Pressure Switch

The pressure sensing element of the SOR pressure switch is a force balanced, piston-actuated assembly.  The sensing element is sealed by a flexible diaphragm and a static o-ring (the o-ring is eliminated on welded diaphragm systems).  Media pressure on the area of the piston counteracts the force of a range spring (adjustable by an adjusting nut) which moves the piston shaft only a few thousandths of an inch to directly actuate the electrical snap-action switching element. This principle of operation virtually eliminates friction and resultant wear.

Learn more on Pressure Switch Operating Principles

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

Absolute pressure – Pressure measured against a reference of full vacuum.  Absolute pressure is normally referred to as PSIA.

Adjustable range – The span of pressure between upper and lower limits within which the pressure switch may be adjusted to actuate/deactuate.

Dead band – The difference in pressure between the increasing set point and the decreasing set point. Also known as differential, hysteresis, reset value and reset.

Differential – The difference between two sensed pressures.

DPDT switching element – Attained using two synchronized SPDT switching elements which actuate together at increasing set point and deactuate together at decreasing set point. Discrete SPDT switching elements allow two independent circuits to be switched; i.e., one AC and one DC.

Firesafe – The ability of a welded seal pressure sensor to contain the process at elevated temperatures up to 1900°F at the rated overrange pressure, unsupported by the body of the pressure switch.

Gauge pressure – Pressure measured against atmospheric pressure. It is the pressure sensed above ambient pressure.

Hermetically sealed -The method of sealing an electrical switching element so that it is unaffected by ambient surroundings. The enclosure is completely sealed from the environment using fusion techniques such as welding, soldering, brazing or glass-to-metal fusion.

Overrange – The maximum input pressure that may be continuously applied to the pressure switch without causing permanent change of set point, leakage or material failure.

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

Process –Liquid or gas that is being sensed by a pressure switch. Also known as “process media”.

Proof pressure – The maximum input pressure that may be continuously applied to the pressure switch without causing leakage or catastrophic material failure.

Repeatability – The ability of a pressure 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 three consecutive cycles to establish a pressure profile.

Repeatability on SOR switches will be smaller than 1% of full scale per ISA/ANSI S51.1.

Reset point – The pressure value at which the switching element changes state. It is normally expressed in a unit of measurement and as increasing or decreasing.

Set point – That discrete pressure at which the pressure switch is adjusted to actuate/deactuate on rising or falling pressure. It falls within the adjustable range and the set point value is usually called out as increasing or decreasing pressure.

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

Vacuum – Pressure measured that is less than atmosphere pressure.

Wetted parts – Components of a pressure sensor that are exposed to the process media.