Monitoring & metering


is that the human nose is extremely sensitive, and of course humans are the receptors for this form of pollution, so the method is appropriate. However, the major disadvantage, apart from the time delay and the cost, is that the gas sample represents one moment in time, so it cannot be truly representative. This is because odour levels may have been much higher or lower before or after the sample was taken, but there would be no data to show this. Continuous monitoring is therefore preferable, but only if the data are representative of the local odours.


A further complication with odour is that it is rarely the result of one individual chemical. Frequently, odours are caused by a mixture of chemicals, and there may be synergistic effects between different odorous compounds. For this reason, the standard method (grab samples) is based on human perception. In order to be able to monitor odour continuously, it is necessary to utilise an ‘electronic nose’ or IOMS (Instrumental Odour Monitoring System), with the capability to measure all of the most common odorous compounds. For this reason, the ELLONA monitors employ a suite of monovariant and multivariant sensors that are capable of measuring volatile organic compounds (VOCs), sulphurous compounds such as hydrogen sulphide, mercaptans and other odorous compounds such as amines or aldehydes. The detection of specific gases, however, is not sufficient on its own, to effectively monitor odour; it is also necessary to be able to identify site-specific odours and to conduct multidimensional mapping so that sources can be identified. Prior to the configuration of Green Star monitors, ELLONA collected representative Tedlar bag samples from the site, and conducted comprehensive laboratory and dynamic olfactometry analysis of the potentially odorous compounds present. Odour threshold limits were established using the dilution method outlined in ASTM E679. This enabled ELLONA to create a quantitative fingerprint for


Continued on page 22... Instrumentation Monthly April 2023 21


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