JPH05156863A - Device for obtaining position of movable structure - Google Patents

Abstract

PURPOSE: To make it possible to determine an absolute position of a mobile structure along a track by providing an electric switching means, an actuator means and circuit means and making the circuit means give a voltage signal having a magnitude proportional to a distance between an end part of a resistance means and a switching means. CONSTITUTION: A sash position detecting means comprising a switching system 80 is provided adjacently to a sash door, and an actuator 94 is supported on the sash door and preferably is urged by a spring. When the sash door is moved, the actuator 94 is moved along the switching system 80 and a contact point between a resistance layer 84 and a conductor layer 88 is formed and is detected by a single electric circuit. A voltage proportional to a distance between a connection end of the resistance layer 84 and the contact point by the actuator 94 provided on another sash door in a track is generated. By this method, installation becomes easy, signals are generated by using a minimal number of parts and an excellent contribution to cost efficiency improvement in manufacturing phase can be realized.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates generally to a device for determining the position of one or more structures, such as sliding doors or windows, mounted in a truck, and more particularly movable in an attached truck of a laboratory fume hood. Apparatus for determining the position of one or more sash doors.

[0002]

Fume hoods are used in a variety of laboratory environments that provide a work location where potentially dangerous chemicals are used, and these hoods allow workers access to the interior of the enclosure for experiments. Where possible, it has an enclosure with a movable door in the front that can be opened in different amounts. Typically, the enclosure is connected to an exhaust system that removes harmful fumes to prevent personnel from being exposed to harmful fumes while working in the hood.

The fume hood controller, which controls the flow rate of air through the fume hood, has become increasingly sophisticated in recent years and now accurately maintains the desired flow characteristics and provides the desired average surface velocity of the fume hood opening. As a function, it allows for efficient exhaust of fumes from the enclosure. Mean areal velocity is generally defined as the flow rate of air flowing into the fume hood per square foot of open face area of the fume hood, where the size of the open face area is one of the enclosures or the front of the fume hood. More movable door positions and, in most types of enclosures, also depend on the amount of bypass openings provided when one or more doors are closed.

The fume hood is evacuated by an exhaust system which generally includes a blower that can be driven at variable speeds to increase or decrease the flow rate of air from the fume hood to compensate for the varying size of the opening or open surface. .. Alternatively, one for the exhaust manifold connected to each duct of multiple fume hoods.
Two blowers may be connected, and a damper may be provided in each duct to control the flow rate from each duct and modulate the flow rate to maintain a desired average surface velocity. Furthermore, you may combine both types mentioned above.

The door of such a fume hood can be opened by raising it vertically to a position often referred to as the sash position, and some fume hoods are usually slidable along two sets of tracks. It has a plurality of mounted doors. In addition, some doors have multiple tracks mounted in a vertically movable frame assembly and are movable in both horizontal and vertical directions.

The conventional fume hood controller includes a detecting means for measuring the absolute position of the vertical door or the relative position of the horizontal door and changing the speed of the blower or the position of the damper using a signal proportional to the detected position. ..

[0007]

The main object of the present invention is to:
An object is to provide an improved sensing means for measuring the absolute position of a movable structure along a track.

Another object is to provide an improved sensing means for determining the absolute position of a sash door in a laboratory fume hood in which one or several sash doors are movable along one or more sets of trucks. ..

A related object is to provide an improved sensing means which is broadly suitable for providing an absolute position indication signal for a door movable either vertically or horizontally.

Another object is to provide a single elongated switching means which can be placed along a track on which one or two sash doors can be moved, with separate signals indicating the respective absolute positions of the one or two sash doors. It is to provide an improved sensing means that is generated.

Yet another object of the present invention is to provide separate signals indicative of the absolute position for each of the two doors movable along a set of tracks, with the sensing means having a sufficiently small cross section and structural construction. It is an object of the invention to provide an improved detection means which is simple and can be placed out of the way in the truck without interfering with the movement of the sash door.

The above and other objects will become apparent from the following detailed description of the invention with reference to the accompanying drawings.

[0013]

To achieve the above object, the device according to the invention uncovers an opening in a fume hood having at least one movable sash door which selectively covers the opening in response to movement of the sash door. In a device used to control the air flow rate through the fume hood so as to maintain a relatively constant average surface velocity through the section, such as a device providing a position indicating signal indicative of the absolute position of each sash door,
An electrical switching means arranged parallel to at least one possible direction of movement of said sash door and adjacent to said sash door and comprising elongated electrical resistance means having a predetermined resistance value per unit length; both when said sash door moves Actuator means attached to each one of the sash doors for movement, the actuators being positioned to operate the switching means at one individual switch location which varies along the length of the resistance means as the sash door moves. Means; and circuit means for providing a substantially constant current through the resistance means portion between the end of said resistance means and the nearest switch location, proportional to the distance between the end of said resistance means and said switch location. It is characterized in that it comprises circuit means for providing a voltage signal having a magnitude.

[0014]

To begin with, as a general rule, the fume hood controller controls the flow of air through the fume hood and the effective size of all openings to the fume hood, including those openings not covered by one or more sash doors, is It should be understood that it is intended to provide a relatively constant average surface velocity of the air moving into the hood. This means that regardless of the area of the uncovered opening, the average amount of air per unit surface area of the uncovered part is
Means moving into the fume hood. As a result,
Air constantly flows into the fume hood and out of the exhaust duct, protecting workers in the laboratory from exposure to harmful fumes and air flow.
It is preferably controlled at a predetermined rate of approximately 75-125 cubic feet per minute per square foot of effective surface area of uncovered openings. In other words, if one or more sash doors are moved to the maximum open position and workers gain maximum access to the interior of the fume hood for experiments, etc. The flow of must be increased. There are significant differences in the capabilities and effectiveness of various prior art controllers.

Broadly speaking, sensing means suitable for determining the absolute position of one or more structures, such as doors or windows movable in one or more sets of trucks, are obtained with the present invention. Be done. However, since the present detection means can be used in many types of structures that can move along a predetermined path, the application of the present invention is not limited to doors and windows. The invention is particularly
Exactly the flow rate of air through the fume hood, so as to maintain an average surface velocity of the air entering the fume hood, which is partly a function of the sash door position of the fume hood, which is a function of the size or area of the opening. Suitable for use in laboratory fume hoods that have a controlling controller. The fume hood controller device provides extremely quick and effective control of the fume hood average face velocity, as well as the desired average face velocity within a few seconds after one or more doors over the fume hood's front opening have been moved. Since it is highly desirable to achieve and maintain the position of each door, it is necessary to quickly and accurately determine the position of each door, which is why the present invention provides a substantially continuous voltage signal that is input to the fume hood controller,
The controller uses these signals to control the flow of air into and out of the fume hood.

That is, the device of the present invention is provided adjacent to one of the sash doors, and is attached to the sash door to operate the switching means, the electrical switching means including elongated electrical resistance means having a predetermined resistance value per unit length. Actuator means and circuit means for providing a substantially constant current through the portion of the resistance means between the end of the resistance means and the nearest switch location, the circuit means between the end of the resistance means and the switch location. A voltage signal having a magnitude proportional to the distance is provided.

[0017]

1 is a block diagram of a room controller 22, an exhaust controller 24 which may be part of the room controller itself, and several fume hood controllers 20 interconnected with a main control console 26. The figure is shown. The fume hood controller 20 is interconnected with a room controller 22, an exhaust controller 24 and a main control console 26 by a local area network (LAN) indicated by line 28 which may be formed of a multi-conductor cable or the like. Room controller 22, exhaust controller 24 and main control console 26
Is usually part of the main HVAC system of the building in which the laboratory room containing the fume hood is located. The fume hood controller 20 is powered via line 32 under appropriate voltage via a transformer 32 or the like.

Referring to FIG. 2, a fume hood controller 20 is shown explicitly showing each function of its input and output connector ports, which is connected to an operator panel 34. It should be understood that each fume hood has a fume hood controller 20 and an operator panel is provided for each fume hood controller 20. That is, an operator panel 34 is provided for each fume hood and is interconnected with the fume hood controller 20 by a line 36 which preferably comprises a multi-conductor cable having eight conductors. The operator panel 34 is, for example, a 6-line RJ
It has a connector 38 such as a 11-inch phone jack, and a laptop personal computer or the like can be connected to the connector 38 in order to input information regarding the configuration and operation of the fume hood at the time of initial setting, or to change some operation parameters if necessary. Can be connected. Also, the operator panel 34
Is preferably attached to the fume hood in a convenient location for easy observation by an operator working in the fume hood.

The fume hood controller operator panel 34 includes a liquid crystal display 40 which is selectively energized to provide a visual indication of various aspects of the fume hood, including a three digit 42 which provides an average face velocity. Display 4
0 also indicates low face velocities, high face velocities, emergency conditions, and other conditions such as an indication of controller failure. The operator panel 34 may also have an alarm 44 and an emergency purge switch 46 that the operator can press to purge the fume hood in the event of an accident. The operator panel also has two auxiliary switches 48 that can be used for various customer needs, including day / night mode of operation. It is conceivable that the nighttime mode of operation will have a different, preferably lower, average surface velocity than the daytime mode of operation. This is because it is assumed that no one will work indoors late at night and such a low average surface velocity will save energy. An alarm silence switch 50 is also preferably provided.

The fume hood can be of many different styles, sizes and shapes, including those with one or multiple sash doors, with the sash doors being movable vertically, horizontally or in both directions. Also, various fume hoods have different bypass flow rates, i.e., different flow rates through the openings that are present even when all sash doors are closed as far as possible by design.

Referring to FIG. 3, there is shown a fume hood, generally indicated at 60, which is movable to provide access to the fume hood and is in a generally closed position as shown. It has a vertically actuable sash door 62 movable to. Some fume hoods have a bypass opening located above the door sash, while others are located below. In some of the fume hoods, the initial movement of the sash door increases the opening at the bottom of the door shown in FIG. 3, for example, but the door blocks the bypass opening as it rises, and the total opening of the fume hood increases. The effective size is kept relatively constant during the first approximately 1/4 of the travel along the path of movement of the sash door 62.

Another type of fume hood includes several horizontally movable sash doors 66, as shown in FIGS. 4 and 5, and upper and lower pairs of adjacent tracks 68.
Each door may be movable along. When the door is positioned as shown in FIGS. 4 and 5, the opening of the fume hood is completely closed and the operator can move the door horizontally to gain access to the inside of the fume hood. Smoke hood 6
Both 0 and 64 have an exhaust duct 70 that typically extends to an exhaust system that may be for an HVAC device as described above. Fume hood 64 includes a filtering structure, shown generally at 72, which prevents harmful fumes and other contaminants from flowing out of the fume hood into the exhaust system.
Referring to FIG. 6, there is shown a combined fume hood 74 having a horizontally movable door 76 similar to the door 66, which fume hood 74 supports a frame structure 78 supporting the door 76 along a suitable track. And the frame structure 78 is vertically movable within the fume hood opening.

In FIG. 6 there is an abbreviated portion, indicated by dashed line 73, which allows the frame structure 78 to be raised sufficiently to allow proper access to the interior of the fume hood by the operator. It is intended to show that the height of the hood can be made larger than shown. Generally, there is a bypass area shown as vertical area 75,
There is also typically a top lip 77, which can be about 2 inches wide. This dimension is preferably determined so that its influence on the calculation of the open surface area can be taken into account.

Although not specifically shown, much like a casement window for a home, one to the other along the width of the fume hood opening, such as two or more vertically movable sash doors along adjacent tracks. Other combinations are possible, including multiple sets of vertically movable sash doors located adjacent to each other.

The fume hood controller can control virtually any of the various types and styles of fume hoods that are commercially available, and as such, has a large number of input / output ports (connectable to the various sensors used in the controller). Line, connector or connection, which are all considered equivalent for the purposes of this description). As shown in FIG. 2, 5
One sash position sensor port can be used to detect the position of the sash, which can be moved both horizontally and vertically.

In order to determine the position of the sash door, the sash position detecting means embodying the present invention is provided adjacent to each movable sash door, which is schematically shown in FIGS. 7, 8 and 9. .. Referring to FIG. 8, the door sash position sensing means comprises an elongated switching mechanism 80 of relatively simple mechanical design, which preferably comprises a relatively thin polyester base layer 82, which is known per unit length above the base layer 82. The electric resistance ink 84 having a constant resistance is printed in a strip shape. Another polyester base layer 86 is provided, on which conductive ink 88 is also printed in strips. Two base layers 82 and 8
6 are adhesively bonded to each other by two beads of adhesive 90 located on both sides of the strip. Both base layers are preferably approximately five-thousandths of an inch thick, both beads are approximately two-thousandths of an inch thick, and these beads are conductive layer 8.
8 and the resistance layer 84 provide a space region. The switching mechanism 80 is preferably attached to the fume hood by a layer 92 of adhesive.

The polyester material is sufficiently flexible that one layer can move toward and contact the other layer,
The movement of the sash door causes the actuator 94 to move along the switching mechanism 80, since the strips are supported by corresponding sash doors arranged side by side and the contacts are formed in response to a preferably spring-loaded actuator 94. A contact is created between the resistive layer and the conductive layer, which is sensed by an electrical circuit described below and provides a voltage output indicative of the position of actuator 94 along the length of switching mechanism 80. That is, the actuator 94 is supported by the sash door to give a voltage indicating the position of the sash door.

The actuator 94 receives sufficient pressure on the switching mechanism 80 as the sash door moves, and
The two base layers are abutted, and the resistive layer and the conductive layer are preferably spring biased toward the switching mechanism 80 so as to make an electrical contact with each other and to provide a voltage level in response to the electrical contact. .. The sash position can be accurately determined by making the switching mechanism 80 long enough to cover the entire range of movement of the sash door as shown in FIG.

It should be noted that the switching mechanism 80 shown in FIGS. 3 and 5 is merely an outline, and in practice, the switching mechanism 80 is preferably arranged in the sash frame itself, and as shown in the drawing, from the outside. can not see. Due to the small width and thickness dimensions of the switching mechanism 80, interference with the operation of the sash door is virtually non-issue. The actuator 94 may be placed in a small hole drilled in the sash door, or mounted outside one end of the sash door so that the switching mechanism 80 is in a position to operate. In the vertically movable sash door shown in FIGS. 3 and 6, it is preferable to provide the switching mechanism 80 on one side or the other side of the sash frame, while in the fume hood having the horizontally movable door, the movable door is used. It is preferable to place the switching mechanism 80 on top of the track 68 so that the weight thereof does not affect or damage the switching mechanism 80. In addition, the actuator 94 is referred to by Ahmed et al. In the above-mentioned cross-referenced application as “Method and apparatus for determining uncovered size of openings covered by a plurality of movable doors”, serial No. 52498
Preferably, it is located at one end of each door for the reasons described in.

Referring to FIG. 9, there is shown a preferred electrical circuit for producing the position indicating voltage, the electrical circuit indicating the position of two sash doors within a truck.
Suitable to give two separate voltages. Referring to the cross-sectional view shown in FIG. 5, each supporting two sash doors
There are two horizontal tracks, and for each track the same circuit shown in FIG. 9 is provided, including the switching mechanism 80, to provide a separate voltage for each of the four sash doors shown.

The switching mechanism 80 is preferably attached to the fume hood with a layer of adhesive 92, with actuators 94 abutting it at each location along the length of the switching mechanism 80. Referring to FIG. 7, 2 shown in FIG.
A schematic diagram of a pair of switching mechanisms 80, as used in one truck, is shown. The switching mechanism 80 is provided in each track, and the four arrows in the figure represent the contact points formed by the actuators 94. As a result, a signal is given to each end of each switching mechanism, and the magnitude of this signal is It represents the voltage proportional to the distance between each end and the closest arrow. Thus one switching mechanism 80 provides position indicating signals for the two doors located on each truck. The circuitry used to implement the above voltage generation is shown in FIG. 9, with one circuit for each track. Resistance element (layer)
84 and the conductive element (layer) 88 is shown with one end connected to ground and the two arrows at the other end are the resistances formed by respective actuators 94 attached to two separate doors. It represents the contact point between the element and the conductive element. The circuit shown includes an operational amplifier 100 having an output connected to the base of a PNP transistor 102, the emitter of PNP transistor 102 via resistor 104 to a positive voltage source and directly to operational amplifier 100. And the positive input of operational amplifier 100 is also preferably connected to a positive voltage source of about 5 volts. The collector of the transistor 102 is connected to one end of the resistive element 84 and has an output line 106, at which a voltage indicating the position of the door is generated.

The above circuit operates to provide a constant current to the resistive element 84, which produces a voltage proportional to the resistance between the collector and ground that changes as the closest contact point along the resistance changes. Occurs on output line 106. Operational amplifier 100 operates to drive the negative input to equalize the voltage level of the positive input, resulting in a current at the output of the operational amplifier that varies in direct proportion to the effective length of resistive element 84. The lower part of the circuit operates in the same manner as described above, providing a voltage proportional to the distance between the connecting end of the resistive element 84 and the point of contact by the actuator 94 attached to the other sash door in the track.
8 occurs similarly.

[0033]

As will be appreciated from the foregoing detailed description, there is provided improved sensing means for determining the position of a sash door which provides advantages over the prior art. 1 for each track
Only two switching mechanisms are provided, but for two sash doors that can move along one set of tracks,
The position indicating voltage can be generated by a single circuit. The device is easy to install, can generate signals with a minimum number of components,
This contributes to extremely excellent cost efficiency in manufacturing.

While various embodiments of the present invention have been shown and described,
It should be understood that various alternatives, substitutions and equivalents can be used, and the present invention should be limited only by the description of the claims and equivalents thereof.

Various features of the invention are set forth in the appended claims.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an apparatus according to the present invention, which is shown together with a room controller of a monitor and control system for heating, ventilation and air conditioning of a building.

FIG. 2 is a block diagram of a fume hood controller, shown connected to an operator panel shown in a front view.

FIG. 3 is a front schematic view of an exemplary fume hood having a vertically actuatable sash door.

FIG. 4 is a front schematic view of an exemplary fume hood having a horizontally actuatable sash door.

5 is a cross-sectional view substantially taken along line 5-5 of FIG.

FIG. 6 is a schematic front view of an exemplary combined sash fume hood having sash doors operable in both horizontal and vertical directions.

FIG. 7 is an electrical schematic diagram of a plurality of door sash positions showing switching means.

FIG. 8 is a sectional view of door sash position switching means.

FIG. 9 is a schematic diagram of an electric circuit for determining the position of the sash door of the fume hood.

[Explanation of symbols]

 20 Fume Hood Controller 22 Room Controller 24 Exhaust Controller 60, 64, 74 Fume Hood 62, 66, 76 Sash Door 78 Frame Means 80 Switching Means (Mechanism) 82, 86 First and Second Base Members (Layer) 84 Resistance Means 94 Actuator Means 100 comparator means (operational amplifier) 102 PNP transistor

Claims (17)

[Claims] 1. A fume hood for maintaining a relatively constant average surface velocity through an uncovered portion of an opening of a fume hood having at least one movable sash door that selectively covers the opening in response to movement of the sash door. In a device used to control the air flow rate through the sash door, such as providing a position indicating signal indicative of the absolute position of each sash door: parallel to at least one possible direction of movement of the sash door and adjacent to the sash door. Electrical switching means, each of which has a predetermined resistance value per unit length, including elongated electrical resistance means; actuator means attached to each one of the sash doors so as to move with the sash door when the sash door moves. Operating the switching means at one discrete switch location which varies along the length of the resistance means as the Actuator means positioned in such a way; and circuit means for providing a substantially constant current through the resistance means portion between the end of said resistance means and the nearest switch location, said end of said resistance means and said switch location A circuit means for providing a voltage signal having a magnitude proportional to the distance therebetween. 2. The apparatus of claim 1 wherein said circuit means connects said resistance means to ground at each switch location. 3. The electrical switching means comprises: a first elongated thin base member provided with the elongated electrical resistance means, the resistance means being printed on the base member and having a known length per unit length. A strip of resistive ink having a constant resistance; a second elongated, thin base member located substantially overlapping the first base member, the second base member being in the strip form. Of resistive ink in close proximity to the second base member, the strip-shaped conductive ink being applied to the second base member; the first and second base members being in close proximity by a pair of spaced apart beads of adhesive. However, the resistance ink contacts the conductive ink at the switch location according to the urging means. Bent in a direction which divides an amount only phase approaches apparatus of claim 1. 4. The apparatus of claim 3 wherein said electrical switching means has a length that corresponds at least to the travel length of said sash door. 5. The sash door moves vertically to open an opening, the electrical switching means is vertically disposed adjacent to one side of the sash door, and the sash door is attached thereto at a predetermined location. A height of said actuator means when said sash door is opened to substantially its full range of movement from said actuator means at least when said sash door is substantially completely closed, comprising said actuator means. 4. The device of claim 3 extending to. 6. A plurality of sash doors covering the opening,
At least two sash doors are horizontally movable along each of the two sets of trucks, each of the two sets of trucks has electrical switching means, and each of the sash doors has one of the actuator means. 4. The apparatus of claim 3 wherein said circuit means provides a substantially constant current from both ends of said resistance means to a switch location on each sash door. 7. The circuit means is connected between a power supply means and the resistance means and provides a substantially constant current through the resistance means regardless of the position of a switch location along the resistance means. 7. The device according to claim 6, comprising adjusting means. 8. The apparatus of claim 7 wherein said voltage signal is produced between said current adjusting means and the end of said resistance means to which said current adjusting means is connected. 9. The circuit means is connected between the power supply means and each end of the resistance means and extends from each end to the nearest switch place regardless of the position of the one or more switch places. 8. The apparatus of claim 7 including current regulating means for providing a substantially constant current through said resistive means portion. 10. The current regulating means comprises comparator means having one input connected to a standard voltage and the other input connected to the power supply means through an emitter of a PNP transistor and also through a resistor. 8. The apparatus of claim 7, wherein the base of the transistor is connected to the output of the comparator means and the collector of the transistor is connected to the resistance means. 11. A fume hood for maintaining a relatively constant average surface velocity through the uncovered portion of the fume hood opening having at least two movable sash doors selectively covering the opening in response to movement of the sash door. In a device used to control the air flow rate through the sash door, such as providing a position indicating signal indicating the position of each of the sash doors: arranged parallel to one possible direction of movement of the sash door and adjacent to the sash door. Electrical switching means including elongated electrical resistance means having a predetermined resistance value per unit length, the switching means being switchable to ground at individual switch points which are changeable along the length; associated with each said sash door , An actuator means that moves with the sash door as it moves, one individual switch along the length of the switching means. Actuator means each positioned to operate the switching means at a switch point; and circuit means for providing a substantially constant current through the resistance means portion between each end of said resistance means and the nearest switch point, An apparatus comprising circuit means for providing to each end a voltage signal having a magnitude proportional to the distance between the end of said resistance means and said nearest switch location. 12. The fume hood includes a vertically movable sash door, the electrical switching means is vertically disposed, and the circuit means provides a voltage signal proportional to the vertical position of the sash door. 11. The device according to item 11. 13. Maintaining a relatively constant average surface velocity through the uncovered opening of a fume hood having at least three sash doors movable at least horizontally within adjacent sets of parallel tracks. A device used to control the air flow rate through a fume hood, covering a portion of the opening when the sash door is selectively positioned along the parallel tracks, and showing the horizontal position of each sash door. In a device for providing position indication signals: electrical switching means attached to each of the sets of trucks,
Switching, each adjacent to and parallel to each set of tracks, including elongated electrical resistance means having a predetermined resistance value per unit length and switching to ground at individual switch points that are changeable along the length. Means; actuator means attached to each of said sash doors and moving together with said sash doors, each actuator means positioned to operate said switching means at one individual switch location; and attached to each said switching means Circuit means for providing a substantially constant current through the resistance means portion between at least one end of said resistance means and the nearest switch location, at least one of the ends of said resistance means and the switch location closest to said end. A circuit means for providing a voltage signal having a magnitude proportional to the distance between and. 14. Two sash doors are moveable within a set of trucks, said circuit means providing a substantially constant current through the resistance means portion between the ends of said resistance means and the nearest switch location, 14. An apparatus according to claim 13, whereby the circuit means provides a voltage signal proportional to the distance between each end of the resistance means and the nearest switch location. 15. The fume hood has at least four sash doors, at least two sash doors moveable within each set of trucks, each of the circuit means being closest to both ends of the resistance means. 15. A substantially constant current is provided through a portion of the resistance means between and so that the circuit means provides a voltage signal proportional to the distance between each end of the resistance means and the nearest switch location.
The described device. 16. The apparatus of claim 15 wherein one of said actuator means is attached to each sash door at a predetermined known location along the width of said sash door. 17. The apparatus of claim 15, wherein each set of trucks is located within a common frame means, the frame means being vertically movable with respect to the opening of the fume hood.