Some critical facets in air pollution

1. Some Critical Facets in Air Pollution Monitoring
Dr. Rajendra Prasad
Managing Director
Envirotech Instruments Pvt. Ltd.
A-271, Okhla Industrial Area, Phase -1, New Delhi - 110020 (email: envirotech@vsnl.com)
1.0 BACKGROUND
The prime objective of air quality monitoring is to find out prevailing
concentration of air pollutants with accuracy of 1 Dg/m3
. This accuracy is
depend upon several factors. Man and machine are the two basic facets
equally responsible for reliable data generation which can meet aforesaid
accuracy expectations.
Qualification, training and experience of man power can improve quality of
generated data. However if monitoring personnel are motivated absolute
reliability in collected data can ensured without further efforts even in existing
setup. Similarly the monitoring instruments have to be accurate, precise and
regularly calibrated without which quality data generation is out of question.
CPCB have already specified following accuracy limits for critical parameters
recorded by a monitoring system used for AAQM.
Flow rate accurate upto + 5 %
Time accurate upto ± 1 min.
Above parameters required to be critically examined in monitoring
instruments and must be ensured within specified accuracy limits for
maintaining desired accuracy of measurements.
1.1 PRESENT SCENARIO
1. About Man power :
The manpower deployed for regular air quality monitoring either in NAAQM
or in any other monitoring program most of time are not adequately qualified
and experienced. Formal training has never been given to these staff. Most
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2. critical part is that the people engaged in these monitorings are not motivated
for the assigned jobs.
Besides this there are several other serious limitations with man power like
they are overloaded with work due to less staff, have no clear cut instructions
about monitoring for acceptance and rejection of sample, have no freedom to
put there problems and view point with senior and experienced officers. At
last there salaries are neither attractive not revised since long hence
whenever there are opportunities people are escaping.
About the Machines :
Monitoring instruments used for assessment of air pollution are prone to drift
and may show variation with time in measured parameters. Hence it is a
false expectation that their calibrations are stable over long period of time.
As per standard requirements calibration need to be done.
• When instrument is manufactured.
• When instrument is transported to a new location and in different climatic
condition.
• If instrument is operated after interruption of several months.
• After repairs or overhaul of key components.
• After observing significant drift either in zero or span value or in recorded
value.
The monitoring instruments used in regular air pollution monitoring either
under NAAQM or in other project are not calibrated as per above
requirements. Even when it is known that regular calibration of monitoring
instrument is a basic need for reliable data generation.
• In general life of a HVS/RDS is about 5 years. But instruments are not
retired even after crossing age of 10 years and more with old machine
(not regularly calibrated with age of 10 years and above) reliability of data
are expected similar as expected with new calibrated machines.
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3. Table below show the age distribution of installed HVS at various NAAQM
stations.
S.No. Age (years) No. of Machine
1. 10 and above 32
2. 7 - 1 0 75
3. 5 - 7 28
4. Less than 5 06
Total 141
(Source Status Report on Servicing of HVS Installed at Various NAAQM Station in India - 1995)
Over all it can be said that conditions of machines used in air quality
monitoring are critical and are not good for reliable data generation. In such
situation when conditions of man and machine are weak reliable data can not be
generated as per requirements.
1.2 Difficulties of Calibration :
Users face following difficulties while calibration of instruments.
• Calibration facilities are not available at number of places in the country.
Further all required set-ups related to the calibration of air pollution
monitoring are not available with one agency.
• Reference set up at calibrating agencies are not portable enough hence
calibration at site is not possible. Thus monitoring system need to be taken
to calibration agencies for calibration. It is not easy to transport HVS/RDS
from distant places. Besides this cost of transportation is also significant
where possibilities of damages are always there.
• Time taken for calibration is several weeks since spare machines are not
available hence calibration is postponed due to this limitation.
• Cost of calibration is very high most of the time more than the cost of
instrument to be calibrated. Some calibration costs are given below.
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4. S.No, Name of the item
Cost of item
calibrator (Rs.)
Cost of calibration
(Rs.)
1.
Portable calibrator for flow
rate calibration
15,000 28,000
2. Thermometer (6 points) 100 1,200
3. Dry gas meter 5,000 25,000
4. Pyrometer + thermocouple 4,000 10,000
5. Pitot Tube 1,500 3,000
6. Stop Watch 1,000 2,500
7. Weights (5 Nos.) 1,000 3,000
• Calibration of supplied instrument is done as per understanding of the
scientist undertaking the calibration. Number of situations have been faced
when calibrations are not done as per requirements and significant errors
are observed in calibration data.
• Once instrument is calibrated user do not try to look into the calibration data
and graph. Further values are not corrected for observed errors indicated in
calibration. Calibration graph and reports are only used to show that this
instrument is calibrated.
• There is no clarity about frequency of calibration. For one type of instrument
one organization says recalibration is due after one year while other says it
is due after 3 years. It appears that frequency of calibration is decided by
calibrating agency arbitrarily.
1.3 SUGGESTED METHODOLOGY:
(A) For field calibration.
Keeping above difficulties in mind Envirotech has setup Roots meter, a
primary standard device for finding out possitive displacement of air. This
unit is traceable to NISST and used for calibration of portable Top Loading
calibrator and also monitoring instrument. The portable calibrator is useful in
the field for calibration of machines at site of their operation. How calibrator
is calibrated? How sampler is calibrated using calibrated calibrator?
Procedure has been discussed in the methodology of calibration given in
Annexure -1.
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5. Since calibration is required only for those parameters which are taking part
in the calculation. In HVS/RDS following units need to be calibrated:
for flow - orifice - manometer and rotameter
for time - time totalizer
All these units are possible to calibrate in the field using calibrated portable
Top Loading Calibrator, Dry Gas Meter and Stop Watch.
(B) For Calibration at Our Works.
Machines which can be brought at our works are calibrated using Roots
meter for flow rate at various pressure drop equal to the clean filter and dust
loaded filter. Rotameter calibration is carried out using soap bubble meter or
using dry gas meter depending upon the capacity of Rotameter.
For establishing cut off of the cyclone unit of RDS, collected dust on filter
paper and cyclone cup is analysed for particle size distribution on lesser
based particle size analyser. Rough estimate of cut-off of the cyclone can
also be done using manual method with help of projection microscope.
1.4 OTHER CRITICAL FACTS OF AIR POLLUTION MONITORING
RESULTING UNRELIABLE DATA GENERATION
1. Filter handling is not done as per recommended procedure.
2. Filter are not checked for pin holes. Experiences of this author say that in
every box of 100 sheets 4-5 filters are found damaged with pin holes need to
be rejected and should not be used in sampling.
3. At some places it has been observed that filters are dryed at 105°C for few
hours prior to weighing instead of putting them in desiccator with active
absorbent at room temp (17-27°C) and at low humidity (0-50 % R.H.) as per
standard procedure notified by BIS/CPCB.
4. Cleaning of filter gaskets, top cover, inlet pipe, cyclone assembly is not done
at periodic intervals.
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6. 5. Manometer tube water is not replaced at specified intervals and there are
situation, when tap water is found filled in the tube responsible for erroneous
recording of flow readings.
6. Initial zero level in manometer is not maintained accurately.
7. Carbon accumulated in blower is not cleaned at periodic interval causes
break down and variation of rpm of armatures. Resulting flow fluctuations.
8. Manual of instrument most of time neither available with operator nor read by
operators. Thus precautions suggested by manufacturer are never followed
resulting errors in sampling and break down of the machine due to
mishandling.
9. In case of gaseous attachment ice is not kept in ice box, flow setting is done
casually, Cleaning of manifold is never done where fine particulates escaping
from impinger solution found to be deposited on the body of needle valve
ultimately responsible for reduction of flow rate because pressure drop at
needle valve is changed during sampling of gaseous pollutants.
10. Impingers are not properly greased and leakages are common in impingers
in present practices. This ultimately responsible for escaping a part of gases
without absorption in the reagents.
11. It has been seen that impingers used for sampling of gaseous pollutants do
not meet BIS/CPCB published specifications. Due to change in dimensions
concentration of gaseous pollutants absorbed in the reagent influenced
significantly.
12. Cleaning of impinger tip with pore size of 1mm is not done after each
sampling. At tip deposition of salts and particulates while sampling causes
reduction of flow rate due to change in pressure drop at the tip.
13. In case of Respirable Dust Sampler collected dust in cyclone cup must be
removed with every filter change otherwise if accumulated dust in cyclone
cup is collected beyond the capacity it shall re-entrained with air stream and
may reach to filter paper and effect collection of Respirable Dust.
14. It has been observed that flow reading are taken immediate after switching
on the blower. Which results recording of inaccurate flow rate because
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7. steady flow is only achieved after 4-5 minute of operation. This time is
needed to set the motor brushes.
15. Availability and quality of power is the most critical parameter influencing
monitoring duration and thus measured concentrations also. Frequency of
power (50 cycles/sec) become critical about electrical components like Time
Totalizer where readings are known to be influenced significantly if frequency
of power is changed.
1.5 RECOMMENDATIONS :
« Man power operating instruments need to be qualified, trained and motivated.
• Periodic check of monitoring methodology must be done by an experienced
team through field evaluation on each site where actual sample collection
analysis and calculation of results must be supervised and operators are advised
. Such evaluation must be done once in year for all locations.
• Regular calibration of monitoring instruments must be done as per requirements
of calibration.
• Calibration requirements and frequency of calibration must be made a part of the
standard as method of sampling and analysis is mentioned in air quality
standard.
e On going practices which do not match with standard procedure need to be
stopped so that generation of reliable data can be ensured.
a CPCB should set-up calibration centres at their various zonal laboratories where
calibration of HVS/RDS and other air pollution monitoring instruments is done for
various users in the area on payment basis. While calibration freedom need to
be given to the user to witness the method of calibration and obtained data.
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8. Annexure -1
METHODOLOGY OF CALIBRATION OF HVS/RDS
• Flow metric devices used in HVS/RDS are orifice and manometer which indicate
flow in m3
/min.
• Flow measurements are based on observed differential pressure. Thus for flow
calibration, procedure has to be performed against a known air flow.
• Top loading calibrators where a calibration orifice unit with resistance plates
(load plates) are used for calibration of flow rate. Calibration orifice resistance
plates and manometer is shown in figure 1.0.
• Calibration unit is consist of a modular pipe with static pressure tap located at
one end (closed to calibration orifice). Farther end away from static tap is
flanged. In this portion resistance plates are accommodated while calibration.
Near end of the static pressure tap is fitted with a metal plate with an air inlet
which has a fix dia hole. This hole is the orifice on which the calibration
procedures are based. Five resistance plates are used to create resistance of
filter with varying particulate loading. The resistance plates have 5,7,10, 13 & 18
holes. The plate with 18 holes represents a clean filter while other plates
represents a filter with increasingly heavy dust loading.
• Series of steady flows are drawn through the orifice unit for each resistance
plate. The air flow is recorded along with corresponding pressure differential
from the manometer attached to the pressure tap of the orifice unit. Placement
of orifice unit before the primary standard (Roots meter) reduces the inlet
pressure to this meter below atmospheric pressure. To compensate for this a
second manometer is attached to an inlet pressure tap of the primary standard.
After recording barometric pressure, the true volume of air drawn though primary
roots meter is calculated.
P b a r o m e t r i c —
P m e a s u r e d
V t r u e ~ [ -I * ^ m e a s u r e d
P b a r o m e t r i c
Where,
P b a r o m e t r i c
=
Barometeic Pressure at place of calibration mmHg.
P m e a s u r e d
=
Measured Pressure mmHg
Vmeasured = Value measured
Vtme =
True flow rate at ambient conditions
True flow rate can be calculated by
VT rue(m3
)
Q (m3
/rain) =
Time of flow (min)
A plot of orifice manometer pressure reading versus true floW' rate is generated
which become calibration curve for calibrator which is used to calibrate HVS/RDS.
This Top Loading Calibrated Calibrator is fitted at the inlet of sampler with 18 hole
resistance plate in place properly so that there is no leakages. Calibrator pressure
tap is connected to one end of manometer and other end left open to atmosphere.
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9. Sampler is switched on and allowed to run for 5 minute for steady flow (this time is
needed to set the motor brushes). Reading indicated by the sampler and pressure
reading of the manometer is recorded. This is done for different resistance plates.
Find out true flow rate reading against recorded pressures in manometer. A plot of
sampler flow rate versus true flow rate reading is plotted and best fit curve not more
than one point of inflection is used as calibration curve of HVS/RDS.
When using the Orifice Calibration unit to calibrate a HVS/RDS correction must be
made to indicated flow if actual atmospheric temperature and pressure vary from
calibration conditions. Corrected flow can be calculated as.
Qcorrcct - QlJncorrcct [(T2 Pl)/(Tj P2)]
Ti
T2
Pi
P2
Corrected flow rate
Uncorrected flow rate reading from orifice manometer
calibration curve.
Absolute temp at orifice calibration condition.
Absolute temp when calibrating the sampler.
Barometric pressure at orifice calibration conditions mmHg.
Barometric pressures when calibrating the sampler
monitoring mmHg
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