Olfactometry

What is Olfactometry?

The effect of odor on humans cannot be measured by physical sensors. It is also not possible to describe odor detection in terms of the number of substances that create the odor. There are many chemicals that can create odors. Therefore, the effect of an odor on the sense of smell can vary greatly depending on which of the odor-causing chemicals are involved and in what proportions. It is very difficult to find out what these chemicals are by analysis. It is not possible to find a relationship between the intensity of the odor and the concentration of it in the mixture by determining some of the main components in the mixture. In addition, it is impossible to evaluate the odor as pleasant or unpleasant or to distinguish between pleasant and unpleasant with technically developed sensors. Only the human nose can make this distinction. For these reasons, the human nose is considered the best-known sensor for measuring odors.

Olfactometry is basically a method of presenting a controlled concentration of an odorous gas to the human nose and measuring the effect of this odor on the human olfactory sense. The method is described in VDI Guideline 2449, Part 2 (Std.6879).
 
Principles of Olfactometry

Olfactometry is based on 2 main principles:

  • Calibrating the nose’s ability to smell by presenting a particular (familiar) odor to the human nose in specific (pre-determined) concentrations and measuring its effect,
  • Determining unknown odor concentrations using the human nose as a detector.
    Devices that measure odor concentration using the olfactometry method are called “Olfactometers”.
 
Where to Use Olfactometer Results?
  • Odor concentration was measured with an olfactometer,
  • It can be used to model the dispersion of odorous gases in the atmosphere and to calculate expected odor emission rates,
  • The odor intensity measured by an olfactometer helps place the odor on a scale ranging from “not noticeable” to “very strong odor.” The measured odor intensity depends on the odor source.
  • The hedonic quality of an odor (pleasantness/unpleasantness) depends on the odor source. Depending on the odor concentration, an odor that is pleasant at low concentrations may be unpleasant (irritating) at high concentrations.
 
Definitions

Odor (OU-Odor Unit):
The amount of odorous substance dispersed in 1 m3 of neutral air at the moment when the human nose senses the odor is 1 OU. 1 OU/ m3 is a starting point on the odor concentration scale.
Neutral Air:
It is an air that is thermodynamically described in terms of T, P, and humidity. Neutral air should not contain any substance that creates a certain sense of smell or affects the perception of smell.
Odor Threshold:
The concentration of an odor at the moment when it is perceived by 50 out of 100 people exposed to the odor is the “Odor Threshold” value and is defined as 1 OU/m3.
Olfactometer:
An olfactometer is a device in which a sample of an odorous gas is diluted with neutral air at certain rates and presented to the noses of panelists and their reactions are recorded.

 
Sources of Odor

The sources of odor can be very different. The following sources can be given as examples of odor sources:

  • Wastewater treatment plants
  • Lacquer-Varnish-Paint factories
  • Paint shops
  • Food processing facilities
  • Animal feed factories
  • Livestock farms
  • Refinery and petrochemical plants
  • Fertilizer factories
 
Sample Collection

Basically, there are two different sampling techniques for odor measurements:

  • Continuous Sampling – A gas stream taken from the odorous gas source to be examined continuously provides odorous gas from the source to the olfactometer and the olfactometer makes a measurement.
  • Discrete Sampling – The gas taken from the odorous gas source to be examined is filled into a “sampling bag”. It is brought to the laboratory and measured with an olfactometer.

In order to avoid some important errors during sampling, it is necessary to pay attention to the following points:

  • To prevent condensation of some gases during sampling, pre-dilute with dry and neutral air should be performed during gas sampling from the source.
  • No particulate matter should enter the olfactometer.
  • The sampling system should be odorless.
  • During sampling, transfer from the odor source to the olfactometer, chemical reactions between components in the odorous gas sample or adsorption of some components on the walls of the gas sampling system, should be prevented.

Such problems should be avoided by testing the sampling system as much as possible in advance and by testing different time periods between sampling and analysis.

Measurement method

When measuring with an olfactometer, the sample bag containing the gas whose odor concentration is to be measured (a single or multi-component mixture) is first connected to the olfactometer. The odorous gas absorbed from the bag is diluted with neutral air in the olfactometer until it reaches the odor threshold. The numerical value of the odor concentration depends on the ratio between the volumes of odorous gas and neutral air required to reach the odor threshold. The dilution ratio found when the odor threshold is reached is called the “Threshold value”. The unit of odor concentration is “Odor unit”, i.e. OU/m3.

In the olfactometer, a gas sample with a concentration above the “odor threshold” concentration is diluted with neutral (odorless) air at certain rates. The diluted odorous gas is sent to the panelists sitting around the olfactometer in order, starting with very low concentrations. The number of panelists is 4. In front of the panelists, there are “olfactory detection ports” (olfactory masks) through which they will smell the odorous gas sent. The panelists perceive and evaluate the odor through these ports.
Gas samples are sent to the panelists at different dilutions. The dilution rate is the highest at the beginning, then it is gradually reduced. That is, the first gas mixture sent is very dilute, and the odor is very low. As the dilution rate decreases, the odor increases. During the experiments, a 5th person acts as the “Leader” and sets the dilution rates to ensure that a sufficient number of odor concentrations around the odor threshold are measured.

In comparative odor measurements, for example, when it comes to reducing odor by technical measures taken at a source of odor, a panelist group is used to compare the odor before and after the evaluation and to examine the odor concentration reduction efficiency.

As a rule, only those who have been proven to have a “standard nose” when tested with certain chemicals can serve as “panelists”. Although it has been proven that the range of sensitivity of individuals to a single type of odor is wider than the range of sensitivity to the odor produced by a mixture of many substances, such a standard mixture has not yet been prepared. For this reason, “n-butanol” is used to test and calibrate the olfactory abilities of panelists’ noses. In tests with n-butanol, a panelist’s “odor threshold concentration” must be around 40 ppb and in the range of 20-80 ppb. Persons falling outside this range cannot be panelists.

How to Measure with an Olfactometer?

The TO7 Model Olfactometer used in odor measurements is a semi-automatic device that can be controlled by a computer and works with 4 panel members and an experiment leader. The measurement results obtained in the device are automatically transferred to the computer and all calculations are made with a computer program.

The gas pump in the device works with compressed air that does not contain any odor. Neutral air is provided by an air compressor or a compressed air cylinder. If neutral air is to be provided from a compressor, the compressed air is first passed through a filter system containing silica gel, active carbon, and microfilter and this air is given to the olfactometer. The flow rate of the air is controlled with flow meters. The gas sample to be measured for odor concentration is connected to the olfactometer and the odorous gas is sent to the olfactometer by suction from the sample bag with the gas pump. The amount of this gas is also controlled with flow meters. If the odor concentration is too high, the odorous gas is subjected to pre-dilution.

In the gas pump, the odorous gas is thoroughly mixed with neutral air. This mixture is sent to the “olfactory detection ports” with the help of a central distribution valve. The central distribution valve sends either odorous gas or neutral air to the panelists in the olfactory detection ports. The panelists are notified of the sniffing moments with an optical signal. This process (breathing and sniffing process) is carried out in parallel for 4 panelists.

The neutral air and odorous gas mixture is directed to the panel members one after the other. Therefore, the panel members have the opportunity to make a direct comparison between odorous and neutral air. In this way, the reliability of the results is also increased. The measurement period can be adjusted as desired. However, care should be taken to ensure that the initial concentrations in the measurements are below the odor threshold and the dilution rate of the odorous gas should be adjusted accordingly. After the panelists receive neutral air at least four times, the tests with the odorous gas mixture should be started. This gas mixture is given to the panelists in a staggered manner, no specific order is followed. After each panelist is tested once in the first round, the second round is started. In the second round, the odorous gas concentration is adjusted to a higher level. Therefore, more odorous gas is mixed into the neutral air with the help of needle valves. The mixing ratio is shown to the panel leader on the computer screen beforehand.

The entire measurement period is controlled by the computer program and is applied automatically. The program also includes different measurement periods applied one after another. The results obtained are recorded directly on the computer. The test results are evaluated with the help of the program in the computer and the program prepares a test report as output. This report is printed on the printer and the measurement process ends.

Determination of Odor Level in Ambient Air by Field Inspection

Determination of the odor level in the ambient air with an “olfactometer” does not give accurate results, because the concentration of odor-causing compounds in the ambient air is much lower and varies greatly depending on meteorological conditions. For this reason, the determination of the odor level in the ambient air is done with “area measurements”. For area measurements, the area where the odor map will be created is divided into squares with a side of 125-250 m. This work is defined as the creation of a “grid system”. Odor measurements are made at the corner points of these squares within a certain program.

The parameter measured in field measurements is the determination of “% odor time”. This parameter determines how often the odor in the ambient air exceeds a specified “threshold value” and can be detected by a “receptor”. For the determination of % odor time, the “receiver” goes to the measurement point in the area and draws the air of the environment in for a certain period of time (10 seconds). If it detects a smell, it writes 1, if not it writes 0. This process is repeated 60 times for 10 minutes. Therefore, this method is used to detect an existing situation. Also, determining whether the smell is “pleasant” or “unpleasant” by questioning is an important part of field measurements.

Measurements made at a “single point” to measure the odor characteristics present in the ambient air are repeated for one year for each grid point in a grid system created in a specified measurement area. If there is no existing source and it is desired to predict how the odor from a future planned source will spread, then dispersion models or measurements of odors emitted from previously existing similar sources can be used to determine the odor characteristics in the ambient air.

A “single point” measurement is defined as a measurement of the odor emission at a specific defined point during a specific measurement period. In order to make an assessment that can represent the odor situation at a certain point at a certain span of time with at least 80% reliability, it is necessary to make a measurement at that point for at least 10 minutes. Single-point measurements are made either to determine the odor characteristics in the ambient air at that point in grid measurements or to determine the frequency of odor detection at that point in an odor plume.

Single measurements are made by panel members. Each panel member goes to the point where the measurement will be made and inhales the air at that point to test whether they can detect an odor. Two methods are used to calculate the % odor detection time:

  • The panel member takes a breath every 10 seconds at the point where the measurement is made and notes whether or not he/she perceives a smell. These measurements are repeated for 10 minutes. The air is sniffed 60 times in 10 minutes. The smell frequency (% smelling time) is the ratio of the number of breaths in which the smell is perceived to the total number of breaths (60).
  • The panel member tests whether there is an odor in every breath he/she takes at the point where the measurement is made. He/she starts the stopwatch in his hand when he smells it, and stops the stopwatch when he/she does not smell it. This process continues for 10 minutes. The number of the span of time when the odor is smelled are added together on the stopwatch and measured. The ratio of the total time the smell is smelled to the total measurement time at the end of the measurement gives the % smell time.

According to the definition of odor characteristics in the ambient air, the panel member must definitely test for a perceptible odor in the environment. The panel member must have a good capacity to test perceptible odors. Before the testing process begins, the panel member should be introduced to the types of odors that may be present at that point. In this way, a reference is created for the panelist in terms of odor. The types of odors depend on the purpose of the measurement (test). These need to be determined in advance. Before starting the test program, the panel member should be familiar with the possible types of odors and should be made to smell them.

However, if the panel member already knows these odors, then there is no need to introduce them. If an unknown odor is detected by the “panelist” at the measurement point, this odor should also be reported as “other odors.” Such odors can be described by the panelist in a few words and recorded in the report. It is only possible to describe unknown odors with open-ended descriptions when general odor maps are prepared. Apart from this, open-ended descriptions should be avoided.

In addition to the % odor detection time, the panelist may also be asked to determine the intensity of the odor. The panelist evaluates the perceived odor intensity with each breath taken according to the odor scale specified in VDI 3882 Part 1. This evaluation is used to calculate the distribution of odor intensities and the % frequency of odor detection at each intensity.

The odor intensity scale specified in VDI 3882, Part 1 is given below:
0 : No odor
1 : Very little
2 : Little
3 : Evident
4 : Intense
5 : Very intense
6 : Extremely intense

The results perceived by a panelist while making single measurements should be recorded in the same way as the results of measurements made with a measuring device. Generally, the results are either written on paper or recorded electronically. Here too, the results obtained from the panelists are reported either as “Yes/No” or as the intensity/and quality of the odor.


Results Collected with an Integrating Chronometer

During the measurements, if a panelist starts the stopwatch at each odor detection and stops it at the end of the odor detection, the total time odor detection within the total measurement period can be read from the stopwatch. If the odor detection is done in this way, the % odor detection time is:
It is calculated with the formula AT=TT/T
AT : % olfactory time
TT : The span of time during which the odor is perceived
T : Total measurement time.