LO ARTIFICIAL: EL PREDOMINIO DEMOGRAFICO Y ECONÓMICO

This discussion is divided into four sub-sections:

1 ) literature review of particulate monitoring techniques (Section 4.5.1.1 ); 2) the particulate monitor for this research (Section 4.5.1.2) ;

3) working principle (Section 4.5.1.3): and 4) justification of selection (Section 4.5.1.4).

4.5.1.1 Literature Review of Particulate Monitoring Techniques

This literature review is aimed at finding the most up-to- date and practical instrumentation for monitoring particulates. Vesilind et al (1988) divide the development of monitoring instrumentation into three generations. The first-generation instrumentation is abandoned since they are no longer considered accurate. The limitation of second- generation instrumentation is that they could not provide continuous data. Although the third-generation instrumentation are capable of providing continuous data, the duration of continuous monitoring is limited since the particulate deposit has to be removed frequently.

Light scattering is probably the only practical technique used in the third-generation particulate monitors. Grot et al (1991) used a light-scattering particle counter in monitoring an office building. Here the concentration of the solid particulate is determined from the scattering of infra-red radiation when radiated onto the particulate. The major difficulty in continuous monitoring of solid particulate is to frequently clean the particulate deposit. Woskie et al (1994) reported their experience in using Miniran, a light-scattering particle counter, for a one calendar year environmental monitoring of a mine site. The monitor was cleaned, zero-checked and downloaded daily.

In this paragraph, the techniques used in the second- generation particulate monitors are described briefly. Two of the monitoring techniques used in the second-generation instrumentation are gravimetric and piezobalance. In both types of particulate monitoring, a known volume of air is passed through a sampling head by means of a pump. In the gravimetric technique, the sampling head consists of a filter. To determine the concentration of particulates in air, the filter is weighed before and after the air monitoring. The filter is normally made from a material which is stable in weight (Collison and Baum, 1992). Cellulose ester (Grimaldi et al, 1990), polyvinyl chloride (Goyer, 1990), glass fibre (Purnell and 1RS Staff, 1987) and silver membrane (Purnell and 1RS Staff,

1987) are some of the examples of the air monitor filters.

In the gravimetric technique, the particulate monitor monitors total suspended particulate instead of instantaneous concentration of the particulate. The concept of total suspended particulate is used in this case since the rate of the particulate deposited decreases with time. The decrease is due to the increasing thickness of particulates on the filter which consequently reduces air flow through the filter. For example, if the particulate monitor is left running for fifty minutes, the particulate collected over the first five minutes is less than that collected over the last five minutes. Therefore the total particulate collected during the entire fifty minutes of monitoring is less than ten times that collected during the first five minutes. Based on the same argument the total particulate collected during the entire fifty minutes of monitoring is more than ten times that collected during the last five minutes. A problem with all gravimetric technique is that it does not distinguish between particle sizes or number nor does it identify the nature of the particles.

In the piezobalance technique, the particulate is collected electrostatically onto a vibrating piezoelectric quartz crystal. The concentration of the particulate is determined from the change in the resonant frequency of the crystal. For accuracy, the piezobalance particulate monitor should not be used beyond the resonant frequency limit, the frequency at which the piezoelectric quartz crystal is recommended for cleaning.

This literature review suggests that for long monitoring duration, the scattering technique is no better than the piezobalance technique. Both techniques require periodic cleaning of the monitor.

4.5.1.2 The Particulate Monitor for This Research

As suggested in the above literature review, the piezobalance technique is selected for particulate monitoring. The particulate monitor is Model 8510 Piezobalance Respirable Aerosol Mass Monitor manufactured by TSI Inc. USA. The monitor, which carry a serial number 151 6-90, was installed with the piezoelectric quartz crystal number 3347.

4.5.1.3 Working Principle

The particulate monitor consists of two main components; the impactor and the precipitator. Particulate separation occurs at both components. Air from the inlet of the monitor is initially passed through the impactor which filters out the larger particulates. This is the first stage separation. The air containing smaller particulates is then passed through the precipitator which separates and collects the solid particulate remaining in the air. The solid particulate collected in this second stage separation is weighed by the piezobalance technique. The air then leaves the particulate monitor. The movement of air over the impactor and precipitator is maintained by means of a pump.

The impactor and the precipitator are described in detail below.

a. Impactor

An impactor consists of a nozzle and an impaction plate. The principle is that if a stream of air containing particulates is directed perpendicular to a plane, in this case the impaction plate, a partial separation due to centrifugal force will occur. Due to its relatively large centrifugal force, the larger particulate will collide with the impaction plate and remains there. The smaller particulate will be able to follow the air streamline.

The concept of fifty percent cut-off size (sometimes called cut-off diameter) is required to understand the separation of particulates at the impactor. The fifty percent cut-off size of the impactor of the monitor used in this research is 3.5 micron. Ideally all particulates above 3.5 micron will be collected on the impactor plane and all particulates below the cut-off diameter will follow the air stream to the precipitator. Practically only fifty percent of the particulate of 3.5 micron in diameter will be collected on the impactor and the other

fifty percent will pass through the impactor, polarised, collected, and affect the resonant frequency of the quartz crystal.

b. Precipitator

The particulate that passed through the impactor is polarised for collection on a vibrating quartz crystal. The air containing particulates from the impactor is passed through a nozzle. In the centre of the nozzle, a needle is placed axially to the air stream. The needle is supplied with a high voltage so that a negative-polarity corona passes from its tip to the sensing crystal plate. Under this electrical condition the particulate are charged. The electric field causes the particulate to be collected on the quartz crystal. The particulate collected changes the resonant frequency of the crystal. From the change in resonant frequency the concentration for the particulate is determined.

4.5.1.4 Justification of Selection

The first consideration in using this particulate monitor is its availability. The second consideration is the compatibility between the size range of the monitor and the size range of the particulate to be measured. The range of the particles size that the above particulate monitor could measure is between 0.01 to 10 micron. In an investigation of health hazard problems in office buildings this range is considered justified for three reasons. Firstly, as noted in Chapter 3, this is the range considered by the World Health Organisation as having the greatest effect on human beings (United Nations, 1979).

Secondly, the indoor concentration in a large number of buildings, including offices, in previous researches are reported to be between 0.1 to 0.5 mg/mS (ASHRAE, 1993).

Thirdly, the use of third-generation instrumentation seems to be no better than this particulate monitor. In principle, the third generation is preferred over the second- generation because the former is capable of providing continuous reading. This may not be true in this case. The monitoring period involved is relatively long during which time particulate removal may be required.