Liquid presence detector

A liquid presence detector having a light-transmissive chamber for the liquid to be detected. A light emitting diode is positioned outside of the chamber for transmitting light through the chamber and a photosensitive transistor is positioned for receiving the light that has been transmitted through the chamber. The light emitting diode is located at a distance from the chamber that is equal to the focal point of a lens that is created by the chamber and liquid when the liquid is present. The photosensitive transistor output voltage is compared with a reference voltage and an output signal is provided in response to a predetermined difference between the detector output voltage and the reference voltage.

BACKGROUND OF THE INVENTION 
The present invention concerns a novel liquid presence detector. 
In certain applications it is essential that the presence of a colorless 
liquid be detected. For example, in blood processing, including blood 
fractionation, a colorless anticoagulant supply is often fed during the 
processing of the blood. The absence of the anticoagulant during the blood 
processing may cause serious consequences and for that reason it is 
important that an effective detector be provided for detecting the 
presence of the anticoagulant. It is to be understood, however, that the 
detection of the presence of anticoagulant is but one of the many uses of 
the liquid presence detector to be described herein. 
An object of the present invention is to provide a liquid presence detector 
that is capable of detecting accurately the presence or absence of a 
colorless liquid. 
Another object of the present invention is to provide a liquid presence 
detector that is simple in construction and is efficient to manufacture. 
Another object of the present invention is to provide a liquid presence 
detector that is reliable and requires a relatively small number of parts. 
A further object of the present invention is to provide a liquid presence 
detector that includes safety provisions in the event of component 
failure. 
Other objects and advantages of the present invention will become apparent 
as the description proceeds. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a liquid presence detector is 
provided which comprises a light-transmissive chamber for the liquid to be 
detected. A light source is positioned outside the chamber for 
transmitting light through the chamber and a light detector is provided 
for receiving the light that has been transmitted through the chamber. The 
light source is located at a distance from the chamber that is equal to 
the focal point of a lens that is created by the chamber and liquid when 
the liquid is present. 
In the illustrative embodiment, a reference voltage level is provided. The 
detector output voltage is compared with the reference voltage level and 
an output signal is provided in response to a predetermined difference 
between the reference voltage and the detector output voltage. 
In the illustrative embodiment, the light-transmissive chamber comprises a 
flexible plastic tube, the light source comprises a light emitting diode 
and the light detector comprises a photosensitive transistor. A 
potentiometer is coupled to the light emitting diode for calibrating the 
detector output voltage. 
In the illustrative embodiment, four comparators are provided, and the 
reference voltage is provided by a voltage divider. A first point on the 
voltage divider is coupled to an input of the first comparator, a second 
point on the voltage divider is coupled to an input of the second 
comparator, and a third point on the voltage divider is coupled to inputs 
of the third and fourth comparators. The detector output is coupled to the 
other inputs of the four comparators. In this manner, an output signal is 
provided by at least one of the comparators when the detector output 
voltage differs a predetermined amount from the reference voltage. 
A more detailed explanation of the invention is provided in the following 
description and claims, and is illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
Referring first to FIGS. 1-3, the liquid presence detector assembly 
includes a housing 10 defining a slot 12 for receiving a transparent 
chamber 14 through which the liquid to be detected flows. In the 
illustrative embodiment, transparent chamber 14 comprises flexible plastic 
tubing which is pressure-fitted into slot 12. Thus slot 12 is dimensioned 
to receive the plastic tubing 14 with a snug fit. 
An opening 16 is provided for receiving a light source 18 in the form of a 
light emitting diode (LED) and an opening 20 is provided for receiving a 
light detector 22 in the form of a photosensitive transistor. LED 18 and 
photosensitive transistor 22 may be obtained in the form of a single 
package, which is sold by General Electric Company under component No. 
H23BL. LED 18 and phototransistor 22 are positioned in the openings 16, 
20, respectively, as illustrated in FIG. 3, on opposite sides of slot 12, 
and the space remaining in the openings 16, 20 is filled with an epoxy 
resin 24. As illustrated in FIGS. 1 and 3, openings 16 and 20 are 
contiguous to provide an air space 24 separating the light source 18 from 
chamber 14 and separating the detector 22 from chamber 14. 
A feature of the present invention is the location of LED 18 at a distance 
from chamber 14 that is equal to the focal point of a lens that is created 
by chamber 14 and the liquid therein when the liquid is present in the 
chamber. 
Referring now to FIG. 4, a source 30 of direct current is coupled via line 
32 to LED 18 and photosensitive transistor 22 (which is in the form of a 
Darlington pair). LED 18 is coupled in series with a resistor 34 and a 
potentiometer 36, while the emitter 38 of transistor pair 22 is coupled to 
ground via resistor 40 and is coupled to a comparator circuit via line 42 
and resistor 44. 
A voltage divider is provided including resistor 46, resistor 48, resistor 
50 and resistor 52. A point 54 on the voltage divider between resistor 46 
and 48 is connected to an input of comparator 56, a point 58 on the 
voltage divider between resistor 48 and resistor 50 is connected to an 
input of each of comparators 60 and 62, and a point 64 on the voltage 
divider between resistors 50 and 52 is connected to an input of comparator 
66. The other inputs of comparators 56, 60, 62 and 66 are connected to 
line 42 via line 68. The outputs of comparators 56, 60, 62 and 66 are 
coupled to a common output point 70. A resistor 72 is connected between 
output point 70 and line 32. 
Although no limitation is intended, the following is a specific example of 
parameters that have been found to provide a satisfactory circuit: 
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Component Value or Model No. 
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18, 22 GE H23BL 
Resistor 34 510 ohms 
Potentiometer 36 0-1K 
Resistor 40 10K 
Resistor 44 10K 
Resistor 46 4.99K 
Resistor 48 10K 
Resistor 50 10K 
Resistor 52 4.99K 
Comparators 56, 
60, 62 and 66 LM339A 
Resistor 72 10K 
DC supply 30 15 volts 
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In the operation of the FIG. 4 circuit, potentiometer 36 is adjusted with 
liquid being present in the chamber so that there is a voltage at point 74 
of nine volts. With source 30 providing 15 volts, it can be seen that 
point 54 will be approximately 12.5 volts, point 58 will be approximately 
7.5 volts and point 64 will be approximately 2.5 volts. The system is thus 
operative to provide a low output signal at point 70 if the voltage at 
point 74 is less than 7.5 volts or is greater than 12.5 volts. Thus if the 
voltage at point 74 is less than 7.5 volts, comparators 60 and 62 will 
provide a low output signal while if the voltage at point 74 is greater 
than 12.5 volts, comparator 56 will provide a low output signal. 
Comparators 60 and 62 are duplicates of each other, for safety purposes. 
Comparator 66 is also provided for safety purposes, in the event that 
comparators 60 or 62 do not operate and the voltage at point 74 is below 
2.5 volts. In such event, comparator 66 will provide a low output signal. 
Output 70 may be connected to suitable circuitry as is well-known in the 
art, for further processing, such as terminating operation of the system. 
Although an illustrative embodiment of the invention has been shown and 
described, it is to be understood that various modifications and 
substitutions may be made by those skilled in the art without departing 
from the novel spirit and scope of the present invention.