Fluid flow direction and velocity monitor

A fluid flow detection monitor (80), particularly useful in monitoring pressure differentials between a controlled environment and its surroundings, includes a heat source (1) situated between an upstream and downstream thermal sensor (2,3) within a channel (5). The electrical resistance of the downstream sensor varies as the downstream sensor is heated from fluid carried past the heat source. Circuitry is used to detect the electrical resistance differential between the thermal sensors, thereby detecting the presence of fluid flow.

SPECIFICATION 
FIELD OF THE INVENTION 
This invention relates generally to fluid flow direction detectors, and 
more specifically to a ventilation related monitor for detecting the 
direction and velocity of gas flow in a tube or channel connecting two 
rooms. 
BACKGROUND OF THE PRESENT INVENTION 
A number of flow measurement and detection arrangements have been developed 
over the years. These devices may be generally categorized as one-, two- 
and three-element devices. Single sensor devices can calculate fluid 
velocity, but are generally not suitable for determining fluid flow 
direction. 
Two- and three-element devices generally operate on the principle of adding 
heat to a flowing fluid and measuring the heat transfer functions of 
sensors placed along the fluid flow stream. The difference between the 
heat transfer functions of the upstream and downstream sensors is used to 
calculate flow direction and velocity. 
For example, U.S. Pat. No. 4,787,251 reports a two-element configuration. 
The configuration in U.S. Pat. No. 4,787,251 requires that both sensors 
produce a significant thermal wake. To produce these thermal wakes, both 
sensors are typically 30.degree.-100.degree. C. above that of the 
surrounding fluid temperature, a significant power requirement. The 
thermal wake is sensed by the downstream sensor. This two-element 
arrangement also requires thermally insulated flow sensors and greater 
complexity in the circuitry used to measure the electrical resistance or 
heat transfer functions of the thermal sensors. Use of the sensors as both 
thermal sensors and heating elements also results in unnecessary power 
dissipation in the measuring circuit. Additionally, the heat added by the 
downstream element is not utilized. 
U.S. Pat. No. 3,196,679 describes a three-element configuration which uses 
a heated element situated between an upstream and a downstream thermal 
sensor. This three-element configuration requires a high heat source and 
relies on the mechanical movement of spring-type thermal sensors. Because 
spring-type thermal sensors are not appropriate for detecting small 
temperature differentials of a surrounding gas flow, this configuration is 
not suitable for use in measuring low volume gas flow. 
Thus, there exists a need for an inexpensive, reliable, and low-power fluid 
flow direction and velocity monitor for use in laboratories, hospitals and 
other ventilation applications where airborne contaminants must be 
isolated within or outside of a controlled space. Reliable fluid flow 
direction monitors may be used to maintain minimal pressure differentials 
between the controlled and non-controlled spaces. Reliable detection of 
minimal pressure differentials allows for the use of lower volume air 
supply systems, therefore, reducing energy requirements. 
SUMMARY OF THE INVENTION 
To detect whether fluid is flowing in a preselected direction within a 
fluid flow channel, a heat source is situated between two thermal sensors. 
The heat source and thermal sensors are positioned within the fluid flow 
channel so that one thermal sensor is always downstream from the heat 
source when fluid flow is present. The downstream thermal sensor will thus 
be exposed to greater heat than the upstream thermal sensor. This exposure 
heats the downstream sensor and varies its electrical resistance. The 
difference between the electrical resistance of the upstream and 
downstream thermal sensors is monitored and used to determine whether 
fluid is flowing in the pre-selected direction. 
To monitor the electrical resistance values of the thermal sensors, each 
sensor is electrically coupled to opposite arms of a Wheatstone bridge. 
Voltages at cross-nodes of the Wheatstone bridge are amplified by a 
differential amplifier. The amplified signal is compared to a voltage 
level, preferably a user-selected voltage level, to determine whether the 
fluid in the fluid flow channel is flowing in the preselected direction. 
It is an object of the invention to provide a simple, fluid flow direction 
monitor with low power requirements. 
It is another object of this invention to provide a fluid flow direction 
monitor for determining the direction of fluid flow utilizing a single 
heating element and two thermal sensor elements, wherein the heating 
element is located between the two thermal sensor elements such that the 
downstream sensor will be heated to a higher temperature than the upstream 
sensor. 
It is another object of this invention to provide a fluid flow detection 
monitor utilizing a single heating element and two thermal sensor elements 
located in a channel connecting two rooms or spaces. The fluid flow 
direction monitor is used to verify that one of the rooms or spaces is 
maintained at a higher pressure than the other space. Such pressure 
differentials are critical in hospital operating rooms, isolation rooms, 
and clean room environments. 
It is a further object of this invention to provide a fluid flow velocity 
monitor for determining the velocity of fluid flow within the fluid flow 
channel. A pulse circuit is used to provide a pulse of power to the 
heating element. A timing circuit is connected to the thermal sensors to 
measure the delay between the pulsing of the heating element and the 
change in electrical resistance of the downstream thermal sensor. The 
fluid flow velocity may then be calculated by knowing the delay and the 
distance between the heating element and the downstream thermal sensor.

DETAILED DESCRIPTION 
Preferred embodiments of the present invention will now be described with 
reference to the drawings. 
Referring to FIG. 1, the fluid flow direction monitor includes a fluid flow 
channel 5 having a first opening 30 and second opening 31. A first thermal 
sensor 2 and a second thermal sensor 3 are disposed within the fluid flow 
channel 5. A heating element 1 is disposed between the thermal sensors 2 
and 3, and heated above ambient temperature. Fluid flowing over the 
heating element 1 will be heated and will flow over one of the thermal 
sensors: either 2 or 3, depending upon which sensor is downstream from 
heating element 1, and heat the downstream thermal sensor. The fluid flow 
channel 5 need not be horizontal and has been implemented at a 30 to 45 
degree angle relative to horizontal. 
The heating element 1 is typically a resistor through which a current is 
driven, thereby causing the resistor to heat the surrounding fluid flow 
above ambient temperature. Typically the resistor is between about 530 and 
630 ohms and preferably between 608 and 620 ohms. In a preferred 
embodiment, the heating resistor is powered by a transformer with 
approximately 125 mW of power. This resistor heats the fluid that comes in 
contact with it. The resistor is typically maintained at approximately 
10.degree.-50.degree. C. above the ambient fluid temperature, and 
preferably 10.degree.-30.degree. C. above the ambient fluid temperature. 
The electrical resistance of each of thermal sensors 2 and 3 varies 
rapidly and predictably as the temperature of the sensor increases. 
Preferably, thermal sensors 2 and 3 are thermistors. When the downstream 
thermal sensor is heated by the heated fluid flowing past it, its 
electrical resistance decreases. Thermal sensors 2 and 3 are connected to 
circuitry that determines whether the electrical resistance of the thermal 
sensor that should be downstream is greater than the electrical resistance 
of the thermal sensor that should be upstream. In a preferred embodiment, 
the magnitude of the difference in thermal sensor resistance is user 
selectable and is typically in the range of 100 to 600 ohms. The presence 
of this difference in electrical resistance indicates fluid flow in the 
preselected direction. When the circuitry does not detect this difference, 
in preferred embodiments, the fluid flow direction monitor produces a 
sensible output, typically a sound or light signal. 
FIG. 2 illustrates a fluid flow direction monitor constructed according to 
the principles of the present invention. The fluid flow direction monitor 
includes heating element 1, and thermistors 2 and 3, mounted within fluid 
flow channel 5. Fluid flow channel includes a first opening 30 and second 
opening 31. The illustration shows two rooms, 7 and 8. Fluid flow 
direction monitor 80 is mounted on wall 85 in room 8. Flow tube 70 
connects the first opening 30 of fluid flow channel 5 to room 7. Each room 
has a nominal pressure. Fluid may flow to or from rooms 7 and 8 through 
fluid flow channel 5 and flow tube 70. The direction of fluid flow through 
channel 5 will be toward the room with the lower pressure. 
FIG. 3 illustrates a fluid flow direction monitor constructed in accordance 
with a preferred embodiment of the present invention. A printed circuit 
board 51 forms one face of the fluid flow channel 5. A first and second 
thermal sensor, 2 and 3 respectively, are thermistors mounted on the 
printed circuit board 51 within the fluid flow channel 5. The heating 
element 1 is a resistor, mounted on the printed circuit board 51 between 
the thermal sensors 2 and 3. The heating element 1 and thermal sensors 2 
and 3 are preferably standard through-hole, leaded, components, but could 
alternatively be surface mounted. Channel cover 61 forms the opposing 
three faces of fluid flow channel 5 allowing fluid to flow through fluid 
flow channel 5, past the thermal sensors 2 and 3 and heating element 1. 
FIG. 4 illustrates a simplified electrical schematic diagram of a circuit 
constructed in accordance with a preferred embodiment of the present 
invention. The thermal sensors 2 and 3 are each part of separate legs of a 
single bridge circuit 10. Bridge circuit 10 is formed from thermal sensors 
2 and 3, resistors 13 and 14, and potentiometers 11 and 12. The 
potentiometers 11 and 12 are used to compensate for component inaccuracies 
and may be eliminated by improved component tolerances. A bridge voltage 
23 is applied to node 15 of the bridge circuit 10. Voltage measurements 
are taken at bridge nodes 16 and 17 by voltage followers 18 and 19. The 
output voltages 20 and 21, of voltage followers 18 and 19, are 
electrically connected to dip switch 65. Dip switch 65 is used to 
preselect the direction in which fluid flow may be detected by setting a 
selected output voltage 70 and a nonselected output voltage 71 from output 
voltages 20 and 21. The selected 70 and nonselected 71 output voltages are 
electrically connected to differential amplifier 22, which amplifies the 
difference of the selected output voltage minus the nonselected output 
voltage and produces a differential voltage 62. Comparison amplifier 24 
compares the differential voltage 62 to reference voltage 63. Reference 
voltage 63 can be either preset or user selectable. Comparison amplifier 
24 then produces a logic signal 25, indicating whether there is fluid flow 
in the downstream direction. 
In a preferred embodiment, logic signal 25 is electrically connected to a 
programmable logic chip which controls a audible alarm and LEDs to signal 
a lack of flow in the preselected direction. 
FIG. 5 shows an alternate embodiment of the invention. Thermal sensors 2 
and 3 are connected in series, but are not a part of a bridge circuit. The 
thermal sensors 2 and 3 form a junction 41. Device 40 provides a stable 
reference voltage and is electrically connected to the thermal sensor in 
series at junction 42. A first operational amplifier 47 is connected to 
junctions 41 and 42 and produces an output 43 proportional to the voltage 
loss across thermal sensor 2. Temperature across thermal sensor 3 is 
available by measuring the voltage 46 at junction 41. A second operational 
amplifier 44 compares the differential voltage of the output 43 and 
voltage 46 of junction 41 and outputs a logic signal 50, indicating 
whether fluid is flowing in the desired direction. 
FIG. 6 illustrates an electrical schematic diagram of a circuit constructed 
in accordance with the invention. Heating element 1 is heated by a pulse 
of voltage generated by pulse generating circuit 101. Heating element 1 
heats the fluid that comes in contact with it, causing the downstream 
thermal sensor to be heated. Thermal sensors, 2 and 3, generate an 
interrupt when heated. When the pulse generating circuit 101 pulses 
heating element 1, the timing circuit 100 begins counting clock ticks 
produced by clock 102, until an interrupt generated by the downstream 
thermal sensor, either 2 or 3, interrupts the timing circuit 100. Fluid 
velocity within fluid flow channel 5 is determined from the known distance 
of the downstream thermal sensor, either 2 or 3, from the heating element 
1 and the number of clock ticks between the pulse on the resistor and the 
interrupt at bridge nodes 16 or 17. The pressure differential between 
rooms 7 and 8 may be determined from the fluid velocity. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of the present invention. Accordingly, it is not intended 
that the scope of the claims be limited to the description set forth 
herein, but rather that the claims be construed as encompassing all 
features of patentable novelty that reside in the present invention, 
including all features that would be treated as equivalents by those 
skilled in the art.