GB2119932A

GB2119932A - Low pressure sensor for monitoring respiration

Info

Publication number: GB2119932A
Application number: GB08311853A
Authority: GB
Inventors: Richard Anthony Riley
Original assignee: BRITISH IND BIOLOG RESEARCH AS
Current assignee: BRITISH IND BIOLOG RESEARCH AS
Priority date: 1982-05-07
Filing date: 1983-04-29
Publication date: 1983-11-23
Also published as: GB2119932B; GB8311853D0

Abstract

A low pressure sensor comprises a compartment, defined by a housing (1) and a closure piece (2), which, in use, is in fluid communication with a fluid whose pressure is to be sensed. The compartment has a latex wall (5) which flexes in response to pressure changes in the compartment. A Hall effect sensor (17) is provided for connection in an electrical detection circuit. A permanent magnet (14) is coupled to the latex wall (5) and the Hall effect sensor (17) is positioned so that the density of the magnetic flux generated by the magnet (14) and sensed by the Hall effect sensor (17) varies in response to flexure of the latex wall (5), the Hall effect sensor producing a corresponding voltage change in the electrical circuit. The sensor can be coupled to a housing containing an animal or to bellows strapped to the chest of an animal or human being for monitoring respiration. <IMAGE>

Description

SPECIFICATION Low pressure sensor The invention relates to a low pressure sensor.

In many scientific applications, particularly monitoring human or animal respiration, it is necessary to sense low (i.e. small) pressure changes typically occurring at atmospheric pressure. A wide variety of pressure sensors are already known but those which can be used for detecting low pressure changes are very expensive and complex. Furthermore, known pressure sensors normally have to be positioned near to the area of interest. This is often difficult to achieve and is particularly undesirable when that area is potentially hazardous to the operator.

In accordance with the present invention, a low pressure sensor comprises a compartment which, in use, is in fluid communication with a fluid whose pressure is to be sensed, the compartment having a wall part which flexes in response to pressure changes in the compartment; a Hall effect sensor for connection in an electrical circuit including detection means; and magnetic means, one of the sensor and magnetic means being coupled to the wall part and the other being positioned so that the density of the magnetic flux generated by the magnetic means and sensed by the Hall effect sensor varies in response to flexure of the wall part, the Hall effect sensor producing a corresponding voltage change in the electrical circuit.

This sensor is particularly simple in construction and therefore low in cost and may also easily be adapted for use at a position remote from the area of interest simply by utilising a probe in fluid communcation with the compartment. The use of a Hall effect sensor simplifies considerably the electrical circuit needed while allowing high rates of change of pressure to be sensed. In principle, the sensor is capable of detecting pressure change rates of up to 100 kHz, this figure being determined by the sensitiv ityofthe Hall effect sensor.

Conveniently, the Hall effect sensor has a differential output so that one output increases linearly in voltage whil the other decreases for a linear increase in magnetic flux density.

Preferably, the sensor is sensitive to pressure changes down to 1 mum of water at a mean pressure of one atmosphere. The sensitivity can be varied by adjusting the surface area, tension and/or thickness of the wall part and by selecting a suitable material forthewall part.

Preferably, the wall part is made of latex and/or may be planar. A planar wall part is preferable to a corrugated wall part since a more accurate response is obtainable.

Preferably, the degree of flexure of the wall part is substantially the same for rates of change in pressure up to at least 250 per minute. This is one of the main advantages of this invention since it enables a fairly constant response to be obtained over a wide range of rates of pressure change.

The magnetic means preferably comprises a permanent magnet coupled to the wall part. This is a convenient construction since the Hall effect sensor can be incorporated into a fixed printed circuit board mounted adjacent the permanent magnet. It is particularly convenient if the permanent magnet is coupled directly to the wall part so that wall part flexure is directly transduced into changes in magnetic flux density.

The sensitivity of the sensor is most conveniently maximised by coupling the magnetic means to that part of the wall part which undergoes the greatest flexure for a given change in pressure.

One important application of the sensor is in apparatus for monitoring animal respiration, the apparatus comprising an animal body plethysmograph; a sensor in accordance with the invention, the sensor compartment being in fluid communication with a body section of the plethysmograph; and an electrical circuit including detection means connected to the Hall effect sensor to detect the sensed changes in air pressure in the body section of the plethysmograph.

Typically, a plethysmograph comprises a housing divided into a head section and a body section which are isolated from each other. When an animal placed in the plethysmograph respires, changes in air pressure occur with each breath, this variation in air pressure being sensed by the sensor. The sensor in accordance with the invention is particularly suited for this application since the absolute pressure is typically atmospheric and the rate of change in pressure is normally high. For example, a rat normally respires at between 100 and 150 breaths per minute while a mouse whill respire at between 240 and 250 breaths per minute.

In another example, apparatus for monitoring human or animal respiration comprises a sensor in accordance with the invention, a belt connected to the sensor for securing the sensor to a human or animal body; electrical circuitry including detection means connected to the Hall effect sensor; and a bellows connected to the sensor, the bellows engaging the human or animal body in use and arranged such that movement of the body in response to respiration is transduced by the bellows into pressure changes in the sensor compartment.

This apparatus is particularly applicable for monitoring the respiration of large animals or humans and the use of a bellows provides a convenient way in which to transduce body movements into pressure changes in the sensor compartment.

Preferably, the electrical circuitry includes an alarm and timing means, the timing means actuating the alarm when no pressure change is detected after a preselected time interval. This is particularly useful for monitoring a baby's breathing rate so that if the baby should stop breathing for a preselected time interval, for example 10 seconds, an alarm will be sounded.

Some examples of sensors and monitoring apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which :- Figure 1 is a side elevation, partly in section, of one example of a sensor; Figure2 illustrates a portion of an electrical circuit for use with the sensor shown in Figure 1; Figure 3 is a side elevation, partly in section, of an animal plethysmograph for attachment to the sensor shown in Figure 1; and, Figure 4 is a partial cross-section through apparatus for monitoring human or animal respiration.

The sensor illustrated in Figure 1 comprises a plastics or perspex hollow cylindrical body 1 closed at its lower end by a perspex or plastics closure piece 2 bonded to the body portion 1. The closure piece 2 has a central aperture 3 which is connected to an elongate probe 4 partly shown in Figure 1.

The other end of the body portion 1 is covered by a latex sheet 5 providing a flexible wall part which is secured in position by an "O"-ring 6 located in a groove 6a in the portion 1. The outer diameter of the body portion 1 is about 40 mm while the inner diameter is about 30 mm, the height of the body portion 1 together with the closure piece 2 being about 50 mm. A typical thickness for the latex sheet 5 is 0.02 inches (0.5 mm).

A perspex support block 7 is cemented to the body portion 1. The support block 7 has a bore 8 in which is located a guide pin 9 and a second bore 10 positioned, in use, vertically above the bore 8. The bore 10 is internally screw threaded. An "L"-shaped brass arm 11 having an elongate slit (not shown) is mounted on the support block 7. The guide pin 9 and a clamp nut 12 extend through the slit in the arm 11 so that the arm 11 is movable vertically, as seen in Figure 1, but may be secured in any desired position by tightening the clamp nut 12. The clamp nut 12 and the guide pin 9 together prevent the arm 11 from tilting.

The arm 11 supports a downwardly extending tubular guide 13 into which extends a permanent magnet 14. The permanent magnet 14 is glued to the outer surface of the sheet 5. The arm 11 also supports a mounting block 15 to which is cantilevered a printed circuit board 16 including a Hall effect sensor 17. The Hall effect sensor 17 is arranged to be directly in line with the permanent magnet 14.

A suitable Hall effect sensor 17 is supplied by R.S.

Components Limited of 13-17 Epworth Street, London under the stock number 304-267.

The Hall effect sensor 17 is connected in an electrical circuit part of which is shown in Figure 2.

The sensor 17 has four leads 18,19,20,21. The leads 18, 19 are connected between a 4-10 volt dc supply while output signals from the sensor 17 are fed in use through the leads 20, 21 to conventional detection apparatus (not shown). The output signals are generated across resistances 22,23 each of which typically has a resistance of 2000 ohms.

In use, the probe 4 is positioned in the fluid whose pressure is to be sensed and which position may be considerably spaced from the sensor position, while the leads 20, 21 are connected to detection means which may be for example a chart recorder or a suitably programmed computer.

One important application of the invention is in monitoring animal respiration and for this purpose an animal, such as a rat or mouse, is placed in an animal plethysmograph such as that shown in Figure 3. The plethysmograph is conventional in form and comprises a housing 24 which is divided into a head section 25 and a body section 26 by a collar assembly 27. The collar assembly 27 comprises an annular collar 28 sealed to the housing 24 by means of a pair of "O"-ring seals 29. A pair of rubber seal rings 30 having central apertures (not shown) are mounted across the central opening of the collar 28 and are secured in position by means of a pair of "O"-rings 31.

An inlet for the passage of air to enabldethe animal to breath is provided at 32, a corresponding outlet (not shown) also being provided.

The body section 26 of the plethysmograph includes a wire mesh tray 33 on which the animal stands and an animal restrainer 34 having a cut out portion 35 for locating the animal's tail. The restrainer 34 is fixed to an adjustable rod 36 which is sealed to the housing 24 by means of a tightening nut 37.

In use, an animal is placed in the plethysmograph with its head in the head section 25, its body in the body section 26 and its neck surrounded by the collar assembly 27. Foam, commonly shaving foam, is injected through a bore 38 in the collar assembly 27 to seal the head section 25 from the body section 26. An opening 39 is provided in the housing 24 in communication with the body section 26 and this is connected with the probe 4 of the sensor shown in Figure 1. Thus, in use, as the animal respires, changes in pressure in the section 26 will occur with each breath. The pressure amplitude is proportional to the volum of inspired and expired air breathed by the animal and is related to the changes in volume occupied by the chest cavity of the animal in body section 26.

The pressure changes will be communicated through the probe 4 to the pressure sensor and consequently cause movement of the rubber sheet 5. This movement will cause corresponding movement of the permanent magnet 14 in the guide 13 and thus the magnetic flux density experienced by the Hall effect sensor 17 will change. This will result in a differential output in the leads 20, 21 from the Hall effect sensor 17 and can be detected by conventional detection means such as an oscilloscope.

In an experiment to test the response of the sensor, the maximum movement of the permanent magnet 14 in Figure 1 was monitored by introducing air samples equivalent to tidal volumes up to 2ml for increasing 'respiration' rates up to above 200 breaths/minute. The maximum movement was found to be substantially constant at about 18 mm for all measured frequencies and for tidal volumes from 0 to 1.8ml. This experiment was carried out with a mean pressure equal to 1 atmosphere.

Figure 4 illustrates another example of a sensor for use in monitoring animal or human respiration. The sensor comprises a perspex housing 1', one wall of which is provided with an opening 40 having an integral tubular portion 41 extending into the housing 1'. A rubber sheet 5' is mounted on the tubular portion 41 by an "O"-ring 6' located in a groove 6a' and a magnet 14' is glued to the sheet 5'. The magnet 14' extends into an opening 42 in a guide 43 mounted in the housing 1',a Hall effect sensor 44 being mounted within the guide housing 43. The sensor 44 is connected to electrical circuitry similar to that shown in Figure 2. A rubber bellows 45 is fixed to the housing 1' while a belt 46 is also connected to the housing 1'.

In use, the belt 46 is fastened around the chest of the human or animal to be monitored with the bellows 45 resting against the chest. When the human or animal respires the bellows 45 will expand or contract appropriately and cause corresponding movement of the rubber sheet 5'. The electrical circuitry can either monitor this movement as in the previous example or can include timing means so that if no movement of the rubber sheet 5' is detected in a preset time interval, for example 10 seconds, an alarm will be sounded.

Typical dimensions for the sensor shown in Figure 4 are: inner diameter of housing 1'-65mm; height of housing 1' - 15 mm; inner diameter of tubular portion 41 - 20 mm.

Claims

1. A low pressure sensor comprising a compartment which, in use, is in fluid communication with a fluid whose pressure is to be sensed, the compartment having a wall part which flexes in response to pressure changes in the compartment; a Hall effect sensor for connection in an electrical circuit including detection means; and magnetic means, one of the sensor and magnetic means being coupled to the wall part and the other being positioned so that the density of the magnetic flux generated by the magnetic means and sensed by the Hall effect sensor varies in response to flexure of the wall part, the Hall effect sensor producing a corresponding voltage change in the electrical circuit.

2. A low pressure sensor according to claim 1, wherein the wall part is made of latex.

3. A low pressure sensor according to claim 1 or claim 2, wherein the wall part is planar.

4. A low pressure sensor according to any of the preceding claims, wherein the magnetic means comprises a permanent magnet coupled to the wall part.

5. A low pressure sensor according to any of the preceding claims, wherein the degree of flexure of the wall part is substantially the same for rates of change in pressure up to at least 250 per minute.

6. A lower pressure sensor according to any of the preceding claims, the sensor being sensitive to pressure changes of 1 mm of water at a mean pressure of one atmosphere.

7. A low pressure sensor substantially as described with reference to the accompanying drawings.

8. Apparatus for monitoring animal respiration, the apparatus comprising an animal body plethysmograph; a sensor in accordance with any of the preceding claims, the sensor compartment being in fluid communication with a body section of the plethysmograph; and an electrical circuit including detection means connected to the Hall effect sensor to detect the sensed changes in air pressure in the body section of the plethysmograph.

9. Apparatus for monitoring human or animal respiration, the apparatus comprising a sensor in accordance with any of the preceding claims; a belt connected to the sensor for securing the sensor to a human or animal body; electrical circuitry including detection means connected to the Hall effect sensor; and a bellows connected to the sensor, the bellows engaging the human or animal body in use and arranged such that movement of the body in re sponge to respiration is transduced by the bellows into pressure changes in the sensor compartment.

10. Apparatus according to claim 9, wherein the electrical circuitry includes an alarm and timing means, the timing means actuating the alarm when no pressure change is detected after a preselected time interval.

11. Apparatus for monitoring human or animal respiration substantially as described with reference to any of the examples shown in the accompanying drawings.