Could You Please Explain the Difference Between Isokinetic and Anisokinetic Sampling?
During the course presentations, we viewed a video tape that explained the term “Isokinetic Sampling.” Isokinetic sampling conditions exist when the velocity of the
particles and gases entering the probe nozzle tip (Vn) is exactly equal to the velocity of the stack gases (Vs), that Vn = Vs. Percentage isokinetic is then calculated:
percent isokinetic (% I) = Vs/Vn X 100
When Vn does not equal Vs, we have anisokinetic conditions where sample concentrations can be biased because of the inertial effects of particles in the gas stream. Depending on the particle composition and sizes in the gas stream depends upon how much effect there will be on the final pollutant mass rate (pmr) from the facility. In general, the following conditions exist in a stack gas stream:
- Small particles (< 1 micron) tend to follow the stream lines of the gas stream. If the source is composed of only small particles, then really little effect on whether you sample above or below isokinetics, thus little effect on the pmr.
- Large particles (> 5 microns) tend to move in their own initial direction. For under isokinetic sampling (the nozzle is bring in gas at too low a rate), the gas stream “bunches up” at the nozzle inlet. The large particles tend to “punches through” the “stream lines” (due to their own inertia) and into the nozzle area (they should have gone around the nozzle), thus biasing the pmr higher associated with a lower sample volume. We consequently have too many large particles for the small volume sampled. For over isokinetic sampling (the nozzle inlet velocity is greater than the passing gas stream velocity), the nozzle brings in gas not directly infront of it. The large particles, due to their inertia, do not follow the stream lines and continue in the same direction. Thus, the nozzle samples a non-representativeness of large particles in the gas stream, but for twice the volume of gas sampled through the nozzle (the larger particles enter the nozzle as if 100 % isokinetic sampling was occurring). The small particles enter the nozzle outside the effective area of the nozzle (the small particles follow the bent stream lines into the nozzle). Consequently, with the combination of the effects of the large and small particles, the pmr increases.
- Intermediate particles are somewhat deflected for the stream lines of the gas stream.
How do we use this information in determining if you should reject or accept the stack test if the percent isokinetics are outside the 90-110 % limits and the source pmr were within their limits. If the source test report shows that the percent isokinetics was under 90 % and majority of the particles were < 1 micron in size, then the test should be accepted since fine particles effect pmr very slightly. In the same manner, if the particles are > 5 microns and the percent isokinetics are less than 90 %, then the test should be definitely accepted since the results are bias high anyway due to large particles (more large particles for a smaller sample volume). Therefore, accept the results even so the isokinetics were below 90 %
Now, if the pmr is above the emission limit and we have < 90 % isokinetics, one can multiply the pmr by the factor %I/100 and recalculate pmr. If this adjusted pmr is still higher than the standard, then the test is accepted even with the percent isokinetic over 110 (the results are in favor of the EPA). On the other hand, if the adjusted pmr is lower than the standard (thus bringing them into compliance), then reject the test and require a retest to be performed.
If the pmr is above the emission limit and the % I is above 110 %, then the test should be accepted because the pmr is equal to the true value or bias low relative to it; thus, the pmr is definitely over the standard. If pmr is below the standard and the % I is above 110 %, then perform the same correction as above to the pmr (multiply by %I/100), and if is still below the emission limit, the test should be accepted. The pmr meets the standard even though the maximum adjustment (biases due to large particles) have been made. On the other hand, if the adjusted pmr exceeds the standard, you can accept the test results even though they did not meet the 90-110 % I because they still exceed the standard.