Page 34 of catalogue Fluid sensors and diagnostic systems of ifm Electronic GmbH

Flow Sensors

Flow Sensors and transmitters, Flow meters

System description

Introduction In almost all fields of process and plant engineering liquids or gases are used for coolant and lubricant supply of Machines and aggregates, ventilation of installations and buildings and the processing of products. Even the product itself may be the medium which is to be monitored permanently. In case of no flow of these media considerable damage and downtime may result. Thus it is very important to monitor that these media are at the right place at the right time and in sufficient quantities. In modern installations electronic flow monitors are used for this purpose. They work without wear and tear and without mechanical components. This guarantees reliable monitoring even in case of difficult media over a long period.

Electronic sensor: Wear-free monitoring of flow.

Use of flow monitors for monitoring seal water on Pumps.

M1

HZG

M2

Operating principle Many applications require a simple binary flow monitoring. This means that the states “medium flowing” and “medium not flowing” or value below or above a defined threshold must be detected safely. Electronic flow monitors operating on the basis of the calorimetric principle are perfectly suited for this. They use the physical effect that a flowing medium absorbs heat energy and conducts it away. The sensor tip contains two temperature-dependent measuring elements as well as a heat source. The heat source generates a local temperature rise in the medium which is detected by one of the measuring elements. If the medium flows, energy is conducted away from the heat source, i.e. it is cooled. The resulting temperature change is an indication of flow. To avoid a falsification of the result of the measurement by a change in the medium temperature, a second measuring element is used for temperature compensation. The difference in the measured values of the measuring elements results in the signals “medium flowing” or “medium not flowing” being provided by the control monitor after comparison with the set preset and limit values. Sudden high temperature changes of the medium can only be compensated for within the data given in the data sheet. In order to avoid a temporarily false output signal, these values should not be exceeded. As these systems work without any mechanically moved parts the user can mount them independent of mounting position and flow direction. For certain applications and environments preferred positions are recommended.

Principle of the transducer: Two temperaturedependent measuring elements (M1 / M2) and a heat source (HZG).

value in % of the measuring range limit value

100

80 calorimetric principle

60

40

20

0

20 40 60 80 flow velocity in % of the measuring range limit value

100

Preferred applications: The system is particularly sensitive in the area of low flow.

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