Low volume flow meter

The low flow monitor provides a means for determining if a fluid flow meets a minimum threshold level of flow. The low flow monitor operates with a minimum of intrusion by the flow detection device into the flow. The electrical portion of the monitor is externally located with respect to the fluid stream which allows for repairs to the monitor without disrupting the flow. The electronics provide for the adjustment of the threshold level to meet the required conditions. The apparatus can be modified to provide an upper limit to the flow monitor by providing for a parallel electronic circuit which provides for a bracketing of the desired flow rate.

BACKGROUND OF THE INVENTION 
There is often a need to monitor the flow of liquids or gases when the rate 
of flow is very low or when the need to detect a flow has added 
restrictions that limit the degree of interaction between the means for 
measuring the flow rate and the fluid itself. Typical applications of 
restrictive flow monitoring environments include chemical processing 
plants, where the fluids may be highly combustible or corrosive, or in 
facilities where there is a danger of electrical shock. A further example 
involves an X-ray laser, where there is a need to measure the flow of the 
cooling water through a high field pulsed magnet. Under typical conditions 
the rate of flow of water through the magnet is approximately 0.333 
gallons per minute and the magnetic field coils are electrically pulsed to 
voltages exceeding several thousand volts; these conditions subject the 
flow monitor to very low rates of flow and a very high electric field. 
There are several methods of monitoring fluid flow in a conduit or pipe. 
One of the methods of determining flow employs a magnetic switch pickup 
which uses a magnetic washer that is free to move over a short distance 
under the influence of a pressure differential across the washer. A 
magnetic reed switch is placed near the material, and when the fluid is 
flowing and the pressure differential is sufficient to cause the washer to 
move, the switch is engaged and closes indicating a fluid flow. This 
method allows for the measurement of flow in an opaque fluid but is 
usually restricted to high flow rates. Also, since the washer is imbedded 
in the flow, it provides a constriction to the path of flow and in 
addition, may prove to be position sensitive. 
Another method of measuring flow, which is applicable to low flow rates, 
involves the use of a thermistor and a heating element. This method places 
a heating element upstream from a thermistor. The temperature detected by 
the thermistor is a function of the power supplied to the heating element, 
the specific heat of the fluid and the flow rate. Thus, knowing the power 
supplied to the heating element and the specific heat of the fluid, one 
can calibrate the fluid flow rate. One problem with this method is that 
both elements, the thermistor and the heating element, invade the flow 
stream. In addition, this method often encounters calibration problems. 
Another method employs the doppler effect. With this method, a sound wave 
is transmitted into the fluid flow, and the frequency of the returning 
sound is detected. Movement of the fluid shifts the frequency of the 
returning sound by adding a component to the frequency. This is a very 
accurate method of determining flow and can measure very low flows, but it 
is much more costly than applicant's method or the other methods 
described. This method may also be susceptible to noise or mechanical 
vibration. 
Applicant's invention involves an apparatus capable of detecting the flow 
in a fluid stream having a flow rate less than 0.333 gallons per minute. 
Further, with applicant's apparatus, there is only limited interference 
with the flow of the fluid stream. The apparatus for monitoring low flow 
is also capable with a minor modification to bracket a desired flow rate 
such that the apparatus will monitor a low rate of flow and also monitor 
if the fluid flow rate exceeds a predetermined upper limit. 
Accordingly, it is an object of this invention to provide a means of 
measuring the existence of a fluid stream having a low volume rate of 
flow. 
It is still a further object of this invention to provide a low flow meter 
capable of having a variable low flow trip point so that the user can 
select the low flow "trip" level at which flow will be detected. 
It is still a further object of this invention to provide an apparatus with 
limited interference with the fluid stream. 
It is a further object of this invention to provide for modified circuitry 
which will monitor fluid flow within a set of high-low fluid flow rate 
conditions. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. The objects and 
advantages of the invention may be realized and attained by means of 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
SUMMARY OF THE INVENTION 
To achieve the foregoing and other objectives and in accordance with the 
purposes of the present invention, as embodied and broadly described 
herein, the present invention provides a means for determining whether the 
fluid flow rate associated with a fluid stream meets a predetermined low 
flow rate. The apparatus also possesses a means of varying the threshold 
at which a flow is monitored. By modifying the circuit, one can bracket 
the flow so that the user is informed if the flow exceeds a specified 
upper level or falls below a specified lower level.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a means for monitoring low volume fluid 
flows and bracketed flows with limited invasion of the fluid stream by the 
measuring probe. 
FIG. 1 depicts the flow detection device, 1. It is essentially a waterwheel 
type apparatus having a positive rotational displacement relative to the 
flow of the fluid. For the apparatus currently in use, a Cole-Palmer Co., 
model N-06297-01 was used. This unit is capable of measuring flows ranging 
from 5 ml/sec to 95 ml/sec. The detection device, 1, consists of three 
opaque paddle wheel sections each encompassing sixty degrees of arc, 2, 
where each of the opaque sections is separated by an empty space 
encompassing 60 degrees of arc. This provides a high degree of contrast 
for light transmitted through the device, and thus, allows the flow 
detection device, 1, to be used with a variety of clear fluids. 
FIG. 2 depicts the low flow apparatus which includes the flow detection 
device, 1, and the instrumentation used to detect the rotation of the 
paddle wheel sections, 2, within a specified frame of reference. In this 
apparatus, a light emitting diode (LED), 3, emits an infrared beam which 
is focused, through the use of an internal integral focussing lens 15, on 
an area of the flow detection device, 1, which is alternatively occupied 
by one of the paddle wheel sections, 2, and a clear area during the 
rotation of the wheel; in constructing the apparatus, a LED55C was 
employed. A high gain, photo-Darlington detector, 4, is used to sense the 
portion of the infrared beam transmitted when the infrared beam transcends 
the clear space between the opaque paddles, 2. All of the components used 
to construct the device are external to the flow stream and flow detection 
device, 1. This allows the components to be replaced without disrupting 
the fluid flow. As the flow detection device, 1, turns, it modulates the 
infrared beam generated by the LED, 3, at a rate of three cycles for each 
360 degree rotation. The rate of modulation is sensed by the 
photo-Darlington detector, 4, and the pulsed cycle is transmitted to a 
first retriggerable mono-stable multivibrator system, 5. The period of the 
first retriggerable mono-stable multivibrator system, 5, is adjusted using 
the potentiometer, 6, so that its logic output, Q, remains at a logic one 
as long as the pulse period as detected by the photo-Darlington detector, 
4, is shorter than a set drop-out time, FIG. 4, 21; if the pulse period 
exceeds the drop-out time, the Q output cycles to a logic 0, FIG. 5, 31. 
Normally, the second retriggerable mono-stable multivibrator system, 7, 
remains in the Q=0, Q-bar=1 state, FIG. 4, 22. Thus, under normal 
conditions with the flow above the entered minimum value both 
multivibrator systems, 5 and 7, would convey a logic 1 to the AND gate, 8, 
which would respond with a logic 1 to an outside monitoring device, 9. If 
the pulse rate provided by the photo-Darlington detector, 4, drops to a 
very low value, such that the output of the first retriggerable 
mono-stable multivibrator system, 5, is cycling at a low rate, FIG. 5, 31, 
the second retriggerable mono-stable multivibrator, 7, output goes to a 
logic 0, FIG. 5, 32, and causes the output of the AND Gate, 8, to go to 
logic 0 which in turn causes the monitoring device, 9, to detect a low 
flow. In the circuit shown the retrigger period of the second 
multivibrator, 7, is set to 10 seconds which is considered to be 
essentially negligible flow. Zero flow causes the output of the first 
multivibrator system, 5, to go to logic 0, which causes the output of the 
AND Gate, 8, again to go to 0 with the monitor device, 9, again reading a 
low flow rate. In an alternate form, the pulse output from the 
photo-Darlington detector, 4, can be inputted to an external counting 
scheme, 10, which can be calibrated to provide a measure of the actual 
flow rate. 
FIG. 3 depicts a parallel modification to the invention displayed in FIG. 
2. The parallel modification yields an apparatus which will detect both a 
low flow condition and a high flow condition. In FIG. 3, M1, 5, M2, 7, M3, 
11, and M4, 12, are all retriggerable mono-stable multivibrator systems. 
M1, 5, and M2, 7, serve the same functions as the first and second 
retriggerable mono-stable multivibrator systems, and 7, described in the 
low flow indicator, FIG. 2, while M3, 11, and M4, 12, represent the third 
and fourth retriggerable mono-stable multivibrator systems, which serve to 
detect the high flow case. The period limits for each of the 
multivibrators can, for example, be set as follows: M1=1 sec, M2=10 sec, 
M3=0.1 sec, and M4=1 sec. Using these settings, as long as the input 
frequency pulses to M1, 5, and M3, 11, are between 1 and 10 pps (pulses 
per second), M1, 5, will remain constantly set at Q=logic 1, and Q-bar=0, 
FIG. 4, 21 and 23, and the M2, 7, Q-bar output will be logic 1, FIG. 4, 
22. M3, 11, will emit pulses at the input rate, FIG. 4, 24, and cause M4, 
12, to remain set with Q=logic 1 and Q-bar=logic 0. Therefore, all inputs 
to the AND Gate, 13, will be at logic 1 and the output from the AND gate 
will be a logic 1. This will convey an indication of a valid flow rate to 
the flow monitor, 14. For pulses with a frequency less than 1 pps, M1, 5, 
FIG. 5, 33, will emit pulses at its output and M2, 7, will set causing its 
Q-bar output to go to logic 0 blocking the AND Gate, 13, and thus, 
indicating an invalid flow to the flow monitor, 14. For pulses greater 
than 10 pps, M3, 11, will remain constantly at a Q-bar output of logic 0 
which will cause the trigger input to M4, 12, to cease thereby causing the 
Q output to go to the logic 0 state after one second, FIG. 6, 41, thus, 
blocking the AND Gate, 13, yielding a logic 0 from the AND gate, 13 and 
indicating an invalid flow rate to the flow rate monitor, 14. The status 
of the multivibrators M2, 7, and M4, 12, for the various flow regimes are 
indicated below: 
Normal flow: M2, 7, Q-bar=1; M4, 12, Q=1 
Low flow: M2, 7, Q-bar=0; M4, 12, Q=1 
High flow: M2, 7, Q-bar=1; M4, 12, Q=0