The presence of particulate is in many situations at least a nuisance and at worst catastrophic or illegal. Particulate can carry impurities into locations where its presence is undesirable. Such locations include industrial plant and the environment including air quality monitoring. Thus in the manufacture for example of electronic components the presence of particulate can lead to impurities being included inadvertently in the component, for example, a chip, so rendering the component faulty. Many testing procedures are carried out in areas which are intended to be sterile and particulate free; the presence of particulate may lead to test results from such contaminated areas worthless. In a hospital environment, for example, it is necessary that many wards and, in particular, operating theatres, be kept essentially particulate free so that conditions be maintained as sterile as possible. In electricity generating stations, for example, particulate in the inlets to turbines must be kept to a minimum in order to reduce particulate build up on the turbine blades; such build up has to be removed, generally by water spraying, or, if not carried out, leads to a reduction in turbine performance and ultimately blade disintegration with obvious destructive results. In either event, generating time is reduced.
Particulate free conditions in the examples given above should exist in the inlet of gas, often air, into the relevant area. However particulate should not be fed through the outlet of an area. For example, exhaust from power stations, industrial processing including chemical plant processes, should not emit particulate into the atmosphere. Such a practice is environmentally unacceptable and particulate emissions must be kept within approved maximum or legal limits.
Particulate entering or leaving an area is generally reduced by the use of a range of abatement systems, often located in a duct through which gas is supplied to an area or removed from an area. Such abatement systems include, for example, filters, combination of filters, electrostatic precipitators, wet arresters. If the abatement system has been fitted incorrectly or erroneously or in time the abatement system degrades, the efficiency of the abatement system in reducing particulate passing through the abatement system is reduced. It is common practice to replace an abatement system after a given period which is determined by experience, of acceptable abatement system performance. It is also found however that an abatement system may fail catastrophically before that period has been exhausted and allow unacceptable passage of particulate through the filter system. This is a particular problem where, for example, the gas flow is very high or where the abatement system comprises a set of filters and one filter in the set should prematurely fail.
Presently available particulate detectors only detect the presence of particulate in a very narrow band or area in a duct. In one such detector, a conductive rod is located in a duct and any increase in charge caused by charged particulate passing close to or striking the rod induces a current in the rod which is detected. Such a detector measures particulate over a very small volume of the duct and the rod acts as an obstruction to gas flow in the duct. In another detector, a very narrow, pencil like beam of light is directed across the duct and opacity measured over a single cross duct measurement is made. In yet another detector, a very narrow beam is projected into a duct and a detector is focussed on a small portion of the beam, typically 15 cm, and reflection back from the portion of the beam monitored. All these detectors measure presence of particulate over a very small cross sectional area of a duct and presence of particulate over more of the cross section can only be estimated by extrapolation and are wholly reliant on the assumption that turbulence is present, which causes an even distribution of particulate across the cross sectional area of the duct which is not always the case, especially where true laminar flow exists.
There is therefore a requirement for an apparatus and a method for detecting of particulate suspended in gas in such ducts over the cross section of the duct. Detecting of particulate on a regular basis also leads to continuous or regular monitoring of a duct so as to detect the presence of transient particulate above its normal zero or low acceptable level and the apparatus of the invention provides such a facility. In these ducts, the normal level of particulate is essentially zero (i.e. particulate free) or at a very low and acceptable level when the abatement system is performing efficiently. The apparatus of the invention avoids an assumption that a small sample of the cross section represents the whole of the cross section
Other detectors are known such as those described in PCT Application 03/012375 where quantitative measurement of large concentrations of particulate in a gas environment are to be determined, preferably by microwave technology, in pipework; however such detectors, and those in the patent publication and patent specification described below, only sample selected cross sectional portions of the pipework and assume approximate homogeneity of particulate concentration therefrom. Accordingly such a detector does not function where such approximate homogeneity does not exist. Furthermore, such a detector does not provide monitoring of low levels of particulate. In U.S. patent publication 2002/0105645 A1 there is described an apparatus for monitoring particulate in water or oil; however the apparatus provides only point to point measurement in pipework. The apparatus of U.S. patent specification No. 6,055,052 measures particle size, and not unexpected presence of particulate in air within pipework. The problem solved and information gathered from the apparatus of the foregoing patent specifications are wholly different from the problem to be solved by the apparatus and method of the present invention.