4. Sampling and Measurement of Concentration Isokinetic sampling - Velocity entering the sampling probe is equal to free stream velocity. - No particle loss at inlet. - Does not insure no particle loss elsewhere.

is the sampling flow rate is the flow rate of aerosol is the probe diameter is the duct diameter

Anisokinetic sampling Misalignment Superisokinetic Subisokinetic

Case 1: Miasalignment Maximum error is concentration in the probe is concentration in free stream

Effect of misalignment of the sampling probe

Case 2 & 3: Superisokinetic & subisokinetic Maximum error

Superisokinetic and subisokinetic sampling (q = 0)

Superisokinetic and subisokinetic sampling (q = 0)

Case 4: Velocity mismatch and misalignment Maximum error

Sampling from still air Two sources of error 1. Settling - sampling tube facing upward-> overestimation of concentration. - sampling tube facing downward-> underestimation of concentration. 2. Inertia - Same as superisokinetic case->loss of large particles.

Criteria to avoid settling error Davies’s criteria For standard condition in mm in m3/hr

Criteria to avoid inertia error Davies’s criteria For standard condition in mm in m3/hr

Davies’s criteria for upper and lower limits of probe diameter.

Maximum air velocity that still-air sampling criteria can be used Agarwal & Liu’s criteria: less restrictive with less than 10% error No restrictions for particle smaller than 100 micron.

Maximum ambient air velocity that still-air sampling criteria can be used

Transport losses: in tubing and fittings

1. Settling loss is the tube length is the inclination of the tube in radians

2. Inertia loss at the bend Laminar flow Turbulent flow is the bend angle

3. Other losses - Diffusion: Previous chapter - Interception: small - Electrostatic: will be discussed later - Thermophorsis: sample of hot aerosol stream in a cooler tubing. Will not be discussed.