Mass balance
(See Mass balance in emissions overview.)
Extractive techniques
To measure the quantity of material leaving a process, a sample of the material can be trapped in the duct or chimney before it is released. The sample of material can be extracted and measured by weighing it. This gives a direct measurement of the pollutant.
To establish the concentration of the pollutant in the effluent gas stream, the quantity of exhaust gas which contained the sample must be established. The main problem with extractive sampling is that the temperature and pressure of the gas stream is not the same as ambient conditions and adjustments must be made accordingly.
Isokinetic sampling
Sampling of solid particles such as dust or grit poses specific problems. To be representative of the particle concentration, a sample must be extracted isokinetically, i.e. the sample must be taken at exactly the same velocity as the flow of effluent materials in the duct.
If the flow in the sample tube is too rapid, an excessive amount of dust will be sucked in. If the flow of the sample is too slow, some dust particles will by-pass the sample nozzle. Careful measurement of the gas flow and setting of the pump which is part of the sampling equipment is nowadays done by computer control.
To complicate matters further, solid particles entrained in the effluent in a duct are not uniformly distributed across the duct. To compensate for this, samples should be taken at intervals right across the diameter of the duct in two right-angled planes.
These techniques have been set out in a series of British Standards.
Note that these complications only apply to the sampling of solid particulate in ducts. Gases are more uniformly distributed and are more easy to sample.
In situ sampling
For particulates there are several techniques for taking samples inside ducts. They rely on physical phenomena which alter in proportion to the quantity of solid materials flowing by. They have to be calibrated against extractive techniques at regular intervals. Examples of such techniques are:
- triboelectric probes measure the static charge which builds up as dust is deposited on a protrusion on the inside of a chimney; and
- light scatter involves projecting a beam of light across a duct . Obscuration of the beam is proportional to the quantity of dust.
For gaseous pollutants there are other techniques such as:
- flame ionisation detection (FID) for organic solvents; and
- photo ionisation detection (PID).