Introduction to Industrial Automation

Figure 2.46  Paddle-type liquid flow switch.

Flow

Calorimetric flow switch

housing and electronics

RTD 1 RTD 2 and heater

 

No flow

(a)

(b)

(c)

Thermal dispersion

around heater

Figure 2.47  Calorimetric flow switch mounted on a pipe (a) and thermal dispersion around its 

heater probe for fluid at rest (b) and for existence of flow (c).

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Hardware Components for Automation and Process Control 

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thermography around the sensor RTD 2 when the flowing medium is at rest or the medium is 

flowing is shown in Figures 2.47b and 2.47c correspondingly, and illustrates the heat dispersion 

principle. Therefore, the temperature difference is inversely proportional to the flow rate and pro-

vides a primary signal that is converted to an SPDT relay switching output.

A basic advantage of the thermal dispersion flow switch is that it has no mechanical moving 

parts providing precise and fast detection of flow and no flow. Compared to the traditional paddle 

type flow switch, thermal dispersion flow switches offer higher sensitivity and no limitation of 

installing locations, while are capable of responding under low flow rates.