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Parallax: An optical illusion which occurs in analog meters and causes reading errors. It occurs when the viewing eye is not in the same plane, perpendicular to the meter face, as the indicating needle.

Parallel Transmission: Sending all data bits simultaneously. Commonly used for communications between computers and printer devices.

Parity: A technique for testing transmitting data. Typically, a binary digit is added to the data to make the sum of all the digits of the binary data either always even (even parity) or always odd (odd parity).

Peltier Effect: When a current flows through a thermocouple junction, heat will either be absorbed or evolved depending on the direction of current flow. This effect is independent of joule I2 R heating.

Perfectly Balanced Rotor: A rotor is perfectly balanced when its mass distribution is such that it transmits no vibratory force or motion to its bearings as a result of centrifugal forces.

Peripheral: A device that is external to the CPU and main memory, i.e., printer, modem or terminal, but is connected by the appropriate electrical connections.

pH Junctions: The Junction of a reference electrode or combination electrode is a permeable membrane through which the fill solution escapes (called the liquid junction).

pH(S) (Standard pH Scale): The conventional standard pH scale established on the basis that an individual ionic activity coefficient can be calculated from the Debye-H¸ckel law for primary buffers.

Phase Difference: The time expressed in degrees between the same reference point on two periodic waveforms.

Phase Proportioning: A form of temperature control where the power supplied to the process is controlled by limiting the phase angle of the line voltage.

Phase: A time based relationship between a periodic function and a reference. In electricity, it is expressed in angular degrees to describe the voltage or current relationship of two alternating waveforms.

PID: Proportional, integral, derivative. A three mode control action where the controller has time proportioning, integral (auto reset) and derivative rate action.

Piezoelectric Accelerometer: A transducer that produces an electrical charge in direct proportion to the vibratory acceleration.

Piezoresistance: Resistance that changes with stress.

Pixel: Picture element. Definable locations on a display screen that are used to form images on the screen. For graphic displays, screens with more pixels provide higher resolution.

Plane Separation: Of a balancing machine, is the operation of reducing the correction plane interference ratio for a particular rotor.

Platinel: A non-standard, high temperature platinum thermocouple alloy whose thermoelectric voltage nearly matches a Type K thermocouple (Trademark of Englehard Industries).

Platinum 6% Rhodium: The platinum-rhodium alloy used as the negative wire in conjunction with platinum-30% rhodium to form a Type B thermocouple.

Platinum 10% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type S thermocouple.

Platinum 13% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type R thermocouple.

Platinum 30% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with platinum 6% rhodium to form a Type B thermocouple.

Platinum 67: To develop thermal emf tables for thermocouples, the National Bureau of Standards paired each thermocouple alloy against a pure platinum wire (designated Platinum 2 prior to 1973, and currently Platinum 67). The thermal emf's of any alloy combination can be determined by summing the "vs. Pt-67" emf's of the alloys, i.e., the emf table for a Type K thermocouple is derived from the Chromel vs. Pt-67 and the Alumel vs. Pt-67 values.

Platinum: A noble metal which in its pure form is the negative wire of Type R and Type S thermocouples.

Poisson Ratio: The ratio between the strain of expansion in the direction of force and the strain of contraction perpendicular to that force v = -Et/E1.

Polarity: In electricity, the quality of having two oppositely charged poles, one positive one negative.

Polarization: The inability of an electrode to reproduce a reading after a small electrical current has been passed through the membrane. Glass pH electrodes are especially prone to polarization errors caused by small currents flowing from the pH meter input circuit and from static electrical charges built up as the electrodes are removed from the sample solution, or when the electrodes are wiped.

Port: A signal input (access) or output point on a computer.

Positive Temperature Coefficient: An increase in resistance due to an increase in temperature.

Potential Energy: Energy related to the position or height above a place to which fluid could possibly flow.

Potentiometer: 1. A variable resistor often used to control a circuit. 2. A balancing bridge used to measure voltage.

Power Supply: A separate unit or part of a circuit that supplies power to the rest of the circuit or to a system.

PPM: Abbreviation for "parts per million," sometimes used to express temperature coefficients. For instance, 100 ppm is identical to 0.01%.

Primary Device: Part of a flowmeter which is mounted internally or externally to the fluid conduit and produces a signal corresponding to the flowrate and from which the flow may be determined.

Primary Standard (NBS): The standard reference units and physical constants maintained by the National Bureau of Standards upon which all measurement units in the United States are based.

Primary Standards: Aqueous pH buffer solutions established by the National Bureau of Standards within the 2.5 to 11.5 pH range of ionic strength less than 0.1 and which provide stable liquid junction potential and uniformity of electrode sensitivity.

Principal Axes: The axes of maximum and minimum normal stress.

Probe: A generic term that is used to describe many types of temperature sensors.

Process Meter: A panel meter with sizeable zero and span adjustment capabilities, which can be scaled for readout in engineering units for signals such as 4-20 mA, 10-50 mA and 1-5 V.

Program: A list of instructions that a computer follows to perform a task.

Prom: Programmable read-only memory. A semiconductor memory whose contents cannot be changed by the computer after it has been programmed.

Proof Pressure: The specified pressure which may be applied to the sensing element of a transducer without causing a permanent change in the output characteristics.

Proportioning Band: A temperature band expressed in degrees within which a temperature controller's time proportioning function is active.

Proportioning Control Mode: A time proportioning controller where the amount of time that the relay is energized is dependent upon the system's temperature.

Proportioning Control plus Derivative Function: A time proportioning controller with derivative function. The derivative function senses the rate at which a system's temperature is either increasing or decreasing and adjusts the cycle time of the controller to minimize overshoot or undershoot.

Proportioning Control plus Integral: A two-mode controller with time proportioning and integral (auto reset) action. The integral function automatically adjusts the temperature at which a system has stabilized back to the setpoint temperature, thereby eliminating droop in the system.

Proportioning Control with Integral and Derivative Functions: Three mode PID controller. A time proportioning controller with integral and derivative functions. The integral function automatically adjusts the system temperature to the set point temperature to eliminate droop due to the time proportioning function. The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.

Protection Head: An enclosure usually made out of metal at the end of a heater or probe where connections are made.

Protection Tube: A metal or ceramic tube, closed at one end into which a temperature sensor is inserted. The tube protects the sensor from the medium into which it is inserted.

Protocol: A formal definition that describes how data is to be exchanged.

PSIA: Pounds per square inch absolute. Pressure referenced to a vacuum.

PSID: Pounds per square inch differential. Pressure difference between two points.

PSIG: Pound per square inch gage. Pressure referenced to ambient air pressure.

PSIS: Pounds per square inch standard. Pressure referenced to a standard atmosphere.

Pulse Width Modulation: An output in the form of duty cycle which varies as a function of the applied measurand.