SAMPLING AND MEASUREMENT OF CONCENTRATION
Lin Shou04/13/2010
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OUTLINE
Aerosol Sampling System Isokinetic Sampling Anisokinetic sampling Sampling from Still Air Transport Losses Measurement of Mass Concentration Direct-reading Instruments Sampling Pumps
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AEROSOL SAMPLING SYSTEM Aerosol measurement frequently requires that an
aerosol sample be conveyed to a measurement device, which is accomplished by withdrawing a sample from its environment and transporting it through sample lines to the device.
An aerosol sampling system generally consists of 1. A sample inlet 2. A sample transport system consisting of the
necessary plumbing to convey the aerosol sample to the measuring instrument or to a storage chamber.
3. A sample storage volume (optional) that will have an additional sample inlet and transport system to the measuring instrument.
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AEROSOL SAMPLING SYSTEM
(Ref: Aerosol Measurement - Principles, Techniques, and Applications , 2nd edition)
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ISOKINETIC SAMPLING• What is isokinetic sampling?
• A procedure to ensure that a representative sample of aerosol enters the inlet of a sampling tube when sampling from a moving aerosol stream.
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ANISOKINETIC SAMPLING A failure to sample isokinetically Result?
C: concentration in the probe;C0: concentration in the free stream;
(a) Misalignment
• Stk>6:
• 0.01<Stk<6:
• Stk<0.01:
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ANISOKINETIC SAMPLING
(b) Superisokinetic sampling, U>U0
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ANISOKINETIC SAMPLING
(c) Subisokinetic sampling, U<U0
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ANISOKINETIC SAMPLING
U=U0
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ANISOKINETIC SAMPLING
When both a velocity mismatch and a misalignment of the probe are present,
Assumptions: 1) thin-walled tube; 2) sharpedged inlet; 3) laminar flow.
How about blunt samplers? Turbulent flow?
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SAMPLING FROM STILL AIR Possible bias? 1) Particle settling velocity 2) Particle inertia The criterion for negligible bias due to
particle settling for smapling probes in any orientation:
The criterion to ensure negligible sampling loss due to particle inertia:
Davies
(1968)
TSU 25V s2 QD5 g
1/3
sQD 104
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SAMPLING FROM STILL AIR
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SAMPLING FROM STILL AIR
Criterion for the size of an upward-facing probe, Agarwal and Liu (1980)
(Error in ± 10%) Advantage of this criterion: 1) relatively simple and independent of the
inlet velocity; 2) much less restrictive than Davies’ criteria 3) no practical restrictions on still air
sampling for particles less than 100 µm in aerodynamic diameter.
2sD 20 g
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TRANSPORT LOSSES Even with efficient entry of particles into a
sampler inlet, particles may be lost in the tubing and fittings between the inlet and the collection or measuring device.
What mechanisms can happen during transport?
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TRANSPORT LOSSES
• Settling Loss:
• Bend Loss:
1) Laminar flow:
2) Turbulent flow:
• Diffusion Loss• Electrostatic depositions• Thermophoretic depositions
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TRANSPORT LOSSES
Ways to reduce transport losses: 1. make the transport path as short and
straight as possible; 2. use the same diameter tubing throughout 3. dilute the aerosol stream with clean air to
reduce coagulation or temperature or to prevent condensation.
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MEASUREMENT OF MASS CONCENTRATION Gravimetric analysis Filter Factors need to be considered in using filters
to measure mass concentration?
Weigh the filter; Sampling flow rate; Sampling time; Filter sizes; Mass on the filter; Temperature; Humidity; …
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MEASUREMENT OF MASS CONCENTRATION
(Ref: Aerosol Measurement - Principles, Techniques, and Applications , 2nd edition)
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MEASUREMENT OF MASS CONCENTRATION
Personal sampling Commonly used in the field of occupational
hygiene to evaluate workers’ exposure to aerosols by measuring their concentration in the worker’s breathing zone.
Breathing zone: the region within 0.3m of the worker’s mouth
http://jsec.com.sg/advanced_search_result.php?keywords=filter+holder
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MEASUREMENT OF MASS CONCENTRATION
Stack Sampling
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IMPINGER
The jet is immersed in water or alcohol.
Particles larger than about 1µm are captured by inertial mechanisms and end up suspended in the liquid.
If particles are smaller than 1 µm, collection efficiency drop rapidly.
http://www.skcinc.com/prod/225-36-1.asp
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DIRECT-READING INSTRUMENTS Quartz crystal
microbalance Collect aerosol particles
by electrostatic precipitaiton or impaction onto the surface of a piezoelectric quartz crystal.
Crystal oscillate at its resonant frequency (5-10 Hz), which decreases linearly with the mass deposited on the surface of the crystal. TSI Piezobalance respirable
aerosol mass monitor model 3500
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DIRECT-READING INSTRUMENTS Tapered Element
Oscillating Microbalance (TEOM)
0 2 2
1 1
f i
m Kf f
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DIRECT-READING INSTRUMENTS
Beta gauge Beta detector
measures the radiation from the source before and after the sample is collected.
The difference in these measurements is proportional to the deposited particulate mass and is converted to mass concentration and displayed digitally.
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SAMPLING PUMPS Personal sampling pumps: used to sample
the atmosphere to which a worker is exposed.
Features: Lightweight, portable, battery-powered; Can be worn on a person’s belt; Can last for 8 hours at 1-4 L/min.
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REFLECTION
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QUESTIONS?
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