GB2382669A - Personal air sampling system with flow rate control means - Google Patents

Abstract

A personal air sampling system comprises a pump 58 for drawing air through a sampling device, a flow restriction downstream of the pump 58, a transducer for measuring a pressure drop across the restriction and producing a corresponding signal, input means to allow a user to set a desired flow rate and means for controlling the pump 58 to produce a pressure drop across the restriction which corresponds to the desired flow rate. The system may also include means to calculate the actual flow rate, and a display means which is able to display the actual flow rate. The system may also include means for alerting a user if the actual flow rate differs from the desired flow rate by more than a predetermined amount. The local restriction may be an adjustable needle valve, and the system may further comprise pulsation dampers 60, 62 and a temperature sensor 64. The pump 58 may be a diaphragm pump.

Description

Personal air sampling system and pump for use therein This invention

relates to a personal air sampling system, and a pump for use in the system.

Personal air sampling systems typically are carried by workers exposed to dusty s or otherwise contaminated atmospheres. In such a system a pump develops a partial vacuum to pull air through a filter or other device which collects airborne particles or otherwise traps or senses a contaminant in the atmosphere. Provided that the volumetric flow rate of air through the device is known, operation of the system for a defined period of time yields information on the concentration of lo contaminant in the atmosphere to which the worker has been exposed.

However, with the passage of time, particulate material collects on the filter and increases its resistance to flow, with the result that the flow rate decreases unless the power input to the pump is increased. A known method of doing this (Peck US 5107713) makes use of an empirical relationship between motor speed and 15 flow rate. Another (Betsill US 5163818) maintains a constant motor speed by varying the motor voltage. Both of these methods however depend on the very indirect indication of flow rate afforded by motor speed. The present invention in one aspect seeks to provide a method of control more closely related to actual flow conditions.

20 In one aspect the invention provides an air sampling system comprising a pump for drawing air through a sampling device, a local flow restriction downstream of the pump, a transducer for measuring a pressure drop across the restriction and for producing a signal indicative thereof, input means for enabling a user to set a desired flow rate, and control means containing a correlation between the 25 pressure drop and the volumetric flow rate of air through the restriction, the control means being configured to control the pump to produce a said pressure drop corresponding to the desired flow rate set by the user.

There may be means for calculating the actual flow rate, and there may be further means for alerting a user if the calculated actual flow rate differs from the flow rate set by the user by more than a predetermined amount.

The system may comprise a display, preferably integral with the pump, for s displaying the actual and/or desired flow rate.

The input means may be integral with the pump. By "integral" we mean incorporated into the same housing or casing.

The local flow restriction may be adjustable; it can for example be a needle valve.

An adjustable restriction allows the pressure generated by the pump (and hence 10 its power consumption) to be minimised for a given flow rate; typically, personal sampling systems are set to an exact flow rate before use. In known systems this has been by calibration using an external reference flow meter or a bubble meter, whereas in the preferred embodiments of this invention the user can set the required flow rate without the need for additional equipment.

15 Alternatively the local flow restriction may be fixed, eg. a pin-hole orifice or a porous plug eg. of a gauze material.

A pulsation damper (eg. a cavity with a wall formed of a flexible membrane) may be provided upstream and/or downstream of the pump.

The system may include a temperature sensor, for sensing the temperature of the 20 air passing through the restriction, the control means adjusting the said correlation according to the sensed temperature.

The temperature sensor may be located anywhere in or close to the airflow through system, but conveniently may be in the downstream pulsation damper.

As a personal sampling system is worn by the user, the size and weight of the pump are important. Also, the efficiency of the pump (how much electrical power is required to drive the motor in order to produce a given flow) is important because the sampling pump is powered by batteries and if the pump is more efficient then the same run-time may be achieved using smaller, and hence lighter, batteries.

Thus preferred embodiments of the invention use a pump in which high output is combined with compact dimensions.

The preferred forms of pump (aspects of which are novel per se) has a pumping lo chamber having a displaceable wall, a motor, and drive means driven by the motor for reciprocating the wall, the wall having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.

The major dimension of the motor may be parallel to an axis of rotation thereof.

15 There may be two pumping chambers, each with a displaceable wall disposed on opposite sides of the motor.

The drive means may comprise an eccentric mounted on a shaft of the motor.

Each wall may have semicircular ends joined by straight sides.

The or each wall preferably may be a diaphragm.

20 The or each diaphragm may comprise a membrane connected around its edge to a wall defining the remainder of the chamber by a roll section. This can enable the entire membrane to be moved bodily through a distance equal to the reciprocating stroke of the drive means thereby maximising the volume of air displaced by each stroke.

The invention now will be described merely by way of example with reference to the accompanying drawings, wherein: Figures 1 A and 1 B are longitudinal sections through two pumps according to the invention; S Figure 2 is a cross-section on line A-A through the pump of figure 1 B; Figures 3A and 3B show a diaphragm of the pump of figure 1 B. figure 3B being a section on line A-A of figure 3A; Figure 4 shows diagrammatically a personal air sampling system according to the invention; and o Figures 5 and 6 show a prototype production version of a system according to the invention. Referring to figures 1A, 1B and figure 2, a pump according to the invention comprises a generally cuboid body 10 defining one (figure 1A) or two (figure 1 B) pumping chambers 12. Each pumping chamber is defined by walls 14 of fixed 15 structure and a displaceable wall 16 formed by a diaphragm having a peripheral semicircular section roll 18 the outer edge of which is anchored between outer 20 and middle portion 22 of the fixed structure.

The fixed wall 14 of the chamber includes outlet 24 and inlet 26 nonreturn valves, formed of discs which are lightly sprung-biased against respective valve seats.

20 The inlet valve(s) communicate(s) with an inlet plenum chamber 30, and the outlet valve(s) communicate with an outlet plenum chamber 28. These chambers in turn communicate with inlet and outlet ports of the pump (not shown).

Attached to the centre of the diaphragm 16 by upper and lower plates 33 of the same shape as but smaller than the diaphragm is a yoke or connecting rod 32 25 having a laterally extending slot 34. In this slot is disposed an circular cam 36 eccentrically mounted on the shaft 37 of a motor 38 which is mounted within the

body 10. Operation of the motor 38 rotates the eccentric 36 which reciprocates the yoke 32 vertically with a stroke twice the offset of the centre of the cam 36 relative to the motor shaft.

The yoke thereby displaces the or each diaphragm 16 relative to its chamber in 5 the manner of a piston, the roll section 18 permitting the central portion of the diaphragm to move as a whole relative to its fixed edge. The cyclic change in volume of the chamber 12 thus is maximised for each stroke. Enlargement of the chamber 12 causes the inlet valve 26 to open and admit air. Upon reversal of the diaphragm movement the inlet valve closes and the pressure in the chamber 10 opens the outlet valve 24 permitting the air in the chamber to be expelled.

Referring to figures 3A and 3B the diaphragm 16 is a single piece of elastomeric material such as silicone rubber having around its edge an approximately semicircular roll section 18 and a peripheral lip 40 which is received in a corresponding groove in the portions 20, 22 of the pump body. The diaphragm 15 thus is firmly clamped around its edge in an air tight manner. A central hole 42 permits a fixing eg. a screw to pass through the diaphragm and clamp the diaphragm between the plates 33 to the yoke 32. Alternatively two locating holes eg. at opposite ends of the major axis 48 of the diaphragm may be provided to improve mechanical alignment during assembly.

20 The diaphragm is generally oval in shape, having semicircular ends 44 joined by straight sides 46. The diaphragm is fixed to the yoke 32 with its major (longer) axis 48 parallel to the shaft of the motor 38. The minor axis 50 of the diaphragm is not significantly greater in length than the diameter of the motor 38. Thus the diaphragm does not project sideways materially beyond the motor, and the 2s thickness of the casing 10 is kept within reasonable limits.

A conventional diaphragm-would be circular in shape, with the result that if it were of diameter equal to the minor axis 50 of the figure 3 diaphragm it would be of only small area, and the pump output would be limited. Conversely, if it were circular of diameter equal to the major axis 48, the pump output would be greater 30 but the thickness of the casing would be undesirably increased.

The diaphragm of figure 3 provides an advantageous compromise: by having an extended major axis 48, a usefully larger diaphragm area is achieved without an increase in the thickness of the casing 10.

The personal air sampling system of figure 4 comprises a sampling head provided s with a filter 56. Air is drawn from an inlet 54 through the filter by a pump 58 so that airborne particulate matter is captured on the filter for subsequent measurement and analysis. The pump 58 preferably is (but need not be) as previously described. A pulsation damper 60, in the form of a vessel or cavity with a flexible elastomeric wall, smooths fluctuations in the partial vacuum applied 10 to the filter 56 by the pump 58. Without this damper, variations in air velocity through the filter may cause particles to be dislodged from the filter material.

Other types of sampling head, eg. a size-selective device such as a cyclone, may be subject to increased error if operated in an air flow which is not smoothed by a pulsation damper.

15 A further pulsation damper 62 smooths pressure pulses in the flow of air output from the pump, in order to improve the accuracy of flow measurement and control. A temperature sensor 64 may be incorporated into the damper 62 or elsewhere in the circuit to enable a microprocessor 66 (discussed further hereafter) to compensate for variations in temperature of the air.

20 The pulsation damper 62 exhausts to atmosphere via a local flow restrictor 68.

Preferably this is an adjustable valve such as a needle valve. By "local" we mean the restrictor is of only small extent in the flow direction, thereby reducing the possibility of a difficult-to-clear blockage. Altematively it can be a fixed local restrictor such as a pin-hole orifice plate or a porous plug of gauze or similar filter 2s material.

A pressure sensor 70, typically of a silicon micro-machined type, measures the pressure at the inlet to the restriction 68 relative to atmosphere and thus, since the restrictor discharges to atmosphere, the pressure drop across it. The sensor provides an analogue electrical signal representative of that pressure difference.

The signal is passed through a signal conditioning circuit 72 and thence to an analog to digital converter 74. The digital signal is supplied to the microprocessor 66, which is provided with a display 67.

The microprocessor compares the digital signal with a previously-stored target s value equivalent to a desired flow rate and generates a pulse width modulated (PWM) signal to the motor drive circuitry. The PWM signal is controlled within a software control loop such that the motor speed is controlled to maintain a constant signal from the pressure sensor 70 and hence a constant flow rate.

The initial values relating pressure drop to flow rate are stored in the 10 microprocessor's memory and are established during an initial calibration routine.

This initial procedure involves measuring the downstream pressure at two different flow rates and calculating all other values of pressure at the differing flow rates. The basic calibration curve fits a quadratic equation of the form: Pressure = A* (flown B*(flow) 15 where A and B are coefficients obtained from the two initial calibration points.

These calculated values of pressure for given flow are approximate. As operating standards for the use of personal air sampling systems require routine calibration of the system using a flow tube or bubble meter, the calculated value at any particular calibration point may be overwritten by exact values. We have found 20 that in a prototype device, after integral calibration and subject to subsequent infrequent recalibration, the user can select any flow within the operating range with an accuracy typically within 4%.

When an adjustable valve is employed as the local restriction 68, the load on the pump can be minimised. For high flow rates, the valve is opened more than for 25 low flow rates. Thereby the back pressure on the pump, and thus its power requirement is reduced. The microprocessor is recalibrated when the valve 68 is adjusted, so that it operates with the appropriate constants A and B in the

quadratic calibration curve.

Referring to figures 5 and 6, a system according to the invention is housed in a casing 80 having on its reverse side a clip enabling the device to be fixed to a user's belt or pocket. The casing prominently features the display 67 which when the system is in a standby mode displays at 82 the required flow rate, and when in operation displays the actual flow rate. Keys 84 permit the required flow rate to be set.

Within the casing is a printed circuit board (PCB) upon which the display 67 is set, together with the microprocessor 66 and other electronics, and the pressure 10 sensor 70. The PCB is fixed in the front portion of the case and overlies the pump and other items of the flow circuit, which are disposed in the top part of the rear casing 86. The lower part consists of a battery pack (not shown).

Referring to figure 6, the inlet 54 is disposed at the top of the casing so that the inlet is less likely to be obstructed by the wearer's clothing. The inlet air passes 15 through the filter 56 to the inlet pulsation damper 60, which comprises a sealed cavity having two sides formed of an elastic membrane material 90. Improved damping performance may be achieved by optionally separating faces for the membrane using an internal spring 91 which may be a compression spring eg. a helical spring as illustrated, or any other suitable spring eg. a bow spring.

20 The inlet valves 26 of the pump communicate with the damper 60 via passageways moulded into the casing. The outlet valves 24 are connected by similar passageways to the downstream pulsation damper 62 which has an elastic wall or walls 92. From the damper 62 the air passes to the adjustable restrictor 68. In this example the restrictor is a needle valve, the opening of which is 25 adjustable by a screw 94. The flow passing through the needle valve exhausts immediately into the interior of the casing at 96, and thence through apertures in the casing to atmosphere.

A pressure tapping is taken from immediately upstream of the needle valve and thence via a pipe 98 to the pressure sensor 70. The end 100 of the pipe 98 projects towards the pressure sensor and is connected to it on the circuit board before the front and rear halves of the casing are fitted together. Because there s is negligible back-pressure presented to the needle valve by the exit of the flow through the casing the pressure relative to atmosphere sensed by the sensor 70 is effectively the pressure difference across the needle valve.

Each feature disclosed in this specification (which term includes the claims)

and/or shown in the drawings may be incorporated in the invention independently lo of other disclosed andlor illustrated features.

Statements in this specification of the "objects of the invention" relate to preferred

embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

The text of the abstract filed herewith is repeated here as part of the specification.

15 In a personal air sampling system, flow control is achieved by monitoring the back pressure created by a local flow restriction (eg. an adjustable valve) located at the sampler's outlet. The pressure drop across the restriction is monitored by a pressure sensor and is directly related to inlet flow rate. The use of an adjustable restriction on the outlet provides increased dynamic range of the flow 20 measurement system without excessive loading to the pump. A signal conditioning circuit takes the signal from the pressure sensor and feeds it to an analogue to digital converter which in turn feeds it to a microprocessor. The microprocessor controls the pump drive circuitry such that a constant inlet flow rate is maintained. The user can select the desired operating flow rate to an 25 accuracy such that an additional external flow measuring device is not required.

A pump of the system makes use of non-circular diaphragms within the pump assembly to provide a compact, space saving design. Both upstream and downstream dampers are employed to reduce pulsation in the air flow.

Claims (1)

1. An air sampling system comprising a pump for drawing air through a sampling device, a local flow restriction downstream of the pump, a transducer for measuring a pressure drop across the restriction and for 5 producing a signal indicative thereof, input means for enabling a user to set a desired flow rate, and control means containing a correlation between the pressure drop and the volumetric flow rate of air through the restriction, the control means being configured to control the pump to produce a said pressure drop corresponding to the desired flow rate set by the user.

10 2. A system as claimed in claim 1, further comprising display means.

3. A system as claimed in claim 2, wherein the display means is integral with the pump.

4. A system as claimed in any one of claims 1 to 3 further comprising means 15 for calculating the actual flow rate.

5. A system as claimed in claim 4, wherein the display means is arranged to display the actual flow rate.

6. A system as claimed in claim 4 or claim 5, further comprising means for alerting a user if the calculated actual flow rate differs from the flow rate set 20 by the user by more than a predetermined amount.

7. A system as claimed in any preceding claim, wherein the input means is integral with the pump.

8. A system as claimed in any preceding claim wherein the local flow restriction is adjustable.

25 9. A system as claimed in claim 8 wherein the local flow restriction is a needle valve.

- 11 10. A system as claimed in any preceding claim comprising a temperature sensor for sensing the temperature of the air passing through the restriction, the control means adjusting the said correlation according to the sensed temperature.

s 11. A system as claimed in any preceding claim comprising a pulsation damper downstream of the pump.

12. A system as claimed in claim 11 wherein the temperature sensor is located in the downstream pulsation damper.

10 13. A system as claimed in any preceding claim comprising a pulsation damper upstream of the pump.

14. A system as claimed in any preceding claim wherein the pump comprises a pumping chamber having a displaceable wall, a motor and drive means driven by the motor for reciprocating the wall, the wall having a major 15 dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.

15. A system as claimed in claim 14 wherein the major dimension of the motor is parallel to an axis of rotation thereof.

16. A system as claimed in claim 14 or claim 15 wherein there are two 20 pumping chambers, each with a displaceable wall disposed on opposite sides of the motor.

17. A system as claimed in any one of claims 14 to 16 wherein the drive means comprises an eccentric mounted on a shaft of the motor.

18. A system as claimed in any one of claims 14 to 17 where the or each wall 25 has substantially semicircular ends joined by straight sides.

19. A system as claimed in any one of claims 14 to 18 wherein the or each wall - is a diaphragm.

20. An air sampling system substantially as herein described with reference to the accompanying drawings.