Measuring natural gas odorization: tests and certificates

Detailed study
21 November 2022

Odorability tests for biomethane, for its release into distribution networks, must comply with UNI 7133:2019 and must be repeated every three years

Biomethane, before being fed into distribution networks, must undergo odorability tests, asUNI/TS 11537:2019 prescribes.The indication on which tests to carry out to odorize gas may be found in Attachment G of UNI 7133-2:2019. This is the standard providing the definition of the main terms used in fuel gas odorization (or rhino-analysis) for domestic and similar use and describing how to perform a rhino-analytical test.      

Capiamo quindi meglio:

  • the elements that characterize rhino-analysis;
  • how the rhino-analytic test is performed;
  • when tests are to be performed;
  • whatthe issued test reportmust contain.

Rhino-analysis elements

Rhino-analysis is a sensory technique, regulated and defined by the UNI 7133 standard, designed to verify the correct degree of odorization of gases and biomethane in particular.The rhino-analytical test must be performed in an Accredited laboratory (UNI CEI EN ISO/IEC 17025) and consists of a sensory analysis in which the most important instrument used is the human nose, trained and calibrated for each test. 

To perform a complete rhino-analytic analysis, several elements are required.



These are selected individuals, trained according to very specific requirements described in the standard. Their characteristic is their average olfactory sensitivity giving consistent responses over time to a reference odor. Their analyses are validated and constantly monitored based on the odor intensity scale divided into 10 olfactory grades (DecaSales scale).  

DecaSales Scale

Each degree on this scale corresponds to an increasing intensity starting from zero odor (equal to level zero on the scale) to a maximum odor, also called the "upper limit of sensation" (equal to level 10 on the scale). 

The scale takes into account the "perceptibility limit" which is the lowest intensity of odor perceptible by an individual and corresponds to grade 1. 

Grade 4 (average odor) is very significant and defined as "perceptibility safety". This represents the intensity of odor that any individual with average olfactory faculties and under average efficiency conditions cannot fail to detect.    


It is an organic chemical, or the combination of chemicals, having an intense odor, added to gas in small concentrations and capable of imparting an alerting (usually unpleasant) characteristic and distinctive odor

The most common odorants in Italy are THT (tetrahydrothiophene, C4H8S) and the mercaptan mixture TBM/IPM/NPM.    

Rhino-analytical chamber

This is a room completely lined with odor-insulating material where to perform olfactory evaluations on odorizer-added gas.  


Test report

This is the document produced by the laboratory performing the tests required for odorization control. The issued certificate reports the compliance rating corresponding to a value of degree 4, or more, on the Decasales Scale, as well as an affirmative response of odor type recognition for at least 85% of the examiners taking part to the experiment.

How to conduct the rhino-analytic experiment

The examination team consists of two rhino-analysts and a panel leader. The latter is in charge of setting up the test, preparing the mixtures and processing the collected data. The test foresees flushing unscented sample gas inside the chamber.

Through a plate heated up to high temperatures, the odorizer transforms into vapor, and thanks to a mixing system, the two components (gas and odorizer) get equalized to create a single matrix. Then, examiners perform the sensory analysis taking the proper precautions and working in compliance with safety rules.

To assess the type of odor in the odorous fluid, examiners make qualitative and quantitative odor assessments based on the DecaSales scale and the known reference odorizer.

When to perform olfactive tests

Olfactive tests must take place at the first feed-in and are to be repeated, according to UNI 7133-2:2019, whenever the biogas upgrading process changes or a variation in the type of biomass quantity and/or odorizer occurs. 

Anyway, they must be repeatedevery three years.

Accredited test report

Also according to UNI 7133, the test report issued following the rhino-analytical experiment must include identifying data regarding:

•sample, client and laboratory that performed the test;

•type of odorizer used;

•assessment of conformity according to Attachment G of UNI 7133-2:2019.

The biomethane sample is acceptable according to the odorizability test if two results are obtained:

•an odor intensity at the alarm concentration (1% gas in the air) equal to at least 4 olfactory degrees on the DecaSales scale;

•at least 85% affirmative responses to the question "Is the odor similar to that presented as a reference?"

The test should be repeated for all odorants used downstream of the plant.

Two tests (both with THT and TBM) are to be carried out in the case of feeding into the transport network.

The analytical technician, who performed the tests and the laboratory manager, sign the test reports.

LOD is the only private laboratory accredited according to UNI ISO 17025 and UNI 7133-3:2019 and is in a position to perform rhino-analytical tests for gas odorization, including domestic and similar gases, issuing the accredited test reports.  

Example results


The method of minimum squares allows determining the equation of the interpolating line segment.

The odor intensity value at the alarm concentration rate is approximated in its entirety.

What's new in the field of biomethane and hydrogen

Studies on blending biomethane and hydrogen in varying H2 concentrations ranging from 2 percent up to 30 percent have intensified in the past year.

Pilot projects and research on blending levels and the possibility of managing and feeding the blend into the network are currently underway.

Lod is actively taking part to a large experimental project. And it is in charge of odorability tests on the hydrogen-methane matrix.

Through such projects, our company contributes to decarbonization and the development of alternative and environmentally friendly energy.

Normative references

  • Technical Specification UNI/TS 11537:2019 - Feeding biomethane into natural gas transmission and distribution networks
  • UNI 7133-1,2,3,4:2019 Standards –Odorization of gas for domestic and similar use
  • UNI/TR 11722:2018 Technical Report - Guidelines for the preparation of risk analysis by producers of biomethane from biomass

LOD: Laboratorio Olfattometria Dinamica

Spin-off dell’Università degli Studi di Udine