- Flow sensors
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- Thermal flow sensors
Thermal flow sensors
Thermal flow sensors enable reliable velocity measurement (m/s) of liquid and gaseous media in pipes based on the calorimetric measuring principle. By stating the internal pipe diameter, it is also possible to make an approximate calculation of the flow rate (l/min).
ifm offers the sensors with robust housings made of materials such as stainless steel, titanium, ceramic and Hastelloy. They offer a high level of reliability even in harsh environmental conditions. Thanks to a broad selection of process adapters, the sensors can be used in nearly all industrial applications.
In general, thermal flow sensors are suitable for a wide range of media and applications. The basic prerequisite for the functional principle is heat removal from the medium. A thermal sensor can then be taught to a specific medium using the teach function.
Hydrous media
Thanks to their good thermal connection, thermal flow sensors are ideal for monitoring cooling and water supplies, for example in industrial pumps and machines or in building technology.
- Server cooling
- Pump monitoring / dry run protection
- Cooling of machine tools
- Consumption measurement / trend monitoring
Glycols and oils
The integration of calculated media curves enables use in viscous oils. Our sensors calibrated in water can also realise simple trend detection of the plant status.
- Run-dry protection
- Filter monitoring
- Cooling circuit monitoring
- Industrial air conditioning systems HVAC
Gases
Specially calibrated thermal flow sensors have been developed for the supply of fresh air and the extraction of aerosols, exhaust air or vapours. The sensors are to be used in constant flows with constant temperatures.
- Air supply monitoring
- Extraction systems
In principle, thermal flow sensors are compatible with a variety of media, which is also evident from the wide range of possible applications. Gaseous media such as air, water-based media, glycol and oils can be monitored with thermal sensors. The peculiarities of the individual media are described in more detail in the slider.
Turbulence
Laminar vs turbulent flow profile
Thermal flow sensors are designed to operate in a stable flow profile. A laminar flow profile (1) is characterised by a uniform, orderly movement of the medium whereby the flow velocity is highest in the centre of the pipe. A turbulent flow profile (2), on the other hand, is characterised by uneven movements of the medium, which can lead to vortices or disordered flows that distort the flow profile (2).
Installation before and after pipe bends
Structures in the pipe, bends, valves or reducers generate different turbulences in the system that lead to distortion of the flow profile and must be taken into account when interpreting the measurement results.
For this reason, the installation situation must be selected in such a way that the medium can calm down via an inlet section and a stable flow profile is present at the measuring point.
Mounting behind a source of interference
For this purpose, the recommended distances between the sensor and the interference source must be maintained as a calming distance and an immersion depth of at least 15 mm, preferably in the centre of the pipe.
Temperature
Temperature stratification
Particularly with oils, serious temperature stratification can occur within the pipes. In this special case, please note the following:
A central pipe installation is recommended to minimise the influence of the ambient temperature on the sensor tip. We offer sensors with measuring tips of different lengths for this purpose.
Temperature differences
Due to the heat transfer between the sensor housing and the sensor tip, the media and ambient temperature should only have a small temperature difference, especially in air and gas applications. Due to the thermal operating principle, the sensor tip placed in the medium must assume the temperature of the medium in order to achieve the specified accuracy.
Temperature jumps
Rapid and uneven temperature changes can cause temporary misinterpretation. The temperature gradient indicates how much the temperature changes in a defined time interval. The higher the temperature gradient, the faster the temperature in the medium rises or falls. A temperature gradient > 0.5 k/min can significantly influence the measuring accuracy of thermal sensors, as the sensor and reference element have different thermal connections to the medium due to temperature stratification.
Conclusion on mounting
Due to the large number of applications, media and installation situations, the notes in the operating instructions represent the minimum requirements for the inlet and outlet sections (D = diameter) in order to achieve reproducible measurement results. For the best possible performance: the greater the distance from the interference source (S) to the sensor, the more stable the flow profile.
It is important to understand that every thermal flow sensor, regardless of its design, only measures the flow velocity at one point in the pipe.