sharp Model No. GP2A200LCS, although other reflective sensors could be used equally as well. The sensor  14  includes a cut-out area  33  in which is mounted an infrared light emitting diode  38  which turns “on” and “off” with a 50% duty cycle approximately 8000 times per second. An infrared photo diode sensor  40  is also mounted in cut-out area  33  and senses the light received at its surface. During the “off” cycle of the light emitting diode  38 , the infrared sensor  40  measures the ambient light received at its surface. During the “on” cycle, the infrared photo diode  40  again measures the light received at its surface. An electronic circuit within the sensor  14  then subtracts the “off” signal from the “on” signal. If a reflective object is not present the difference is zero and the sensor module  14  produces a logic “1” output. If a reflective object is present within the range of the photo diode  40 , (the subtraction “on”−“off”) will yield a positive value and the sensor will produce a logic “0” output on the data lead  22 . When a logic “0” output appears on the data lead  22 , the diode  36  turns on indicating a drop has passed. Because this measurement is repeated approximately 8000 per second, an object need remain in the infrared beam only 150 microseconds for the sensor to record its passage. The subtraction of “off” background light from the “on” cycle measurement effectively makes the sensor immune from changes in background light arriving at the sensor. 
   As shown in  FIGS. 2 and 3 , an elongate support rod  42  has one end fixedly attached to the enclosure  12 . The support rod  42  is mounted to the enclosure  12  with a screw  44 . The free end of the rod  27  may be mounted to a conventional ring stand (not shown) so as to support the drop counter  10  in a fixed relationship with the stream of drops  44  coming from a burette  46  as best seen in  FIG. 1 . In operation, a conventional burette  46  is used to provide titrant drops  44  for insertion into an unknown solution  48  as shown in  FIG. 1 . Unknown solution  48  is often stirred by a conventional stir plate  50  so as to mix thoroughly titrant drops  44  and the unknown solution. Since drops  44  from burette  46  are quite constant in size, knowing the number of drops added to an unknown solution  48  provides a way of calculating the volume of known titrant added to the unknown solution. The rate of drops leaving burette  46 , however, does vary considerably as the burette is emptied. But, by counting drops  24 , an accurate, instantaneous measure of the volume of titrant delivered may be ascertained. 
   When counting drops, drops may fragment when they leave the tip of the burette to fall through the air to solution  48 . Further, when a drop hits the solution, a splash may occur. Both of these situations may cause the counter to record false counts. To compensate for these two problems, a 20 millisecond “dead time” is created immediately after the presence of a drop is recorded, during which the drop counter is insensitive to pulses generated by sensor  14 . This permits both fragmented drops  44  and splashes to fall into the unknown solution before the circuit again becomes sensitive and is ready to count a following, fresh drop. This “dead time” can be created with a 20 millisecond delay in the software of the counting device that is triggered at the arrival of a drop signal. It can also be created with a multi-vibrator circuit through which the sensor&#39;s signal is passed to the interface counting circuit. 
   The output of the drop counter  10  is directed to the lab interface equipment such as a computer via data transmission lead  20  for recording and further processing. By counting drops  24 , the amount of titrant supplied to the unknown solution  48  can be calculated. 
   While the fundamental novel features of the invention have been shown and described, it should be understood that various substitutions, modifications and variations may be made by those skilled in the art without departing from the spirit or scope of the invention. Accordingly, all such modifications or variations are included in the scope of the invention as defined by the following claims.