Equal velocity in sample taking of a gas for dust measurement.
To obtain a representative sample independent of particle size, it is necessary to remove the sample stream isokinetically, i.e. with the same velocity as the main stream. The following Figure shows the pattern of the flow lines in the vicinity of a thin-walled sampling probe, with w the velocity of the main stream and v the velocity of the sample stream:
Fig. 25: Sampling of gas with dust particles
In the isokinetic case (w=v, Fig. 25), all particles flowing toward the intake opening are equally collected. If the sample taking velocity is too low (w>v), heavy particles can enter the probe even if the flow line on which they were located passes by the probe. Thus too many large particles are collected. If the sample taking velocity is too high (w<v), heavy particles fail to adhere to the flow lines and end up bypassing the probe. So too few large particles are collected. Isokinetic sampling is often susceptible to faults since the sampling can easily be a super- or sub-isokinetic sampling. The resulting measurement error is significantly higher in case of sub-isokinetic sampling (w>v). In SIGRIST dust measurements, the diameter of the sampling is adjusted to the velocity of the main stream such that a super-isokinetic sampling (w<v) results. The resulting measurement error is small as long as the particle size distribution of the dust particles is constant.