Absolute Pressure:
Absolute pressure measurements are referenced to zero pressure, (a perfect vacuum.)
Absolute Pressure Transducer:
A transducer that has an internal reference chamber sealed at or close to zero pressure (full vacuum) when exposed to atmosphere a reading of approximately 14.7 psi results.
Boyle's Law:
The volume of a gas is inversely proportional to the pressure of the gas at constant temperature: V=1/P.
Charles' Law:
Essentially states for a fixed volume of gas, if the temperature is raised, the pressure will increase. P = Constant x T.
Common Mode Pressure:
The underlying common pressure (or static pressure) within a system from which a differential measurement is being made.
D/P: Differential Pressure, (pronounced DP):
Other names used to mean the same thing are d/p cell, d/p transmitter and DP transmitter (where D is delta or differential). This is the most common type of transmitter used in most process industries. It can be used to measure level, flow, pressure, differential pressure, and density or specific gravity. With some modifications, it can measure such things as temperature and oxygen purity. The d/p transmitter can be pneumatic, electromechanical, or solid state. It can also be a smart transmitter. A typical large process plant can have hundreds or thousands of d/p transmitters in service.
Gage Pressure:
The pressure relative to atmospheric pressure. Gage pressure = absolute pressure minus one atmosphere.
Gage Pressure Transducer:
A transducer that measures pressure relative to atmospheric pressure.
Ideal Gas Law:
Combining Boyle's Law and Charles' Law, results in the Ideal Gas Law: PV=nRT, where nR is constant for a particular gas analogous to the number of molecules and the relative size of the molecule.
I/P (I to P):
A current to pressure transmitter. A common instrument in modern industrial plants. A typical large paper mill or refinery could have 5,000 I/Ps in use.
Line Pressure:
The maximum pressure in the pressure vessel or pipe for differential pressure measurement.
Orifice Plate:
A very low cost and common primary sensing element (PSE) for measuring flow. It must be used in conjunction with a d/p cell. It creates a venturi and a resulting P is developed across the plate whose square root is proportional to flow.
P/I (P to I):
A pressure to current transducer.
Pneumatic Relay:
Refers to a pneumatic instrument that performs a function to its input and provides the result on its output (Example: square root extractor, adder, etc.).
PSI:
Pounds per square inch (same as psig).
PSIA:
Pounds per square inch absolute.
PSID:
Pounds per square inch differential.
PSIG:
Pounds per square inch gage (same as psi).
Square Root Extractor:
An instrument or software program that takes the square root of input and puts the result on its output. Square root extraction is needed to linearize many flow signals. Example: orifice plates, venturis, target flow meters, and pitot tubes all require the transmitter's output signal to be linearized. Mag flow meters, turbine flow meters, Doppler flow meters, and vortex shedding flow meters don't require square root extraction.
Static Pressure:
The zero-velocity pressure at any arbitrary point within a system.
Wet/Dry Differential:
A differential pressure transducer or transmitter that uses a metal diaphragm at the wet port where fluids can be applied, and no diaphragm at the dry port. The dry port exposes the sensor material to the medium, so only clean dry gas can be applied to this port.
Wetted Parts:
The diaphragm and pressure port material that comes in direct contact with the medium (gas, liquid).