Introduction to Industrial Automation

Figure 2.45  Optical level switch is non-refraction state (a), and refraction through the liquid 

state (b).

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  Introduction to Industrial Automation

from a heating source due to a medium flow. This sensing technology can be reliably applied 

almost in any liquid or gas. As shown in Figure 2.47a, the flow switch consists of two resistor tem-

perature detectors (RTD) acting as temperature sensors and the required housing with electronics. 

The unheated RTD 1 sensor measures the temperature of the fluid where the probe is immersed. 

The RTD 2 sensor is heated from an embedded, constant, low-power heater and hence measures the 

temperature of the heater. This creates a temperature difference between the two sensors, which 

varies according to the existence or absence of flow, since the flowing medium cools the heated 

sensor. Particularly at high flow rates, the temperature difference is lower as flow removes more 

heat; at low flow rates, the temperature difference is higher, since flow removes less heat. The 

No flow

Flow

SPDT output

terminals 

Paddle

Flow switch

(pipe cross view)

Side view

Side view

Spring return

rotation 

NO

NC