How Temperature Switches Work ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How Temperature Switches Work

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A temperature switch works just like a typical electrical switch for on /off application. In this case, the temperature switch operates to switch on or off at discrete process temperatures. A temperature switch consists of two basic parts that you will find in all designs:
(a) A sensing part immersed in the process whose temperature is required to be controlled. The sensing part can either be a sensing bulb filled with a fluid –liquid, gas or a bimetallic strip that uses the differential expansion of two dissimilar metals.
(b) Snap-action contacts that act to switch on electrical power to the device controlling process temperature.

How a Temperature Switch Works
Liquid filled temperature switches comprises a sensing bulb and a bellows element. The bulb is immersed in the process whose temperature is being controlled. The bellows element senses fluid pressure (liquid or gas)
as temperature increases within the process. Below is a schematic showing the basic operating principle of a temperature switch:

As shown above, the fluid in the sensing bulb reacts to temperature variations and increases the pressure in the bellows element when temperature rises. A rise in the sensing bulb temperature compresses the bellows and moves the main spindle upwards until spring force and bellows pressure are in equilibrium. This movement of the spindle is transferred to the switch and causes on or off action depending on set point of the temperature switch.

Suppose we are controlling the temperature of a water bath being heated by a burner system powered by an electrical circuit and the water bath temperature is set at 75 degree C. The temperature switch will not activate so long as the water bath temperature is below 75 degree C. However when the temperature exceeds set point, the temperature switch activates switching off the electrical circuit controlling the burners.
A temperature switch having a bimetallic trip operates in similar fashion to the liquid filled system but the designs are markedly different.

Typical Terms/Specifications of Temperature switches:

Adjustable Range
The span of temperature between upper and lower limits within which the temperature switch can be adjusted to actuate or de-actuate. It is typically expressed for increasing temperature.

Set Point
That discrete temperature at which the temperature switch is adjusted to actuate or de-actuate on rising or falling temperature. It must fall within the adjustable range and can be expressed as increasing or decreasing  temperature.

The difference in temperature between the increasing set point and decreasing set point. It is normally fixed (not adjustable).

The ability of a temperature switch to successively operate at a set point that is approached from a starting point in the same direction and returns to the starting point over consecutive cycles to establish a temperature profile. The closeness of the measured set point values is normally expressed as percentage of full scale (maximum adjustable range temperature.)

SPDT Switch Element
Single-Pole, Double-Throw (SPDT) switching element has three connections: C-Common, NO-Normally Open and NC-Normally Closed, which allows the switch to be electrically connected to the circuit in either NO or NC state.

DPDT Switch Element
DPDT is two synchronized SPDT switching elements which actuate together at increasing set point and de-actuate together at decreasing set point. Discrete SPDT switching elements allow two independent circuits to be switched; i.e. one AC and one DC.

For fluid filled temperature switches, overrange temperature is that temperature to which the sensing bulb can be continuously exposed without causing permanent change of set point or distortion sufficient to cause leakage or significant degradation of the fill fluid. Temperatures greater than overrange could cause permanent damage and render the temperature switch inoperative.

Maximum Process Pressure.
The maximum process pressure to which the temperature sensing bulb should be exposed without being protected by a thermowell.