Sampling and Measurement of Concentration Lin Shou 04/13/2010
Outline • Aerosol Sampling System • IsokineticSampling • Anisokinetic sampling • Sampling from Still Air • Transport Losses • Measurement of Mass Concentration • Direct-reading Instruments • Sampling Pumps
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.
Aerosol Sampling System (Ref: Aerosol Measurement - Principles, Techniques, and Applications , 2nd edition)
IsokineticSampling • 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.
Anisokinetic sampling • A failure to sample isokinetically • Result? (a) Misalignment • Stk>6: • 0.01<Stk<6: • Stk<0.01: C: concentration in the probe; C0: concentration in the free stream;
Anisokinetic sampling (b) Superisokinetic sampling, U>U0
Anisokinetic sampling (c) Subisokinetic sampling, U<U0
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?
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)
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.
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?
Transport Losses • Settling Loss: • Bend Loss: • Laminar flow: • 2) Turbulent flow: • Diffusion Loss • Electrostatic depositions • Thermophoretic depositions
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.
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; …
Measurement of Mass Concentration (Ref: Aerosol Measurement - Principles, Techniques, and Applications , 2nd edition)
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
Measurement of Mass Concentration Stack Sampling
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
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 Piezobalancerespirable aerosol mass monitor model 3500
Direct-reading instruments • Tapered Element Oscillating Microbalance (TEOM)
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.
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.