- Flow sensors
- Technology
Measuring principles of ifm flow sensors and flow meters
Magnetic-inductive
The SM type flow meter operates according to Faraday’s law of induction. The conductive medium flowing through a pipe in a magnetic field (M) generates a voltage which is proportional to the flow velocity (v) or volumetric flow quantity. This voltage is tapped via electrodes (E) and converted in the evaluation unit. Its resistant materials mean the sensor is suitable for a multitude of media. A high protecting rating and a robust, compact housing distinguish the sensor in the field.
The measuring principle is suitable for liquids with an electrical conductivity of min. 20µS/cm. Typical values for the electrical conductivity are 0.5µS/cm for distilled water, 50µS/cm for drinking water and 50,000µS/cm for salt water.
ISO calibration certificate for SM type flow meters: ZC0052
ISO calibration certificate for SM ATM type flow sensors: ZC0054
Vortex measuring principle
Behind a blunt/bluff body integrated in the measuring pipe, the flowing medium (water with and without conductivity) generates swirling vortices depending on the velocity. These vortices are detected by a piezoceramic sensor. If the cross-section is known, the number of the vortices allows to determine the flow rate.
This flow rate measurement principle, known as vortex principle, is virtually independent of pressure and temperature fluctuations of the medium.
Mechatronic measuring principle
The flow sensor works to the principle of spring-supported piston: The piston, located in the valve seat in the housing, is lifted by the flowing medium against the spring resistance.
The piston position is monitored via a magnetic field sensor and is output as an analogue signal. The spring resistance forces the piston to return to its original position with decreasing flow. This ensures position-independent installation of the flow sensor preventing backflow.
Another robust mechanical design (SBT) allows use at high temperatures up to 180°C and in harsh industrial environments.
Ultrasonic measuring principle
Ultrasonic sensors consist of transducers (on the right in the image) that transmit and receive sound pulses and reflectors (on the left in the image) that direct the pulses from one transducer to the other. A pulse is sent through the medium and a sensor measures the time of flight from one transducer to the other. Then a pulse is sent in the opposite direction. The sensor measures the difference in time (in the nanosecond range) and calculates the flow rate.
Calorimetric measuring principle
The types SA and SI have two measuring elements and a heat source.
The reference element which is attached 10mm above the ground measures the medium temperature and is used for temperature compensation. The temperature difference to the element on the ground is kept constant by means of the heat source located there. The power needed to keep this difference constant is proportional to the flow velocity. An increasing flow velocity generates a higher heat dissipation.
The SD type compressed air meter uses the same thermal principle. One of its ceramic measuring elements is heated (measuring element), the other one is not heated (reference element). The voltage difference which results when heat is carried away by the flowing medium is an indication of flow.
The standard volume flow (to ISO 2533) is directly detected.
Calibration certificate for SD type flow sensors: ZC0020
DAKKS calibration certificate for SD type flow sensors: ZC0075
Using both flow and pressure measurements, the SDP air gap sensor measures the distance in absolute distance values [mm]:
The closer a workpiece is to a measuring nozzle, the lower the quantity of air that flows through the air gap between the workpiece and the measuring nozzle. This makes it possible to secure the position of the workpiece and to clearly detect a zero gap or a clogged nozzle.