Electricity + Control September 2017

SENSORS. SWITCHES + TRANSDUCERS

Economical

Monitoring

Oleg Greber, WIKA

Flow switches based on the calorimetric measuring principle can represent a genuine alternative compared to classic flow meters.

Take Note!

The latest generation of flow switches is the most powerful and in- telligent yet. A flow switch based on the calorimetric meas- uring principle consists of a measuring probe with two temperature sensors. Unlike other switch types, calorimetric flow monitors have no mov- ing parts in the medium.

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W hen it comes to flow control in pipe- lines, flow switches are an efficient and economical solution. They are increas- ingly based on the calorimetric measuring princi- ple. Users can choose between digital output sig- nals and continuous analogue outputs. That classic flow meters can also perform the role of a flow switch is beyond dispute. However, this sensor type is mainly used to measure the ex- act flow rate per time unit, for example in order to determine water or fuel consumption. Volume or mass flows are recorded with an error of 2% or less. Flow switches are unable to match this extreme precision: on the other hand, flow meters require an appreciably higher investment owing to their specific design, extensive electronics and comparatively complex installation. If the sole objective of the application is to mon- itor whether or not, and if so to what extent, a me- dium is flowing, flow switches or flow monitors represent a much cheaper – not to mention more compact – alternative. They detect the flow and trip a switching mechanism if the measured value exceeds or falls below a defined velocity. Precision is not a key priority: between 2 and 5% non-re- peatability is the norm for contact switching. In

addition to flow monitors with a float body, turbine or impeller, a growing number of users meanwhile resort to devices featuring the calorimetric or ther- mal measuring principle, for instance for cooling circuits, the cooling lubricant systems of machine tools, filter units or dry run protection in pumps. Calorimetric flow switches follow the physical laws of heat transport in flows. They are based on one of two different methods, depending on the manufacturer, each leading to the same result re- gardless of the application. A flow switch which is based on the calorimetric measuring principle consists of a measuring probe with two temperature sensors integrated into it (see Figure 1 ). One of these sensors is heated continuously with the aid of a built-in heating ele- ment (wire-wound) with a constant heating power and measures the temperature at the sensor tip. The second sensor determines the temperature of the medium in the pipe. Consequently, a tem- perature difference occurs between the two sen- sors, which is registered by the electronics. Continuous heating (constant power method)

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The higher the flow velocity of the medium in the pipeline, the smaller this temperature differ- ence will be. The basis for this is a law of thermodynamics: A body with a temperature higher than its surroundings sup- plies a medium flowing past that body with energy in the form of heat. The molecules in the me- dium flowing past the probe tip collect heat and transport it away. The more molecules flow past, the greater the cooling effect. The num-

Figure 2: Flow switch, type FSD-3, with built-in temperature measurement and optional diagnostic function.

Figure 1: Schematic illustration of a measuring probe for a calorimetric flow switch.

42 Electricity + Control

SEPTEMBER 2017