Electricity + Control September 2017

Figure 3: Calorimetric flow switches – also referred to as flow monitors – are often used to monitor the presence of a flow in a piping system.

to its precision. Apart from this, calorimet- ric flow switches are extremely tolerant of changes in the media properties such as tem- perature, density, dielectric constant or con- ductivity. Hence, they are equally practical in both stable and non-stable conditions. The ability to choose between digital and analogue output signals makes these devices even more flexible. If an analogue output is selected, for instance 4 … 20 mA, informa- tion about the change in flow can be inferred from the continuous signal via the span. When monitoring a filter, say, blocking can be de- tected in good time before it becomes a problem. Conclusion The latest generation of flow switches are the most powerful and the most intelligent yet. Ver- sions are available with built-in temperature meas- uring and diagnostic functions. Today’s users, in other words, are also in a position to monitor the temperature of the medium using these de- vices, so that a second measuring point can be dispensed with. The diagnostic function outputs a warning if a sensor defect is detected. In case of damage, the switching output can be used to trigger a downstream safety function.

When it comes to flow control in pipelines, flow switches are an efficient and economical solution.

ber of molecules passing by increases continuous- ly with increasing flow velocity.

Regulated heating (constant tempera- ture difference method) The measuring probe is basically identical in de- sign. With this method, however, the heating power is regulated so that the temperature differ- ence between the two sensors is kept constant throughout. Consequently, as the flow velocity increases, the heating power must be increased accordingly. It is thus a direct measure of the flow velocity in the medium. Unlike other switch types, calorimetric flow monitors have no moving parts in the medium. They therefore also work reliably wherever there is a likelihood of contaminated media, for exam- ple in cooling circuits. What’s more, they are vir- tually maintenance-free. The probe only requires periodic cleaning when used in very sticky media because adhesive residues could be detrimental

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Oleg Greber is in Product Management, Electronic Products, Industrial Instrumentation, at WIKA.

+27 (0) 11 621 0000 sales.za@wika.com www.wika.co.za