Instrumentation Engineering Professional Website

Level Measurement Instruments

Level measurement instruments are used to provide the exact level of liquids or solids in atmospheric or pressurized vessels, tanks or hoppers that can lead to calculate their weight or volume.

Level measurement instruments could be used for inventory control, custody transfer, leakage monitoring, overfill protection, batch control, dosing systems, mixed liquids separation and many other precise controls.

Level measurement instruments could be categorized as the following:

Level gauges are used for visual and local level indication in the field and level transmitters are used for continuous level monitoring and control from control room.

Level switches are usually used for protection controls.

Level Gauge

The most common industrial types of level gauges are:

Level Gauge

Sight Glass

Sight glass is a glass window or tube installed on a vessel or tank that allows the operator to observe the inside of that vessel or tank.

Operator might needs to observe liquid level or check the process inside of the vessel to ensure optimal quality or safety.

Sight glass could be in form of a glass disk with a flanged metal frame which will be installed on the vessel nozzle or could be a vertical tube with flange connections at top and bottom.

Sight Glass

Tube sight glasses are similar to contemporary level gauges, but only provides level indication at a specific level or a very small range.

Glass Level Gauge

There are three types of level gauges; Reflex, Transparent and Bi-Color.

Glass Level gauges have four major parts; metal chamber, glass, two cock valves on top and bottom and two valves or plugs for drain and vent.

Tubular is another type of glass level gauge but is not applicable in industrial applications.


Reflex Level Gauge

Reflex Level Gauge works based on light reflection and refraction laws.

In Reflex Level Gauge, front of the metal chamber is covered with a glass that has prismatic grooves with section angle of 90° that forms an angle of 45° with the liquid.

Reflex Level Gauge

When the liquid enters the gauge chamber, based on communicating vessels principle, its level in the gauge settles at the same level of liquid in the vessel or tank.

The liquid level is distinguished by different brightness of the glass between the liquid and gas/vapor zones, because of the difference in the refractive indices of liquid with gas/vapor.

Reflex Level Gauge Working Principle

Gas/vapor region light reflection degree is less than 45°, therefore in gas/vapor region the light is reflected back to the glass surface so it appears silvery white.

In other words, the difference in the index of reflection between the gas/vapor and the glass is large, hence the light cannot pass the from the glass prismatic grooves and reflects back.

Liquid region light reflection degree is greater than 45°, so the light is not reflected back, therefore the glass appears darker in liquid region.

It means that due to small difference in the index of reflection between the glass and the liquid, the light passes through the prismatic area of the glass.

Reflex level gauge does not require illumination, and day light is sufficient for visualizing the liquid level.


Reflex Level Gauge Applications

Reflex Level Gauge is used for clean and clear liquids, and should not be used for steam or corrosive services, because using mica shield which is necessary for glass protection cannot be installed with prismatic glass.

Reflex Level Gauge is not suitable for services of two liquids with different densities because the interface level between the upper and lower liquids cannot be distinguished clearly.

Reflex Level Gauge could be used for C3 and lighter hydrocarbon if they are not dark and dirty.

Heavier hydrocarbons, dark or dirty liquids might coat the glass with a dark layer, hence the lower liquid levels could not be distinguished properly.


Transparent Level Gauge

Transparent Level Gauge has two flat glass panels in front and behind of the chamber.

Transparent Level Gauge

The light ray enters from the rear glass and passes through the fluid and exits from the front glass.

Difference between transparencies of mediums distinguishes the level.

Transparent Level Gauge Working Principle

Usually, an illuminator is installed behind the gauge for better level indication.

Because liquids with different transparencies could be distinguished, transparent level gauge is used for indication of interface level between two liquids with different densities.

In order to protect the glass from steam, hot alkaline or acidic solutions, amines, hydrofluoric acid, caustic and other corrosive services, transparent level gauge should be equipped with mica shield as a protective film behind the glass.

Transparent level gauge illuminator should be suitable for the hazardous area classification of the installation area.


Transparent Level Gauge Applications

Transparent Level Gauge can be used for the following applications:

  • Dirty or dark colored liquids
  • Acid or caustic services
  • Interface services
  • Services with viscosity higher than 10 cP.
  • Corrosive services where mica shield should be used

Bicolor Level Gauge

Bicolor Level Gauge is used for indication of condensate water and steam levels on high pressure boiler steam drums.

Bicolor Level Gauge indicates Water (lower part) in green and steam (upper part) in red.

Bicolor Level Gauge

This instrument consists of a stainless steel body which has a trapezoid cross section, two glass sets in front and back of the body, a bicolor filter and an illuminator behind the back glass.

The glass faces are not parallel because of trapezoid cross section of the body.

A red and green filter is located behind the back glass that converts the illuminator light into red and green light rays.

Red rays and green rays have different index of refraction when passing obliquely through glass, water and steam.

Bicolor Level Gauge Working Principle

In lower part where water condensate is accumulated, the red rays are deviated, and because of non-parallel glasses could not pass through the front glass, but green rays can reach the front glass, therefore the observer sees this part of level gauge in green.

In upper part where contains steam, conversely the green rays are deviated and could not pass through the front glass, but red rays pass through the front glass, and this part of level gauge is observed in red.

Because of high temperature, front and back glasses are protected by mica shield.

Because bicolor level gauge is used for high pressure boiler steam drums, it should be designed in accordance with ASME BOILER AND PRESSURE VESSEL CODE - SEC. I. standard.

Bicolor level gauge illuminator should be suitable for the hazardous area classification of the installation area.


Glass Level Gauge Parts

Glass Level gauges have three major parts; metal chamber (Body) , glass and valves (two cock valves on top and bottom and two valves or plugs for drain and vent).


Level Gauge Body

Chamber, the metal part of the body, with the glass which is bolted and fastened to the chamber, form the whole gauge body.

Because of limitations in length of the glass, chambers are manufactured in standard lengths, based on standard sizes of the glass.

There are 9 standard sizes for glass, therefore there would be 9 sizes of body sections.

Manufacturers provide a table including the visible length and connections center-to-center (C-C) length relevant to different numbers of sections for each standard glass size.

In order to keep physical and mechanical strength of the level gauge, it is recommended not to use glass level gauge with C-C length of more than 2000mm.

If the required C-C length is more than 2000mm, then two or more level gauges with at least 50mm overlap in visible length could be used.

Body material is usually ASTM A105 carbon steel for noncorrosive services, but for corrosive and cryogenic services ASTM A316 stainless steel is the best solution.

For services that contain an aggressive species such as chloride (Cl-) like seawater, Hastelloy C which is Nickle based alloy with molybdenum content or Duplex or Superduplex alloys could be the suitable materials for body.


Level Gauge Glass

Strength of glass decreases by increase in length; therefore, the glass length could not be as long as the measuring length.

Therefore, the gauge chamber should be constructed by connecting definite numbers of standard sections proportional to standard glass sizes to provide the gauge visible length.

There are 9 standard sizes for glass from 115mm to 340mm.

Standard Sizes for Level Gauge Glass
Size 1 2 3 4 5 6 7 8 9
Length (mm) 115 140 165 190 220 250 280 320 340

Size 9 is the most common glass size, used in industrial applications.

Glasses are available in three widths of 34mm,30mm and 25mm, but 34mm is the standard width for level gauges glass.

The glass is the weak point of level gauge and should be manufactured from special materials that could withstand the industrial temperature and pressure conditions.

Tempered glass is the most frequent glass used for level gauges which is thermally and physically stronger than normal glass due to special thermal and chemical treatments.

Borosilicate glass is a kind of tempered glass which is manufactured from silica and boron trioxide in high temperature.

Borosilicate glass has a very low thermal expansion coefficient and has the best chemical resistance to acidic solutions also is very resistant to sudden temperature changes, but is less resistant to alkaline solutions.

Borosilicate glass could be used for services with temperature up to 315 ℃.

Aluminosilicate glass that contains aluminum oxide is more heat resistant that can tolerate temperatures up to 425℃, and has higher pressure rating and better chemical resistance especially to alkaline solutions in comparison to Borosilicate glass.

Quartz which is made by fusing quartz crystals is more abrasive resistant material than glass and can be used for services with temperature up to 560℃, but it is more brittle.


Level Gauge Glass Protection

In order to protect the glass from steam, hot alkaline or acidic solutions, amines, hydrofluoric acid, caustic, and other corrosive services, transparent level gauge should be equipped with a protective film.

Mica shield is the best and most frequent protective film used for transparent level gauges.

Mica shield is not applicable for reflex level gauges, because of the prismatic grooves on inner side of the glass.

Mica is natural silicate (phyllosilicate) mineral and has extremely high temperature resistant, good transparency and clarity, acceptable flexibility and corrosion resistance against hot alkaline or acidic solutions.

Mica has different grade from V1 to V10 for different applications, but there are two mica grades that are suitable for level gauges glass protection:


V2 Ruby/Green Clear and Slightly Stained

V2 Clear and Slightly Stained mica grade is hard, and has uniform color, but may contain slight crystallographic discoloration, and is free from all vegetable and mineral stains, cracks, buckles, other similar defects.

It is also free from foreign inclusions except for a few tiny air inclusions in not more than one-fourth of the useable area.

This grade is suitable for steam services from 600 to 1500 PSI (41 to 103 bar).

It is available in two color groups; ruby and green.


V4 Ruby/Green Good Stained

V4 Good Stained mica grade is hard, and has uniform color, but may contain slight crystallographic discoloration, and is free from vegetable and mineral stains, cracks, buckles and other similar defects.

It is also free from foreign inclusions, except somewhat wavy but not rippled, and may contain some air inclusions in not more than two-thirds of the usable area.

This grade is suitable for steam services from 300 to 600 PSI (20 to 41 bar).

It is available in two color groups; ruby and green.


Standard mica shield sizes and thicknesses are as the following tables:

Standard Mica Shield Sizes (mm)
No Mica Shield (Type A/B) Mica Shield (Type A) Mica Shield (Type B/H) Mica Shield (Type TA-28)
Length x Breadth Length x Breadth Length x Breadth Length x Breadth
0 95 x 24 95 x 30 95 x 34 133 x 47
I 115 x 24 115 x 30 115 x 34 158 x 47
II 140 x 24 140 x 30 140 x 34 183 x 47
III 165 x 24 165 x 30 165 x 34 208 x 47
IV 190 x 24 190 x 30 190 x 34 238 x 47
V 220 x 24 220 x 30 220 x 34 268 x 47
VI 250 x 24 250 x 30 250 x 34 298 x 47
VII 280 x 24 280 x 30 280 x 34 338 x 47
VIII 310 x 24 320 x 30 320 x 34 358 x 47
IX 320 x 24 340 x 30 340 x 34 378 x 47
X 340 x 24 370 x 30 370 x 34
XI 360 x 24 400 x 30 400 x 34
XII 370 x 24 430 x 30 430 x 34
XIII 400 x 24 460 x 30 460 x 34
XIV 430 x 24 500 x 30 500 x 34
Standard Mica Shield Thicknesses (mm)
0.10 - 0.15 0.13 - 0.17 0.15 - 0.20 0.18 - 0.22 0.20 - 0.30 0.30 - 0.40

For Hydrofluoric Acid services primarily, PCTFE shields (Kel-F ®) are used which can operate in temperature range of -240℃ up to 150℃.


Level Gauge Valves

Glass Level gauge is connected to the vessel or tank with two cock valves at top and bottom of the level gauge.

Cock valve is an angle valve that the angle between the inlet and outlet of the valve is 90 deg.

The outlet of the cock valve which is connected to the vessel nozzle is usually flanged.

Level Gauge Cock Valves

Cock valve connection to the level gauge is normally 1/2" or 3/4" NPTF thread connection, but 3/4" NPTF is preferred due to blocking risk reduction.

The top cock valve has another 1/2" connection for vent plug or vent valve.

Similarly, the bottom cock valve has a 1/2" connection for drain plug or drain valve.

Using plug for vent / drain is an economical solution, but after years of operation and maintenance it becomes the level gauge and vessel weak point and leakage source.

Therefore, it is recommended to select 1/2" vent /drain valve instead of plug.

Cock valve should be offset type in which the valve stem has an offset to the gauge body axis in order to allow glass cleaning with bottle brush without opening the cock valve and removing the level gauge.

But the offset could be left-handed or right-handed which needs to be defined before order in data sheet, considering the level gauge nozzles location on the vessel and the access point or ladder location.

Also, if the top cock valve is a right-handed offset valve, then the bottom cock valve should be left-handed offset valve.

This problem could be solved by selecting a rotatable head cock valve which could be installed at left side or right side of the gauge body during installation.

Offset Cock Valve

Cock valves should be equipped with a safety device which is called ball check.

Ball check prevents vessel content loss in case of level gauge failure which could be leakage or glass break.

In case of level gauge failure, the level gauge pressure decreases and the vessel liquid content tends to move inside the level gauge and bushes the ball check to cock valve seat, hence the level gauge entry will be blocked.

According to ASME BOILER AND PRESSURE VESSEL CODE - SEC. I. standard, ball check is not necessary for glass level gauges on boiler drums , because in prevents steam passing through the gauge during periodic steam blowdown.

Otherwise for steam applications, it should be vertical rising ball check and equipped only for the bottom cock valve.

Cock valve body due to small size is usually forged material and its metallurgy should be the same as level gauge body material.

For example, if the body material is cast carbon steel, ASTM A216 WCB, the cock valve material should be the equivalent forged carbon steel, ASTM A105.

Cock valve plug, needle, seat, stem and ball check should be AISI 316 Stainless steel or better.


Level Gauge Illuminator

Transparent and bicolor level gauges require illuminator.

Contemporary illuminators are LED illuminators, but incandescent bulbs and fluorescent lamps were the most frequent used illuminators.

Incandescent bulbs and fluorescent lamps require a plastic diffuser in order to spread the light across the gauge.

LED illuminators have low power consumption and longer life cycle.

Illuminator Type Incandescent Bulb Fluorescent Lamp LEDs
Life Cycle 1000 hr 15000 hr 100000 hr

Therefore, Incandescent bulb and fluorescent lamp illuminators could be equipped with a spring return power switch to ensure that the illuminator is turned off after use for longer life time and minimum maintenance.

LED illuminators have a long-life cycle and more reliability, because if a LED fails the nearby LEDs lighting overlap, covers the failed LED area.

Illuminator in hazardous areas should be explosion proof.

Electrical department is responsible for providing the required electrical power for illuminators; hence this department needs to be informed about operating voltage type and level, power consumption and location of the illuminators.


Glass Level Gauge Installation

Strength of glass decreases by increase in length; therefore, the glass length could not be as long as the visible length.

Therefore, the gauge chamber should be constructed by connecting definite numbers of standard sections proportional to standard glass sizes to provide the gauge visible length.

As mentioned formerly, there are 9 standard sizes for glass from 115mm to 340mm.

Standard Sizes for Level Gauge Glass
Size 1 2 3 4 5 6 7 8 9
Length (mm) 115 140 165 190 220 250 280 320 340

Size 9 is the most common glass size, used in industrial applications.

The following table provides recommend number of sections and sizes for different visible lengths:

No. of Sections Glass Size Level Gauge Size Visible Length (mm) Approximate C-C Length (mm)
1 9 (340mm) 1x9 320 600
2 9 (340mm) 2x9 680 950
3 9 (340mm) 3x9 1040 1300
4 9 (340mm) 4x9 1400 1650
5 9 (340mm) 5x9 1760 2000

Level gauge visible length should cover the measuring levels of all the level transmitters and switches on a vessel or tank.

It is recommended to select the visible length of level gauge to be 50mm higher than the highest measuring level (HH) and 50mm lower than the lowest measuring level (LL).

Glass Level Gauge Installation

After selecting the visible length of the level gauge approximate C-C length could be derived from the above table or the manufacturer catalogue.

In order to keep physical and mechanical strength of the level gauge, it is recommended not to use level gauge with C-C length of more than 2000mm.

If the required C-C length is more than 2000mm, then two or more level gauges with at least 50mm overlap in visible length could be used.

Multi-Glass Level Gauge Installation

But using two or more level gauges requires more nozzles on the vessel which could affect its mechanical strength and could become a weak point.

Therefore, installation of a standpipe beside the vessel or tank for the two or more level gauges is an appropriate design.

Standpipe diameter could be 4" or 6" and it should be equipped with isolating valve.


Magnetic Level Gauge

Magnetic Level Gauge consists of a float with permanent magnets inside a non-magnetic metal chamber and an indicator with rotating magnetic flags.

Magnetic Level Gauge

Magnetic Level Gauge does not have glass, therefore it does not have length limitation of glass level gauges, and can be used in higher pressure and temperature applications.

Another advantage of magnetic level gauge in comparison to glass level gauges is that the level can be visible from far distance.


Magnetic Level Gauge Applications

  • High temperature or high pressure services that glass might fail
  • Liquified C4 and lighter hydrocarbons which are colorless and flammable
  • Flammable services where fire hazard exists in case of glass failure
  • Dirty or viscose services that glass becomes coated and loses its transparency
  • Interface level indication especially on separator drums
  • Very corrosive or acidic services
  • Cryogenic services

Magnetic Level Gauge Working Principle

Liquid level in the chamber of magnetic level gauge is the same as the vessel, based on the principle of communicating vessels.

The float will be remaining buoyant on the liquid's surface based on positive buoyancy.

There is a magnetic assembly inside of the float, and when float moves up and down with level on the liquid's surface, it rolls a series of rotating bi-color magnetic flaps or flags on an indication case which is fastened to the chamber.

Magnetic Level Gauge Working Principle

As a result, the indicator color from the liquid's surface level to the bottom is different from liquid's surface level to the top.

Magnetic level gauge could be used for interface level indication, too.

In this case float will be designed considering the lower liquid density in a way that, it remains buoyant on the heavier liquid's surface which is the interface level.


Magnetic Level Gauge Chamber

Magnetic Level Gauge Chamber is a standpipe with minimum diameter of NPS 2 with Schedule of 40.

NPS (Nominal Pipe Size) is an American standard for pipe size.

NPS 2 is equivalent to European standard pipe size, DN 50, and identical with 60.33mm.

Pipe schedule describes the pipe wall thickness and Sch.40 determines 3.912mm wall thickness for PNS 2 pipe.

Chamber material must be from non-magnetic metal that could pass the magnetic field of the float

Austenitic Stainless Steels (316SS, 304SS …) are non-ferrous stainless steels and can pass magnetic field.

AISI 316 / 316L Stainless Steels are the best and the most common material for chamber material due to proper resistance against corrosive services and acceptable mechanical properties with temperature range of -254 to 816°C.

For services that contain an aggressive species such as chloride (Cl-) like seawater, 316SS or 316L SS could not be used and Hastelloy C which is a Nickle Alloy with PREN (Pitting Resistance Equivalent Number) greater than 40 is recommended for such services but the price is considerable.

In order to provide the possibility of float replacement, the bottom of the chamber has a flanged connection.

A flanged valve could be used at the bottom of the chamber to drain the gauge during the maintenance.

It is recommended not to use drain plug instead of drain valve, because plug threads are subject to wear due to several opening and closing and could become a leakage point.

There should be a vent plug or valve at the top of the chamber to vent the accumulated gases; vent plug is an economic selection.

If the magnetic level gauge is used in boiler service the chamber should be designed according to ASME B31.1 and B31.3.

For other applications on pressurized vessels, the chamber should comply with ASME B&PV Code, Section VIII, Division 1.

For interface level indication application, the chamber should have more connections to the vessel in addition to the top and bottom connections and at least one of the connections should be higher than the interface level, otherwise the lighter liquid might be trapped inside the vessel and could not enter the chamber.

Chamber should be equipped with stop springs at the bottom and top of the chamber to avoid hard impact between float and the chamber, during fast drain or fast level increase.


Magnetic Level Gauge Float

The float is designed based on process pressure, temperature, fluid's specific gravity and fluid's corrosivity.

The magnetic assembly inside of the float rolls the magnetic bi-color flaps or flags of the indicator as the float rises and falls with liquid's level.

The magnetic assembly should be placed in the float in such a way that the indicated level coincides with the actual liquid's level.

The float length should be less than 250mm.

The float material should be non-magnetic material so that it could pass the magnetic field of magnetic assembly inside the float.

Process temperature is very important for selecting material for magnet assembly and designing the float.

The fluid temperature should not be above the magnet's Curie temperature.

Curie temperature (TC), is the temperature that magnetic materials lose their permanent magnetic properties above that temperature.

For application with operating temperature above 150 ℃, high temperature magnets should be used.

Please note that the float must be removed from the chamber before hydrostatic test of the vessel.

After the hydrostatic test special care should be taken for reinstallation of the float as the following:

  • Float serial no. must be checked, because float is designed exclusively for each level gauge according to liquid specific gravity
  • Float should not be installed upside down
  • First, top isolating valve should be opened slowly for pressure equalization, then bottom valve in order to fill the chamber gradually

Magnetic Level Gauge Indicator

Indicator consists of a column of magnetic bi-color flags or rollers in a nonmagnetic case which is fastened or clamped to the chamber.

Alongside with the flags column there is a stain less steel scale marked usually with 10mm divisions.

Flaps color is usually yellow/black or red/white.

Indicator is not in contact with the medium; its case could be made of aluminum alloys even for corrosive applications, but in case of aggressive and harsh environment conditions stainless steel case is recommended.

The flags columns should be hermetically sealed in order to be protected from dost or condensation in humid environments.

The flags column design should prevent over rotation of flags due to fast level changes or vibration; it could be provided with mechanical stop or magnetic interlock.

Magnetic level switches can be clamped to the indicator in order to provide discreate signals as alarms or trips at specific levels.

There is another type of indicator which consists of a magnetic follower in a transparent tube instead of bi-color flags that rises and falls with floater movements in the chamber.

This type of indicator is not recommended for applications with high vibration or fast liquid level changes, because in those applications follower might be decoupled from the float, also the follower cannot be visible from far distance.


Magnetic Level Gauge Installation

Magnetic level gauge installation is mostly the same as glass level gauges installation, but magnetic level gauges could be top mounted which is not possible for glass level gauges.

Top mounting magnetic level gauge is useful for underground or covered vessels.

The most common magnetic level gauges installation configurations are; side-side, top-side, top-bottom and top mounting.

Magnetic Level Gauge Installation

For interface level indication, float will be designed considering the lower liquid density that it sinks into the lower density liquid and remains buoyant on the heavier liquid's surface which is the interface level.

In this application more than two process connections in different levels is necessary, otherwise the interface level in the chamber and vessel may not be the same.

At least one more process connection should be considered higher than the maximum interface level.

Magnetic Level Gauge Installation

The most important issue regarding the installation of magnetic level gauge is that it should be installed far from the equipment that affect its magnetic field.

Therefore, magnetic level gauge should be installed at least 20cm (8 in.) far from ferrous materials such as ladders, supports, carbon steel pipelines, floor grating, etc.

Location of transformers, electrical motors, magnetic separators, heat tracing cables around the vessel forming a coil or other equipment that produce electromagnetic field should be considered for magnetic level gauges installation.


Magnetic Level Gauge with Transmitter or Switch Assembly

Magnetic level gauge could be equipped with a magnetostrictive level transmitter for continuous level measurement in addition to level indication.

Magnetostrictive level transmitter consists of a wire made of magnetostrictive material which is held under tension inside a tube.

The tube is fastened to the magnetic level gauge chamber.

A current pulse produces a circular magnetic field along the wire.

Superposition of this magnetic field and the float magnetic field creates a torsional force in the wire that produces a mechanical wave in the wire proportional to the location of the float and can be converted to an electrical signal using a piezoceramic sensor.

Magnetic Level Gauge with Magnetostrictive Transmitter

When more accuracy is required for continuous measurement wave guided radar level transmitter could be combined with magnetic level gauge.

Radar level transmitter could be installed on top of the level gauge chamber or on another chamber connected to the level gauge chamber.

The advantages of this combination are more accuracy, more reliability, and less process connections on the vessel.

Magnetic Level Gauge with Radar Transmitter

Magnetic level switches can be clamped to the indicator in order to provide discreate signals as alarms or trips at specific levels.

Magnetic Level Gauge with Magnetic Switch

Switches latch into predefined position (open/close) by magnetic field of the float when it passes the switches position and unlatch by the float return.


Float Level Gauge

Float Level Gauge is usually used for large storage tanks level indication.

It consists of float, counterweight, scaled gauge board, rope or chain, guide pipes and movement adjusting pullies or gears.

Float is located inside the tank which rises and falls with liquid level based on positive buoyancy.

Float is connected to a counterweight outside the tank by rope or chain through pipe guides on top of the tank.

Counterweight up and down movement in front of the scaled gauge board outside the tank is adjusted by sets of pullies or gears to keep it at the same level of the float accurately.

Float Level Gauge

In some other types, adjusting pullies and gears are in a local indicator at bottom of the tank which has better accuracy and could be equipped with transmitter or switches to provide analog or digital signals for control system.

Float Level Gauge with Local Indicator

Float level gauge applications

  • Large storage tanks
  • Mediums with high viscosity such as liquid asphalt
  • Petroleum and food products storage tanks such as Kerosene, crud oil, fuel oil and vegetable oil
  • Liquid gases in spherical tanks

Float level gauge advantages

  • High reliability
  • Visible from distance
  • Simple construction
  • Low cost in comparison to other gauges for large tanks
  • Low maintenance

Level Transmitter

Level transmitters are used for continuous level monitoring and control from control room.

Level transmitter output is a 4-20 mA signal in which 4 mA represents 0% level and 20 mA represents 100% level of the controlled equipment.

The most common level transmitters used in industrial applications are as the following:

Differential Pressure Level Transmitter

DP level transmitter is in fact a Differential Pressure transmitter which is used for measuring the level of liquid in a tank or vessel.

Differential Pressure transmitter has two pressure inlets (LP and HP) for connecting to the two measuring point pressures.

LP and HP connections are located at opposite sides of the diaphragm.

Difference between the LP and HP pressures cause the diaphragm to fold to the lower pressure side.

This movement will be sensed by sensor and transferred to an electrical output signal.

The most common pressure sensing element types for pressure transmitters are:

  • Electromechanical strain gauge
  • Variable Capacitance
  • Piezoresistive
  • Piezoelectric

Their working principle and construction are described in pressure transmitter article.

In level measurement applications the nozzle with lower elevation is connected to HP side and the nozzle with higher elevation is connected to LP side.

DP Level Transmitter

In a closed vessel, pressure of higher elevated nozzle, PL, is equivalent to vapor pressure above the liquid in the vessel, Pvapor.

Pressure of lower elevated nozzle, PH, is equivalent to liquid pressure, PLiquid, plus Pvapor.

The differential pressure measured by DP transmitter is subtraction of, PL from PH.

It means that the differential pressure is equal to liquid head pressure, ρgh.

DP Level Transmitter Calculation

Then the level of liquid in the vessel can be derived from the above equation.

Vessel nozzles are connected to differential pressure transmitter's pressure inlets (LP and HP) by stainless steel tubes, which are called impulse lines.

If the vapor above the liquid tends to condense into liquid form, then the LP (Low Pressure) impulse line should be filled with liquid before the operation, which is called wet leg.

If the vapor does not condense into liquid form then, dry vapor of the vessel enters the LP impulse line, which it is called dry leg.

DP Level Transmitter Wet leg & Dry Leg

In wet led design, the sensed pressure at LP side of DP level transmitter is higher than the sensed pressure at HP side, due to head pressure of liquid in LP impulse line.

It means that the transmitter out put value for level is always a negative value.

DP Level Transmitter Zero Elevation

In order to solve this problem, the transmitter should be positively biased to elevate the transmitter output signal to compensate the LP impulse line head pressure.

It is called "Zero Elevation".

Most of the time, elevation of DP level transmitter is lower than the 0% level, therefore the sensed pressure at high side of DP level transmitter is higher than the required liquid head pressure because of head pressure for the remained liquid in HP impulse line.

In this situation if the vessel is fully empty the transmitter shows a value for vessel level.

DP Level Transmitter Zero Suppression

The solution is subtraction of impulse line liquid head pressure from the HP side sensed pressure which means that the transmitter is negatively biased.

It is called "Zero Suppression".

Another solution to eliminate impulse line problems is to use diaphragm seal with capillary tubes.

Diaphragm seal consists of an external sensing metal diaphragm that is connected to DP transmitter with liquid-filled capillary.

Diaphragm Seal

In this design, pressure sensor is isolated from the medium, and vessel pressure deflects the diaphragm membrane, therefore the measuring pressure is transferred to the filled liquid and finally to the sensor.

Diaphragm Seal with Capillary Tube

Capillary tubes are welded to diaphragm seal and transmitter by manufacturer to provide a hermetically sealed system.

Different types of diaphragm seal are described in diaphragm seal article.

It is recommended to use diaphragm seal for the following services:

  • Highly corrosive services
  • Leakage is dangerous and unallowed (toxic, explosive… services)
  • Very high temperature services
  • Cryogenic services
  • Hygiene services (food and drug industries)
  • Dirty services
  • Services containing solid particles
  • Slurry services
  • Very viscous services
  • Services that fluid tends to be crystallized or polymerized

Diaphragm Seal Filling Liquid

Diaphragm seal filling liquid should have low viscosity, low thermal expansion coefficient, low vapour pressure and most importantly should be stable in the operating temperature range.

At high temperature, filling liquid might be vaporized and affect the true pressure measurement.

At very low and frigid temperature, filling liquid might become very viscous or even turn into solid.

Therefore, the most important point in filling liquid selection is its stability operating temperature.

Filling liquids are available with temperature range from -90 ℃ up to 350 ℃.

Glycerin and Silicone Oil are the most common filling liquids.

In cryogenic ambient conditions Silicone Oil has better performance due to having low viscosity even in cold temperatures.

But Glycerin and Silicone Oil shall not be used for strong oxidizing agents such as Oxygen, Chlorine, Nitric Acid and Hydrogen Peroxide.

For these process fluids, Fluorinated or Chlorinated liquids such as Halocarbon or Fluorolube should be used.


Ambient Temperature Effect on Capillary

Ambient temperature changes result in expansion or contraction of capillary tube filling liquid, causing changes in internal pressure of capillary tube.

This pressure change is the measurement error of DP level transmitter with diaphragm seal and capillary system.

One solution is to consider capillary tube length of the LP side and HP side exactly the same in which the error in both sides are the same and would be eliminated automatically by ΔP calculation.

Second solution is to provide temperature sensor in DP level transmitter to compensate filling liquid density change.

The third solution is to use the contemporary DP level transmitter design, Electronic or Digital Remote Sensor.


Electronic Remote Sensor DP level transmitter

In Electronic Remote Sensor DP level transmitter design, two individual pressure sensors are used.

One installed on HP nozzle and the second one installed on LP nozzle.

The two sensors are connected to each other using a special communication cable.

One of the sensors calculate the differential pressure and converts it to 4-20ma signal.

Electronic Remote Sensor DP level transmitter

In this new design impulse lines and capillary tubes and their problems are eliminated.

Advantages of electronic remote sensor design

  • Ambient temperature affect is eliminated
  • Zero Suppression or Zero Elevation are not required
  • Impulse lines leakage risk eliminated
  • Fast response
  • Easy installation
  • Low maintenance

Electronic remote sensors design is suitable for tall vessels, where there is long distance between LP and HP nozzles.

In cold climate applications, heat tracing in necessary for conventional impulse lines which in this new design will be eliminated.


Hydrostatic Level Transmitter

Hydrostatic Level Transmitter construction is the same as DP level transmitter, but the LP (Low Pressure) side is open to the atmosphere.

Hydrostatic level transmitter is usually used for open vessels and tanks level measurement.

Hydrostatic Level Transmitter

Submersible type of hydrostatic level transmitter is available that can be used as top mounting installation.

This type of hydrostatic level transmitter is useful for underground atmospheric tanks and sumps.

Submersible Hydrostatic Level Transmitter

In this design sensor will be submersed in the liquid and located at 0% level point using an adjustable sensor cable or rod, and transmitter will be installed at top of the vessel.


Ultrasonic Level Transmitter

Ultrasonic Level Transmitter uses ultrasound waves and operates based on Time-of-Flight principle.

Ultrasonic Level Transmitter

Its sensor produces an ultrasonic wave and sends it to the medium in the tank which could be liquid or solid and detects its reflection from the medium surface.

The travel time of the wave between emission to reception of its reflection which is called Time-of-Flight, is proportional to the distance between sensor and the medium surface.

Ultrasonic Level Transmitter analyzes the wave Time-of-Flight and provides the medium level in the tank as a 4-20 mA output signal.

Ultrasonic Level Transmitter Working Principle

Ultrasonic Wave Frequency Range

Sonic is the sound wave in human hearing range, 20Hz up to 20KHz.

Ultrasonic is a sound wave above human hearing range.

Some animals like bats and dolphins use ultrasonic waves between 20KHz to 200KHz and echolocation to recognize objects and food in dark.

Ultrasonic Wave Ranges

Ultrasonic waves are used for different industrial applications with frequency range of 20KHz up to 10MHz.

The ultrasonic wave frequency range used for ultrasonic level transmitters is usually within a range of 40 to 200 KHz.


Ultrasonic Level Transmitter Limitations

Temperature Variation

Sound wave speed depends on density of the medium. The higher the density, the higher the sound wave speed.

Sound travels very slowly in gasses, faster in liquids and travels with highest speed in solids

Sound speed is 343 m/s in air, 1481 m/s in water and 5120 m/s in iron.

In vessels the density of the gas above the liquid changes with temperature, therefore the speed of ultrasonic wave sent by sensor defers in different temperatures.

It causes inaccuracy in measurement for systems or ambient with fast temperature variation.

This problem could be solved with an integrated temperature sensor inside the ultrasonic level transmitter for density compensation.

In closed vessels for the best accuracy result, pressure and compressibility factor of the vapor above the liquid should be considered for density compensation.


False Echoes

The other problem of ultrasonic level transmitter refers to the construction of the vessel that could make false echo and affect the level measurement.

Thermowells, agitators, stilling wells, meshes, steam heating pipes… are obstructions in ultrasonic level transmitter point of view and produce false echoes of the submitted ultrasonic wave.

False echo problem could be solved by software modification that let the ultrasonic level transmitter to ignore undesired echoes; and it is called mapping or false echo suppression.

For mapping, the ultrasonic level transmitter will be turned on when the vessel is empty and it sends an ultrasonic wave and receives the echoes.

All the echoes from obstructions above the minimum considered liquid level will be considered as false echo and level transmitter learns to ignore them in futures.

Ultrasonic level transmitter made by some of manufacturers can do the empty vessel mapping automatically in addition to operator manual mapping.


Sensor Blockage

Ultrasonic level transmitter sensor might be blocked by dirt moving up with the medium vapor or dust.

This problem is very common in solid level measurement in hoppers or in food industry for oil level measurement.

Manufacturers have solved this problem with self-cleaning vibrator that removes dirt or dust from the sensor.


Foam on Liquid

In some applications foam accumulates on liquid surface; and echoes the ultrasonic wave back to the sensor or sometimes absorbs the wave causing false level measurement.

This problem could be solved by using a guide pipe from the sensor to the bottom of the vessel in a way that medium enters the pipe from the bottom and prevents foam accumulation on liquid surface inside the pipe.

But this solution is not 100% reliable, therefore it is recommended not to use ultrasonic level transmitter for mediums with foam, instead radar level transmitter can be helpful.


Ultrasonic Level Transmitter Advantages

  • Non-contact measurement
  • Simple installation and commissioning
  • Loop powered (no external power supply needed)
  • Low maintenance
  • High accuracy and reliability