Annual Guide 2019 I SOURCE TESTING ASSOCIATION
The common standards followed although not always in their entirety are:
1. BS EN 13284-1: 2017: Low Level Dust Measurements 2. ISO EN 9096:2017: High Level Dust Measurements
3. US EPA Method 5 (more for the application of equipment than as the Standard).
It is this latter following of a part of a Standard and applying the US EPA Method 5 type train where a number of problems tend to arise.
Each of the methods for particulates worked well when we were sampling high-level dust at 50mg/Nm3
and above. The
imprecision and inaccuracies generally were minor if normal procedures were followed. However, as we came lower and lower in concentrations and levels approached the 2-5mg/Nm3 levels then the uncertainties associated with the measurement have become much more important.
Why is sampling isokinetically important and is it always required?
Isokinetic Sampling and Why?
The main issues are that we have gravity, buoyancy and aerodynamics to worry about with particulates. Gases, if given enough time, will mix in a homogenous fashion, but dust will not.
If we think of larger sizes of dust, if these were contained in a vessel and there were no air movement large particles would settle out on the bottom of the container, lighter particles of dust would not. These smaller and lighter particles may be attracted to themselves and the walls of the container due to electrostatic effects, and minor air currents and thermal effects will help to keep them airborne. Just think of a well-lit room and as the sun streams in, you see dust dancing in the air.
So, the aim of sampling is always to obtain a representative sample from the medium you are sampling in, or if there is any bias to understand the bias and apply corrections.
As a generalisation any particle below 1 µm (micron) acts as a gas; as the particle sizes increase, the characteristics associated with particulates increases.
To minimise the effects of an uneven distribution of particulates in a gas medium and to reduce the biases caused by sampling inappropriately, isokinetic sampling was adopted where we sample the dust at the same velocity as the gas is travelling in the duct.
In this way the fl ow lines of the dust in the gas stream are not affected acutely and a more representative sampling of the dust is achieved. The problem that a sampling specialist has is that the dust usually occurs as a range of sizes and quantities and this size-distribution is rarely if ever known before sampling is undertaken. Indeed, it is the sampling being conducted that is potentially determining the distribution. Thus, if we are unsure as to whether the dust is all sub-micron or is a mixture of sizes including sizes that are above the 1 µm threshold, then it is
(a) The Good Ugly
(a) The Good Vs = Stack Gas Velocity Vn =Velocity in the nozzle during sampling
(a) Sampling Isokinetically (b) Sampling too Slow (c) Sampling too Fast
(b) The Bad (b) The Bad
In (a) the particle distribution is maintained In (b) the heavier particles are sampled at the expense of the smaller lighter ones In (c) the lighter particles are sucked into the nozzle
Vs = Stack Gas Velocity Vn =Velocity in the nozzle during sampling (a) Sampling Isokinetically (b) Sampling too Slow (c) Sampling too Fast
If we look at the American system, we see Methods 1-4 which also support Method 5. The molecular weight of gases has an effect as does the moisture determination.
In (a) the particle distribution is maintained In (b) the heavier particles are sampled at the expense of the smaller lighter ones
In (c) the lighter particles are sucked into the nozzle
Pitot Traverse Velocity
Table 1. US and EU Standards for Determining the gas characteristics prior to particulate sampling
Density and Molecular Weight of Stack Gases and Reference Conditions
Water Vapour Species/Analyses US EPA Method No
Species/Analyses US EPA Method No EU Standard Sample Location and sample points
If we look at the American system, we see Methods 1-4 which also support Method 5. The molecular weight of gases has an effect as does the moisture determination.
US EPA Method 1 BS EN 13284-1 or BS ISO 9096: BS 15259
US EPA Method 2 Above and BS EN ISO 16911-1
US EPA Method 3 Gas Analysis Data; O2: BS EN 14789: CO2: BS ISO 12039 and CO: BS EN 15058
US EPA Method 4 BS EN 14790
Table 1: US and EU Standards for Determining the gas characteristics prior to particulate sampling
EU Standard
Sample Location and sample points
Pitot Traverse Velocity
Density and Molecular Weight of Stack Gases and Reference Conditions
Water Vapour
US EPA Method 1 BS EN 13284-1 or BS ISO 9096: BS 15259
US EPA Method 2 Above and BS EN ISO 16911-1
US EPA Method 3
Gas Analysis Data; O2: BS EN 14789: CO2: BS ISO 12039 and CO: BS EN 15058
US EPA Method 4 BS EN 14790 (c) The Ugly (c) The
best to assume that the dust is inhomogeneous and sample isokinetically. I cannot think of an occasion that it is incorrect to do so! The amount of effort to sample isokinetically rather than anisokinetically is not usually worth the risk as it is the time taken in preparing fi lters and weighing them that is usually the time limiting factor, not the sampling, and a revisit to site because it was done incorrectly is damaging to reputations, expensive and time consuming.
The effect of Isokinetic Sampling on Different Particle Sizes The effect of Isokinetic Sampling on Different Particle Sizes
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