GB2291195A - Monitoring personal dust exposure - Google Patents

Abstract

An integrated dust monitoring system comprises a miniature passive electret ionisation chamber to be worn by individuals, and a dedicated computerised electronic electret potential reader unit which non-invasively interrogates the chamber and measures the personal exposure of the user to all or particular species of dust. The chamber is planar and miniature and contains 3 or more electret films which have been charged to stable high voltages. These form a complex electric field distribution within the chamber which promotes the flow of air and which selectively attracts ionised and unionised dust particles to the respective electrets. A non-invasive measurement of surface potential and distribution enables the level of dust exposure to be calculated for several different species independently. The system may find application in industry and in the avoidance of allergies. <IMAGE>

Description

PATENT APPLICATION "A Miniature Electret lonisation Chamber and Reader System for the Discrimination and Monitoring of Personal Dust Exposures" This invention relates to a system, comprising a passive miniature ionisation chamber, and a dedicated microprocessor-controlled reader, which is capable of providing the user with a personal measurement of the levels of various types of dust that he or she may be exposed to during specific periods of time.

Dust, in its various forms, especially when present in the work-place, is now established as a major cause of industrial diseases of the chest including bronchitis and lung cancer. There is increasing evidence that asthma attacks may be triggered by various polluting dust particles whether organic or inorganic. A reduction of personal dust exposure has therefore become a corner-stone of Health and Safety legislation both in the UK and in Europe in general. There is a need for new and inexpensive monitoring technologies.

It is unfortunately the case that dust levels can vary significatly in every working environment not only with respect to time but also with respect to particle density and species. The result is that general monitoring of dust levels in rooms fails to provide the individual with the definitive personal exposure that might allow correlations between dust dose and the onset of breathing diseases to be made.

Electrets are thin films of insulating plastics on which a high surface potential has been established by one of several methods including corona charging. They can be used to produce a high electric field in a chamber without the need for electronic components. Electret ionisation chambers (elms) have been used to measure the cumulative dose of gamma and x-radiations for a number of years; more recently the inventor of the present system produced a miniature electret ionisation chamber (MEIC) which was shown to be capable of performing similar measurements in a small badge thanks to the use of electret films with stable, very high surface potentials.

Cylindrical electret ionisation chambers have been employed to measure the ionic content of the environment using a fan to draw the air through the active chamber.

Electrets have been used in various configurations as a filter of small particles but have not previously been used to provide personal dust monitoring because: (i) dust may not always carry a charge (ii) personal ionisation chambers need to be small and of a form which allows their non-intrusive use by the individual (iii) personal ionisation chambers should be small and flat and should not contain electronic components because of the possibility of the expiry of the battery, and (iv) the flow-rate required for consistent and accurate air sampling may be insufficient without the use of mechanical means such as a fan.

High voltage electret ionisatibn chambers are now being employed for the measurement of radon gas levels in specific rooms in buildings and in water supplies; they rely on the natural air exchange between the chamber and the environment to provide the radon molecules which will eventually decay inside the chamber to provide the air ions which will neutralise the electret charge and which will enable the measurement of radon gas to be calculated..

The present invention employs similar principles to measure the personal dust levels to which individuals are exposed. Dust particles are normally charged triboelectrically and this results in different particles carrying a different charge. Some dust particles are more likely to be the cause of specific ailments than others and it is therefore necessary to distinguish between different types of dust. It has been found that some dangerous airborne contaminants carry a net positive charge while others carry a negative charge. Neutralisation may also occur when both species are present with the result that a simple charge measurement system using the electret principle may underestimate the level of dust.

The Desian & Operation of the Electret Dust Monitorina Chamber (or Badae) The present invention overcomes these deficiencies by using a triple electret as shown in Figure (a). The film strip of polytetrafluoroethylene or FEPTTFE copolymer is electroded with evaporated aluminium (or another suitable conductor) on the underside of the two outer sections label led 1 and 3 respectively. The top side of the central region (labelled 2 is electroded). One of the outer regions and the central region are charged to a positive potential while the other region is charged to a comparable negative potential in such a way that the charges are long-lived and stable.

In the ionisation chamber (Figure b), the evaporated aluminium electrodes are connected to the body of the chamber (labelled 4) which forms the ground for the system. This is insulated from the wearer by a non-conducting sheet of plastic which is attached to the clothing of the wearer in a manner which prevents rubbing and triboelectric charging. Air is admitted through the holes (5) at one end of the chamber and leaves through the holes (6). Filters are provided to ensure that large particles are not admitted to the chamber.

The electric field within the chamber will be in the general directions shown in Figure c and will sustain the movement of air through the chamber. The attraction of dust particles to the electret films will depend on the polarity and magnitude of the charges and also on their relative masses. Thus, highly negatively charged and lightweight dust particles will be attracted to the nearest side of the electret (marked x in Figure c). Heavier particles (and those with a smaller charge to mass ratio) will land further in to the electret film while those with zero charge may land in the central (electroded and grounded) region. Similarly, positively charged particles will be attracted to the negatively charged electret. Thus, a distribution of dust particles will be collected and these will modify the surface charges on the electrets according to the charges carried by the dust.It may be evident that a measurement of the distribution of dust on the electret films may provide information on both the type and the relative distribution and level of dust contamination in the environment. It may also be the case that other more complex electret arrangements may be configured to provide greater particle discrimination. The dust collected on the central (electroded) electret may not neutralise the charge on that electret but may provide an added mass to the film which will reduce its efficiency as an electret microphone in the presence of an exciting pressure wave.

The electrets may be replaced after high exposure to dust.

The Measuring Technoloav In order to provide a personal record of dust exposure it is essential that the user can read his or her electret badge simply and as often as necessary by an automatic method which is accurate and which records the dose history. In this invention, this is achieved using an adaptation of the field nulling method. Figure d shows a mechanically vibrating plate above the electret film. An a.c. signal is induced in this plate provided that an electric field exists between the plate and ground. The magnitude of the induced signal is dependent on the surface potential and on the respective separations of ground, the electret and the vibrating plate. This leads to potentially large inaccuracies in the estimate of surface potential, especially when non-uniform charge distributions are involved.By ramping the potential of the ground or of the vibrating plate it is possible to null the field between the plate and the electret; the value of the ramped voltage corresponds directly to the surface potential of the electret irrespective of the dimension tolerances involved.

In our arrangement, the vibrating head will contain three separate conducting zones insulated from each other (see Figure e), one covering each of the three electret zones shown in Figure a. The outer two parts of the head will be manufactured from a metallic gauze which will ensure that no mechanical perturbation occurs; the central region will be solid so that it may produce a longitudinal wave capable of creating a measurable electret-microphone type response. The voltage ramp will be generated by a microcomputer which will also read the magnitude of the induced voltage. From the minima of the two induced signals it will be possible to determine the surface potentials of the positive and of the negative electrets respectively and, by comparison with previous data, determine the dust exposure since the previous measurement. The activity of the electret microphone can also be compared to previous activity in order to determine the level of damping of response which will depend on the dust mass collected.

If the surface potential on the electret films was uniform then the induced a.c. signal at the nulling potential would be zero. In practice, the surface potential will vary across the film surface according to the distribution of the sizes and charges of the dust particles. This distribution can be calculated from the width and the level of the detected null using purpose-designed software, enabling the dust monitoring to provide further and valuable information.

The measuring head will be hand-held and remote from the electronics and microcomputer and display. It will be capable of entering the ionisation chamber without contacting the electrets and will interface to the measurement unit through appropriate electrical connections.

Claims (10)

1) A system comprising a miniature passive electret ionization chamber and a purpose-designed electronic reader unit capable of measuring an individual's exposure to dust.

2) A system as described in claim 1 including a miniature passive electret ionization chamber which contains three or more separately charged electret elements.

3) A system as described in claim 1 which employs electrets with stable high surface potentials.

4) A system as described in claim 1 with a miniature passive electret ionization chamber in which dust particles in the air are subjected to a complex electric field distribution.

5) A system as described claim 1 which measures either or both positive or negativeky charged dust concentrations through the reduction of electret surface potential.

6) A system as described in claim 1 which measures the level of uncharged (or neutralised) dust using the reduction of sensitivity of an electret microphone.

7) A system as described in claim 1 which can measure the relative charge to mass ratio of dust species.

8) A system as described in claim 1 which employs the field-nulling technique to measure electret surface potential accurately and non-invasively.

9) A system as described in claim 1 which uses an adaptation of the field-nulling technique to estimate dust mass/charge distribution.

10) A system as described in claim 1 which employs a computerised control and analysis system for the calculation and recording of dust data and for displaying dust levels to the user.