Sensor for detecting air/liquid transitions in a transparent tubing

A non-intrusive optical transmission liquid monitoring system that detects bubbles in a transparent liquid flowing through a transparent tubing. The system dynamically compensates for changes in optical transmission efficiency of the monitored liquid and distinguishes between the transition from liquid to air and air to liquid. A system comprising a light transmitter and a light sensitive receiver secured on opposite sides of a transparent tubing. The output of the receiver is fed into a self-referencing and drift compensation circuit. The integrated output is connected to circuitry sensitive to a change in the integrated output and triggers one of two possible alarms to indicate a detected transition from liquid to air, or air to liquid.

BACKGROUND 
The present invention relates to the detection of bubbles or air in 
transparent tubing and, in particular, a sensor for detecting both 
air-liquid and liquid-air transition. 
Bubbles or air pockets in transparent tubing result when liquid passing 
through the tubing runs out or a break in the tubing causes the 
introduction of air into the tubing system. Many modern laboratory 
operations require liquids to be pumped or passed through tubing 
connecting a solvent reservoir to an instrument or other operation. 
Pumping air through many instruments or introducing air in certain 
reactions can cause damage to instrumentation or in some cases create a 
significant safety hazard. In these situations, it is very important that 
tubing transporting the solvent be monitored for the presence of air just 
before the solvent enters an instrument or is added to the operation, such 
as a chemical reaction, so that action can be taken to prevent the 
introduction of the air. 
A typical setup to monitor for air in solvent lines involves placing an 
energy source on one side of the tubing carrying the solvent and a 
receiver sensitive to the energy emitted by the energy source on the other 
side. For example, see U.S. Pat. Nos. 4,314,484 and 4,487,601, the entire 
disclosures of which are incorporated herein by reference. One common 
energy source that can be used as an example is a light emitting diode and 
its appropriate energy receiver is a photo transistor. Such a bubble 
detector works on the theory that when liquid is present in the tubing the 
photo detector produces a set output signal, the level of which can be 
measured to establish a "liquid present" output level. A reference voltage 
(or current) is then set which is slightly offset from the "liquid 
present" voltage and the two voltages are continuously compared. If a 
bubble develops in the tubing and passes between the light emitting diode 
and the photo detector, the output voltage of the photo detector changes 
causing it to fall below (or above) the reference voltage, thus signaling 
the presence of air. 
This prior art detection approach has three principle disadvantages. First, 
it requires that the reference voltage be adjusted, usually manually, 
every time any part of the system that affects the output of the photo 
detector is changed. The output of the photo detector is affected by many 
properties such as the clarity of the liquid and the material of which the 
tubing is made including its transmittivity, wall thickness, and diameter. 
Also, other factors such as alignment of the transmitter/receiver pair and 
their relative efficiencies affect the output voltage of the photo 
detector. This heavy dependence on the system configuration requires that 
the reference voltage of the bubble detector be adjusted for each use. 
A second disadvantage is that the prior art detection approach does not 
compensate for slow drift of the "liquid present" signal level. For many 
operations the liquid monitored in the tubing changes over the course of 
time, such as with gradient column chromatography where the starting 
solvent can be water but changes to acetonitrile over a certain time 
period, typically 30 minutes. Changing the solvent monitored in the tubing 
causes a change in the level of the "liquid present" voltage. If the 
change is significant enough the "liquid present" voltage can cross the 
reference voltage causing a false indication that air is present. 
The third disadvantage of the prior art detection approach is that the 
difference between the output voltage from the photo detector when liquid 
is present and when air is present can be very small. This situation 
requires that the reference voltage be set very close to the "liquid 
present" voltage, close enough that even normal amounts of electrical 
noise can cause false detection of air. 
An additional disadvantage is present in the design of some bubble 
detection devices, namely that they only detect the transition from liquid 
to air and not the transition from air to liquid. This is important for 
applications that do not turn off equipment when air is detected, but 
rather open a valve to purge air from the system. These applications 
require information not only that air is present to initiate the purge 
operation, but also that the transition from air to liquid has occurred to 
indicate the purge is complete. 
SUMMARY OF THE INVENTION 
Among the objects of the invention is the provision of a bubble detection 
apparatus which detects air in transparent tubing by attaching a photo 
transmitter and photo detector on opposite sides of the tubing and 
monitors for change in light transmission; the provision of a bubble 
detector which is insensitive to the absolute output voltage of the photo 
detector and automatically references to any detector output voltage 
between the limits of the positive and negative supplies; the provision of 
a bubble detector providing alarm output which is unresponsive to gradual 
changes in the photo detector's output level but is responsive to rapid 
changes; the provision of a bubble detection apparatus that produces 
separate signals for detected transitions from liquid to air and air to 
liquid; the provision of a bubble detection device which may be used with 
transparent tubing of varying diameters; the provision of such a device 
that automatically references to the output signal level of the photo 
detector and allows for gradual change of the reference level to 
compensate for drift in the photo detectors output signal without 
triggering the bubble detection alarm; the provision of such a device that 
is insensitive to the absolute output voltage of the photo detector, but 
responsive only to change in the photo detectors output; and the provision 
of such a device with two output lines, one that is activated when the 
transition from liquid to air is detected and the other when the 
transition from air to liquid is detected. 
In one form, the invention comprises an apparatus for sensing transitions 
from liquid to gas or from gas to liquid in a tubing through which the 
liquid and the gas are flowing. The apparatus includes a transmitter, a 
receiver, and a detecting circuit. The transmitter is adapted to be 
adjacent the tubing and generates energy which propagates through the 
tubing and through the liquid and gas flowing through the tubing. The 
receiver is associated with the transmitter for detecting the energy 
transmitted through the liquid and gas flowing through the tubing. The 
receiver generates a detected signal representative of the transmitted 
energy. The detecting circuit detects a change in magnitude of the 
modified detected signal and generates a first alarm signal when the 
detected signal increases in magnitude and a generates a second alarm 
signal when the modified detected signal decreases in magnitude. The first 
alarm signal is representative of one of either a transition from liquid 
to gas or a transition from gas to liquid and the second alarm signal is 
representative of the other transition. 
Other objects and features will be in part apparent and in part pointed out 
hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a block diagram of an apparatus 1 according to the 
invention for detecting a transition from liquid to air (or other gas 
mixed with the liquid) and air (gas) to liquid in transparent tubing 2. 
Apparatus 1 has a detection head 3 consisting of a photo transmitter 4 
such as a light emitting diode and a photo detector 5 such as a photo 
sensitive NPN transistor secured to opposite sides of the tubing 2. Light 
from the photo transmitter 4 travels through the tubing 2 and is received 
by the photo detector 5 which produces a proportionate output signal which 
in turn is feed to the referencing and drift compensation circuit 6. 
In particular, when the proportionate output signal increases, such an 
increase suggests a transition from gas to liquid. This is because the 
tube filled with clear liquid transmits more light than the tube filled 
with gas so that the receiver receives more light after the transition and 
generates a first output signal having a proportionately higher output. 
Similarly, when the proportionate output signal decreases, such a decrease 
suggests a transition from liquid to gas. This is because gas transmits 
less light to the photo detector than liquid so that the receiver receives 
less light after the transition and generates a second output signal 
having a proportionately lower output. 
Referencing and drift compensation circuit 6 performs three functions. 
First, circuit 6 shifts the relative output voltage sensed from receiver 5 
to a known voltage level. Second, circuit 6 compensates and references for 
slow changes in the output signal of receiver 5. Third, circuit 6 supplies 
a positive going signal when a transition from liquid to air occurs and a 
negative going signal when a transition from air to liquid occurs. 
The output of compensation and referencing circuit 6 is applied to an 
integrator 7 which integrates the output of 6 in either a positive or 
negative direction. If the time-voltage product of integrator 7 overtakes 
the reference voltage of amplifier 9 in a positive going direction, the 
output voltage of amplifier 9 rises providing a first alarm signal 
indicating a transition from liquid to air was detected by detector head 
3. The positive going signal produced by amplifier 9 is the alarm signal 
that a liquid to air transition was detected in the solvent stream in 
delivery tube 2. If the time-voltage product of integrator 7 overtakes the 
reference voltage of amplifier 10 in a negative going direction, the 
output voltage of amplifier 10 rises providing a second alarm signal 
indicating a transition from air to liquid was detected by detector head 
3. The positive going signal produced by amplifier 10 is the alarm signal 
that an air to liquid transition was detected in the solvent stream in 
delivery tube 2. 
FIG. 2 is a schematic diagram of the detection head 3, the referencing and 
drift compensation circuit 6, the integrator 7, the voltage reference 8, 
and the output amplifiers 9 and 10. The detection head 3 comprises a light 
emitting diode 20 with its anode connected to a +12 vdc power supply and 
its cathode connected to a grounded current limiting resistor 21. 
Detection head 3 also includes a NPN photo transistor 22 with its 
collector connected to the +12 vdc supply and its emitter connected to a 
grounded current limiting resistor 23. 
Under normal use, the light emitting diode 20 and the photo transistor 22 
are positioned on opposite sides of a piece of transparent tubing 
containing the liquid and gas to be monitored such that light 24 emitted 
by the light emitting diode 20 passes through the tubing, its contents 
(liquid and/or gas), and is received by the photo transistor 22. The 
detected signal output 34 of the detection head 3 is connected to the 
input of the referencing and drift compensation circuit 6 on one side of 
decoupling capacitor 25. Circuit 6 consists of a decoupling capacitor 25 
which acts to decouple circuit 6 from the absolute output voltage 34 of 
monitor head 3 and make the circuit reactive only to changes in the output 
voltage 34 of monitor head 3. 
By making circuit 6 responsive only to changes in the output voltage 34 of 
detector head 3, apparatus 1 eliminates the need for manual referencing 
typical with prior art bubble detectors. This effect is shown in FIG. 3A 
and 3B where it can be seen that the output of circuit 6, lines 52 and 55, 
are identical whether the output 34 from detector head 3 occurs at a high 
voltage 50 or at a low voltage 57. The only factor affecting the output of 
circuit 6 is the change in monitor head 3 voltage, not its absolute 
voltage. The other end of decoupling capacitor 25 is connected to resistor 
26 which is connected to the inverting input of amplifier 28. Amplifier 28 
has a feedback resistor 27 connected between its output and inverting 
input and then a 6.0 volt reference connected to its non-inverting input. 
Referencing and drift compensation circuit 6 works as shown in FIG. 4A. 
When the circuit is at equilibrium (such as during a constant flow of 
liquid or gas through tubing 2), monitor head 3 applies an arbitrary, but 
constant, voltage 70 to decoupling capacitor 25. Amplifier 28 has charged 
or discharged capacitor 25 until the voltage across 25 is such that the 
voltage at the inverting input of amplifier 28 equals the reference 
voltage at the non-inverting input. At equilibrium, amplifier 28 has an 
output voltage of 6.0 volts. If at time 76 a transition from liquid to air 
occurs in the tubing between diode 20 and transistor 22, the output 
voltage 70 of monitor head 3 decreases because less light is transmitted 
through the air and received by transistor 22 than the light transmitted 
through the liquid and received by the transistor 22. This decrease in 
voltage 70 causes a voltage 71 to develop across resistor 26 of a 
proportionate amount. Amplifier 28 responds by increasing its output 
voltage 72 until capacitor 25 discharges to the point that circuit 6 
reestablishes equilibrium at time 78. 
Referring to FIG. 4B, if monitor head 3 senses a transition from air to 
liquid in the tubing 2 at time 86, the circuit responds identically to the 
previous example, but in the opposite direction as can be seen from the 
shape of the voltage curves. Monitor head 3 has the voltage profile shown 
for curve 80, the voltage across resistor 26 is curve 81 and the output of 
amplifier 28 is curve 82. 
Proportional reference voltages are derived from a voltage divider 8 
consisting of resistors 29, 30, 31, and 32 in series. Resistor 29 is 
connected to the positive power supply then to resistor 30 which is 
connected to resistor 31 which is connected to resistor 32 which is 
connected to ground. The junction of 29 and 30 form a 6.2 volt reference, 
the junction of 30 and 31 form a 6.0 volt reference and the junction of 31 
and 32 form a 5.8 volt reference. The output of the referencing and drift 
compensation circuit 6 is applied to the input of integrator 7. Integrator 
7 comprises a resistor 33 in series between the output of amplifier 28 and 
a grounded capacitor 34. The voltage across capacitor 34 is applied to the 
non-inverting input of amplifier 9 and to the inverting input of amplifier 
10. The inverting input of amplifier 9 is connected to a 6.2 volt 
reference from voltage divider 8 and the non-inverting input of amplifier 
10 is connected to a 5.8 volt reference from voltage divider 8. 
Referring to FIG. 4A, it can be seen that the output voltage 73 across 
capacitor 34 is the integral of the output voltage 72 of amplifier 28. 
When the output voltage 72 of amplifier 28 increases at time 76, the 
voltage 73 across capacitor 34 increases with the associated RC time 
constant of resistor 33 and capacitor 34. If the voltage increase 72 of 
amplifier 28 is great enough for a sufficient period of time that the 
voltage 73 across capacitor 34 exceeds 6.2 volts, which occurs at time 77, 
then the output voltage 74 of amplifier 9 is driven high supplying the 
signal for a liquid to air transition while the output voltage 75 of 
amplifier 10 remains unchanged. 
Referring to FIG. 4B, when an air to liquid transition is detected by 
monitor head 3 at time 86, the output voltage 82 of amplifier 28 
decreases. If the voltage decrease is great enough for a sufficient period 
of time that the voltage 83 across capacitor 34 falls below 5.8 volts 
which occurs at time 87, then the output voltage 85 of amplifier 10 is 
driven high supplying the signal for a detected air to liquid transition 
while the output voltage 84 of amplifier 9 remains unchanged. 
FIG. 3C shows the automatic drift compensation characteristics of automatic 
referencing and drift compensation circuit 6 for the situation where the 
output voltage from the detection head 3 decreases with time. The output 
voltage of detector head 3 decreases (i.e., less light is detected by the 
receiver) for a number of reasons including change in solvent composition 
from a solvent that adsorbs less light to a solvent that adsorbs more 
light, slow decrease in the efficiency of light emitting diode 20, or the 
efficiency of photo transistor 22. At times 60 and 62 the output voltage 
59 of detector head 3 drifts in a negative direction which results in 
amplifier 28 increasing its output voltage 66 proportionate to the rate of 
voltage drift. As long as the change in detector voltage 59 decreases at a 
rate slow enough that the output voltage 66 of amplifier 28 never exceeds 
the reference voltage at the non-inverting input of amplifier 9, the 
output voltage of amplifier 9 remains at ground thus generating no alarm 
signal. At times 61 and 63, when the output voltage of detector head 3 
stops changing, the output voltage of amplifier 28 returns to its resting 
value of 6.0 volts, even though the absolute output voltage 59 of 
detection head 3 has decreased. The property of circuit 6 to return to its 
resting voltage of 6.0 volts following a change in the output voltage of 
the detection head 3 is referred to as automatic drift compensation. At 
time 64, the output voltage 59 of detection head 3 decreases at a rate 
great enough that the output voltage 66 of amplifier 28 exceeds the 
reference voltage at the non-inverting input of amplifier 9. If this 
condition persists long enough for capacitor 34 to increase its voltage 
above the reference voltage at the non-inverting input of amplifier 9, 
then an alarm signal is generated by the output of amplifier 9 in the form 
of a positive pulse. The action of circuit 6 is identical for the 
situation where the output voltage of the detector head 3 increases with 
time, except the change in voltage levels occurs in the opposite direction 
to those shown in FIG. 3c and for a fast enough rise in voltage, amplifier 
10 supplies a positive pulse. 
In view of the above, it will be seen that the several objects of the 
invention are achieved and other advantageous results attained. 
As various changes could be made in the above methods without departing 
from the scope of the invention, it is intended that all matter contained 
in the above description and shown in the accompanying drawings shall be 
interpreted as illustrative and not in a limiting sense.