Determination
of respiratory dust concentration by MRE 113A.
Theory:
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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