Olfactometry is an effect related measurement method. The effect on the human sense of smell is the unit of measurement. The effect relation cannot be represented with the technical or physical sensors. The odour sensation cannot be described by the quantity of the odorants. Odour originates from a wealth of chemical substances. The effect to the sense of smell can vary enormously, depending on the different components and on their proportion. Due to the large numbers of different substances an analysis of these odorous substances is exceptionally difficult. By measuring guide components a correlation to the odour intensity and concentration can - in most cases - not be found and technical sensors are unsuitable for a qualitative evaluation of pleasant/unpleasant. The human nose is the only possible sensor for odour measurement.
Olfactometry is the controlled presentation of odorants and the registration of the resulting sensations in man. It is a complete measuring method as defined by Guideline VDI 2449 Part 2. Standard DIN 6879. Olfactometry is an effect-related measuring method which has two objectives, i.e.
a)
to determine the power of the human sense of smell, with known odorant concentrations
serving as defined stimulus intensities and thus as scale increments,
or
b) to determine unknown odorant concentrations with the help of the human sense of smell as the detector.
The odorant concentration of the gas sample to be measured (single compound or mixture) is determined by dilution with neutral air down to the odour threshold. The numerical value of the odorant concentration results from the volume flows of the gas sample and the neutral air at the moment when the odour threshold is reached (dilution number at the threshold or odour number). The unit of the quantity `odorant concentration` is odour unit (OU) divided by volume unit (m³), thus OU/m³ .
With the olfactometric method three parameters, which are necessary for the characterisation of an emission source, can be determined:
1. Odorant concentration can be used to calculate the spread of odorous substances in the atmosphere and the resulting forecast of odour immissions to be expected.
2. Odour intensity describes the odour sensation in accordance to increasing concentrations on the scale 'not noticeable' to 'extremely strong'. Therefore it is source specific.
3. Hedonic quality (pleasant/unpleasant) is also source specific. With high concentrations it can turn over from pleasant to unpleasant.
Odour
unit (OU)
Based
on the definition of the odour threshold, 1 OU is the very quantity (number
of molecules) of odorants which just induces an odour sensation when dispersed
in 1 m³ of neutral air. 1 OU/m³ is also the bench mark of the odorant concentration
scale.
Odour
threshold
The
concentration of odorous substances at threshold level leads to an odour impression
with 50 % of the defined population. The concentration at the threshold is
1 OU/m³ by definition.
Olfactometer
Olfactometers
are instruments in which a gas sample (odorous sample) is diluted with neutral
air in a defined ratio. Neutral air is air in a defined thermodynamic state
(temperature, pressure, particularly humidity). It must not contain interfering
components at concentrations which induce odour sensations or influence the
sense of smell .
 |
Figure
1. A picture of an Olfactometer |
Measuring
procedure
An
odorous sample at above-threshold concentration is diluted with odourless
air. In this way, different low concentrations reach the nose of one or several
panellists simultaneously, either direct via one or several sniffing tubes
or indirect though a nose mask or a breathing mask, thus enabling an odour
assessment.
The diluted gas samples are presented to test persons (panellists) as a smell sample. The panellists are offered several dilution steps. One of the panellists at the olfactometer examine the number of dilution steps. The number of dilution steps must be chosen in such a way as to ensure that the range around the odour threshold is sufficiently taken into account.
The assessment of the presented dilution of the odorous sample is done by the appropriate panellists who form the panel. A panel for basic measurements, for example, should consist of 8 to 15 persons. As a rule, smaller panels may be sufficient for comparative measurements aiming at an assessment of the degree of the odorant concentration reduction achieved with technical measures for emission control.
Only persons who have been able to verify that they have a `standard nose` with the aid of test substances may be used as test panellists. Although it has been proven that the range of sensitivity to individual odours is significantly wider than for blends of many different substances (preferably using n-butanol), as a reproducible odorant mixture is currently not available. According to this standard, the odour threshold concentration of a group of panellists should be approximately 40 ppb in the range of 20 and 80 ppb.
Basically, there are two different sampling techniques:
1.
A partial flow of the odorous gas under investigation is conveyed directly
and continuously from the source to the olfactometer.
2. A sample of the odorous gas from a source is filled into an odourless vessel
and then examined with the olfactometer.
In order to avoid the main sources of error, the following has to be observed in the course of these procedures:
a)
There is a possibility to avoid the formation of condensate by predilution
with dry and odourless air.
b) No particles should enter the olfactometer.
c) One has to ensure the absence of odour in the sampling system.
d) Chemical reactions between the components as well as sorption an the walls
of the sampling system are to be avoided during the transportation from the
sampling place to the olfactometer.
If applicable, these important problems have to be solved in advance tests by varying the time interval between sample collection and investigation.
An odour emission source can be characterised in quantity and in quality for example from the following sources:
•
Waste water plants • Refinery plants • Food processing plants
• Chemical Plants • Fat melting plants • Animal keeping plants
• Lacquer plants
The
olfactometer type T07 which is selected to be used for the odour measurements
during this project will be supported from Emission Measurement Technique
and Consultation Mannebeck Ltd. (Ecoma Ltd.). One olfactometer for the university
in Ankara and one olfactometer for the university in Izmir will be purchased.
The price of the instrument includes the necessary equipment which are summarised
below:
1. Olfactometer
3. Sampling Device
4. Filter system, steel bottles with pressurised air or an oilfree compressor
alternative: Synthetic air in steel bottles
4. Notebook to run the Olfactometer T07
Operational Principle of the Olfactometer Model TO7
Model T07 of olfactometer is a computer controlled semi-automatic olfactometer with four panel members places and automatic calculation of the test results. A gas jet pump is operated by odourless compressed air or synthetic air. The air sample is sucked from the sample-bag via the pre-dilution unit and via the flowmeters. The olfactometer can be operated by synthetic air from steel gas bottles or by odourless air pressurised by a compressor with a filter system for air purifying purposes containing silicagel, active coal and a micro filter. The flow-rate of odorous air is controlled in steps by the needle valves.
 |
Figure 2. Connection of sampling bag to the Olfactometer |
In the gas jet pump the odorous air will be mixed intensively thoroughly with the odourless air. The mixture flows via the stepper motor during central switch-over-valve to get sniffing ports. The central switch-over-valve switches over from clean air to the mixture (during exhaling) for each panellist. The breathing frequency is given by optical signals. The procedures are run simultaneously for the panellists. The duration of a measurement sequence is thereby held to a minimum.
 |
Figure 3. Panelist group and the leader (before the measurement) |
 |
Figure 4. Panelist group (during measurement) |
Due to the fact that clean air and mixture are offered within two consecutive breathing periods the panel members have the possibility of direct comparison. The reliability of the results is considerably increased. Testing periods start with a freely selectable starting step which should -especially when measuring unknown samples- be chosen rather high to ensure that the measurement starts below the threshold. After at least four breathing cycles of pure fresh air the panel members are alternately supplied with purified air and the air mixture to be tested. After each round the air-mixture to be tested is adjusted to the next higher concentration by setting the next higher dilution step and thereby the quantity of flow through the needle valve. The dilution steps are indicated on the monitor in advance.
The total measurement programme is computer controlled and runs automatically. It includes several measurement sequences, recovery breaks for the panel members and includes clean air flushing functions for the dilution unit.
DETERMINATION OF ODORANTS IN AMBIENT AIR BY FIELD INSPECTIONS
The olfactometric measurements of odorous immissions do not give relevant results, because the concentration of odorous compounds are very low and vary strongly due to the wind affect in ambient air. Therefore, the measurement of odorous immissions are accomplished through the field inspections.
The measurement parameter is the `percentage odour time`, which represents how often the odorants in the outdoor air exceeds a recognition threshold so that it is definitely identifiable by the observer. Each assessor (observer) goes to the measurement point and tests the ambient air for odours regularly inhaling the air during a defined measurement duration (single measurement). Therefore, the method is appropriate to describe an existing status. Survey of annoyance through questioning is also an important part of the field inspections.
Depending on the task definition (assessment of individual sources, survey mapping in a multiple-source area, decisions on planning schemes, calibration of calculation models for odorants dispersion etc.), single measurements will be part of a grid measurement if an area-related statement of the existing odorant immission is needed and they will be part of plume measurement, if a plume-related statement is required.
To measure the ambient odour characteristic, based on the method of TA Luft (German Technical Instructions for Clean Air), single measurements are performed repeatedly at each of a grid of measurement points within an assessment area over a measurement point within an assessment area over a measurement period of one year. Where the task involves a future proposed emitter of odorants, it is possible to use dispersion models to predict the ambient odour characteristic, or to make a forecast based on measurements made on existing similar plants.
The single measurement is defined as the measurement of odorant imission at a defined measurement point within a defined measurement duration. A duration of at least ten minutes is needed to give with at least 80 % reliability a representative assessment of odour situation at a particular hour. The single measurements are performed to find either the ambient odour characteristic for a grid measurement, or the frequency distribution of odour recognition within an odour plume.
To measure the odour plume, single measurements are made in a defined dispersion situation, to measure the extent of the plume or the distribution of percentage odour times within the plume. The applications of the odour plume method are for calibrating a dispersion model, and for estimating the ambient odour characteristic with the aid of suitable statistical methods .
The single measurement is performed by the panel. Each assessor goes to his allotted measurement point, and tests by inhaling the air. Two methods are used for calculating the percentage odour time:
a)
The assessor tests the ambient air at regular intervals, e.g. every 10 seconds,
which gives 60 samples in ten minutes. The odour frequency is the number of
positive responses divided by the total number of samples.
b) The assessor tests every breath, and switches the stopwatch on at the beginning
and off at the end of each perceived odour episode. The percentage odour time
is the sum of the odour episode times divided by the total measurement duration.
In accordance with the definition of the ambient odour characteristic, the assessor must test the ambient air for a definitely recognisable odour. The odour is definitely recognised if the assessor is capable of definitely identifying its quality.
The assessors are given a code of odour qualities (types), against which they can check the odour. The odour code is to be chosen depending on the objective of the test. It must be specified beforehand. Before commencing the test programme, the assessor should be given the opportunity of becoming familiar with samples of the various odours concerned. This is not however necessary if the assessor is already familiar with the odours concerned. Odours identified but not listed in the odour code must be reported as `Other odours`. The assessor describes in a few keywords the type of odour. Open-ended descriptions of the type of odour should only be used if the objective of the measurements is general odour mapping of the area, where an advance description of the odours is neither sensible nor possible.
In addition to measuring the percentage odour time, the assessor can also be asked to assess the intensity of the odour. He assesses the intensity of each inhaled sample on the rating scale of Guideline VDI 3882 Part 1. This allows calculation of the distribution of intensity findings and the percentage time for which each intensity level is present.
Intensity
rating scale of Guideline VDI 3882 Part 1:
0 no odour 4 strong
1 very slight 5 very strong
2 slight 6 extremely strong
3 distinct
While the assessor is performing a single measurement, his perceptions must be recorded as if recording test results from an instrument. The recording methods range from writing on paper to using an electronic data recording instrument. The method chosen also depends on whether `yes / no` answers or also intensity and/or odour quality are to be recorded.
Data collection using an integrating Stopwatch
During
the measurement duration the assessor stands at the measurement point, breathes
naturally and calmly, and starts the integrating stopwatch each time a definitely
recognised odour is perceived and stops it when the odour is no longer definitely
perceived. At the end of the measurement duration, the stopwatch shows the
cumulated time during which odour was definitely recognised. The results are
entered on a paper record form on the same principle as when using a timer
and simple counting device. Instead of the number of positive samples, the
cumulative time for which odour was perceived is entered. The percentage odour
time is then calculated by the following formula:
A+ = T+ / T