7 Main Types of Level Sensing Methods - How do they differ?
Liquid level sensors have been around for decades, in markets such as food and beverage, industrial, medical and
domestic, printing, agriculture, automotive and white goods for leak detection or level measurement.We often wonder
why customers choose one technology over the other and it is a common question we are asked. Some equipment
manufacturers may also be surprised at both the variety and intelligence of level sensing alternatives available on the
market.
Processes that used to involve expensive pieces of equipment can now be achieved using creative, innovative and
intelligent technologies that can be cost effective, reliable, robust, highly accurate and simple to install. Fluids that have
historically been known to be extremely challenging to detect such as soap containing bubbles/foam, milk, and sticky
substances such as glue and ink are now proving possible and easier to detect with the variety of level sensing
technologies available.
But is there need for such technology—or any level sensing device is the question that many people may ask. However,
with the competitive nature of the industry and the consistently wanting to improve quality, reduce costs, inefficiencies
and waste kind of mentality, no company wants to take the risk at offering solutions that are not performing as best as
they could be.
1. Optical Level Switches
Pros – Compact, no moving parts, high pressure and temperature capability, can detect tiny amounts of liquids
Cons – Invasive as the sensor requires contact with the liquid, requires power, certain thick substances can cause
coating on the prism.
Applications – tank level measurement and leak detection applications
There are a range of technical terms used to describe this type of level sensing technology. Optical prism, electro-optic,
single-point optical, optical level switch…the list goes on. For this purpose, we will use the term Optical Level Switch. The
switch operates very simply. Inside the sensor housing is an LED and a phototransistor. When the sensor tip is in air, the
infrared light inside the sensor tip is reflected back to the detector. When in liquid, the infrared liquid is refracted out of
the sensor tip, causing less energy to reach the detector. Being a solid-state device, these compact switches are ideal for
a vast range of point level sensing applications, especially when reliability is essential. Optical liquid level switches are
suitable for high, low or intermediate level detection in practically any tank, large or small. They are also suitable for
detecting leaks preventing costly damage. Reflected light, such as in a small reflective tank, mirrored tanks, bubbles, milk
or coating fluids can often cause issues with delayed readings.
2. Capacitance
Pros – Solid-state, can be non-invasive, compact, accurate
Cons – May require calibration, can only be used in certain liquids
Applications – Tank level monitoring in chemical, food, water treatment, power and brewery industries.
Capacitance level sensors operate in the way that process fluids have dielectric constants, significantly different to air.
They measure the change in capacitance between two plates produced by changes in level. Two versions are available,
one for fluids with high dielectric constants and one with low dielectric constants.
Capacitance level sensors work with a range of solids, liquids, and mixed materials. They are also available in contact and
non-contact configurations meaning some of which can be attached outside the container/tank. When selecting a
device, it is important to know that not every capacitance senor works with every type of material or tank. In addition,
the sensor needs to be calibrated to the specific material to excuse the varying dielectric constants and differences in
the tank design. As this type of technology is contact based, the reliability of these sensors can be heavily influenced by
fluids sticking to the probe.
3. Ultrasonic
Pros – No moving parts, compact, reliable, not affected by media properties
Cons – expensive, invasive, performance can be affected by various elements in the environment
Applications – Non contact applications with highly viscous and bulk solids. Used in systems that require remote
monitoring
Ultrasonic sensors measure levels by calculating the duration and strength of high frequency sound waves that are
reflected off the surface of the liquid and back to the sensor – the time taken is relative to the distance between the
sensor and the liquid. The length of time in which the sensor takes to react is affected by various elements in the
atmosphere above the media such as turbulence, foam, temperature etc. Hence why the mounting position is critical in
these devices.
4. Microwave/Radar
Pros – very accurate, no calibration required, multiple output options
Cons – expensive, can be affected by the environment, limited detection range
Applications – Moist, vaporous and dusty environments. They are also used in systems in which temperatures vary
In principle radar works in a similar way to ultrasonic, but the pulses travel at the speed of light and again; the reliability
and repeatability can be affected – but this time by the dielectric constant of the fluid. However, radar can provide very
precise level information and also compensate for fixed structures within the container. The downside can be that the
initial cost of the sensor is relatively high, but several manufacturers are making this technology more accessible to the
wider market. These sensors are among the handful of technologies that work well in foam and sticky substances.
5. Vibrating or Tuning Fork
Pros – Compact, cost effective
Cons – Invasive, number of uses are limited
Applications – level control of liquid, powders and fine grained solids within mining, chemical processing and food and
beverage industries.
The vibrating sensor technology is perfect for solid and liquid level control, including sticky materials and foam, as well
as powders and fine grained solids. However, the types of applications that can use tuning forks is limited to overfill or
run dry type applications and they do not provide continuous process measurement. However can be used in
conjunction with continuous level detection systems, acting as alarm points for over-filling and leaks.
6. Conductivity or Resistance
Pros – No moving parts, easy to use, low-cost
Cons – Invasive, liquids need to be conductive, probe erosion
Applications – Tank level measurement for boiler water, reagent monitoring, highly corrosive liquids
Conductive sensors are used for point-level sensing conductive liquids such as water and highly corrosive liquids. Simply
put, two metallic probes of different lengths (one long, one short) insert into a tank. The long probe transmits a low
voltage, the second shorter probe is cut so the tip is at the switching point. When the probes are in liquid, the current
flows across both probes to activate the switch. One of the benefits to these devices is that they are safe due to their
low voltages and currents. They are also easy to use and install but regular maintenance checks must be carried out to
ensure there is no build up on the probe otherwise it will not perform properly.
7. Float Switch
Pros: Non-powered, direct indication, relatively inexpensive, various outputs
Cons: Invasive, moving parts, large in size, large amount of liquid has to be present before the float makes contact.
Applications: Tank level applications where water, oil, hydraulic fluids and chemicals are being used.
Float switches are one of the most cost effective but also well proven technologies for liquid level sensing. A float switch
includes a magnet within a float and a magnetic reed switch contained within a secure housing. The float moves with the
change in liquid and will cause the reed switch to either open or close depending on if it’s in air or liquid. Although
simple in design, this technology offers long-term reliability at an attractive price point.
Depending on what mounting style the user chooses heavily depends on the design and construction of the tank or
container the switch will be situated. Typically, suppliers will offer a range of mounting options, with the most common
being horizontal/side mounting and vertical mounting.
A) Importance

production equipment and result in undesirable change in the quality of product or loss of production. (Example
– In case of steam generating boilers, measurement & control of water level in the boiler drum is very critical.
Loss of water level or excessive level of water are detrimental for the life of boiler and related auxiliaries)

Monitoring and control of inventory:- Economic efficiency of any operation depends on adequate control of
inventory. This is particularly important for high value items, since excess storage of such items adds to the
“inventory carrying cost”. However low levels of inventory can starve production activity. Hence accurate
measurement of level of liquid / solid inventory is essential.

Custody transfer:- Accurate measurement of level of liquids and solids facilitates transaction of the items with
trust and confidence to ensure faster settlements of deals. Estimation of volume of costly liquids like petroleum
products can always be done more accurately by measurement of level rather than by flow metering.

Human safety:- Spillage of hazardous liquid can cause harm to personnel working in the area and potentially
may create safety and environmental hazards resulting in financial losses. Hence overfill protection is very vital.
Also disproportionate mixing of two or more ingredients can generate harmful products.
B) Measurement Technology
The technologies deployed for measurement and control of liquid / solid level are broadly classified as follows.
1) Contact Type instruments
– These instruments make direct contact with the liquid/solid whose level is being monitored. They are based on various
principles such as Archimedes principle, Magnetism, Hydrostatic pressure, Capacitance, Conductivity and Piezo electric
vibrations.
While making use of contact type level measuring instruments, compatibility of the Material Of Construction (MOC) of
the instrument must be taken into account. Also adequate attention must be paid to the operating pressure and
temperature.
1.1 Archimedes principles

A body, whose density is less than that of a liquid, will float in that liquid and the weight of liquid displaced is
equal to the weight of the body.

A body submerged in the liquid will displace volume of liquid equal to the volume of the body and the buoyant
force experienced by the submerged body is equal to its loss of weight.
1.2 Magnetism
Magnetic force can be used to actuate reed switches and micro-switches without physical contact. Also magnetic force
can be transmitted through non-magnetic materials.
1.3 Conductivity
The current flows through the conductive liquid to complete the circuit between reference electrode and
sensing/control electrode.
1.4 Hydrostatic pressure
This method depends on measurement of pressure exerted on pressure sensor by liquid column
when placed at the bottom of the tank. The pressure exerted by the column of liquid is equal to the height of liquid
column, h, times the specific gravity, sg, of the same.
(P = h x sg.).
Thus from this we can calculate the level of liquid, ‘h’ in a tank, if we know the pressure of the liquid at the bottom and
the specific gravity ‘sg’ of the liquid.
1.5 Capacitance principle
The capacitance relationship is expressed by the following equation:
C = 0.225 K (A / D)
where:
C = Capacitance in Pico Farads
K = Dielectric constant of material
A = Area of plates in square inches
D = Distance between the plates in inches
Capacitance type switches & transmitter make use of this principle where liquid between two electrode acts as dielectric
constant.
1.6 Piezo electric effect
Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical
stress. One of the unique characteristics of the piezoelectric effect is that it is reversible, meaning that materials
exhibiting the direct piezoelectric effect (the generation of electricity when stress is applied) also exhibit the converse
piezoelectric effect (the generation of stress when an electric field is applied).
All vibrating fork switches make use of piezo electric crystal and works on this principle.
2) Non-contact type instruments
– They do not make physical contact with the liquid/solid whose level is being measured. These instruments are mainly
based on ultrasonic, radar and laser principles. Also adequate attention must be paid to the operating pressure and
temperature.
2.1 Ultrasonic principle
Sound travels through air at velocity of 343 meters / sec. at 20 0 C. Echo of ultrasonic pulses at frequencies 30 KHz and
above is used to calculate the distance between sensor and target. The range from the sensor to the target is computed
by:R(t) = ct/2
where: R(t) is the distance to the target
c is the speed of sound
t is the time from the transmission until the echo received.
2.2 Radar principle
This non-contact radar technology has two different versions: Pulse radar and Frequency
Modulated Continuous Wave (FMCW) radar.
The pulse radar – An electromagnetic wave having frequency between 1 and 100 GHz is sent from the antenna toward
the process surface in search of a change in impedance, which will reflect the signal back to the transmitter.
The intensity of pulses reflected back depend upon dielectric constant of the product. The transit time of
electromagnetic pulse is used to calculate the distance and in turn the level of process liquid/solid.
The FMCW radar – It uses high frequency RF signal upto 100 GHz and transmitted towards the target and reflected from
the target surface at time delayed frequency. The difference in transmitted and received frequency of signal is directly
proportional to the distance of target from the antenna which is then converted into level signal of 4-20mA DC.
C) Types of level measuring instruments
1) Visual indication:
– It is achieved using mechanical level gauges which do not depend on electricity for their operation.

They are used for continuous level indication of liquids. Broadly level gauges are classified as glass gauges and
float type gauges.

The glass gauges are based on principle that “liquid seeks its own level” whereas float type gauges are based on
Archimedes principle.

Level gauges are available in wide variety of materials such as PP, CS, SS304, SS316, PVDF, PTFE lined SS304 or
Hastalloy C to suit the type of liquid and operating conditions.

In several applications, provision of direct reading level gauges is mandatory as statutory requirement.
Advantages:–

‘Seeing is believing’, hence reliable and do not depend on electricity.

Glass gauges provide direct indication of liquid level.

Easy to install and operate.

Suitable for high temperature and pressure applications.
Limitations: –

They are suitable for free flowing liquids not containing heavy suspended particles.

Glass gauges have a fragile element like glass tube or flat glass.
Following are the types of level gauges which are commonly used in industry.
1.1 Tubular Level Gauge
The gauge is mounted on the side of a tank so as to form close loop with the content of vessel. Liquid level can be
observed in the glass tube of the gauge. Guards are provided to protect the glass from accidental breakage.
Suitable for maximum temperature 200 ⁰C and pressure 10 Kg/cm2
Generally these gauges are used for low pressure application, and in general, for ‘open to atmosphere’ tanks.
Tubular Level Gauge
1.2 Reflex / Transparent Glass Gauge
Liquid level is observed in flat reflex or transparent gauge glass. Reflex type glass has prismatic corrugations on its front
face by virtue of which liquid portion appears as black and portion above it white.
Transparent gauge is similar to reflex type, except that the gauge glass is smooth finished and a pair of glass is fitted on
front and rear side.
Suitable for maximum temperature 400 ⁰C and pressure 200 Kg/cm2
Tubular Level Gauge
1.3 Magnetic Level Gauge
It consists of a magnetic float inside a chamber made up of non-magnetic material and indicator is clamped externally on
the chamber. Indicator is available in two types, follower capsule or bicolour flapper.
Float follows liquid level and links with capsule/ flapper to indicate liquid level. This gauge is multi-purpose, as it can be
provided with switches and transmitter which are clamped externally on the chamber.
It is suitable for corrosive, toxic and hazardous liquids where glass gauges are considered unsafe. Suitable for max temp
400 ⁰C and pressure 100 Kg/cm2
Magnetic Level Gauge
1.4 Float & Board Gauge
It is the simplest gauge consisting of float connected with wire rope and the other end of rope is connected to pointer
outside the tank.
Float follows the liquid level and pointer moves on the scale board attached on side of the tank. It is suitable for bulk
liquid storage tanks under atmospheric pressure.
Float & Board
It is the simplest gauge consisting of float connected with wire rope and the other end of rope is connected to pointer
outside the tank.
Float follows the liquid level and pointer moves on the scale board attached on side of the tank. It is suitable for bulk
liquid storage tanks under atmospheric pressure.
1.5 Float & Tape Gauge
This is the most accurate gauge, which measures liquid level upto ± 2mm accuracy. It consists of perforated tape wound
on drum. Float follows liquid level and tape is wound /unwound on drum through sprocket wheel.
The motion of sprocket wheel is transferred to pointer through gear mechanism to indicate level on the dial.
Float & Tape Gauge
2) Level detection or control at preset points: –
This is done by using level switch, which provide on/ off contact for operations such as alarm annunciation, start/stop of
pumps/auxiliaries, interlocks etc.

Used to detect liquid / solid level in the tank. Single point or multi point level switches are available.

Available in wide variety of materials such as PP, CS, SS304, SS316, PVDF, PTFE lined SS304 to suit the type of
liquid and operating conditions.

Weather proof or flameproof protection for outdoor installation and installation in hazardous area are provided.
Advantages: –

The switches, based on Archimedes principle do not require electricity for their operation. But they provide
potential free electrical contacts, which can be used for further actions.

Since the electrical switch is magnetically coupled with the float, there is complete isolation between the liquid
and the electrical portion of the instrument.
Limitations –

·Some switches may not be found suitable for direct operation of higher loads.
In such cases, use of intermediate relays/contactors may be required.
Following are the different types of level switches generally used in industry.
2.1 Float Guided Switch
It can provide single or multi point switching. It consists of one or more hermetically sealed reed switches located inside
the guide tube and a magnetic float moves freely along it. Float follows the liquid level and actuate the reed switch.
The output reed contacts have low power rating (40, 60, 120 VA) but nevertheless can be directly connected PLC/DCS.
Suitable for max. temp. 150 ⁰C & pressure 10 kg/cm2
Float Guided Switch
2.2 Float Pivoted Switch
It is single point switch generally side mounted to the tank wall. The switch operates through a glandless magnetic
coupling to provide micro switch changeover contacts of 5A, 230VAC.
The switch is ideally suitable for tanks which are inaccessible from top & bottom. Suitable for max. temp. 300 ⁰C &
pressure 20 kg/cm2
Float Pivoted Switch
2.3 Displacer Switch
It is single or multipoint switch. The displacer, is suspended from a wire rope is connected to actuator rod. With rising
level, when displacer is submerged in the liquid, it experiences a buoyant force and actuates the micro switch of 5A,
230VAC rating.
The level set points are site adjustable. Suitable for maximum temperature 300 ⁰C and pressure of 100 Kg/cm2
Displacer Switch
2.4 Conductivity Switch
As the name suggest, it is suitable for conductive liquids only. In this, a small AC signal is passed through a reference
electrode and same is detected at control electrodes at various set points.
The current flowing from reference electrode to control electrode is used to operate relay which can handle more
power. Suitable for max. Temp. 100 ⁰C & pressure 5 kg/cm2
Conductivity Switch
2.5 Capacitance Switch
It measures capacitance formed by two electrodes of the switch where liquid forms the dielectric. Variation of level
results in change in capacitance which is measured and relay is operated. Suitable for max. temp. 60 ⁰C & pressure 5
kg/cm2
Capacitance Switch
2.6 Vibrating Fork Switch
In air its fork vibrates at its resonance frequency created by means of piezo electric crystal. When material touches to
the fork, its damped vibrations are sensed in electronics to operate relay.
It is single point switch and is available for liquid and free flowing granular /powder. Suitable for max. temp. 250 ⁰C &
pressure 10 kg/cm2
Vibrating Fork Switch
2.7 Rotating Paddle Switch
It is solid level detection switch. It consists of synchronous motor which rotates at 1 RPM. The shaft of the motor is
extended to the required length and paddle is attached to it.
In absence of material paddle rotates and when solid level reaches to the paddle it stops rotating and operates a microswitch lever to provide changeover contacts.
Rotating Paddle Switch
3) Continuous level measurement and monitoring
Level transmitters are used for quantitative measurement of liquids/solids which help in inventory control and custody
transfer. They generally provide an electrical output of 4 to 20 mA DC corresponding to 0 to 100 % of the level, which in
turn can be expressed in terms of various units of measurement including volume/weight.
Transmitters are available in material of PP, SS304, SS316, PVDF coated SS to suit process liquids. Inventory control and
custody transfer can be effected by using the transmitter.
Advantages
Exact quantitative measurement facilitates continuous level monitoring.

Output is also available in RS485 MODBUS or HART. Fieldbus compatible.

Multiple alarm points can be assigned from indicator PLC/ SCADA system.

Limitations –

Contact type level transmitters are not suitable solid applications

Indicator/SCADA system is needed to display level in meaningful unit.
Following are the main types of transmitters available for different applications in the industry.
3.1 Float guided transmitter
It consists of magnetic float moving along the guide tube, containing a chain of resisters & reed switches placed across it.
Float moves on the guide tube according to liquid level to actuates reed switch and corresponding voltage across the
resister is converted into current output of 4-20mA.
It can be used for max. temp. 150 ⁰C & pressure 10 kg/cm2
Float guided transmitter
3.2 Hydrostatic transmitter
10 meters of pure water column corresponds to 98.1 Kpa of pressure. Piezo-resistive sensor fitted at bottom of the
probe measures liquid column pressure at the bottom of the tank.
Sensed pressure is converted into mv output which is processed to provide 4-20mA output. Suitable for atmospheric
pressure and temp. 70⁰C
Hydrostatic Transmitter
3.3 Capacitance level transmitter
The construction and working is similar to the capacitance switch except the change in capacitance is converted into 420 mA output signal. Suitable for max. temperature 60⁰C and pressure of 5 kg/cm2
Capacitance level transmitter
3.4 Magnetostrictive level transmitter
It is high accuracy (< 1mm) float type level transmitter A pulse of current sent over the wave guide wire, creates a
circular magnetic field which interacts with magnetic field generated by the float, to produce twist in the wire.
Time delay between starting of pulse and its return is measured and converted into 4-20mA o/p, which corresponds to
float position / liquid level. It can be used for max temp 100⁰C & pressure 10 kg/cm2
Magnetostrictive level transmitter:
3.5 Guided wave radar level transmitter
This is a contact type transmitter based on Time Domain Reflectometry (TDR) principle. Low power electromagnetic
pulses are sent along the probe length down to the product surface and partially reflected back to the electronics.
Transit time of emitted and reflected pulse is proportional to distance, which is then converted 4-20 mA signal. This
transmitter is suitable for max temp. 250⁰C & pressure of 10 kg/cm2
Guided wave radar level transmitter
3.6 Displacer type level transmitter
It consists of a displacer suspended from an operating lever, the other end of which is connected to a torque tube and
further to a lever assembly fitted with magnetic system.
The change in liquid level causes the operating lever to rotate torque tube proportionately to the change in buoyant
force acting on displacer.
Displacer Type Level Transmitter
This rotation is transferred to lever assembly causing angular displacement to change the magnetic field, which is sensed
by sensor and converted into 4-20 mA o/p through signal processing electronics. Suitable for max temp. 300 ⁰C &
pressure 60 kg/cm2
3.7 Differential Pressure Transmitter
These transmitters are based on piezo-resistive principle or capacitance principle and are extensively used for level and
flow measurement applications.
Differential Pressure Transmitter
They are ideally suitable for level measurement in highly pressurised vessels like boiler drum. Transmitter can be
connected to the vessel by means of impulse pipes. Remote diaphragm seal type transmitters are also available for
hazardous, viscous fluids.
3.8 Non-contact Ultrasonic level transmitter
It transmits a short burst of ultrasonic sound toward a target, which reflects the sound back to the sensor. The system
then measures the transit time of transmitted & reflected signal which is proportional to the distance to the target.
The same transducer can be used to both transmit and receive the sound, or separate transducers can be used to
transmit and receive. Suitable for max. temp. 70⁰C & pressure < 1bar
Non-contact Ultrasonic level transmitter
3.9 Non-contact radar level transmitter
As explained earlier, these transmitters can be either “pulse type” or based on “FMCW principle”. Transmitters are
programmable at site. Higher the frequency of operation, lower is the range of the transmitter but better is the
resolution and vice-versa.
Different types of antennas such as Horn Type, Wave Guide, Parabola Type, Patch Antenna or Drop Antenna are
available to suit the applications. Suitable for max temp. 300⁰C & pressure of 35 kg/cm2
Combinations of level gauge, switch and transmitter can also be used. All float type gauges can be provided with
switches and transmitters. Magnetic level gauge can be provided with twin chamber i.e. one for level gauging and other
for radar transmitter.
Non-contact radar level transmitter
D) Selection Criteria
Level measuring instrument is selected with reference to following criteria/conditions.
1. Chemical compatibility with liquid
2. Physical property of service material – Density, conductivity, dielectric constant, viscosity & suspended particles
3. Operating temperature & pressure
4. Foaming /fuming conditions
5. Agitation /turbulence in liquid
6. Angle of repose and particle size
7. Area classification as hazardous/non-hazardous
8. Mounting position & process connection on the tank/vessel
9. Desired accuracy
Not only selection but also proper installation and commissioning of the instrument is equally important for achieving
accuracy in level measurement.
There is not universal instrument for level measurement of liquids/ solids. However appropriate instrument is selected
with given criteria as above.
E) Advent of Digital Era
Similar to other walks of life, advancements in digital technology has beneficially impacted level measurement
technology as well. In addition to the conventional 4 to 20 mA DC output from the transmitters, MODBUS RTU output is
also available which can be directly monitored from the computer.
‘Tank Farm Management System’ which is essentially a SCADA system, facilitates centralised monitoring and inventory
control of chemicals, solvents and petroleum products.
Also several functions related to monitoring of level, such as on/off operations of pumps, open/close operations of
valves, alarm annunciation etc. can be executed from remote by making use of GSM/GPRS network.
Graphic display and animation of level indications on the DCS/SCADA screens makes it convenient for the operator to
monitor several parameters at a glance.
Recently level measurement & monitoring is done using wireless technology for various IIoT applications