Level Sensors: Introduction
Level measurement is one of the most important parameters in industrial process control. Whether it is water in a tank, oil in a refinery vessel, chemicals in a reactor, or powder in a silo, accurate level measurement ensures safety, efficiency, and proper inventory management.
Level sensors help industries prevent overflow, dry running of pumps, material loss, and process shutdowns.
There are different types of level sensors available today such as float type, differential pressure, capacitance, ultrasonic, radar, conductivity, and vibrating fork each working on a different principle and suitable for different applications.
Understanding how these sensors work is very important for instrumentation students, technicians, and engineers.
In this article, we have prepared objective multiple-choice questions (MCQs) on level sensors to help you test and improve your technical knowledge. These questions are useful for interviews, competitive exams, viva preparation, and practical field understanding. Let’s get started.
Q1: What is the primary purpose of a level sensor in industrial applications?
a) To measure pressure
b) To measure temperature
c) To measure the height of liquid or solid material
d) To measure flow rate
Answer: c
Explanation: A level sensor measures the height or quantity of liquids, solids, or slurries inside tanks, vessels, or silos to ensure proper process control and safety.
Q2: Which type of level sensor works on the time-of-flight principle?
a) Float level switch
b) Ultrasonic level sensor
c) Displacer transmitter
d) Sight glass
Answer: b
Explanation: Ultrasonic level sensors send a sound pulse toward the surface and calculate the level based on the time taken for the echo to return.
Q3: Which of the following is a non-contact level measurement device?
a) Float transmitter
b) Conductivity switch
c) Radar level sensor
d) Displacer transmitter
Answer: c
Explanation: Radar level sensors use microwave signals to measure level without physically touching the medium, making them ideal for corrosive or hazardous liquids.
Q4: Differential pressure level measurement is mainly used in:
a) Open tanks only
b) Closed pressurized tanks
c) Gas pipelines
d) Temperature measurement systems
Answer: b
Explanation: In closed tanks, differential pressure transmitters compensate for vapor pressure and measure liquid level accurately.
Q5: Which level switch is commonly used for high-level alarm applications?
a) Radar transmitter
b) Float level switch
c) Nuclear level sensor
d) Ultrasonic transmitter
Answer: b
Explanation: Float level switches are simple and cost-effective devices widely used for high or low level alarms in storage tanks.
Q6: Capacitance level sensors operate based on changes in:
a) Resistance
b) Inductance
c) Capacitance
d) Frequency
Answer: c
Explanation: Capacitance level sensors detect level changes by measuring variations in capacitance caused by differences in dielectric constants between air and the process material.
Q7: Which level sensor is most suitable for high temperature and high pressure applications?
a) Ultrasonic level sensor
b) Radar level sensor
c) Sight glass
d) Conductivity switch
Answer: b
Explanation: Radar level sensors can withstand extreme temperature and pressure conditions and are widely used in harsh industrial environments.
Q8: Conductivity level switches are suitable for detecting level in:
a) Non-conductive liquids like oil
b) Conductive liquids like water
c) Dry powders
d) Steam
Answer: b
Explanation: Conductivity level switches require the liquid to conduct electricity, so they are mainly used for water and other conductive fluids.
Q9: Which of the following provides continuous level measurement?
a) Float switch
b) Vibrating fork switch
c) Conductivity switch
d) Radar level transmitter
Answer: d
Explanation: Radar level transmitters provide continuous real-time level measurement, whereas switches only detect specific level points.
Q10: A displacer type level transmitter works on which principle?
a) Archimedes’ buoyancy principle
b) Electromagnetic induction
c) Piezoelectric effect
d) Optical reflection
Answer: a
Explanation: Displacer level transmitters operate based on buoyancy. The force exerted on the displacer changes with liquid level according to Archimedes’ principle.
Q11: Which level sensor is commonly used for measuring bulk solids like cement or grain?
a) Conductivity level switch
b) Radar level sensor
c) Sight glass
d) Manometer
Answer: b
Explanation: Radar level sensors, especially non-contact types, are widely used for bulk solids like cement and grain because they are unaffected by dust and material buildup.
Q12: Ultrasonic level sensors may give inaccurate readings in the presence of:
a) Stable liquid surface
b) Vacuum conditions
c) Heavy vapor or foam
d) Clean water
Answer: c
Explanation: Heavy vapor, foam, or turbulence can absorb or scatter ultrasonic waves, leading to incorrect level measurements.
Q13: Which device is mainly used for point level detection in powders and granular materials?
a) Differential pressure transmitter
b) Vibrating fork level switch
c) Sight glass
d) Bourdon tube
Answer: b
Explanation: Vibrating fork level switches detect level by sensing changes in vibration frequency when covered by solid or liquid material.
Q14: In an open tank, level measurement using a differential pressure transmitter depends on:
a) Liquid density and height
b) Tank diameter
c) Vapor pressure
d) Temperature only
Answer: a
Explanation: In open tanks, the pressure at the bottom depends on liquid height and density, which is used to calculate the level.
Q15: Which level measurement method is completely unaffected by temperature, pressure, or density?
a) Differential pressure
b) Capacitance
c) Radar
d) Displacer
Answer: c
Explanation: Radar level measurement is generally independent of temperature, pressure, and density variations because it measures the time of microwave reflection.
Q16: Which type of level sensor requires calibration based on dielectric constant?
a) Float switch
b) Capacitance level sensor
c) Sight glass
d) Magnetic level indicator
Answer: b
Explanation: Capacitance level sensors depend on the dielectric constant of the material, so proper calibration is necessary for accurate measurement.
Q17: A magnetic level indicator works based on:
a) Optical refraction
b) Magnetic coupling
c) Capacitance change
d) Sound reflection
Answer: b
Explanation: Magnetic level indicators use a float with an internal magnet that moves with liquid level and magnetically actuates external indicators.
Q18: Which level measurement device is suitable for interface measurement between two liquids?
a) Ultrasonic sensor
b) Displacer transmitter
c) Conductivity switch
d) Pressure gauge
Answer: b
Explanation: Displacer transmitters can measure interface levels because buoyancy changes depending on the density of the two liquids.
Q19: In a closed tank with vapor pressure, level measurement using DP transmitter requires:
a) Single pressure tapping
b) Two pressure tappings (high and low side)
c) No tapping
d) Optical sensor
Answer: b
Explanation: In closed tanks, both bottom liquid pressure and top vapor pressure must be measured to determine the correct differential pressure.
Q20: Which level sensor is best suited for corrosive chemical applications without direct contact?
a) Float switch
b) Radar level sensor
c) Conductivity probe
d) Displacer transmitter
Answer: b
Explanation: Non-contact radar level sensors are ideal for corrosive chemicals because they do not touch the process fluid.
Q21: Which level measurement device is most suitable for very high-pressure boilers?
a) Sight glass
b) Ultrasonic sensor
c) Radar level transmitter
d) Conductivity switch
Answer: c
Explanation: Radar level transmitters can withstand high temperature and high pressure conditions, making them suitable for boiler drum level measurement.
Q22: In capacitance level measurement, the probe and tank wall together form a:
a) Resistor
b) Inductor
c) Capacitor
d) Transformer
Answer: c
Explanation: The probe acts as one plate and the tank wall as the other plate, forming a capacitor. The capacitance changes as the level changes.
Q23: Which level switch is commonly used for low-level protection of pumps?
a) Radar transmitter
b) Float level switch
c) Nuclear level sensor
d) Manometer
Answer: b
Explanation: Float level switches are widely used to protect pumps from dry running by detecting low liquid levels.
Q24: Ultrasonic level sensors cannot be used effectively in:
a) Atmospheric tanks
b) Clean water tanks
c) Vacuum conditions
d) Open reservoirs
Answer: c
Explanation: Ultrasonic sensors require a medium (air) for sound wave transmission. In vacuum conditions, sound cannot travel properly.
Q25: Which level measurement method is based on radioactive emission?
a) Radar level sensor
b) Nuclear level sensor
c) Capacitance sensor
d) Magnetic level indicator
Answer: b
Explanation: Nuclear level sensors use gamma radiation to measure level and are suitable for extreme conditions where other methods fail.
Q26: A vibrating fork level switch detects level by sensing changes in:
a) Pressure
b) Temperature
c) Vibration frequency
d) Capacitance
Answer: c
Explanation: When the vibrating fork is covered by material, its vibration frequency changes, indicating the presence of level.
Q27: Which parameter directly affects differential pressure level measurement accuracy?
a) Liquid color
b) Liquid density
c) Tank shape
d) Surface finish
Answer: b
Explanation: Differential pressure measurement depends on liquid density. Any change in density affects the calculated level.
Q28: Which level sensor is most suitable for sticky or viscous liquids?
a) Conductivity switch
b) Float switch
c) Non-contact radar sensor
d) Sight glass
Answer: c
Explanation: Non-contact radar sensors are ideal for sticky or viscous liquids because they do not come into contact with the process medium.
Q29: In open tank level measurement using DP transmitter, the low-pressure side is connected to:
a) Bottom of the tank
b) Middle of the tank
c) Atmosphere
d) Pump suction
Answer: c
Explanation: In open tanks, the low-pressure side of the DP transmitter is open to atmosphere to measure gauge pressure.
Q30: Which level device provides visual indication without electrical output?
a) Radar transmitter
b) Ultrasonic sensor
c) Magnetic level indicator
d) Capacitance transmitter
Answer: c
Explanation: Magnetic level indicators provide a direct visual display of liquid level without requiring electrical power.
Q31: Which level sensor is commonly used in water treatment plants for tank level monitoring?
a) Ultrasonic level sensor
b) Bourdon tube
c) Thermocouple
d) Orifice plate
Answer: a
Explanation: Ultrasonic level sensors are widely used in water and wastewater treatment plants because they provide non-contact, reliable level measurement.
Q32: Which level measurement method is affected by foam formation on liquid surface?
a) Radar
b) Ultrasonic
c) Nuclear
d) Magnetic level indicator
Answer: b
Explanation: Foam can absorb or scatter ultrasonic sound waves, causing incorrect level readings.
Q33: A sight glass is mainly used for:
a) Continuous automatic control
b) Remote monitoring
c) Local visual level indication
d) Density measurement
Answer: c
Explanation: Sight glasses provide simple, local visual indication of liquid level inside a tank.
Q34: Which level sensor is most suitable for interface measurement between oil and water?
a) Capacitance level sensor
b) Thermocouple
c) Orifice plate
d) Rotameter
Answer: a
Explanation: Capacitance sensors can detect interface levels because oil and water have different dielectric constants.
Q35: In DP level measurement, increasing liquid density will:
a) Decrease differential pressure
b) Increase differential pressure
c) Have no effect
d) Stop measurement
Answer: b
Explanation: Differential pressure is directly proportional to liquid density and height. Higher density results in higher DP.
Q36: Which level switch is commonly used in silos for powder level detection?
a) Conductivity switch
b) Vibrating fork switch
c) Manometer
d) Pressure gauge
Answer: b
Explanation: Vibrating fork switches are ideal for powder and granular materials in silos due to their reliable detection method.
Q37: Which of the following is a contact type level measurement device?
a) Radar level transmitter
b) Ultrasonic level transmitter
c) Displacer level transmitter
d) Laser level sensor
Answer: c
Explanation: Displacer transmitters are contact-type devices that operate based on buoyancy.
Q38: Radar level sensors use which type of waves for measurement?
a) Sound waves
b) Infrared waves
c) Microwave signals
d) Mechanical waves
Answer: c
Explanation: Radar sensors use microwave signals that reflect from the material surface to determine level.
Q39: Which level measurement method is least affected by dust in solid applications?
a) Ultrasonic
b) Radar
c) Sight glass
d) Conductivity switch
Answer: b
Explanation: Radar sensors are less affected by dust compared to ultrasonic sensors, making them suitable for dusty environments.
Q40: A float level transmitter measures level based on:
a) Temperature variation
b) Pressure variation
c) Buoyancy force
d) Electrical resistance
Answer: c
Explanation: Float level transmitters operate on buoyancy principle, where the float rises and falls with liquid level.
Q41: Which level sensor is suitable for measuring molten metal level?
a) Ultrasonic sensor
b) Capacitance sensor
c) Nuclear level sensor
d) Float switch
Answer: c
Explanation: Nuclear level sensors are suitable for extreme conditions such as high temperature molten metal applications where contact methods cannot survive.
Q42: In ultrasonic level measurement, the distance is calculated using:
a) Voltage drop
b) Echo return time
c) Current variation
d) Density change
Answer: b
Explanation: The sensor measures the time taken for the ultrasonic pulse to return after reflecting from the surface and calculates the level accordingly.
Q43: Which level measurement method is best suited for tall storage tanks in oil terminals?
a) Sight glass
b) Radar level transmitter
c) Float switch
d) Conductivity probe
Answer: b
Explanation: Radar transmitters are ideal for tall tanks because they provide accurate long-range measurement without contact.
Q44: A conductivity level switch cannot be used for:
a) Water
b) Acid
c) Oil
d) Salt solution
Answer: c
Explanation: Conductivity switches require conductive liquids. Oil is non-conductive, so it cannot be detected by this method.
Q45: Which factor can cause error in capacitance level measurement?
a) Change in dielectric constant
b) Change in tank color
c) Pipe diameter
d) Ambient noise
Answer: a
Explanation: Capacitance measurement depends on dielectric constant. Any variation in dielectric properties affects accuracy.
Q46: Which device is commonly used for continuous level measurement in slurry applications?
a) Float switch
b) Non-contact radar
c) Sight glass
d) Manometer
Answer: b
Explanation: Non-contact radar is suitable for slurry because it avoids coating and wear problems associated with contact devices.
Q47: In a closed tank, the low-pressure side of a DP transmitter is connected to:
a) Tank bottom
b) Pump outlet
c) Tank vapor space
d) Atmosphere
Answer: c
Explanation: In closed tanks, the LP side is connected to the vapor space to compensate for internal pressure.
Q48: Which level measurement technique is most accurate for clean liquids in atmospheric tanks?
a) Ultrasonic
b) Radar
c) Float transmitter
d) All of the above
Answer: d
Explanation: All these methods can provide accurate measurement in clean liquid atmospheric tanks if properly selected and installed.
Q49: Which level sensor is most affected by temperature changes in air?
a) Radar
b) Ultrasonic
c) Nuclear
d) Magnetic level indicator
Answer: b
Explanation: Ultrasonic sensors rely on sound velocity in air, which changes with temperature, affecting measurement accuracy.
Q50: Which principle is used in magnetic level indicators?
a) Electrical resistance
b) Magnetic coupling
c) Optical reflection
d) Piezoelectric effect
Answer: b
Explanation: Magnetic level indicators use magnetic coupling between an internal float magnet and external indicator mechanism.
Q51: Which level sensor is ideal for applications where the liquid surface is highly turbulent?
a) Sight glass
b) Float switch
c) Radar level transmitter
d) Conductivity probe
Answer: c
Explanation: Radar level transmitters are less affected by turbulence compared to float or ultrasonic devices, making them suitable for unstable surfaces.
Q52: In differential pressure level measurement, the formula used is:
a) P = V × I
b) P = ρgh
c) F = ma
d) E = mc²
Answer: b
Explanation: Differential pressure is calculated using P = ρgh, where ρ is density, g is gravity, and h is height of liquid column.
Q53: Which level sensor is commonly used for overfill protection in storage tanks?
a) Vibrating fork switch
b) Thermocouple
c) Pressure gauge
d) Rotameter
Answer: a
Explanation: Vibrating fork switches are reliable for high-level alarms and overfill protection in tanks.
Q54: Which type of radar level sensor has higher accuracy and is less affected by vapors?
a) Pulse radar
b) Guided wave radar
c) Ultrasonic
d) Conductivity
Answer: b
Explanation: Guided wave radar uses a probe to guide the microwave signal, improving accuracy and reducing vapor interference.
Q55: Which parameter must be known for accurate DP level measurement?
a) Tank color
b) Liquid density
c) Ambient humidity
d) Pipe thickness
Answer: b
Explanation: Since DP depends on density, accurate density value is essential for correct level calculation.
Q56: A float level switch operates using:
a) Capacitance change
b) Magnetic reed switch
c) Sound reflection
d) Radiation detection
Answer: b
Explanation: Most float switches use a magnetic float that actuates a reed switch as the level rises or falls.
Q57: Which level sensor is most suitable for hygienic applications in food industry?
a) Nuclear level sensor
b) Guided wave radar
c) Sight glass
d) Open manometer
Answer: b
Explanation: Guided wave radar with hygienic design is suitable for food and beverage applications due to its sanitary construction.
Q58: Which level measurement method is independent of liquid conductivity?
a) Conductivity switch
b) Radar level transmitter
c) Resistive probe
d) Two-electrode sensor
Answer: b
Explanation: Radar measurement is based on microwave reflection and does not depend on conductivity.
Q59: Which level switch is commonly used in sump pits for pump control?
a) Float switch
b) Nuclear sensor
c) Capacitance transmitter
d) Laser level sensor
Answer: a
Explanation: Float switches are widely used in sump pits for automatic pump start and stop control.
Q60: Which level measurement device is most expensive and used in extreme conditions?
a) Sight glass
b) Float switch
c) Nuclear level sensor
d) Conductivity switch
Answer: c
Explanation: Nuclear level sensors are costly and typically used in extreme temperature, pressure, or corrosive environments.
Q61: Which level measurement device is commonly used in LPG storage tanks?
a) Sight glass
b) Guided wave radar
c) Thermocouple
d) Orifice plate
Answer: b
Explanation: Guided wave radar is commonly used in LPG tanks because it provides accurate measurement under pressure and varying temperature conditions.
Q62: Which level sensor requires compensation for vapor space pressure in closed tanks?
a) Ultrasonic sensor
b) Differential pressure transmitter
c) Float switch
d) Sight glass
Answer: b
Explanation: In closed tanks, DP transmitters must compensate for vapor pressure by connecting the low-pressure side to the vapor space.
Q63: Which level measurement technique can measure level through tank walls without contact?
a) Capacitance
b) Nuclear
c) Float
d) Conductivity
Answer: b
Explanation: Nuclear level sensors can measure through thick tank walls without direct contact with the process medium.
Q64: Which condition can significantly affect ultrasonic level measurement accuracy?
a) Stable surface
b) High foam formation
c) Atmospheric pressure
d) Clean tank walls
Answer: b
Explanation: Foam can absorb ultrasonic signals, leading to false or unstable level readings.
Q65: In capacitance level measurement, increasing liquid level will generally:
a) Decrease capacitance
b) Increase capacitance
c) Stop measurement
d) Reduce voltage
Answer: b
Explanation: As liquid level increases, the dielectric constant between probe and tank wall increases, resulting in higher capacitance.
Q66: Which level sensor is suitable for measuring sticky substances like bitumen?
a) Float switch
b) Conductivity probe
c) Non-contact radar
d) Manometer
Answer: c
Explanation: Non-contact radar avoids direct contact, preventing coating and buildup issues common with sticky materials.
Q67: Which type of level measurement is purely mechanical?
a) Sight glass
b) Radar transmitter
c) Capacitance transmitter
d) Ultrasonic sensor
Answer: a
Explanation: A sight glass provides a direct mechanical visual indication without electronics.
Q68: Which level sensor is commonly used for interface measurement in oil-water separators?
a) Guided wave radar
b) Thermistor
c) Rotameter
d) Pressure gauge
Answer: a
Explanation: Guided wave radar can detect the interface between two liquids with different dielectric constants.
Q69: Which factor does NOT directly affect radar level measurement?
a) Density
b) Dielectric constant
c) Surface reflectivity
d) Vapor composition (minor effect)
Answer: a
Explanation: Radar measurement does not depend on density; it works based on microwave reflection.
Q70: Which level measurement method is best for very small tanks with limited installation space?
a) Large displacer transmitter
b) Compact ultrasonic sensor
c) Nuclear sensor
d) Tall sight glass
Answer: b
Explanation: Compact ultrasonic sensors are easy to install and suitable for small tanks with space constraints.
Q71: Which level sensor is most suitable for corrosive acid storage tanks?
a) Float switch
b) Conductivity probe
c) Non-contact radar level transmitter
d) Displacer transmitter
Answer: c
Explanation: Non-contact radar level transmitters do not touch the corrosive liquid, making them ideal for acid storage tanks.
Q72: Which parameter primarily determines reflection strength in radar level measurement?
a) Density
b) Dielectric constant
c) Temperature
d) Tank height
Answer: b
Explanation: Radar reflection strength depends mainly on the dielectric constant of the medium. Higher dielectric constant gives stronger reflection.
Q73: Which level switch is commonly used for dry run protection of pumps?
a) Float level switch
b) Nuclear sensor
c) Capacitance transmitter
d) Laser level sensor
Answer: a
Explanation: Float level switches detect low liquid levels and prevent pumps from running dry.
Q74: Which level measurement method can operate effectively in vacuum conditions?
a) Ultrasonic
b) Radar
c) Sight glass
d) Conductivity
Answer: b
Explanation: Radar uses electromagnetic waves and does not require air as a transmission medium, so it works in vacuum conditions.
Q75: In DP level measurement, if liquid density decreases, the indicated level will:
a) Increase
b) Decrease
c) Remain unchanged
d) Double
Answer: b
Explanation: Since DP is proportional to density, lower density produces lower pressure for the same height, causing incorrect lower level indication if not compensated.
Q76: Which level sensor is most suitable for high dust environments like cement silos?
a) Ultrasonic sensor
b) Radar level sensor
c) Sight glass
d) Float switch
Answer: b
Explanation: Radar sensors are less affected by dust compared to ultrasonic sensors and are suitable for dusty silo applications.
Q77: Which level measurement device provides both visual indication and transmitter output?
a) Magnetic level indicator with transmitter
b) Sight glass only
c) Conductivity switch
d) Manometer
Answer: a
Explanation: Magnetic level indicators can be combined with transmitters to provide both local visual display and remote signal output.
Q78: Which type of guided wave radar is commonly used for interface measurement?
a) Single rod probe
b) Twin rod probe
c) Horn antenna
d) Parabolic antenna
Answer: b
Explanation: Twin rod guided wave radar probes are commonly used for accurate interface measurement between two liquids.
Q79: Which level measurement method is least suitable for very low dielectric liquids?
a) Radar
b) Capacitance
c) Ultrasonic
d) Nuclear
Answer: b
Explanation: Capacitance measurement relies heavily on dielectric constant, so very low dielectric liquids may cause poor sensitivity.
Q80: Which level sensor selection factor is most critical for accurate measurement?
a) Tank paint color
b) Process conditions (temperature, pressure, medium type)
c) Cable length only
d) Brand name
Answer: b
Explanation: Proper sensor selection depends on process conditions such as temperature, pressure, medium characteristics, and installation environment.
Level sensors are essential for measuring liquids, solids, and slurries in industrial tanks and vessels. Different technologies like float, DP, capacitance, ultrasonic, radar, guided wave radar, nuclear, and vibrating fork are used depending on process conditions such as temperature, pressure, density, foam, dust, and corrosion.
These 80 objective questions covered key principles, applications, and selection criteria to strengthen your fundamentals in level measurement. If you understand these concepts, you have a solid base in industrial level sensor technology.
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