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?