Quickly understand the types of level gauge used in chemical plants

level gauge
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A level gauge is an instrument that constantly monitors the amount of liquid in a tank, such as the liquid level.

The risk within a hazmat factory is proportional to the quantity of hazmat.

In addition to safety, the amount of material is also important in terms of manufacturing management such as production volume.

The liquid level gauge is very important in terms of determining the quantity.

Don’t think that mechanical engineers are irrelevant because they are instruments, and actively acquire knowledge.

The principle is extremely simple.

Liquid level gauges are important for production control.

Differential pressure type liquid level gauge

A differential pressure type liquid level gauge is a mechanism that detects the pressure of a liquid.

As shown in the figure, there are instruments connected by flanges with diaphragm membranes .

Differential pressure type

The tank is filled with liquid across the diaphragm.

This liquid pressure is received by the diaphragm, and changes in the volume of the liquid sealed inside the diaphragm are detected as signals.


  • There is a sense of trust
  • No need to think deeply about nozzle orientation


  • The indicated value fluctuates when the liquid is moving.
  • Accessories vary depending on whether it is an open system or a closed system.
  • This can cause slurry to become clogged.

It is reliable because it uses the classical physical law of receiving liquid pressure.

However, I feel that it has many disadvantages and is surprisingly difficult to use .

Indicated value fluctuates

The disadvantage of differential pressure level gauges is that they can only measure static pressure .

For example, dynamic pressure of the liquid is generated when liquid is taken into or removed from a tank.

Since dynamic pressure fluctuations are added to the liquid static pressure, the reading on the liquid level gauge will fluctuate.

This reading is not very reliable.

open or closed system

Differential pressure type liquid level gauges differ depending on whether they are open or closed systems .

Let’s start with a simple open system .

The open system has a simple structure as shown in the diagram below.

Pressure system

An open system is a state in which the tank gas line is connected to atmospheric pressure and the pressure loss in the gas line can be ignored .

The idea is simple in the sense that since the pressure in the gas phase of the tank is constant, it is sufficient to simply detect the pressure of the liquid.

A closed system is a condition in which the internal pressure of the tank increases or decreases.

Closed system

In batch chemical plants, the overwhelming majority are vacuum systems .

When distilling dangerous reactants, lowering the boiling point as much as possible greatly reduces the risk of a runaway explosion .

The processes and hazardous materials being developed are getting more dangerous every year.

A safe process has long been developed, and today’s chemistry is poking at the corner of the remaining jubako.

The only new processes available are fairly dangerous reactions.

Vacuum distillation is essential for safe use .

In order to measure the liquid level in a vacuum distillation system, the pressure in the gas phase must be detected.

Even though the liquid level should be the same under normal pressure and under reduced pressure, it would be a problem if the indicated value of the liquid level gauge is different.

Capillary tubes are used when handling highly corrosive liquids.

In some cases, the impulse line is simply extended from the gas phase of the tank.

slurry clogs

The differential pressure type liquid level gauge is connected to the tank nozzle.

As shown in the figure below, there will be a pooled area for the tank nozzle.


In a system where crystals and solids are mixed , it may accumulate in the nozzle and make measurement impossible.

Therefore, there is a type with an extended detection part to prevent solids from accumulating in the detection part.

This is the diaphragm protruding type.

radio level gauge

The radio wave level gauge corresponds to the non-contact type level gauge.

Non-contact means that it does not come into contact with the process liquid .

It is different from the type where the detector comes into contact with the liquid, such as the differential pressure type and the float type.

The greatest advantage of not contacting the process liquid is that it is safe even if it leaks .

When the differential pressure type leaks, all the liquid in the tank leaks , but the radio wave type does not have that concern.

This is super important!

I think the real intention is not to use the differential pressure type if possible.

I use a differential pressure type because there is no other liquid level gauge that can measure it. If possible, use radio waves.

This is what it feels like.

Radio wave is versatile

The radio wave type uses microwaves (electromagnetic waves of 1 GHz to 3 GHz).

Electromagnetic waves are not affected by the gas phase .

Sunlight is also one of the electromagnetic waves, but it propagates through the empty space called outer space.

Same as this.

  • Electromagnetic waves do not pass through metal and are reflected
  • Electromagnetic waves are absorbed in water

It is absorbed in water, so it seems that it cannot be used in water, but it is actually used.

I wonder what they are doing.

Ultrasonic type is susceptible to noise

The ultrasonic type emits a sound wave and detects the time of the sound wave that bounces back.


It’s a bit classic, and the idea is simple.

  • The speed of sound traveling through a liquid is different from the speed of sound traveling through a liquid .
  • There is a time difference corresponding to the depth of the liquid between the part of the sound wave reflected on the liquid surface and the part that passes through the liquid and bounces back .

Since this principle is used, several factors of error naturally arise.

  • Receive temperature nutrition. The speed of sound in the gas phase depends on the temperature , and the amount of change is large.
  • Affected by foreign matter, powder, and steam mist in the gas phase.

This is unavoidable because it uses sound waves , the propagation of matter in the gas phase .

Float type level gauge

The float type is a very old type. It’s analog.

A device that uses a float and uses the float’s position as the indicated value .

Wires, magnets, levers, and so on are available as means of communicating the position of the float, but the basics are the same.

It will be limited to large tanks that other types cannot handle .

wire type

Float gauges are one of the most primitive types of measurement.

See the diagram below.


In this method, a float that is lighter than the liquid is placed in the tank, and the float is always placed at the level of the liquid.

It’s a primitive method that you might see in high school physics.

Detect the position of the float as the rotation position of the pulley ,

This can be used visually or as an electrical signal.

It is divided into several types according to the structure of the float.

The example above uses primitive wire.

Wire strength and corrosion resistance . The required conditions, such as being able to respond to fluctuations in the liquid surface, are relatively strict.

There are limits to how long a product can last.

magnetic type

The magnetic type uses a magnet for the float.

See the diagram below.

magnet float

There is a magnet on the float, and the float inside the seal pipe is connected to the wire.

Since there is a sealed pipe , there is no need to consider the corrosion resistance and strength of the wire.

It’s also classic.

The use of magnets makes it a little more modern.

You can solve the wire problem, but there is a problem on the magnet side.

  • magnet deteriorates
  • If the seal pipe is long, the magnetic float will get caught in the middle.
  • A foreign object gets caught between the seal pipe and the magnetic float.

lever type

The lever type uses a lever instead of a wire.

Unless the tank is a tank type with a diameter larger than its height , it will not be possible to measure the entire tank area.

The fulcrum of the lever is the moving part, so corrosion resistance, strength, and longevity must be considered more than wire type.


It acts in the direction of blocking the installation location of the liquid inlet nozzle and other accessories .

It’s a method that offers almost no benefits.

Direct-view liquid level gauge

A direct-view liquid level gauge has a structure as shown in the diagram below.

direct view

You can place a nozzle in the tank, take out some of the liquid from the nozzle, and visually check it from the outside.

The inside of the tank is a sealed container, so you don’t know what and how much is inside.

Being able to visually check this will give you peace of mind!

The direct view type partially uses materials such as glass .

Although it would be difficult to make the entire tank glass in terms of strength, the idea is that it would be OK if it were partially made of glass.

There are several types of direct-view liquid level gauges.

tube type

This method uses a glass tube as is.

Since the tube is cylindrical, its biggest disadvantage is that it is vulnerable to external pressure .

When not in use, it is common to have a cover on the glass to protect it from the outside environment.

Remove the cover when viewing, and put the cover back on when you are finished viewing.

It’s analog…


In the reflective type, the glass is flat and uses one flat glass .

A slit is made in the part where the process liquid enters.

Since the reflectance of light changes depending on whether or not there is a liquid, we aim to differentiate the part with the liquid by making it look black .

Compared to cylinders, flat plates are cheaper and more resistant to external pressure , so they are more reliable.

However, the cover itself is necessary.


In the see-through type, the glass is flat, and two flat glasses are used.

By using two pieces of glass, the purpose can be divided into the viewing side and the light shining side.

It is characterized by easy visibility .

However, in the sense that the number of glass sheets increases, the strength decreases.

The need for a cover is the same as the reflective type.

magnet type

The magnet type is the same idea as the float type.

Attach a float with a magnet to the part that comes into contact with the liquid .

Using the magnet on the float , it is a method to move the magnet on the outside and indicate the liquid level.

Compared to the above three, the glass is not exposed, so it is highly reliable !

It’s an analog method, so it’s good to receive fire legal!

However, like the float type, it will stop working if it gets clogged.

Auxiliary instrument

The direct-view liquid level gauge is not considered as the subject of control.

Other instruments are the main, and the role to support it.

Used when an instrument breaks or when checking for abnormalities during patrol.

Or maybe something like this:

  • A place where constant monitoring of the liquid level is not required, but is used temporarily such as when starting or stopping the liquid.
  • When you want to confirm that a certain amount of liquid is constantly flowing in and out, such as a buffer tank, and that the liquid level is constant, and that it is operating properly.

In any case, its usage is limited.

What are the laws and regulations?

Liquid level gauges are directly connected to laws and regulations.

Laws related to liquid level gauges include high pressure gas, pressure vessels, and the Fire Service Act.

In particular, there are many references to the direct viewing method in relation to qualification examinations, but in practice it is treated as an exception.

However, it is written correctly, so let’s check it out.

high pressure gas

There are many classifications of high pressure gas, but the basic rules, the General High Pressure Gas Safety Regulations, are stipulated as follows.

General High Pressure Gas Safety Regulations

Article 6 22.
 Liquefied gas storage tanks should be equipped with a liquid level gauge (inert gas (excluding specified inert gases)) or round glass for storage tanks other than oxygen ultra-low temperature storage tanks. (limited to liquid level gauges other than pipe level gauges ). In this case, when using a glass level gauge, measures should be taken to prevent damage to the glass level gauge, and storage tanks (limited to those for flammable gas, toxic gas, and specified inert gas) should be taken. ) and the glass liquid level gauge, measures must be taken to prevent leakage of liquefied gas due to damage to the glass liquid level gauge.

We are not denying electrically controlled liquid level gauges, that is, non-contact liquid level gauges.

However, most of the language targets direct-view level gauges.

Round glass is a tube type.

The round shape makes me feel uneasy, so I decided to use a reflective type. Attach the cover.

Legally speaking, this classic description comes first.

labor safety law

The Labor Safety Act also targets direct-view glass liquid level gauges .

Article 69 of Boiler Construction Standards Steam boilers (excluding once-through boilers) shall be equipped with two or more glass water level gauges on the boiler body or water column tube. However, for steam boilers listed in each of the following items, one of them may be a water level measuring device other than a glass water level gauge .
(1) A steam boiler with a shell with an internal diameter of 750 mm or less; (2) A steam boiler
equipped with two remote indicating water level measurement devices
; (2) A glass water level gauge must maintain the lowest point of its glass tube during use of the steam boiler. It must be installed at a position that indicates the lowest water level (hereinafter referred to as the “safety low water level”).
3. Glass for water level gauges for steam boilers must comply with Japanese Industrial Standard B8211 (boiler water level gauge glass) or have mechanical properties equivalent to or better than this.
4 Glass water level gauges must be constructed so that they can be cleaned and inspected at any time.

In view of the recent circumstances, it is unavoidable not to stipulate an automatic liquid level gauge!

(I really don’t like it) I can’t deny that I have no choice but to wear it.

It’s a word that feels a nuance.

From this condition,  it seems that two automatic level gauges = one direct-view level gauge .

I think that one automatic liquid level gauge has about three times the function of a field type liquid level gauge, but that is a Japanese law.

Fire Service Act

The Fire Service Act r¥ has the following regulations.

Article 11 of the Cabinet Order on the Regulation of Dangerous Materials (9
 ) Outdoor storage tanks for liquid hazardous materials shall be equipped with a device that automatically displays the amount of the hazardous materials.

This is stated as a general rule.

The same is true for outdoor storage tanks as well as indoor and underground storage tanks .

Naturally, the No. 20 tank at the factory is the same.

Measuring method

There are direct and indirect measuring methods .

Especially the liquid level gauge is easy to understand.

The same concept applies to thermometers and flow meters.

direct type

The direct type includes the glass type and the float type.

Rather classical and local

There is an instrument that directly indicates the liquid level and it is a method of confirming it.

It is also possible to transmit electrical signals to the DCS.

Since the liquid level information is directly used, there is an excellent sense of security.

indirect type

Indirect types include radio wave type and differential pressure type.

It attempts to obtain information about the liquid level through radio signals and pressure signals rather than the liquid level itself .

There is still a possibility that the indicated value may deviate due to error factors such as noise and density differences.

Confidence will be slightly reduced as a result.


Instrumentation, not just level gauges, is very important in chemical plants.

In particular, if you have one of the following books, it is recommended because you can use it for a considerable part.


We have summarized the contents of the liquid level gauge that appears in the batch chemical plant.

Differential pressure type, radio wave type, float type, direct view type

Liquid level gauges are important for both safety and production management.

Batch plants have many tanks and reactors, so there are many opportunities to use them.

Machine shops should also know.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)

*We will read all comments received and respond seriously.