Testing technical gases using ultrasound

No. An ultrasound camera does not detect the gas itself, but rather the ultrasonic waves generated by the flow patterns at the leak site. Whether a leak becomes visible therefore depends primarily on the pressure difference, the geometry of the leak, the flow rate, the measurement distance and the environmental conditions.

The type of gas primarily influences the economic and safety implications of a leak. With compressed air, the focus is usually on energy losses, whereas with nitrogen, argon, CO₂, oxygen, helium or methane, process stability, media costs, occupational safety or product quality are often the decisive factors.

In industrial leak detection, the distinction between audible sound and ultrasound is crucial. Most production noises fall within the audible, low-frequency range. Leaks in compressed air and gas pipes generate broadband turbulence with high-frequency sound components at small leak openings. Large leaks often generate audible sound as well; however, in production environments, this is frequently masked by machine noise.

The ultrasound generated by a leak is not caused by the gas itself, but by the turbulence that occurs as the gas escapes. The pressure difference, the geometry of the leak and the shape of the outlet opening determine the sound intensity and the frequency spectrum of the ultrasonic waves generated.

Higher frequencies offer better spatial resolution due to their shorter wavelength. However, as the frequency increases, so does atmospheric attenuation, which reduces the achievable detection range. The choice of measurement frequency therefore represents a trade-off between resolution, range and interference suppression. In many industrial applications, a frequency range around 40 kHz has proved to be suitable. In environments subject to interference, however, a different frequency window may yield better results.

Large-scale production environment: Technical gases and compressed air are assessed acoustically based on the flow at the leak point, not on their chemical composition.

Nitrogen leaks may be detectable acoustically if there is a sufficient pressure difference and turbulent outflow. However, the assessment differs from that for compressed air: in addition to media costs, process safety, inerting, protective gas atmospheres and oxygen displacement must be taken into account. Ultrasound identifies the sound source but does not assess the gas concentration in the room.

Argon and CO₂ can generate ultrasonic waves if there is a sufficient pressure difference and the leakage geometry is suitable. Even small leaks can result in significant costs if the gas is used as a process gas or inert gas. In the case of CO₂, additional considerations relating to ventilation and health and safety must also be taken into account. Acoustic leak detection helps to pinpoint the location of leaks, but is no substitute for a safety assessment of the plant.

Oxygen applications require particular attention to be paid to material compatibility, cleanliness, fire safety and system approvals. An ultrasound camera operates passively and does not require a test gas; however, acoustic detection is no substitute for a safety assessment. Operational guidelines and approvals must be adhered to before any measurements are taken.

Helium is frequently used for highly sensitive leak tests. Ultrasonics can quickly locate larger localised helium leaks on site when turbulent outflow occurs. However, extremely small leaks that fall below the acoustic detection limit remain a use case for highly sensitive helium leak detection technology.

In the case of methane or other flammable gases, the following applies: an ultrasound camera detects sound, not the risk of explosion. The method can only detect an acoustic source if there is sufficient pressure and turbulent outflow. Approved gas detection technology, explosion protection concepts and plant approvals take precedence.