Temperature Switch

The SOR temperature switch utilizes a vapor-pressure thermal system. Fluid vapor pressure changes according to the influence of temperature on the sensing bulb. Process temperature changes cause proportional vapor pressure changes in the temperature sensing bulb that act on the diaphragm/piston assembly to actuate and deactuate a snap-action electrical switching element at discrete process temperatures. The instrument’s behavior is determined by the vapor pressure principle.

SOR recognizes that there is no industry convention with respect to terminology and definitions pertinent to temperature switches. This glossary applies to SOR temperature switches.

Adjustable Range The span of temperature between upper and lower limits within which the temperature switch can be adjusted to actuate/deactuates.

Capillary –  Small diameter tubing used to transfer the measured vapor pressure in the temperature bulb to the piston sensor on remote mount units.

Dead Band The difference in temperature between the increas­ing set point and decreasing set point. Also known as hysteresis, or reset.

Direct Mount –  Describes the style of the temperature switch where the temperature bulb and switch housing assembly are closely coupled when installed in the process line or vessel.

DPDT Switching Element – DPDT is achieved using synchronized SPDT switching elements which actuate together at increasing temperature set point and deactuate together at decreasing set point. Discrete SPDT switching elements allow two independent circuits to be switched.

Hermetically Sealed – A welded steel capsule with glass-to-metal, factory-sealed electrical leads that isolates the electrical switching element(s) from the environment.

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

Overrange – 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 device inoperative.

Remote Mount – Describes the style of the temperature switch where the temperature bulb and housing assembly are separated by distance and the vapor pressures from the temperature bulb are transferred to the piston sensor through a capillary tube.

Repeatability – 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.)

Set Point – That discrete temperature at which the temperature switch is adjusted to actuate/deactuate on rising or falling temperature.

SPDT Switching Element – Single-Pole, Double-Throw (SPDT) 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.

Temperature Switch  – A bi-stable electromechanical device that actuates/deactuates one or more electrical switching element(s) at a predetermined discrete temperature (set point) upon rising or falling temperature.

Thermowell – A tubular metal component that protects the temperature sensing bulb from high pressures or corrosive processes.