Olfactometry

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Examiner at an olfactometer

Olfactometry is the measurement of the test subjects' response to stimuli affecting the sense of smell . During a controlled presentation of air laden with odorous substances, the sensory perceptions occurring in humans are recorded. The devices with which the odor samples are presented to the test persons are called olfactometers . The unit of measurement for quantifying odors is the European odor unit , which is also used in the TA Luft . The Olf unit developed in the 1980s , however, is rarely used anymore.

Mathematical and scientific basics

The mathematical and scientific basis for olfactometry is the Weber-Fechner equation , which establishes a connection between the odor intensity I , the concentration of the odor stimulus c and the concentration of the reference stimulus c 0 ( k w is a constant):

Construction and development of olfactometers

An olfactometer essentially consists of a pump, a mixing device, any pre-dilution device, hoses or pipes and one or more odor tubes. The pump conveys an odor sample diluted with neutral air from the mixing device to the olfactory tubes. The pump noise should be minimized as far as possible in order to meet the requirements of occupational safety and to prevent the individual inspectors from being distracted. N -Butanol is used as the reference substance . This is usually provided in test gas cylinders.

The development of olfactometers began in 1888 with the Zwaardemaker olfactometer . In the 1960s, due to increasing neighborhood complaints, there was an increased need to make odor emissions - at the time mainly from agriculture - quantifiable. In the early 1970s, the first measuring systems appeared on the market, which were later called olfactometers.

Dynamic and static olfactometry

There are two different types of olfactometers. They differ in the method for diluting the odorous gases for the testers.

With static olfactometry, a partial volume of the exhaust gas is sucked into a suitable bag made of odorless material and then analyzed in the olfactometer. In this, the sampled exhaust gas is mixed with an odorless gas. The dilution results from the ratio of the gas volumes. Olfactometers that use this method are called static olfactometers.

The dynamic dilution is based on the mixing of two known gas volume flows, the odor sample and the neutral air . The dilution is calculated from the volume flows. Analogous to the term used for static dilution, devices that mix two gas streams and deliver through a common outlet are called dynamic olfactometers and dynamic olfactometry.

Measuring principle

During the measurement, a group of testers - called a panel - is presented with the sample to be examined in various dilutions. The testers are those persons whose respective sense of smell leads to the measurement result. In order to achieve usable results, the testers are required to meet numerous specified requirements, be it that they do not have a cold or that they have not used any odorous personal care products before the measurement. Usually, a group consists of four testers who are involved in the measurement at the same time, which is also the required minimum size.

During the measurement itself, the sample is fed to the panel in increasing concentrations. The dilution takes place with odorless neutral air, e.g. B. compressed air or - under appropriate laboratory conditions - ambient air. From the feedback from the testers, the dilution factor at which 50% of the testers could perceive an odor is determined in several series of measurements : the odor threshold .

For this dilution factor, the basic unit of odor concentration, the European odor unit per cubic meter (UU E / m³, usually for short: UU / m³), ​​is defined, here it is 1 UU / m³. The odor concentration of the examined sample is then a multiple of a UU / m³, corresponding to the dilution set for the odor threshold determination. The odor concentration in UU / m³ can be used just like the mass concentration in kg / m³. In analogy to sound, odor levels are sometimes given in decibels (dB G ), with the threshold concentration of 1 GE / m³ serving as a reference value.

Presentation of the samples

Two different methods are preferably used to determine the odor threshold and other threshold values:

  • Forced choice method
  • Yes / No mode

With the forced choice method , the examiner is offered two or more olfactory tubes , whereby the actual sample air flows from only one of these tubes, while the others are only flowed through by neutral air. The tester is now required to make a decision as to which pipe the sample is flowing from, even if he is not sure and cannot determine a difference. In order to reduce the range of fluctuation, the examiner is asked to indicate whether his statement is a presumption, suspicion or certainty. The tube through which the sample flows is varied.

In the yes / no mode , the tester has to evaluate the gas emerging from a odor tube to determine whether it smells something or not. The examiner is aware that so-called zero samples are also offered at random positions in the performance series. Zero samples are samples that only consist of neutral air. In this mode, the examiner can also be provided with an odor tube with neutral air as a permanent comparison option.

For both presentation methods, each sample may be presented for a maximum of 15 seconds. The pause between two performances must also be at least 30 seconds. The purpose of both time specifications is that the testers do not get used to a smell (adaptation).

Requirements for the device, laboratory and testers

In olfactometry, as in comparable cases, the safety of all persons involved should have priority: The usual safety regulations for laboratories and other workplaces must be observed, as well as the proper handling of substances that may be toxic. It is necessary to check in advance whether the exhaust gas to be sampled could contain toxic substances, since not only the testers but also the samplers could be endangered.

Since the testers are human, the measurement results are subject to certain fluctuations. In order to influence this as little as possible, the DIN EN 13725 standard contains a “code of conduct for testers and test persons” . In addition to the fact that the participants must be motivated, various behaviors are specified there, which are intended to ensure that the smell of the examiner is impaired as little as possible. For example, there are regulations on food intake before and during the olfactometric measurement as well as personal hygiene.

The requirements for the rooms in which the measurement is to be carried out can also be briefly summarized by the fact that all influences on the inspector are to be avoided there too. This applies to both smells and other distractions, such as B. Loud noises. The maximum carbon dioxide content in the olfactory area must not exceed 0.15%.

The main rule for the device is that all influences on the sample and the tester must be avoided. Materials that have a strong odor or tend to react with the odorous substances and thereby change them should not be used. In order to avoid “residual risks” despite careful material selection, one should also strive to keep the cable lengths for the odor sample as short as possible. When using sample bags, they should interact with the sample as little as possible. For example, bags made of polyethylene terephthalate meet these requirements.

In the technical area, the standard DIN EN 13725 stipulates, among other things, that the dilution unit of the olfactometer must cover at least the dilution range from 1: 128 to 1: 16384, with a minimum range of 2 13 (13 dilution levels) being observed between the largest and smallest possible dilution is. Current, commercially available olfactometers have fully automatic dilution systems with a dilution range from 1: 4 to 1: 65536, which corresponds to a range of 2 14 . The air flow to the testers must not be less than 20 l / min, whereby DIN EN 13725 also makes the following comment: “The opening should be shaped so that the air speed in the opening is at least 0.2 m / s. The air speed from the smelling cup is usually kept below 0.5 m / s in order to save the examiner discomfort. "

Measurement uncertainty and detection limit

In olfactometry, some components such as the dilution unit, reference gas, tester and measuring room have a particular influence on the measurement uncertainty . According to DIN EN ISO / IEC 17025, measurement laboratories must have a method for determining the measurement uncertainty and apply it. For a single laboratory this means in practice that double determinations have to be carried out. As this requires, among other things, a second olfactometer, it is usually not possible to determine the measurement uncertainty on the basis of duplicate determinations for individual laboratories.

When comparing several measurements of the same odor sample, there is a large variation in the results, especially if different laboratories are involved. In round robin tests , identical odor samples from different laboratories were measured under controlled conditions according to the specifications of EN 13725. Due to the great differences in quality between the participating laboratories, the results showed an enormous measurement uncertainty. With the standardized instruments of the ' Guide to the Expression of Uncertainty in Measurement ', or GUM for short, there is an expanded measurement uncertainty (at 95% confidence interval ) between four times and a quarter of the measured value. A measured odor concentration of 1000 GE / m³ therefore has a measurement uncertainty range between 250 and 4000 GE / m³. Accredited olfactometer laboratories at a high quality level can significantly reduce this measurement uncertainty. These laboratories deliver reproducible results by complying with the accuracy and repeatability of reference odorants specified in DIN EN 13725 . Today only a few laboratories can demonstrate compliance with these requirements on the basis of successful round robin test results.

The lowest perceptible odorous substance concentration that can be determined to be different from zero with 95% statistical certainty is set as the detection limit of olfactometry. To determine it, at least six independent measurements must be carried out.

literature

  • DIN EN 13725: 2003-07 Air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin

Web links

Wiktionary: olfactometry  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ DIN EN ISO 5492: 2009-12 Sensory analysis - Vocabulary (ISO 5492: 2008); Multilingual version EN ISO 5492: 2009. Beuth Verlag, Berlin. Pp. 17, 25.
  2. ↑ VDI 3477: 2004-11 Biological exhaust gas cleaning; Biofilters (biological waste gas purification; biofilters). Beuth Verlag, Berlin. P. 87.
  3. ↑ a b c Dietmar Mannebeck, Heinrich Mannebeck: Olfactometer development in Europe . In: Air cleanliness commission in the VDI and DIN standards committee KRdL (ed.): Odors in the environment . VDI reports 2195, 2013, ISBN 978-3-18-092195-2 , pp. 15-22.
  4. ↑ VDI 3884 sheet 1: 2015-02 olfactometry; Determination of the odor concentration with dynamic olfactometry; Implementation instructions for the standard DIN EN 13725 (Olfactometry; Determination of odor concentration by dynamic olfactometry; Supplementary instructions for application of DIN EN 13725). Beuth Verlag, Berlin. P. 18.
  5. ↑ VDI 3880: 2011-10 olfactometry; Static sampling (olfactometry). Beuth Verlag, Berlin. P. 2.
  6. ↑ DIN EN 13725: 2003-07 air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin. P. 13.
  7. ↑ DIN EN 13725: 2003-07 air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin. P. 6.
  8. ↑ a b c DIN EN 13725: 2003-07 air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin. P. 33.
  9. ↑ Monika Paduch: The odor level as a key parameter for differentiating between odor reduction and odor reduction . Hazardous substances - keeping the air clean , Volume 73 (2013) 10, pp. 429–434.
  10. ↑ a b c DIN EN 13725: 2003-07 air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin. P. 38.
  11. ↑ DIN EN 13725: 2003-07 air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin. P. 39.
  12. ↑ VDI 3880: 2011-10 olfactometry; Static sampling (olfactometry). Beuth Verlag, Berlin. P. 9.
  13. ↑ VDI 3884 sheet 1: 2015-02 olfactometry; Determination of the odor concentration with dynamic olfactometry; Implementation instructions for the standard DIN EN 13725 (Olfactometry; Determination of odor concentration by dynamic olfactometry; Supplementary instructions for application of DIN EN 13725). Beuth Verlag, Berlin. P. 26.
  14. ↑ VDI 3880: 2011-10 olfactometry; Static sampling (olfactometry). Beuth Verlag, Berlin. P. 12.
  15. ↑ a b c DIN EN 13725: 2003-07 air quality; Determination of the odor concentration with dynamic olfactometry; German version EN 13725: 2003. Beuth Verlag, Berlin. P. 31.
  16. ↑ Frank Müller: Determination of the measurement uncertainty in olfactometric emission measurements based on DIN EN ISO 20988 . Hazardous substances - keeping the air clean , Volume 69 (2007) No. 6, pp. 243–245.
  17. ↑ DIN EN ISO / IEC 17025: 2005-08 General requirements for the competence of testing and calibration laboratories (ISO / IEC 17025: 2005); German and English version EN ISO / IEC 17025: 2005. Beuth Verlag, Berlin. P. 36.
  18. ↑ VDI 3884 sheet 1: 2015-02 olfactometry; Determination of the odor concentration with dynamic olfactometry; Implementation instructions for the standard DIN EN 13725 (Olfactometry; Determination of odor concentration by dynamic olfactometry; Supplementary instructions for application of DIN EN 13725). Beuth Verlag, Berlin. P. 10.
  19. ↑ a b c Peter Boeker, Torsten Haas: The measurement uncertainty of olfactometry . Hazardous substances - keeping the air clean, Volume 67 (2007) No. 7–8, pp. 331–340 Download (PDF; 331 kB).
  20. ↑ Björn Maxeiner: Ring trial olfactometry - ring trial for dynamic olfactometry according to DIN EN 13725: 2003 . In: Air cleanliness commission in the VDI and DIN standards committee KRdL (ed.): Odors in the environment . VDI reports 1995, 2007, ISBN 978-3-18-091995-9 , pp. 31-45.