When installing a flow sensor, there are a few things to consider: Regardless of whether it is a compact device with permanently connected electronics or a remote sensor with a separate evaluation unit.
In this article, we explain what is important when installing a flow sensor in order to achieve optimum measurement results. The focus here is on calorimetric flow sensors for monitoring and measuring gas flows.
Positioning the flow sensor in the air duct
For an accurate measurement result, a laminar flow should be aimed for. This means that the air should flow past the sensor evenly and without turbulence. Sources of turbulence such as bends, fans, ventilation flaps or constrictions should therefore be avoided. These elements can disturb the flow and thus falsify the measurement results. In new systems, such sources of interference can be easily avoided by installing the sensor on a straight section of the duct. In existing systems, you should consider whether it makes sense to relocate the sensor if necessary.
The optimum immersion depth of a flow sensor
A flow sensor should always be installed in the center of the pipe or duct. This central placement allows the aforementioned laminar flow to be measured. The correct positioning of the sensor tip is crucial. If in doubt, it is advisable to choose a slightly longer sensor. The rear end of the sensor may protrude from the pipe or duct without any problems as long as the sensor tip is correctly positioned. By choosing a slightly longer sensor, you have a little leeway to align the sensor if the flow does not run ideally in the middle of the pipe due to a bend, flap or other obstacle in the pipe.
Installation position of the sensor: upright or suspended installation position?
The sensor basically works regardless of the installation position. Nevertheless, there are a few things to bear in mind regarding the installation position.
If water vapor forms in your application, we recommend positioning the sensor in an upright position with the sensor tip pointing upwards. If water vapor forms, it can slowly run down the sensor. This will not damage the sensor. If the sensor is mounted in a suspended position with the sensor tip pointing downwards, the water vapor may drip off. This rapid temperature change can be misinterpreted by the sensor as a flow.
It also plays a role whether the sensor is installed in a horizontal or vertical pipe section. The background to this is the thermal effect, i.e. the rising of warm air masses. If the sensor is installed in a vertical pipe section, air can rise and be registered by the sensor. This is particularly important in applications in which small air flows are to be monitored or measured and in applications in which heated air circulates. If possible, you should therefore always install the sensor in horizontal pipe sections or set the switching point in such a way that small flows caused by thermals do not trigger the monitor.
The importance of inlet sections and outlet sections
Specific inlet sections and outlet sections are specified for each flow sensor, which should be adhered to in order to achieve optimum measurement results. The inlet section refers to the straight section of the duct upstream of the sensor, while the outlet section describes the section downstream of the sensor. Typically, the length of the inlet section is between 5 and 10 pipe diameters, while the outlet section is usually between 3 and 5 pipe diameters. These distances ensure that the flow of the medium is sufficiently stabilized before it reaches or leaves the sensor.
Deviations from the ideal installation specifications: What to do if inlet distances and outlet distances cannot be maintained?
In many projects, the ideal inlet and outlet distances cannot be adhered to. This mainly affects existing systems or complex mechanical systems.
The key question here is which measuring task is to be fulfilled. For simple switching point monitoring - for example, to ensure that a minimum amount of air is supplied to a room or that a fan in an exhaust air system is working - it is often possible to work with shorter distances. Measurement accuracy is less critical in this case. However, if it is a question of measuring the exact air speed or volume flow, all manufacturer specifications should be observed as far as possible, as accuracy is important here.
Modern volumetric flow meters such as the RLSW®9 also offer the option of storing a calibration factor. If there is a deviation in the measurement result due to the installation position, this effect can be counteracted with the calibration factor in order to avoid measurement inaccuracies.
Flow measurement directly at the fan - is that possible?
In some applications, no pipes or ducts are available for monitoring flows. Typical applications here are hazardous material containers, refrigerated containers, battery storage or pumping stations. Although the measuring conditions are not ideal, simply monitoring whether a fan is running properly is no problem.
You can also find more information on common errors in flow monitoring in our white paper: The 6 most common flow monitoring errors and how to avoid them.
Mounting options: Screw-in adapter, mounting flange or direct screw-in
Various options are available for mounting a flow sensor. From screwing the sensor in directly using a thread, to using a screw-in adapter or a mounting flange
Screw-in adapter
Screw-in adapters work like a cable gland. Once installed in the air duct, the flow sensor can simply be clamped in place and firmly positioned using the screw thread. In this way, you can easily adjust the immersion depth of the sensor and simply inspect and maintain the sensor. There are different types of screw-in adapters, including standard versions, ATEX-certified versions for potentially explosive atmospheres and special adapters for environments with high temperatures of up to 600°C.
The installation of these adapters is relatively simple and does not require any extensive modifications to the air duct. Please note, however, that you need access to the inside of the air duct to install the adapter. This may not always be the case in existing installations.
Mounting flange
A mounting flange provides a stable and secure attachment, especially for larger duct diameters. Different flange sizes and shapes enable optimum adaptation to the specific conditions of the air duct. This type of fastening also offers high stability and can be used in various industrial applications. For air ducts and large pipes, a rigid mounting flange is recommended, whereas small pipes require a flexible flange that adapts to the bend of the pipe.
Direct screwing in
Screwing the sensor in directly is a cost-effective method that is particularly suitable for OEM applications. This method also simplifies sensor maintenance as there are no additional fastening elements. Direct screw fittings are particularly suitable in applications where high mechanical loads are not expected. Please also note that the sensor cable must be able to rotate flexibly. You also have no leeway when positioning the sensor: the immersion depth cannot be subsequently changed.
Summary
Precise compliance with the installation specifications for flow sensors ensures accurate measurement results and a long service life for the sensor. Whether it's central positioning in the pipe, avoiding turbulence or selecting the right mounting option - every detail contributes to the optimal functioning of the measuring device. If you cannot comply with the ideal inlet and outlet sections, you should analyze the measurement task carefully and make compromises if necessary.
For further information or specific questions about our flow sensors, please contact our team of experts.
