The measurement of odor concentration is the mostwidespread method to quantify odors. It is standardized inCEN EN . The method is based on dilution of an odorsample to the odor threshold (the point at which the odor isonly perceptible to 50 % of the test panel). The numericalvalue of the odor concentration is equal to the dilutionfactor that is necessary to reach the odor threshold. Its unitis the European Odor Unit, OUE. Therefore, the odorconcentration at the odor threshold is 1 OUE by definition.
To establish the odor concentration, an olfactometer is used which employs a panel oftest persons. A diluted odorous mixture and an odor-free gas (as a reference) arepresented from sniffing ports to a group of panelists. In comparing the gases emittedfrom each port, the panelists are asked to report the presence of odor. The gas-dilutingratio is then decreased by a factor of 1.4 or two (i.e. the concentration is increasedaccordingly). The panelists are asked to repeat their judgment. This continues for anumber of dilution levels. The responses of the panelists over a range of dilutionsettings are used to calculate the concentration of the odor in terms of European OdorUnits (OUE/m3).
Odor intensity can be divided into the following categories according tointensity: 0 - no odor 1 - very weak (odor threshold) 2 - weak 3 - distinct 4 - strong 5 - very strong 6 - intolerableThis method is most often applied by having a dilution series tested by a panelof independent observers who have been trained to differentiate intensity
Hedonic assessment is the process of scaling odorson a scale ranging from extremely unpleasant vianeutral up to extremely pleasant. There is nocorrelation between this method and the methodof measuring the odor intensity. However, thehedonic perception of some odors may changefrom pleasant to unpleasant with increasingconcentration and intensity.
This is a verbal characterization of the sensed odor bythe test person, such asdisgusting, caustic, ruffling, etc. There are no moreapplications needed than a test person to run thismethod. The evaluation of the odor type could be anemission or an immission method. It has a greatimpact on evaluating the source of the odor emission.
The following details have to be differentiated while theemission is measured:1st there is the odor time slice (Result = Part of “odorhours per year” per area). Then there is the olfactory flagscope (Result = Current scope at actual meteorologysituation). And last but not least there is the harassmentexaltation by questionings (Result = differentiatedacquisition harassments).
There are two main odor sampling techniques, the direct odor sampling and the indirect odor sampling technique Direct odor samplingAir will be sampled at the source and fed straight into the olfactometer forassessment by an odor panel. The following problems can be associated with thistechnique:Odor panel members need to be seated in an odor neutral environment, thusthey need to be housed in a separate area. This is difficult to achieve whenassessing odor released from, for example factories, where the odor can beemitted from a stack on the end of a production line. This means that the odorsample collected needs to be transported from the stack to the unit where theodor panel sits. This can sometimes be on the other side of the factory plant. Thesample then must therefore pass through a very long sample line to theolfactometer. This can have influences on the sample quality, can have potentialair blockages due to water condensation or other operational procedures.Therefore most odor annoyance assessment companies use the indirect airsampling method.
Indirect odor sampling is done with the use of odor (air)sampling bags, which are made from an odor neutral materiale.g. Teflon. The odor sample bags are connected to an airsampling line which is then, for example, hooked up to a stack.The air stream is then sampled and stored in the odor samplebag and can then be analyzed in a suitable environment (e.g. inan odor laboratory).The indirect method is used to sample a wide variety of odorsources. From stacks on the end of a factory line, watersurfaces or ambient air surroundings.Each odor source has its own set of problems when sampled;these problems need to be overcome in order to collect arepresentative sample of the odor source. The followingproblems can be encountered:
Vacuum can be overcome by placing the odor sample bag invacuum container which can be placed under vacuum. If thevacuum is higher than the vacuum at the source, the odorsample will collect in the bag.
High temperatures and high moisture contents inside the odorsource leads to complications when sampled. When thesample leaves the source, it will cool down and producecondensate in the sample line and or odor sample bag. Thiscan lead to growth of bacteria or when drying out release moreodor, thus alter the odor concentration of the sample. Thesame is true when sampling in high moisture conditions. Away round the problem is to use a stack dilution probe throughwhich an inert gas (for example dry nitrogen) can be fed thatdries the sample stream. This prevents the moisturecondensing in the sample line and or the odor sample bag.
Sometimes odor sources emit a high concentration ofgases that are lethal to man. These samples must bediluted to a safe level, before being presented to the odorpanel. This pre-dilution can be done in a stack-dilutionprobe, by the addition of an inert gas or on a dilutiondevice for example an extra olfactometer.
More often than not, odor sampled at the source is higher than the ambientodor concentration. In a few cases the odor concentration can be so high thatpanelists will make a positive identification even if the olfactometer is dilutingthe odor sample in its upper dilution range. The sample must then be pre-diluted to make a sensible reading, this pre-dilution can again be done with astack-dilution probe, by the addition of an inert gas or on a dilution device forexample an extra olfactometer.
When a large surface is emitting odor, for example a sewagetreatment plant, a fixed dimension “hood” can be used. Inone end of the hood, clean air is blown in at a knownrate, and on the other end, a sample is collected via theindirect method. If a large land surface is emitting odor, forexample a bio filter (a big concrete basin filled with woodchip through which the factories waste air is pumped), asection can be cornered off with plastic (e.g. Teflon) (ofwhich the dimensions are known). The air from the factorywill inflate the plastic (lift it up) and an odor sample can betaken from under the plastic via the direct air method.