VDI/VDE 3518 Part 4 is a new technical guideline that defines standardized test procedures and test-gas mixtures for VOC detectors used in indoor air quality (IAQ) measurements. Titled "Multigas sensors – Standardized test instructions and test gases for VOC detectors for indoor air quality measurement" it was published in April 2025 under ICS 71.040.40. This 42-page guideline aims to ensure that VOC-detector measurements are comparable and reproducible across different sensor technologies and manufacturers.
The guideline is primarily aimed at manufacturers of VOC detectors and multi-gas sensors, testing laboratories, certification bodies, and experts in indoor air hygiene and building services engineering. It provides a framework for evaluating sensor performance, including sensitivity, selectivity, response time, and long-term stability. By defining standardized environmental conditions, gas mixture compositions, and test sequences, VDI/VDE 3518 Part 4 establishes a methodology designed to enable more reliable assessments of indoor air quality. Does it succeed? Read on to find out.
The guideline series VDI/VDE 3518, developed by the VDI/VDE Multigas Sensorics Technical Committee, provides an approach for the use of multigas sensors.
- Part 1 defines terminology and system architecture for applications in comfort, diagnostics, process monitoring, and safety.
- Part 2 defines performance features, measurement ranges, functionalities, and test procedures.
- Part 3 describes applications for odour-related measurements using multigas sensors (this standard is actually withdrawn).
- Part 4 applies to sensors and measuring systems—collectively termed "detectors"—that detect VOCs in the gas phase and output one or more measured values representing indoor air quality with respect to VOCs.
Section 8.3, Exposure to test gas mixtures, specifies how VOC detectors must be exposed to defined gas mixtures during testing. This section covers gas-mixture composition, concentration levels, flow conditions, and the sequence and duration of exposures. It also requires the use of clean, VOC-free air as a zero-gas reference before and after each exposure step to ensure accurate baseline and recovery measurements.
The guideline defines test-gas mixtures that represent typical indoor-air pollution sources, including alcohols, aldehydes, terpenes, and aromatic hydrocarbons. These substances are chosen because they are commonly found in indoor environments due to cleaning agents, air fresheners, paints, adhesives, and furnishings. Concentrations are set at realistic indoor levels, typically in the low ppm to sub-ppm range, to reflect actual exposure scenarios rather than extreme industrial conditions.
Some mixtures are broad-spectrum, containing many VOCs at once, to test total VOC response and cross-sensitivities. Others are narrow-spectrum or single-component to calibrate or validate sensitivity to specific substances. This dual approach allows manufacturers and laboratories to assess both overall performance and interference behavior, which is key for reliable indoor-air-quality monitoring.
This guideline defines environmental conditions (temperature, humidity, airflow), test-gas mixtures, test sequences, and evaluation criteria for VOC detectors.
To achieve realistic indoor-air conditions, the guideline introduces the concepts of "zero air" and "normal air," where normal air is a defined mixture of zero air, humidity, and typical background gases such as carbon dioxide, methane, nitrous oxide, hydrogen, and carbon monoxide. This avoids overly idealized test setups and ensures that detectors are characterized under conditions that resemble real rooms rather than laboratory-pure atmospheres. The document also specifies how humidity and temperature must be varied within defined ranges during testing to assess temperature-humidity bias and to verify that the detectors remain stable over a broad operating envelope.
A new thing of this VDI/VDE 3518 Part 4 is the introduction of the parameter Total Volatile Organic Carbon, TVOCVDI, a total VOC indicator independent of the underlying sensor principle. According to the standard, there are different definitions for TVOC; instead of tying TVOC to one specific analytical method, TVOCVDI is defined through standardized test gas mixtures and procedures, allowing different detector technologies (MOX, PID, electrochemical, etc.) to be compared on a common basis. This parameter is then linked to an air-quality index that classifies indoor air from "very good" to "heavily polluted" on a numerical scale, which can be displayed to users as colors or discrete classes in building-management systems.
And there is a point here: Standards typically define methodologies, leaving regulators to set compliance limits. This is appropriate because those limits should be public and accessible to everyone in the form of an official bulletin, not contained in a standard that only buyers can access. In fact, I'm not sure whether the German Committee on Indoor Air (GKI) guidance values, which recently released a guideline with publicly available IAQ levels, would agree with the TVOCVDI limits set in this standard.
The standard does not aim to replace toxicological or hygienic evaluation of indoor air; instead, it offers a statistical classification of typical VOC levels. If a detector indicates a persistently poor air-quality class, VDI/VDE 3518 Part 4 recommends follow-up investigations using appropriate analytical methods to identify specific pollutants and their sources. In this way, the guideline positions TVOC-based detectors as early-warning devices while acknowledging the need for detailed chemical analysis in critical cases.
Beyond basic performance, the guideline dedicates several sections to long-term behavior, drift, and potential sensor poisoning. It describes how detectors must be tested over extended periods with realistic VOC mixtures, varying environmental conditions, and repeated exposure cycles. This is particularly relevant for low-cost multigas sensors, whose long-term stability and resistance to contamination are often limiting factors in real-world applications. By harmonizing these endurance tests, VDI/VDE 3518 Part 4 sets specifications to help users distinguish between short-lived gadgets and robust monitoring solutions.
The document also provides guidance on the minimum number of devices under test and on how to determine signal-stabilization time for each detector type. For example, the time constant t63 (time that elapses between a change in the test conditions and reaching 63% of the new, stable signal level) must be determined for the combined system of detector and test chamber. The guideline requires using a multiple of this time constant to ensure that sensors have truly reached stable readings before data are recorded.
Finally, VDI/VDE 3518 Part 4 specifies how test results should be evaluated and reported, including suggested scoring schemes for accuracy, cross-sensitivity, environmental robustness, and long-term repeatability. This structured reporting makes it easier for manufacturers to document compliance and for independent laboratories or certification bodies to compare different products transparently. For the indoor-air community, this guideline marks an important step toward more trustworthy VOC sensor data, better comparability between devices, and ultimately more reliable assessments of indoor-air quality in everyday environments.
But...
If you search for the word "odour" in the text, you won't find it. That's because this standard is not intended for this parameter unless there is a correlation between an odour and the TVOC (or TVOCVDI, if you wish). Such a correlation could make sense when evaluating the impact of an adhesive or a new carpet, but it's far less reasonable in other cases, for example, when trying to detect the smell coming from a restaurant on the ground floor or from wastewater in the baby's room.
A very low concentration of a mercaptan or an indole, for instance, would result in a very low TVOC value, meaning that the IAQ could be rated as "very good" while the smell is actually terribly stinky. So be careful with that. We have often seen in outdoor air monitoring how misleading it can be to report a rating of "very good air quality" while people are protesting outside because of an unbearable stench. Chapter 6 of this interesting paper has more information on the topic of odour, health, and TVOC.
If you're interested in learning more, we recommend purchasing the standard directly from the publisher here.
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