Instrumentation Sensors Book

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Calorie A measure of heat energy, which is defined as the amount of heat required to raise the temperature of 1g of water 1°C.

Coefficient of heat transfer A term used in the calculation of heat transfer by convection.

Coefficient of thermal expansion A term used to determine the amount of linear expansion due to heating or cooling.

Comparator A device that compares two signals and outputs the difference.

Concentric plate A plate with a hole located at its center (orifice plate) that is used to measure flow by measuring the differential pressures on either side of the plate.

Continuity equation A flow equation which states that, if the overall flow rate is not changing with time, then the flow rate past any section of the system must be constant.

Continuous level measurement A level measurement that is continuously updated.

Controlled variable The variable measured to indicate the condition of the process output.

Controller The element in a process control loop that evaluates any error of the measured variable and initiates corrective action by changing the manipulated variable.

Converter A device that changes the format of a signal (e.g., voltage to current) but not the type of energy used as the signal carrier.

Correction signal The signal to the manipulated variable.

Digital to analog converter A device that converts a digital signal into an analog voltage or current.

Data acquisition system The Interface between many log signals and the computer.

Direct digital control Action in which a computer performs all the functions of error detection and controller action.

Dead weight tester A device for calibrating pressure-measuring devices that uses weights to provide the forces.

Derivative action Action that is proportional to the rate at which the measured variable is changing.

Dew point The temperature at which the water vapor in a mixture of water vapor and gas becomes saturated and condensation starts. Dielectric constant The factor by which the capacitance between two plates changes when a material fills the space between the plates.

DP cell A differential pressure sensor.

Dry-bulb temperature The temperature indicated by a thermometer whose sensing element is dry.

Dynamic pressure The part of the total pressure in a moving fluid caused by the fluid motion.

Dynamometer An instrument used for measuring torque or power.

Eccentric plate An orifice plate with a hole located below its center to allow for the passage of suspended solids.

Electromagnetic flow meter A flow-measuring device that senses a voltage between two electrodes induced by a magnetic field when a conductive fluid is flowing.

Error signal The difference in value between a measured signal and a set point.

Feedback (1) The voltage fed from the output of an amplifier to the input, in order to control the characteristics of the amplifier. (2) The measured variable signal fed to the controller in a closed-loop system, so that the controller can adjust the manipulated variable to keep the measured variable within set limits.

Flow nozzle A device placed in a flow line to provide a pressure drop that can be related to flow rate.

Flume An open-channel flow-measuring device.

Form drag The force acting on an object due to the impact of fluid. Foundation Fieldbus Process control bus used in the United States. Free surface The surface of the liquid in an open-channel flow that is in contact with the atmosphere.

Gauge pressure The measured pressure above atmospheric pressure.

Gas thermometer A temperature sensor that converts temperature to pressure in a constant volume system.

Hall effect sensor A transducer that converts a changing magnetic field into a proportional voltage.

Head Sometimes used to indicate pressure (e.g., 1 ft of “head” for water is the pressure under a column of water 1 ft high).

Hot-wire anemometry A velocity-measuring device for gas or liquid flow that senses temperature changes due to the cooling effect of gas or liquid moving over a hot element.

Humidity ratio The mass of water vapor in a gas divided by the mass of dry gas in the mixture.

Hygroscopic A material that absorbs water, and whose conductivity changes with moisture content.

Hysteresis The nonreproducibility in an instrument caused by approaching a measurement from opposite directions (e.g., going from low up to the value, or high down to the value).

Impact pressure The sum of the static and dynamic pressure in a moving fluid.

Impedance An opposition to ac current or electron flow caused by inductance, and/or capacitance.

Incremental position measurement An incremental position measurement from one point to another, in which absolute position is not recorded and the position is lost if the power fails.

Indirect level-measuring device A device that extrapolates the level from the measurement of another variable (e.g., liquid level from a pressure measurement).

Integral action The action designed to correct for long term loads. Ionization gauge A low pressure sensor (<10−3 atm) based on the conduction of electric current through ionized gas whose pressure is being measured. Kirchoff’s Current Law Law that states that the sum of the currents flowing at a node is zero.

Kirchhoff’s Voltage Law Law that states that the algebraic sum of voltages around a closed path is zero.

Ladder logic The programmable logic used in PLCs to control automated industrial processes.

Lag time The time required for a control system to return a measured variable to its set point.

Laser Light amplification by simulated emission of radiation, which is characterized by monochromaticity and high collimation.

Linearity A measure of the direct proportionality between the actual value of the variable being measured and the value of the output of the instrument to a straight line.

Load The process load is a term used to denote the nominal values of all variables in a process that affect the controlled variable.

ment by conversion to a linearly proportional voltage.

Magnetorestrictive element A magnetic field sensor that converts a changing magnetic field into a proportional resistance.

Manipulated variable The variable controlled by an actuator to correct for changes in the measured variable.

Multiplexer An analog or digital device to select one of many input signals.

Newtonian fluid A fluid in which the velocity varies linearly across the flow section between parallel plates.

Nutating disk meter A flow-measuring device using a disk that rotates and wobbles in response to the flow.

Offset The nonzero output of a circuit when the input is zero. Open-channel flow The flow in an open conduit (e.g., as in a ditch). Orifice plate A plate containing a hole that, when placed in a pipe, causes a pressure drop, which can be related to flow rate.

Overpressure The maximum amount of pressure a gauge can withstand without damage or loss of accuracy.

Overshoot The overcorrection of the measured variable in a control loop.

Parabolic velocity distribution Occurs in laminar flow when the velocity across the cross section takes on the shape of a parabola.

pH A term used to indicate the activity of the hydrogen ions in a solution, it helps to describe the acidity or alkalinity of the solution.

Phons A unit for describing the difference in loudness levels.

Photodiode A sensor used to measure light intensity by measuring the leakage across a pn junction.

PlD Proportional control with integral and derivative action, where the feedback signal is directly proportional to the error signal, but the error is further reduced and has a faster response time due to integral, and derivative action.

P&ID Stands for piping and instrument diagrams, representing process and material flow, and signal flow in process control.

Piezoelectric effect The electrical voltage developed across certain crystalline materials when a force or pressure is applied to the material. Piezoresistive effect is a change in the resistance of a semiconductor element when it is under strain.

Pirani gauge A gauge used primarily for low pressures (< 1 atm) based on the resistance of a heated wire whose temperature is a function of the pressure.

Pitot-static tube A device used to measure the flow rate using the difference between dynamic and static pressures.

PLC Programmable logic controller. A microprocessor based system that is easily programmed for the control of industrial processes

Pneumatic system is a system that uses gas for control and signal transmission.

Poise The measurement unit of dynamic or absolute viscosity. Process A sequence of operations carried out to achieve a desired end result.

Process control The automatic control of certain process variables to hold them within given limits.

Processor A digital electronic computing system that can be used as a control system.

Profibus Process control bus used in Europe.

Proportional action A controller action in which the controller output is directly proportional to the measured variable error.

Psychrometric chart A chart dealing with moisture content in the atmosphere.

Pyrometer An instrument for measuring temperature by sensing the radiant energy from a hot body.

Quarter amplitude A process control tuning criterion for stability, where, after a transient change, the oscillations of the control signal decrease by one-quarter of its amplitude with each cycle.

Radiation The emission of energy from a body in the form of electromagnetic waves.

Rate action See Derivative action.

Relative humidity The amount of water vapor present in a given volume of a gas, expressed as a percentage of the amount that would be present in the same volume of gas under saturated conditions at the same pressure and temperature.

Reset action See Integral action.

Resistance thermometer A sensor that provides temperature readings by measuring the resistance of a metal wire (usually platinum).

Resolution The minimum detectable change of a variable in a measurement.

Reynolds number A dimensionless number indicating whether the flow is laminar or turbulent.

Rotameter A flow-measuring device in which a float moves in a vertical tapered tube.

Saturated The condition when the maximum amount of a material is dissolved in another material at the given pressure and temperature conditions (e.g., water vapor in a gas).

Sealing fluid An inert fluid used in a manometer that is used to separate the fluid whose pressure is being measured from the manometer fluid. Segmented plate An orifice plate with a hole that allows suspended solids to pass through.

Sensitivity The ratio of the change in output to input magnitudes. Serial transmission A sequential transmission of digital bits.

Set point The reference value for a controlled variable in a process control loop.

Signal conditioning The linearization of a signal, temperature correction, and the setting of the signal’s reference and sensitivity to set levels.

Sling psychrometer A device for measuring relative humidity.

Smart sensor The integration of a processor directly into the sensor assembly, which gives direct control of the actuator and digital communication to a central controller.

Sone A unit for measuring loudness.

Sound pressure level The difference between the maximum air pressure at a point and the average air pressure at that point.

Span The difference between the lowest and highest reading for an instrument.

Specific gravity The ratio of the specific weight of a solid or liquid material and the specific weight of water; or for a gas, the ratio of the specific weight of the gas and the specific weight of air under the same conditions.

Specific heat The amount of heat required to raise a definite amount of a substance by one degree (e.g., 1 lb by 1°F, or 1g by 1°C).

Specific humidity The mass of water vapor in a mixture divided by the mass of dry air or gas in the mixture.

Specific weight The weight of a unit volume of a material. Stoke The measurement unit of kinematic viscosity.

Strain gauge A sensor that converts information about the deformation of solid objects when the objects are acted upon by a force into a change of resistance.

Sublimation The process of passing directly from solid to vapor, or from vapor to solid.

Telemetry The electrical transmission of information over long distances, usually by radio frequencies.

Thermal time constant The time required for a body to heat or cool by 63.2% of the difference between the initial temperature and the aiming temperature.

Thermocouple A temperature-sensing device that uses dissimilar metal junctions to generate a voltage that is proportional to the differential temperature between the metal junctions.

Thermopile A number of thermocouples connected in series.

Time constant (electrical) The amount of time needed for a capacitance C, to discharge or charge through a resistance R, to 62.3% of the difference between the initial voltage and the aiming voltage. The product of RC gives the time constant in seconds.

Torque The force moment that tends to create a twisting action.

Torr The pressure caused by the weight of a column of mercury 1 mm high.

Transducer A device that changes energy from one form to another. Transfer function An equation that describes the relationship between the input and output of the function.

Transmission The transferring of information from one point to another. Transmitter A device that conditions the signal received from a transducer so that it is suitable for sending to another location with minimal loss of information.

Turbine flow meter A flow-measuring device utilizing a turbine wheel. Ultrasonic probe An instrument using high frequency sound waves to measure fluid levels.

Vapor pressure thermometer A temperature sensor, in which the pressure of a vapor in a closed system is a function of the temperature.

Vena contracta The narrowing down of the fluid flow stream as it passes through an obstruction.

Venturi tube A specially shaped restriction in a section of pipe that provides a pressure drop, which can be related to flow rate.

Viscosity The resistance to flow of a fluid.

Vortex The swirling or rotating motion of fluids when in motion. Weir An open-channel flow-measuring device.

Wet-bulb temperature The temperature indicated by a thermometer whose sensing element is kept moist.

Wheatstone Bridge The most common electrical bridge circuit used to measure small changes in the value of an element.

Zeigler-Nichols method A method of determining the optimum controller settings when tuning a process control loop for stability.