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Olfactometry

WHAT IS OLFACTOMETRY?

The effect of odor on people cannot be detected by any physical devices such as sensor. Furthermore, it is not possible to identify odor perception just based on the amount of odorants. A number of chemical substances may create odor formation. Therefore, the impact of odor on olfactory system may greatly vary to the type of chemicals that cause odor impact as well as their concentration levels. It is very hard to identify the types of chemicals by analyzing. It is not possible to find a relation between odor strength and concentrations in the mixture by means of determining some kind of main elements in the mixture. Technical sensors cannot also detect the pleasant/unpleasant odors or distinguish them from each other The said separation can only be made by human nose. For these reasons human nose is considered as the best sensor for odor measurement.

In principle, olfactometry is a method of presenting odorous gas to human nose in controlled way with certain concentration and measuring the impact of odor on sense of olfaction. Application process of the method is described in 2nd Part (Std. 6879) of VDI Guideline.

Principles of Olfactometry

Olfactometry is based on two main principles. They are:
• To calibrate the olfactory sense of nose by providing familiar odor to human nose at certain concentrations and measuring the impact of provided odor,
• To determine the concentration of unfamiliar odors with the help of human nose acting as detector.
“Olfactometer” is the name of devices measuring odor concentration by virtue of olfactometry method.

Usage Areas of Olfactometry Results

Odor concentration measured by olfactometer
may be used for modeling the dispersion of odorous gases within the atmosphere and calculating the expected odor emission values
Odor strength measured by olfactometer helps the classification of odor ranging from “not perceptible” to “extremely strong”. Measured odor strength depends on the source of odor.
Hedonic tone of the odor (pleasant/unpleasant) condition also depends on the odor source. Based upon the odor concentration, it is possible that an odor may be pleasant at low concentration while being unpleasant at high concentration levels (may be irritating)

Definitions

Odor (OU-Odor Unit)
1 OU equals to the amount of dispersed odorous substance in 1m3 neutral air when odor is perceived by human nose 1 OU/m3 is a starting point in the scale of odor strength.
Neutral Air
It is the air identified thermodynamically in terms of T, P and moisture. Neutral air should not contain any odorous substance or other elements that affect the odor perception.
Odor Threshold
The concentration level of an odor during the time when 50 persons out of 100 perceive the said odor shows “Odor Threshold” value and it is defined as 1 OU/m3.
Olfactometer
Olfactometer is a device used for presenting a sample of odorous gas to panelists by diluting with certain rate of neutral air and then recording the responses of panelists.

Odor Sources

There are various types of odor sources. Some odor sources are mentioned below:
• Waste water treatment facility
• Lacquer-Varnish and Dye Plants
• Paint shops
• Food Production Plants
• Animal Feed Production Plants
• Animal farms
• Refinery and Petroleum Chemical Production Facilities
• Fertilizer Plants

Sampling

Two basic sampling methods are used for odor measurement:
Continuous Sampling: Gas flow coming from the odorous gas source to be analyzed continuously provides odorous gas to olfactometer and it is measured in olfactometer.
Intermittent Sampling: The gas extracted from odorous gas source to be analyzed is filled in a “sampling bag”. Then it is sent to the laboratory. It is measured by means of olfactometer in the laboratory environment.

It is necessary to pay attention to following issues in order to avoid making some critical mistakes.
a) Pre-dilution with dry and neutral air should be carried out while taking gas sample from the source in order to prevent some gases from condensing;
b) No particulate matter should penetrate into the olfactometer;
c) Sampling system should be odorless
d) During the sample transfer from odor source to olfactometer, it is necessary to prevent compounds of gas sample from reacting chemically or prevent some compounds from adsorbing on side walls of gas sampling system.
Such kind of problems should be eliminated by testing the sampling system beforehand for different time frames between sampling and analysis.

Measurement Method

First of all the sampling bag (single or multi-component mixture) containing the gas whose odor concentration is measured is connected to olfactometer. Odorous gas sucked from the bag is diluted until reaching the neutral air level and odor threshold in olfactometer. Numerical value of odor concentration depends on the ratio of odorous gas and neutral air volumes that is required for reaching the odor threshold. “Threshold value” shows the achieved dilution rate when reached to odor threshold. The unit of odor concentration is “odor unit”, that is OU/m3.

The gas sample at a concentration level above the “odor threshold” value is diluted with neutral (odorless) air in a certain ratio. Odorous gas which is already diluted is directed to panelists sitting around the olfactometer in a certain order and starting from low concentrations. There are 4 panelists. Olfactometer is equipped with “sniffing ports” (nose masks) that allow panelists to sniff at odorous gas. Panelists perceive and evaluate the odor by means of these ports.
Gas samples with various dilution rates are provided to panelists. Despite of being highest level at the beginning, dilution rate is gradually lowered. The gas mixture supplied firstly is highly diluted manner and so odor rate is very low. There is an inverse proportion between dilution rate and odor strength. During the tests, the fifth person acts as "Leader" and adjusts the dilution rates. Leader adjusts the dilution rates in such a way that proper measurement of odor concentrations around the odor threshold shall be ensured.

In comparative odor measurement tests, if the strength of an odor source is reduced by technical measures to be taken, a panelist group should compare the odors before and after the said process and assess the efficiency of diminishing odor concentration.

In principle, only the persons, who have "standard nose" according to tests carried out with certain chemicals, may be assigned as "panelist". Although it has been proven that the sansitivity range of the persons to a single type odor is significantly wider than the mixtures of many different substances, such kind of standard mixture has not been able to be made yet. Therefore, "n-butanol" is used in testing the sense of olfaction of panelists and calibrating accordingly. "Odor threshold concentration" of a panelist should range between 20-80 ppb, especially around 40 ppb for the tests carried out with n-butanol. The persons out of the said range cannot be selected as panelist.

How does an Olfactometer work?
The olfactometer type T07 which is used for the odor measurements is a semi-automatic device that is controlled by a computer and requires 4 panelists and 1 test leader. Measurement results obtained by olfactometer are automatically transferred into computer and all calculations are conducted by computer program.

The gas pump of the device is operated with the help of odorless compressed air. Neutral air is supplied by an air compressor or compressed air cylinder. In case that neutral air is provided by compressor, the compressed air is firstly passed through a filter system consisting of silica gel, activated carbon and microfilter and then it is supplied to olfactometer. Flow rate of the air is controlled by flow meters. Gas sample whose odor concentration will be measured is connected to olfactometer and it is sucked by a gas pump from the sampling bag and then odorous gas is transferred to olfactometer. The amount of said gas is also controlled by flow meters. If the odor concentration is very high, odorous gas is subjected to pre-dilution process.

Odorous gas is mixed very well with the neutral air in the gas pump. This mixture is supplied to "sniffing ports" with the help of central distribution valve. Central distribution valve provides either odorous gas or neutral air to the sniffing ports of panelists. Panelist are informed of the time for sniffing with an optical signal. This process (breathing and sniffing) is conducted simultaneously for each (4) panelists.

Neutral air and odorous gas mixture is provided to panelists one after another. So and therefore, this process allows the panelists to compare the odorous air with neutral one directly. Consequently, the reliability of results has been increased. The measurement period may be determined as desired. However, it should be paid attention to keep the starting concentration under the odor threshold and dilution rate of odorous gas should be adjusted accordingly. The test should be started with odorous gas mixture after neutral air has been provided to panelist four times at least. The panelists are provided with the odorous gas mixture on a confused order and a certain order cannot be followed. After odorous content is provided to each panelist in the first round, the second one starts. In the second round, concentration of odorous gas is increased to a higher level. Therefore, further odorous gas is mixed into neutral air via needle valves. Mixing rate has already been watched by the leader on monitor.

Measurement period is controlled by a computer program automatically from the beginning to the end. Program includes different measurement periods following one after another. Obtained results are directly recorded in computer. Computer program assesses the test results and prepares a printable test report. It is printed out and then measurement period ends.

DETERMINATION OF ODOR LEVEL IN THE AMBIENT AIR BY FIELD INSPECTIONS

Determination of odor level in the ambient air by olfactometer does not give reliable results. Because the concentration of compounds that produce odor in the ambient air is far less and it changes considerably depending on meteorological events. Thus, determination of odor level in the ambient air is made by "field inspections" The area to be mapped for the field inspection of odor is divided into squares with one side of 125-250m. It is defined as the formation of "grid system" Odor measurements are performed in the corner areas of the said squares within a program.

The parameter to be measured in field inspections is the determination of "% odor time". This parameter shows in what frequency the odor in the ambient air exceeds the specified "threshold value" and how it can be detected by an assessor (receptor). The assessor goes to the measurement point and inhales the air for a certain time (10 sec.) in order to determine "% odor time". Assessor writes 1 if he/she perceives any odor, and writes 0 if not. This process is repeated in 60 times for 10 minutes. Consequently, this method is applied to identify an existing situation. Also, determination of odor as "pleasant" or "unpleasant" by means of questioning is an important part of the field inspections.

The measurements performed in "a single point" to measure the odor characteristics in the existing ambient air are repeated for each grid point one year long. If no source exists and it is requested to envisage how an odor to be emanated from a possible source will be dispersed in future, then Dispersion models or odor levels already measured from the similar source can be used for determining the odor characteristics in the ambient air.

"Single point" measurement is defined as the measurement of odor immission in a specified point for a certain period of time. It is necessary to make a 10 minute- measurement in order to make an evaluation that will represent the odor level in a specific point and in a certain time with the reliability level of 80% at least. Single point measurements are carried out for either determining the odor characteristics in the specified point during the grid measurements or determining the odor perception frequency in an odor plume.

Single point measurements are carried out by panelists. Each panelist goes to the measurement point and tests whether he/she perceives the odor or not by inhaling the air. Two methods are used for determining % odor time:
a) Panelist inhales for every 10 seconds in the measurement point to detect the odor. These measurements are repeated for 10 minutes. Panelist sniffs 60 times in 10 minutes. Odor frequency (% odor perception time) shows the ratio of breath number - that odor has been perceived - to total number (60) of breath.

b) It is tested by panelist whether there is an odor for each breath. He/she starts the chronometer at the time of perception and stops it when no odor is perceived. This process lasts for 10 minutes. The periods when odor is perceived are added to each other. When measurement ends, the ratio of total time of odor perception to total measurement time shows percentage (%) odor time.

According to definition of odor characteristic in the ambient air, panelist should certainly test a perceivable odor in the ambient air. Panelist should have the high capability of testing the perceivable odors. Possible odor types should be introduced to panelist before test period starts. Thus forming a basis for odor to be perceived during the test. Odor types depend on the aim of measurement (test) to be carried out. The aims should be determined beforehand. Before starting the test program, panelist should become familiar with the possible odor types and he/she has already sniffed at them.

But he/she is already familiar with the said odors, there is no necessity for introducing the odors to panelists. If "panelist" perceives an unknown odor, it should be reported under the title of "other odors". These odors may be identified with a few words by the panelist and recorded in the report. Open ended descriptions of unknown odors is only possible in mapping the general odors. For other cases it should be avoided making open-ended descriptions.

In addition to % odor time, panelist is requested to determine the odor strength. Panelist assess the odor strength of each breath according to odor scale specified in Part 1 of VDI 3882. This assessment is used for the calculation of the dispersion of odor strength and % detection frequency of different odor strengths.

Odor intensity scale specified in Part 1 of VDI 3882 is as follows:
0 : Not perceptible
1 : Very weak
2 : Weak
3 : Distinct
4 : Strong
5 : Very strong
6 : Extremely strong

The results obtained by single point measurement should be recorded as the test results carried out with a measurement device. Generally, the results are indicated on a paper or recorded in electronic environment. The results given by panelist are reported either as "Yes/No" or the strength/and quality of odor.
Data Collection with an Integrated Chronometer

If a panelist starts the chronometer when he/she detects the odor and stops it when odor disappear, total period of odor perception out of total measurement period can be read on chronometer. If the odor measurement is carried out by using this method, % odor time is calculated with the formula of
AT=TT/T Where;
AT : % odor time
TT : Perception time of odor
T : Total measurement period.