Electricity + Control September 2017

Electricity

Control

SEPTEMBER 2017

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.

Continuous heating (constant power

method)

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.

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-

Economical

Monitoring

Oleg Greber, WIKA

Flow switches based on the calorimetric measuring principle can represent a

genuine alternative compared to classic flow meters.

SENSORS. SWITCHES + TRANSDUCERS

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.

Take Note!

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.