Instrumentation Glossary absolute pressure Gauge pressure plus the local atmospheric pressure. alarm A switch that indicates abnormal ranges for process variables. alarms Auditory or visual signals warning personnel of upsets in a process. atmospheric pressure The pressure exerted by the air around you which is approximately 14.7 psi. automated control valves Valves that restrict fluid flow to manipulate a controlled variable as part of a control loop. bellows element A pressure sensing element that uses the elastic deformation of a series of capsules that expand or contract with changes in pressure. Bernoulli’s principle As flow increases, pressure decreases. bias control The addition of a value to the measured variable signal to determine the controller’s output signal. bluff body The part of a vortex flow meter that is placed across a fluid stream. bourdon tube A pressure sensing element that detects changes in pressure by the elastic deformation of a flattened tube that expands or contracts. bubbler system Device that uses a purge gas to measure pressure exerted by a liquid to determine level. bumpless transfer Changing to automatic or manual mode without the controller changing the output signal. bypass switch A device used to override the normal operations of a system. cascade control Two controls loops combined so that one controller provides a set point for the other control loop. closed-loop control A continuous monitoring and adjustment cycle to control a single process variable. compound gauge A gauge that measures pressure both above and below atmospheric pressure. control loop model A simplified diagram showing the flow of information along an instrumentation loop. controllers Instruments that make decisions to control a process variable. coriolis A mass flow meter that measures the rate of a fluid travelling through a moving tube. dead time A measure of time between a manipulated variable signal change and when the resulting response can be observed in the controlled variable. discreet control valves Valves used where the desired flow is either on or off. displacer Instrument used to measure liquid-liquid interfaces. distributed control systems Computer-based systems that are inter-connected to reduce the potential of a catastrophic failure. Emergency Shutdown Systems Devices responsible for shutting down equipment in extremely abnormal situations. error The difference between the target and the measured process variable. feedback control Action the controller will take depending on feedback from the system. feedforward control Taking corrective action before a process variable deviation occurs. flow The measurement of fluid movement. flow rate The measure of the units of a fluid flowing past a specific location over a period of time. flow switch A switch that uses paddles placed in a fluid to detect flow. fluid A substance that has the ability to flow, either a liquid or gas. gain A term describing the proportional relationship between the magnitude of the controlled variable signal and the magnitude of the manipulated variable signal. gauge pressure Absolute pressure minus atmospheric pressure. head pressure The pressure exerted by a liquid. HOA A three position toggle switch for Hand/Off/Auto hunting Cycling of the controlled variable signal above or below the set point. hysteresis A lack of responsiveness to a change in the direction of the manipulated variable signal. indicated variables Variables that are not controlled or manipulated but are measured and monitored. infrared digital thermometers An instrument that analyzes the infrared energy emitted from objects to measure temperature. innage The measurement of a fluid's height from the bottom of the tank to the surface of the material. interlocks Two or more control loops interacting with one another to take either a process or a safety action. lag time The period of time, after the dead time period, that it takes a controlled variable to reach equilibrium following a manipulated signal change. laminar A fluid path which has a smooth flowing pattern. level gauge Device used to visually measure level through a viewing chamber connected to a vessel. level measurement The determination of the height of a process material's surface in reference to a zero point. limit switch A switch which detects the position of an object by direct contact. loop tuning The act of adjusting the proportional, integral, or derivative mode parameters to optimize control for a particular process and its response dynamics. louvers Device used as a final control element to modulate air flow in a process. manipulated variables Variables that are directly adjusted by the controller to correct a controlled variable. manometer A hollow transparent tube partially filled with a liquid and used for measuring static pressure at medium intensity. meniscus curve The interface between the air and the liquid in a chamber caused by surface tension. normal state The condition of minimum stimulus for the sensed variable. nutating disc A common type of positive displacement flow meter. offset The difference between the controlled variable and the set point when the manipulated variable signal has not been changed. open-loop control A control that does not use feedback to determine the actions of a process. orifice plate A metal disc that constricts a pipe’s diameter to accelerate a fluid. oscillation The controlled variable signal cycling above or below the set point as the process returns to a steady state. P = F / A Pressure equals force divided by area. parallax error An error occurring when you view a pointer position from the angled viewpoint instead of from eye level. permissives A set of conditions that must be met before a piece of equipment can be started. pitot tube An L - shaped instrument inserted into a process pipe with its open end facing upstream. pressure One of the most common measurements in process industries, typically expressed as pounds and area in square inches resulting in pounds per square inch. pressure transmitters Devices that measure pressure through the use of a sensor and output the measurement to a control loop. process equilibrium The condition of a process when the system is balanced and is running in a steady state. programmable logic controllers Computer-based systems that use a single microprocessor to manage multiple control loops. proportional band The percentage of the controlled variable signal required for the controller to send its full output signal. proximity switch A switch that detects the presence of an object without physically touching it. ratio control Maintaining a fixed ratio between two or more variables regardless of flow rate changes. relay Device used to pass on information between instrumentation. resistance temperature detectors Temperature sensing elements constructed of fine wire wound around a non-conductive material. Reynolds number A representation of whether a flow profile is laminar or turbulent. rotameter A variable area flow meter that uses pressure to measure flow. shutdown switch A switch used in fail safe system designs to deactivate a circuit. signal conversion A calculation that equates process signals from on signal type to another. simulated input signals Test signals sent to a controller to help isolate or eliminate signal problems. smart transmitter A transmitter with built-in microprocessor circuits. split range control Two or more final control elements used to control a process. switches Sensors that trigger an action in a process. thermistors Very small resistors made of a ceramic/metal oxides mixture sensitive to very small changes in temperature. thermocouples Electronic sensors containing two different metal conductors that are joined together to form a junction. thermowells Pressure-tight sheaths inserted into a process steam to allow temperature sensors to detect the process temperature without coming into contact with the process fluid. transducer A device which changes a signal from one form to another. transmitter An instrument that outputs sensed variable information in a format that is suitable for a controller. troubleshooting A logical, systematic process of thinking used to repair malfunctioning systems. turbulent A fluid path which is continuously changing. ullage The measurement of height from the surface of the process material to the top of the vessel. ultrasonic flow meter A meter that emits high frequency sound waves that reflect off particles in the flow stream. valve positioner Device that compares the valve stem position to a control signal and adjusts pressure to the actuator diaphragm or piston. vena contracta The smallest cross section and the point of lowest fluid pressure after an orifice plate. venturi tube A tube with a smooth cone-shaped inlet an