Electricity + Control September 2017

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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
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.
<<Author>>
Oleg Greber is in Product
Management, Electronic
Products, Industrial
Instrumentation, at WIKA.
+27 (0) 11 621 0000
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.