Pressure, Flow, Level, and Acoustic Sensors

Introduction

Pressure, flow, level, and acoustic measurements connect physical processes to control systems. In a mechatronic installation, these variables are rarely measured in isolation: pressure determines pump loading, flow affects heat and mass transfer, level protects tanks and pumps, and acoustic sensing supports ranging, inspection, and human–machine interaction. The sensing element, signal-conditioning circuit, installation, calibration, and control algorithm must therefore be considered as one measurement system.

A sensor produces a physical or electrical response, while a transducer converts one form of energy into another. A pressure transducer may convert diaphragm strain into voltage; a microphone converts sound pressure into an electrical signal. A complete instrument also includes excitation, amplification, filtering, compensation, analog-to-digital conversion, and communication.

Learning objectives

After studying this chapter, the student should be able to:

1. Distinguish absolute, gauge, vacuum, and differential pressure.
2. Explain the operation and limitations of manometers and electronic pressure transducers.
3. Compare differential-pressure, variable-area, turbine, electromagnetic, ultrasonic, and Coriolis flowmeters.
4. Select level sensors for liquids, solids, and difficult process conditions.
5. Explain microphone, ultrasonic, and sonar principles.
6. Analyze installation effects, compensation, calibration, and uncertainty.
7. Choose an appropriate sensor for an industrial mechatronic application.

1. Pressure measurement

1.1 Pressure concepts and reference conditions

Pressure is normal force per unit area:

Here, p is pressure in pascals (Pa), F is normal force in newtons (N), and A is area in square metres (m²). The relation assumes force is distributed normally and uniformly over the stated effective area. One kilopascal equals 1,000 Pa; one bar equals 100 kPa; standard atmospheric pressure is approximately 101.325 kPa at sea level, although actual atmospheric pressure varies.

The reference pressure must always be specified:

• Absolute pressure, p_abs, is measured relative to a perfect vacuum.
• Gauge pressure, p_g, is measured relative to local atmospheric pressure.
• Differential pressure, ∆p, is the difference between two process pressures.
• Vacuum pressure is often reported as pressure below atmospheric pressure.

The basic relationships are:

where p_atm is local atmospheric pressure and p₁ and p₂ are pressures at two ports. Confusing absolute and gauge pressure can produce serious design errors, particularly in gas laws, vacuum systems, and pump calculations.

1.2 Manometers

A manometer balances an unknown pressure against the hydrostatic head of a liquid. For a simple vertical column containing a liquid of density ρ,

where g is gravitational acceleration, approximately 9.81 m/s², and ∆h is the vertical height difference in metres. This equation assumes a static, incompressible manometric liquid and negligible capillary and meniscus errors.

A U-tube differential manometer can measure pressure difference between two connected points. If the process fluid and manometric fluid have different densities, the complete hydrostatic balance must include every column. In a common arrangement with equal elevations of the pressure taps and process-fluid density ρ_f, manometric-fluid density ρ_m, and observed level difference h, the result is often written, for a suitably arranged U-tube, as:

The sign depends on which side is connected to the higher pressure. The equation should not be applied without inspecting the actual geometry.

Manometers are simple, intrinsically stable, and useful as reference instruments. Their disadvantages include slow response, limited remote transmission, liquid evaporation or contamination, difficulty reading small levels, and possible breakage. Inclined-tube manometers improve resolution for small pressure differences because a small vertical head corresponds to a larger movement along the tube.

1.3 Elastic and electronic pressure transducers

A pressure transducer commonly uses a diaphragm, capsule, or bellows. Pressure produces displacement or strain, which is converted into an electrical signal. Important technologies include:

• Strain-gauge diaphragm: bonded or deposited resistors change resistance when strained. A Wheatstone bridge provides sensitivity and temperature compensation.
• Piezoresistive silicon: semiconductor resistance changes strongly with stress. These devices are compact and sensitive but require temperature compensation.
• Capacitive diaphragm: pressure changes the spacing or overlap of capacitor electrodes. They offer high resolution and low power, but parasitic capacitance and contamination must be controlled.
• Piezoelectric sensor: stress generates charge in a crystal or ceramic. It is excellent for dynamic pressure but unsuitable for true static measurement because charge leaks through the electrical system.
• Resonant sensor: pressure-induced stress changes the resonant frequency of a structure. Frequency measurement can provide high stability and digital compensation.

A transducer may be specified by range, overload pressure, burst pressure, accuracy, repeatability, hysteresis, response time, temperature coefficient, wetted-material compatibility, and electrical output. A nominal 0–10 bar transmitter does not necessarily measure with an error of only 0.01 bar; the accuracy specification must state whether it is percentage of span, percentage of reading, or a combination.

For a linearized transmitter with lower and upper range values p_L and p_H, and output y_L to y_H, the ideal output is:

This equation assumes linear calibration and operation within the specified range. A 4–20 mA transmitter is especially useful in industrial systems because 4 mA represents a live zero: zero current can indicate an open circuit or power failure. For a 4–20 mA instrument, the ideal current is:

1.4 Installation effects and compensation

Pressure measurements are strongly influenced by installation. Pulsating pumps may require snubbers or digital filtering; excessive filtering, however, can delay an emergency response. A pressure tap should face the flow appropriately, avoid dead zones, and be located where the pressure represents the intended process condition. Block valves and bleed valves support safe calibration and maintenance.

In steam service, impulse lines are commonly filled with condensate and arranged to protect the transmitter from excessive temperature. In liquid service, trapped gas bubbles can create errors; in gas service, condensed liquid can produce hydrostatic offsets. Equalizing valves are required for many differential-pressure transmitters during commissioning. Chemical compatibility, vibration, electrical grounding, and electromagnetic interference must also be considered.

Temperature affects diaphragm modulus, bridge resistance, fluid density, and electronics. Compensation may be achieved by matched resistors, a temperature sensor and calibration polynomial, lookup tables, or digital model-based correction. Compensation is only valid over its stated temperature and pressure range.

2. Differential-pressure devices and flow measurement

2.1 Differential pressure as a flow measurement

Many flowmeters infer flow from a pressure difference created by a restriction. For an ideal incompressible flow through an orifice, the relationship is approximately:

where Q is volumetric flow rate (m³/s), C_d is discharge coefficient, A₂ is the throat or orifice area (m²), ∆p is differential pressure (Pa), ρ is fluid density (kg/m³), and β = d/D is the ratio of throat diameter d to pipe diameter D. This form assumes steady, fully developed flow, a known discharge coefficient, negligible elevation difference, and a single-phase fluid. Real installations require standards-based coefficients and corrections for Reynolds number, edge condition, temperature, pressure, and compressibility.

For fixed geometry and constant density, Q ∝ √∆p. Consequently, a transmitter calibrated linearly in pressure difference does not produce a linear flow signal. Square-root extraction may be performed in the transmitter, controller, or supervisory software. Incorrectly applying square-root extraction twice is a common commissioning error.

Orifice plates are inexpensive and robust, but they create permanent pressure loss and require straight pipe lengths. Venturi tubes have lower permanent loss but are larger and more costly. Flow nozzles suit high-velocity and high-temperature service. Differential-pressure systems are versatile, but impulse lines can plug, leak, freeze, or develop unequal condensation heads.

2.2 Variable-area flowmeters

A rotameter contains a tapered tube and a float. As flow rises, the float moves upward until upward fluid force, buoyancy, and weight balance. The float position is approximately proportional to flow for a specified fluid and orientation. Rotameters are inexpensive, require no electrical power, and provide local indication. They are less suitable for remote control, opaque fluids, pulsating flow, or changing density and viscosity unless calibrated for those conditions.

2.3 Velocity and mass flowmeters

Turbine meters use a rotor whose speed is related to flow velocity. They provide good repeatability for clean, relatively constant-flow liquids and gases, but bearings wear and viscosity, swirl, and upstream disturbances affect accuracy.

Electromagnetic flowmeters apply a magnetic field to a conductive fluid. Faraday’s law gives the induced voltage:

where E is induced voltage, B is magnetic flux density, ℓ is effective electrode separation, and v is average fluid velocity. For a full pipe of cross-sectional area A, Q = Av. The meter requires a fluid with sufficient electrical conductivity and a full, electrically continuous pipe. It has no obstruction and measures slurries well, but cannot measure insulating hydrocarbons and can be affected by grounding and electrode coating.

Vortex meters detect vortices shed behind a bluff body. The shedding frequency is related to velocity through the Strouhal relation:

where f_s is shedding frequency, St is the Strouhal number, v is velocity, and d is bluff-body width. The relation is useful over the meter’s qualified Reynolds-number range. Vortex meters measure liquids, gases, and steam, but require adequate flow velocity and are sensitive to vibration and disturbed flow.

Ultrasonic flowmeters use transit-time or Doppler methods. Transit-time meters compare upstream and downstream acoustic travel times and work best with relatively clean fluids. Doppler meters measure frequency shifts from particles or bubbles and are useful in dirty liquids. Clamp-on versions avoid cutting the pipe but depend strongly on pipe-wall condition, coupling, geometry, and correct acoustic alignment.

Coriolis meters measure mass flow from the vibration and twisting of one or more tubes. A simplified relationship is:

where F_c is Coriolis force, m is moving mass, Ω is angular velocity, and v is velocity relative to the rotating frame. Commercial meters use tube phase differences rather than directly calculating this simplified force. They provide highly accurate mass flow and often density, but are expensive, can impose pressure drop, and require appropriate sizing and two-phase-flow management.

2.4 Installation and flow conditioning

The meter must be installed where the pipe remains full and the fluid is representative. Pumps, valves, elbows, reducers, and tees produce swirl and asymmetric velocity profiles. Straight-run requirements depend on meter design; a flow conditioner may reduce disturbances but cannot correct every poor installation. The meter should not be placed immediately downstream of a partially closed valve unless the manufacturer permits it.

For liquid systems, avoid locations where cavitation, flashing, or gas release occurs. For gas systems, account for pressure and temperature because density changes. For steam, dryness fraction and pressure compensation may be necessary. Flow direction, grounding, pipe alignment, gasket intrusion, and sensor orientation should be checked during commissioning.

3. Level measurement

Level instruments measure liquid height, infer level from hydrostatic pressure, or detect the presence of material at a point.

3.1 Hydrostatic level

For a liquid of constant density in a stationary tank:

where h is liquid height above the pressure tap. Thus:

The method assumes the tank pressure is known. In a closed tank, a differential-pressure transmitter measures bottom pressure minus vapor-space pressure. Density changes caused by temperature, concentration, or composition directly affect the inferred level. A bubbler system instead supplies purge gas and measures the back pressure required to bubble from a tube near the tank bottom; it is useful for corrosive or fouling liquids but consumes purge gas.

3.2 Direct and point-level methods

A sight glass provides direct visual indication but needs protection and maintenance. Float-and-cable systems are simple and can drive switches or transmitters, though mechanical fouling is possible. Conductive probes detect liquid contact when the liquid completes an electrical circuit; they are unsuitable for nonconductive liquids and may suffer coating. Capacitance probes infer level from changes in dielectric environment and can measure liquids or solids, but require calibration for material properties and buildup.

Vibrating-fork switches detect changes in resonant behavior when a fork is immersed. They are widely used for high- and low-level alarms and are relatively independent of vessel geometry. Rotary-paddle switches are common for powders but can be affected by bridging and mechanical wear.

3.3 Continuous noncontact level

Radar level sensors emit microwave energy and determine distance from reflected signals. Time-of-flight is conceptually:

where d is distance to the surface, c is electromagnetic wave speed in the medium, and t is round-trip time. The factor 2 accounts for travel to the surface and back. Radar performs well with vapors and changing temperature, although foam, turbulence, low dielectric constant, false echoes, and antenna fouling can reduce reliability.

Ultrasonic level sensors use acoustic pulses. Their range estimate is:

where v is sound speed in the gas above the material. Unlike radar, sound speed varies substantially with temperature and gas composition, so compensation is important. Ultrasonic instruments can be affected by foam, dust, condensation, turbulence, and strong echoes from internal structures.

Laser triangulation and time-of-flight systems provide narrow beams and high resolution for suitable surfaces. They are useful in positioning and some solids applications, but dust and optical contamination are limitations.

4. Acoustic sensors, ultrasonic sensors, and sonar

4.1 Sound and microphones

Sound is a mechanical pressure fluctuation propagating through a medium. A microphone converts acoustic pressure into an electrical signal. Common microphone types are:

• Dynamic microphone: a coil moves in a magnetic field. It is rugged and does not normally require phantom power.
• Condenser microphone: a diaphragm changes capacitance relative to a backplate. It offers high sensitivity and wide bandwidth but needs polarization or electronics power.
• Electret condenser microphone: a permanently polarized material reduces the external bias requirement.
• Piezoelectric microphone: deformation of a piezoelectric element produces charge. It is useful for contact and vibration measurements.
• MEMS microphone: a micromachined diaphragm and integrated electronics provide small size, repeatability, and digital interfaces.

Microphone specifications include sensitivity, frequency response, equivalent noise level, maximum sound-pressure level, dynamic range, directivity, and distortion. Sound-pressure level is commonly expressed relative to p₀ = 20 µPa in air:

Here p_rms is root-mean-square sound pressure and L_p is in decibels. The reference is conventional, not a universal zero of sound. Because decibels are logarithmic, two sound pressures cannot generally be added by ordinary arithmetic; their mean-square pressures must be combined when signals are incoherent.

Microphone placement is part of measurement. Reflections create standing waves; proximity to walls and machinery changes the spectrum; wind produces low-frequency pressure fluctuations; and the housing can shadow or resonate. Windscreens, shock mounts, calibration pistons, and appropriate sampling rates improve results. Anti-alias filtering is essential before analog-to-digital conversion.

4.2 Ultrasonic ranging

Ultrasonic sensors transmit sound above the nominal human hearing range, commonly tens of kilohertz for industrial ranging. A pulse–echo system measures time between transmission and reception. The distance equation assumes a known propagation velocity and a reflection from the intended target. Temperature compensation is often implemented using an approximate relation for air:

where v is sound speed in m/s and T is air temperature in °C, over ordinary atmospheric conditions. Humidity and gas composition also matter, especially in precision systems.

Ultrasonic sensors have a beam angle, a dead zone after transmission, and a minimum target size. Soft, angled, porous, or irregular surfaces may absorb or scatter energy. Multiple sensors can interfere with one another unless they are synchronized or sequenced. Signal processing may use threshold detection, correlation, envelope detection, or time-varying gain.

4.3 Sonar

Sonar—sound navigation and ranging—uses acoustic transmission and reception underwater. Active sonar emits a pulse and analyzes an echo; passive sonar listens without transmitting. For a target at range R, the active-sonar relationship is:

where c is sound speed in water and t is round-trip travel time. Sound speed in water depends on temperature, salinity, and depth. Refraction can bend acoustic paths, while reverberation, multipath propagation, bubbles, and seabed reflections complicate target detection.

Sonar applications include bathymetric mapping, underwater vehicle navigation, fishery research, obstacle avoidance, and pipeline inspection. Acoustic exposure must be managed responsibly because powerful transmissions may disturb marine organisms and interfere with other acoustic systems.

5. Comparison and selection

Selection should begin with the measurand and process, not with a preferred technology. Define range, accuracy, response time, fluid or material properties, temperature, pressure, hazardous-area classification, hygienic requirements, maintenance access, output protocol, and consequences of failure. A lower-cost sensor with poor installation may be less economical than a higher-cost instrument that remains reliable.

6. Calibration, uncertainty, and signal integrity

Calibration compares an instrument with a traceable reference over relevant points. It should include zero, intermediate points, span, hysteresis where important, and environmental conditions. Calibration does not eliminate process variation or installation error. A transmitter can be perfectly calibrated while measuring the wrong pressure because of an incorrect impulse-line elevation.

A simple uncertainty model for independent quantities uses root-sum-square combination:

where u_c is combined standard uncertainty and u_i are standard uncertainties. This approximation assumes independent inputs and a locally linear measurement model. Correlated inputs require covariance terms, and nonlinear models may require sensitivity coefficients or Monte Carlo analysis.

Signal integrity includes shielding, grounding, cable segregation, proper excitation, isolation, filtering, and sampling. A sensor signal should be diagnostically plausible: range checks, rate-of-change checks, redundancy, and sensor-failure detection can prevent unsafe control action. Software must distinguish a genuine process excursion from a disconnected or saturated sensor.

7. Illustrative industrial case: condensate level and flow control

Background

Consider an illustrative steam-generation plant. Condensate is collected in a tank, pumped through a return line, and fed to a boiler. Reliable level and flow measurements are required to prevent pump dry running, maintain boiler feed balance, and detect leaks.

Problem

An initial design used a differential-pressure level transmitter on the tank and an orifice flowmeter downstream of the pump. Operators observed a slowly drifting level, noisy flow, and occasional low-flow trips even though the pump appeared healthy.

Analysis

Inspection showed that the tank was vented, but the impulse line to the level transmitter had a trapped gas pocket. The liquid density also changed with temperature, causing a hydrostatic conversion error. The flow transmitter performed square-root extraction, while the control system applied a second square-root function. The resulting signal was disproportionately low at reduced flow. Finally, the flowmeter was installed close to a partially open elbow, producing a distorted velocity profile.

Approach and solution

The level impulse connection was relocated and routed to avoid gas pockets; a vent and calibration manifold were added. Temperature measurement was used to compensate liquid density within the qualified operating range. The control logic was corrected so that square-root extraction occurred only once. The flowmeter was moved to a longer straight run, and a startup plausibility check compared pump status, valve position, pressure, and flow.

Outcome

The level signal became stable, low-flow trips corresponded to actual process conditions, and maintenance staff gained safe isolation and calibration access. The case demonstrates that sensor performance depends on installation geometry, fluid properties, signal processing, and diagnostics—not merely on the catalog accuracy of the sensing element.

Learning points

1. A hydrostatic level measurement is also a density measurement unless density is controlled or compensated.
2. Differential-pressure flow is nonlinear with respect to flow.
3. Installation disturbances can dominate intrinsic sensor accuracy.
4. Independent engineering review of control-system scaling prevents duplicated transformations.
5. Alarm and trip logic should include sensor diagnostics and process plausibility.

8. Safety, standards, and contemporary developments

Pressure vessels, process piping, hazardous areas, and functional-safety systems are subject to jurisdictional regulations and recognized standards. Engineers must consult applicable pressure-equipment codes, electrical installation rules, electromagnetic-compatibility requirements, calibration standards, and manufacturer instructions. Materials must be compatible with the process, and pressure instruments must be rated for static pressure, overpressure, vacuum, temperature, and chemical exposure.

A sensor used in a protective shutdown should not be selected solely for measurement accuracy. Diagnostic coverage, proof-test interval, safe failure behavior, independence, and the required safety integrity level may be relevant. Cybersecurity also matters for networked transmitters: access control, authenticated configuration, change logging, and secure firmware management help protect calibration and control functions.

Modern instruments increasingly combine sensing, self-diagnostics, digital communication, and local computation. Edge algorithms can detect pump cavitation from pressure and acoustic signatures, classify leaks, or estimate sensor health. Machine-learning models may improve anomaly detection, but they require representative training data, validation against physical models, protection against distribution shifts, and human oversight. AI should supplement—not conceal—the need for sound installation, calibration, and engineering judgment.

Best-practice checklist

• Define the measurand and pressure reference explicitly.
• Select a range that covers normal operation without sacrificing useful resolution.
• Verify fluid compatibility, temperature, pressure, conductivity, density, and phase.
• Follow manufacturer straight-run, orientation, grounding, and mounting requirements.
• Provide isolation, venting, draining, calibration, and safe maintenance access.
• Compensate only for variables that are measured or controlled with adequate accuracy.
• Confirm whether square-root extraction or other scaling is performed once and only once.
• Use filtering appropriate to the process dynamics and safety response.
• Record calibration data, uncertainty, configuration, and environmental conditions.
• Validate the complete measurement chain during commissioning and periodically thereafter.

Chapter Summary

Pressure is force per unit area, but its practical interpretation depends on whether the reference is vacuum, atmosphere, or another process point. Manometers provide direct hydrostatic comparison, while diaphragm, strain, capacitive, piezoresistive, piezoelectric, and resonant transducers provide electrical measurement with different dynamic and environmental characteristics. Differential-pressure devices infer flow through a calibrated pressure–flow relationship that is generally proportional to the square root of pressure difference. Electromagnetic, vortex, ultrasonic, turbine, and Coriolis meters offer alternatives suited to different fluid properties and accuracy requirements.

Level can be measured hydrostatically, by point switches, or through noncontact radar, ultrasonic, and optical methods. Microphones measure acoustic pressure, while ultrasonic sensors and sonar determine distance or detect targets through propagation and echo analysis. Installation effects—including gas pockets, condensation, density changes, pipe disturbances, reflections, foam, and temperature—often dominate performance. Calibration, compensation, uncertainty analysis, signal integrity, diagnostics, safety, and cybersecurity are therefore integral parts of sensor engineering.

Key Terms

Pressure: Normal force per unit area.

Absolute pressure: Pressure referenced to a perfect vacuum.

Gauge pressure: Pressure referenced to local atmospheric pressure.

Differential pressure: Difference between two measured pressures.

Manometer: Hydrostatic instrument for comparing pressure.

Pressure transducer: Device that converts pressure into an electrical or other usable signal.

Strain gauge: Resistive element whose resistance changes with mechanical strain.

Discharge coefficient: Empirical factor correcting ideal restriction-flow equations.

Square-root extraction: Conversion of differential pressure into an approximately linear flow signal.

Coriolis meter: Flowmeter that infers mass flow from vibration-induced Coriolis effects.

Electromagnetic flowmeter: Meter using induced voltage in a conductive moving fluid.

Hydrostatic level: Level inferred from liquid pressure and density.

Radar level sensor: Noncontact level instrument using electromagnetic-wave reflections.

Ultrasonic sensor: Instrument using high-frequency acoustic waves for detection or ranging.

Microphone: Transducer that converts acoustic pressure into an electrical signal.

Sonar: Underwater acoustic navigation and ranging system.

Calibration: Comparison with a traceable reference under defined conditions.

Hysteresis: Different instrument output for the same input depending on input direction.

Uncertainty: Quantified doubt associated with a measurement result.

Live zero: Nonzero signal value representing the lower range limit, such as 4 mA.

Review Questions

Short Answer Questions

1. Distinguish absolute, gauge, and differential pressure.
2. Why is a piezoelectric pressure sensor generally unsuitable for static pressure measurement?
3. State two installation problems that can affect a differential-pressure flowmeter.
4. Why does ultrasonic level measurement require temperature compensation?
5. Define active and passive sonar.

Long Answer Questions

1. Explain the construction, operation, advantages, and limitations of a U-tube manometer and an electronic diaphragm pressure transmitter.
2. Compare orifice, electromagnetic, ultrasonic, and Coriolis flowmeters for industrial service.
3. Explain hydrostatic, radar, ultrasonic, and point-level measurement methods and identify suitable applications for each.
4. Discuss microphone specifications, acoustic interference, calibration, and signal conditioning.
5. Explain how installation, compensation, calibration, and uncertainty determine the performance of a complete sensor system.

Analytical / Application Questions

1. A liquid has density 850 kg/m³ and produces a hydrostatic pressure of 25.0 kPa at a tank sensor. Estimate the liquid height, neglecting vapor pressure and elevation offsets.
2. A differential-pressure flowmeter is installed immediately downstream of a control valve and reports unstable flow. Analyze likely causes and propose a commissioning plan.
3. Select a level technology for a hot, foaming liquid in a closed vessel. Compare radar, ultrasonic, and differential-pressure methods, including compensation and failure modes.

Multiple Choice Questions with Answers

Question 1

Which pressure reference is used for absolute pressure?

A. A perfect vacuum

B. Local atmospheric pressure

C. The downstream process pressure

D. A manometer liquid surface

Correct Answer: A. A perfect vacuum

Question 2

For a fixed-geometry restriction and constant-density fluid, flow rate is approximately proportional to:

A. ∆p

B. 1/∆p

C. √∆p

D. ∆p²

Correct Answer: C. √∆p

Question 3

Which flowmeter requires the fluid to have sufficient electrical conductivity?

A. Turbine meter

B. Electromagnetic meter

C. Coriolis meter

D. Rotameter

Correct Answer: B. Electromagnetic meter

Question 4

A hydrostatic liquid-level measurement is most directly affected by changes in:

A. Liquid density

B. Microphone sensitivity

C. Pipe Reynolds number only

D. Electrode polarity only

Correct Answer: A. Liquid density

Question 5

In a pulse–echo ranging system, why is measured travel time divided by two?

A. To compensate for temperature

B. Because the signal travels to the target and returns

C. To remove atmospheric pressure

D. Because the transducer has two electrodes

Correct Answer: B. Because the signal travels to the target and returns

Question 6

Which sensor is particularly suitable for dynamic pressure but generally poor for static pressure?

A. Piezoelectric sensor

B. U-tube manometer

C. Capacitive level probe

D. Rotameter

Correct Answer: A. Piezoelectric sensor

Question 7

What is the principal purpose of square-root extraction in differential-pressure flow measurement?

A. To convert pressure into temperature

B. To linearize the relationship between signal and flow approximately

C. To eliminate all installation errors

D. To measure fluid conductivity

Correct Answer: B. To linearize the relationship between signal and flow approximately

Question 8

Which instrument directly measures mass flow with high accuracy under suitable conditions?

A. Coriolis meter

B. Sight glass

C. Orifice plate without a transmitter

D. Conductive probe

Correct Answer: A. Coriolis meter

Question 9

For sound-pressure level in air, the conventional reference pressure is:

A. 20 Pa

B. 2 kPa

C. 20 µPa

D. 101.325 kPa

Correct Answer: C. 20 µPa

Question 10

A 4–20 mA transmitter uses 4 mA at the lower range limit mainly to provide:

A. A live zero for fault detection

B. A square-root signal automatically

C. A vacuum reference

D. A temperature reference

Correct Answer: A. A live zero for fault detection