Accuracy and Pressure Measurement

Accuracy and Measurement

• Absolute accuracy is accuracy stated as a definite amount.
• Absolute position measures position referenced to a fixed point.
• Accuracy measures the difference between indicated and true values.
• Resolution is the smallest detectable change in a measurement.
• Precision is the smallest division readable on an instrument.
• Repeatability measures the closeness of consecutive readings.
• Reproducibility is the ability to produce the same reading with repeated measurements.
• Percent of reading is accuracy as a percentage of the actual reading.
• Percentage full-scale accuracy is accuracy divided by full-scale output, expressed as a percentage.

Pressure Measurement

• Absolute pressure uses a perfect vacuum as a reference.
• Atmospheric pressure is due to the weight of air, typically measured at sea level.
• Gauge pressure measures pressure relative to atmospheric pressure.
• Vacuum pressure is the amount by which measured pressure is below atmospheric pressure.
• Dynamic pressure is part of the total pressure in a moving fluid caused by motion.
• Hydrostatic pressure is caused by the weight of static fluid.
• Impact pressure is the total of static and dynamic pressure in fluids
• Pressure is force divided by area, expressed in psi or Pa.
• Torr equals the pressure from a 1 mm high mercury column.
• Pressure differential is the difference in pressure at two locations.
• Overpressure is max pressure a gauge can handle without damage/loss of accuracy.

Sensors and Transducers

• Accelerometer measures acceleration or rate of velocity change.
• Actuators perform actions on process variables based on a controller signal.
• Aneroid barometers use an evacuated capsule as a sensing element.
• Bellows are pressure sensors that convert pressure to linear displacement.
• Bourdon tubes are pressure sensors that convert pressure to movement.
• Capacitance probes measure fluid level using capacitance between metal plates.
• Conductivity probes measure fluid level using two electrodes.
• Dynamometers measure torque or power.
• Hall-effect sensors convert magnetic field changes into proportional voltage.
• Hot-wire anemometry measures gas or liquid flow velocity via temperature changes over a heated element.
• Hygrometers measure relative humidity.
• Load cells measure force.
• Magnetorestrictive elements (MRE) convert magnetic field changes into proportional resistance.
• Photodiodes measure light intensity by measuring leakage across a pn junction.
• Pyrometers measure temperature by sensing radiant energy.
• Resistance thermometers (RTD) measure temperature by measuring resistance of a metal wire, usually platinum .
• Strain gauges convert deformation information into resistance change.
• Thermistors are temperature sensors made of metal oxide with a negative temperature coefficient.
• Thermocouples use dissimilar metal junctions to generate voltage proportional to temperature differences.
• Transducers change energy from one form to another.
• Transmitters condition signals from transducers for remote transmission.
• Ultrasonic probes use high-frequency sound to measure fluid levels.

Flow Measurement Devices

• Rotameters measure flow where a float moves in a vertical tapered tube.
• Turbine flow meters measure flow using a turbine wheel.
• Venturi tubes are restrictions in pipes to create pressure drops related to flow rate.
• Electromagnetic flow meters sense changes in a magnetic field to measure flow
• Nutating disk meters measure flow using a wobbling disk.
• Flow Nozzles are constrictions in flow lines create a pressure drop to determine the flow rate
• Orifice plates creates pressure drops that relate to the flow rate.
  • Concentric plates
  • Eccentric plates allows passage of suspended solids
  • Segmented plates allows passage of suspended solids
• Pitot-static tubes measure flow rate via dynamic and static pressure differences.
• Weirs are open-channel flow-measuring devices.
• Flumes are open-channel flow-measuring devices.

Fluid Dynamics

• Bernoulli Equation is a flow equation that includes velocity, pressure and elevation terms.
• Beta ratio is the diameter ratio of a restriction and its containing pipe.
• Buoyancy is the upward force on a floating object in a fluid.
• Reynolds Number indicates whether a flow is laminar or turbulent.
• Laminar flow is a smooth flow in layers.
• Turbulent flow is an agitated flow with random velocity fluctuations.
• Vena contracta is the narrowing of fluid flow through an obstruction.
• Specific gravity is the ratio of a material's specific weight to that of water (for liquids/solids) or air (for gases).
• Velocity measures speed and direction of liquid movement.
• Viscosity describes a fluid's resistance to flow.
• Stoke is the measurement unit of kinematic viscosity.

Heat Transfer

• British thermal unit (BTU) is the heat needed to raise 1 lb. of water by 1°F at 68°F and atmospheric pressure.
• Calorie is the heat needed to raise 1 g of water 1°C.
• Specific heat is the heat needed to raise a substance by one degree (1 lb 1°F or 1 g 1°C).
• Conduction transfers heat energy through a material by energy transfer from molecules to molecules
• Convection transfers heat through the motion of warm or hot material.
• Radiation transfer energy through electromagnetic waves.
• Free convection is heat movement as a result of density differences.
• Thermal conductivity measures a material's ability to conduct heat.
• Coefficient of heat transfer is a term used in the calculation of heat transfer by convection.
• Thermal expansion is the expansion of a material as a result of its being heated.
• Thermal Expansion
• Thermal time constant represents the amount of time it will take for a body to cool or heat 63.2% of the difference between initial temp and aiming temp

Electrical Components and Principles

• ADC (analog-to-digital converter) converts analog signals to digital signals.
• DAC converts digital signals to analog signals.
• Alternating current flows in alternating directions during a period.
• Ampere is the unit of current or electron flow.
• Amplifiers increase signal magnitude.
• Buffer amplifier matches impedance between circuits.
• Capacitance measures the ability to store electrical charge, the unit is Farad.
• Capacitors store electrical charge.
• Electromagnetism relates magnetic fields and electric current.
• Electromotive force (EMF) is the force that moves electrons, measured in volts.
• Farad is the unit of capacitance.
• Impedance is opposition to AC current from inductance and/or capacitance.
• Inductance opposes changes in current or electron flow, the unit is Henry (H).
• Inductors are devices that exhibit inductance.
• Kirchoff's Current Law: Sum of currents at a node is zero.
• Kirchoff's Voltage Law: Sum of voltages around a closed path is zero.
• LED is a light emitting diode.
• Nodes are junctions of three or more conductors.
• Operational amplifiers amplify electronic signals.
• Reactance opposes AC current from capacitance or inductance.
• Resistance opposes electron or current flow.
• Resistors are components that exhibit resistance.
• Volt is the unit of electromotive force.
• Voltage is electromotive force causing a current to flow.
• Voltage drop is the voltage difference between two points.
• Time Constant (Electrical) is the amount of time it will take for capacitance C to discharge through resistance R by 62.3%; RC = Time Constant in seconds
• Wheatstone bridge is a common electrical bridge circuit to measure small changes to element values

Control Systems

• Analog signals are continuously varying.
• Anticipatory action is derivative action.
• Controlled variables indicate process output conditions.
• Controllers evaluate error and initiate corrective action.
• Correction signal provides feedback to the manipulated variable.
• Derivative action is proportional to the rate of change of variable
• Error signal is the difference between a measured signal and the set point.
• Feedback is a measured variable signal used to adjust the manipulated variable.
• Integral action corrects for long-term loads; resets.
• Manipulated variables are controlled by actuators.
• Measured variables indicate process output conditions.
• Integral action corrects for long-term loads.
• ON/OFF control systems have only two positions.
• PID control combines Proportional, Integral, and Derivative actions.
• Process control maintains variables within given limits.
• Proportional action: controller output is proportional to measured variable error.
• Set point is the reference value for a controlled variable.
• Signal conditioning converts a signal format for transmission.
• Overshoot is overcorrection of a measured variable in a control loop.
• Lag time is the time required for a control system to return a measured variable to its set point.

Other Significant Terms

• Alarm is a warning when a variable exceeds limits.
• Aqueous solutions are solutions containing water.
• Binary uses a base 2 numbering system. -Bit is a binary Digit -Byte is eight bits of information
• Celsius is a common temperature scale.
• Convection moves heat by moving warm material.
• Decibel (dB) compares amplitude or power levels.
• Density is mass per unit volume.
• Dew point is the temperature at which water vapor condenses.
• Effective value is the DC voltage/current producing the same power as the measured AC voltage/current.
• Frequency is the number of cycles per second, measured in Hertz (Hz).
• Humidity is a term indicating water vapor content.
• Relative humidity is the % of water vapor present based on the water vapor amount under saturated conditions with the same temperature and pressure.
• Humidity ratio is the ratio of the mass of water vapor in a gas divided by the mass of dry gas.
• Hydrostatic paradox: Pressure varies with depth in a static fluid but remains constant.
• Hygroscopic materials absorb water, changing conductivity with moisture.
• Hysteresis is nonreproducibility due to approaching a measurement from opposite directions.
• Kelvin is the absolute temperature scale of Celsius.
• Load is the nominal variables affecting the controlled variable
• Meniscus can be convex or concave surface for liquid in a tube
• Newtonian fluids have a velocity that varies linearly across the flow section between parallel plates.
• Noise is unwanted or undesirable sounds.
• Pascal's law: Pressure to an enclosed fluid is transmitted to every part
• Percent of reading provides accuracy given terms of the percentage of the reading
  • Percentage full-scale accuracy is the accuracy determined by dividing the accuracy of an instrument by its full-scale output taken as a percentage
• Phons are units describing differences in loudness levels.
• Pneumatic systems use gas for control.
• Poise measures dynamic. or Absolute viscosity
• Process is a sequence of operations to achieve results.
• Range is difference between lowest and highest readings.
• Rankine scale is Fahrenheit's absolute temperature scale
• Reluctance is the opposition to magnetic flux.
• Saturation occurs when the max amount material is dissolved
• Sone measures loudness.
• Span is difference between the lowest and highest instrument readings.
• Specific heat measures heat to raise substance one degree i.e. 1lb to degree F or 1g to a degree C
• Sublimation is passing from solid to vapor and vapor to solid.
• Telemetry transmits electrical info distances.
• Temperature measures of hotness or coldness
• Thermal conductivity describes a material to conduct heat
• Torque tends to create a twisting action.
• Total flow past given point of time,
• Transducer changes 1 form of energy to another,
• Transmission transfers information.
• Turbulent is an agitated flow with random turbulence on average flow
• Velocity is speed and motion of a liquid
• Viscosity measures resistance to flow