Air flow rate

Luftmeister measurement method for conditioned air

Accurately measuring the air flow rate in air ducts has long been considered an unresolved challenge. While an approximate measurement may still be possible in long, straight, round ducts, serious difficulties arise in most real-world applications. Short inlet sections downstream of bends, as well as rectangular duct geometries, lead to highly asymmetrical flow profiles. Very low air velocities, such as those found in outdoor air ducts, cannot be captured using standard sensors. And how should contaminated air (oil mist, solvents, acidic or alkaline air, etc.) be handled?

Luftmeister has developed innovative solutions for all of these challenges and has proven them at more than 2,000 measurement points over ten years of company history. What these solutions have in common is the use of patented Luftmeister probes. Thanks to an open-channel design, they capture all flow vectors along the length of the probe instead of measuring at individual points. A selection of these probes is installed in the duct, connected symmetrically, and then calibrated using a patented method. This process captures the flow and density distribution across the entire cross-section, providing an accurate representation of the actual operating conditions.

Why all this effort? One example illustrates the reason: if the goal is to eliminate typical waste caused by air flow oversizing (around 15%), it is of little value if the measurement solution itself already exhibits inaccuracies of 10% or even 20% – a situation that is unfortunately common, as is the complete absence of any measurement at all.
The solution is called Luftmeister.

Luftmeister measurement method for process air

Air is a key medium in many industrial processes, whether for transporting heat, enabling drying processes, removing pollutants or supplying combustion air. In many cases, large air volumes are moved, sometimes at high temperatures, and in some applications the air is contaminated. To use these air streams efficiently, minimum flow rates must always be monitored while avoiding unnecessary energy waste.

In practice, duct routing presents a number of challenges, making the measurement of flow rates (standardised volume flow, mass flow) or heat flow far from trivial. Luftmeister has solved this measurement task. The solution combines patented probes that capture flow vectors along their entire probe length (open-channel design) with a calibration method that records the flow and density field across the entire pipe cross-section, based on the actual operating conditions.

Probe solutions are available for use in aggressive air or at temperatures exceeding 1,000 °C, while other variants feature targeted probe purging to ensure high robustness in dusty environments. Outdoor installations naturally require different considerations than indoor applications. Luftmeister equips each measurement point individually according to the specific requirements – not based on a predefined standard.

Luftmeister measurement method for flue gas

Most industrial companies operate numerous burners that emit flue gas. In many cases, significant temperature reserves are still available, meaning that waste heat could be recovered from the flue gas without reaching condensation limits, for example. But how should a flue gas heat exchanger be properly sized?

If measurements are carried out as if the medium were air, additional measurement errors of up to 15% can occur. This is because the density and enthalpy (heat content) of air and flue gas are calculated using different equations and differ significantly.

Luftmeister has addressed this challenge with a pragmatic solution. Flue gas parameters are always available from emissions measurement reports. These parameters are entered into the air energy meter, which then calculates and outputs density, enthalpy and thermal power correctly for flue gas applications.

Luftmeister double-blade measuring sections

In certain applications, the highest levels of accuracy are required. For example, certified measuring sections used for consumption-based ventilation cost billing must measure air flow rate (and heat flow) with high precision across a wide velocity range from 1 to 10 m/s.

This Luftmeister development is based on patented probes with an open-channel design, which capture flow vectors along the entire length of the probe. In the double-blade measuring sections, these probes are arranged in a crossed configuration and tapped on both sides. This ensures that each quadrant of the duct cross-section is equally represented – a crucial factor for achieving accurate measurements even under asymmetrical flow conditions.

Each measuring section is additionally optimally adjusted in the high-precision Luftmeister air flow calibration laboratory. The resulting level of accuracy is not only used in the above-mentioned consumption billing applications. It also pays off quickly in cleanroom applications, where even small differences – for example between a required tenfold air change rate and an eleven- or twelvefold rate – can result in significant cost implications.

Luftmeister air flow calibration laboratory

A calibration laboratory is used to verify the accuracy of measurement solutions and to optimise their adjustment in order to achieve the highest possible level of precision. For the measurement quantity air flow rate, setting up and operating such a laboratory is particularly demanding – especially when very small, medium and very large flow rates must be generated and verified using highly precise reference measurement systems.

Luftmeister has implemented this by establishing a high-end air flow calibration test stand covering a range from 25 m³/h to 36,000 m³/h, with a total measurement uncertainty (k = 2) of less than 1%. This calibration laboratory is certified by the Physikalisch-Technische Bundesanstalt (PTB) as well as by the verification authorities and is re-audited annually – one of the key measures that ensures the continued certification of Luftmeister’s metering systems.

Customised, laboratory-calibrated measuring sections

The in-house air flow calibration laboratory enables Luftmeister to precisely adjust (almost) any customised measuring section design. Both round and rectangular cross-sections are possible, including sizes with circumferences exceeding 3 metres.

Depending on the specific requirements, sensor solutions can be selected that are optimised for very high, medium or very low flow velocities. These solutions may already be equipped with temperature, humidity and absolute pressure sensors for density compensation and can directly output the required measurement quantity – ranging from “simple” volume flow to standardised volume flow, mass flow and even heat flow.

A customised selection of communication interfaces is also available, from Modbus RTU and M-Bus to up to 11 analogue outputs – a wide range of configurations is possible.