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Experiment number 11

Determination of respiratory dust concentration by MRE 113A.

 

Theory:

The gravimetric dust sampler is a self-powered portable instrument for use in the gravimetric (by difference in weights) sampling of respirable airborne dust in mines and other locations. This is hung in a mine roadway at a height of 2/3 rd of the full height of the roadway. The instrument operates for full shift and collects harmful dust particles on a filter. From the difference in the weights of the filter the harmful airborne dust per cubic meter of air is calculated.

                A rechargeable 7.5V, 90 mA battery drives the governed motor and a pump to draw 2.5 l/min of dust laden air through a horizontal elutriator size selector. The harmful dusts are collected on the filter. The amount of air sucked during the operation of the instrument is measured on a count, which registers the number of liters of air inspired by the instrument. The counter is resettable by the knurled accessible through the door of the instrument case. The pump is of diaphragm type and is fitted with two flap valves. A flowmeter has three horizontal lines engraved. The middle line represents the correct flow rate of 2.5 l/min. and the one below represents an underflow of 2.5 l/min. whereas the above line shows an overflow of 2.6 l/min. The filters used are 5.5-cm circles of glass fiber type or membrane type. The membrane filter is much convenient to use.

Calculation of airborne dust content:

If, w = initial weight of the filter, g

    W= final weight of the filter, g

      n = initial reading of the counter.

      N= final reading of the counter.

The amount of dust collected = (W-w), g

The air quantity sucked = (N-n), liter

So, the amount of airborne dust per cubic meter = 1000 ( W-w)     g

                                                                                    (N-n)       

Objective:

To measure the gravimetric (by weight) concentration of respirable dust in air by the MRE 113A gravimetric dust sampler and from that to determine if the dusty atmosphere is hazardous to health.

Description of the instrument:

The MRE 113A gravimetric Dust sampler is a self-powered portable instrument for sampling of respirable dust concentration in mines,  where a rechargeable 7.5V, 90 mA battery drives, a governed motor and a pump to draw 2.5 l/min of dust laden air through a horizontal elutriator. The coarse particles in the dust laden air settle down by gravity in the elutriator while the harmful fine particles in the respirable size range are collected in a filter 55 mm in diameter glass fiber or a membrane filters in a light aluminium holder are used for sampling. The amount of air in litres sucked during sampling is recorded on a counter. The counter is resettable to zero by the knurled knob through the door of the instrument case. The pump is of diaphragm type and is fitted with two flap valves and a flow smoothener at the outlet. A flowmeter is connected to the output side of the pump. The counter and the flowmeter are readable through small windows fitted in the side of the instrument. The window of the instrument has three horizontal lines engraved, the middle line representing the correct flow rate of 2.5 l/min.

 

 

Procedure:

A clean filter paper is selected and mounted in the filter holder. The assembly is there weighed in an accurate balance having a sensitivity of 0.1 mg. It is inserted in the sampler and tightened in position. The battery is charged fully by the charger provided. A nearly discharged battery needs 14 hours charging. The charged battery is inserted in the instrument and the counter is set to zero.

The sampling instrument is then placed in the location where the breathable dust

concentration is to be determined at a height approximately equal to the breathing       level of the workers and switched on.

      Normally the duration for sampling covers a whole shift but the time can be less for high dust concentrations or more for low dust concentrations in order that sufficient amount of dust should be made on the rate of sampling which should be around 2.5 l/min. A lower rate of sampling indicates overloading of the filter paper with dust or discharged battery.

      After the sampling is over, the volume of air sampled is noted and the dust-laden filter paper is removed from the instrument along with its folder and weighed. The difference in weight in mg. Divided by the volume of the air sampled in m3 gives the dust concentration in mg/m3.

 

 

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