Pressure gauge | simple principle on-site

pressure gauge
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Pressure gauge can be seen everywhere in chemical plants.

I think some engineers don’t think deeply about it because it’s such a common instrument.

However, its value is very high.

Together with thermometers, flowmeters, and liquid level gauges, they are also called the four basic instruments.

Great for on-site work with pressurizing equipment such as pumps, depressurizing equipment such as vacuum pumps, high heat equipment such as steam, and removal equipment such as strainers .

The higher the pressure, the more dangerous it is, so it is used to indicate a dangerous situation.

In addition, it can also be used to indicate whether something is present in equipment or piping.

There is not much value in engineering in the sense of instrument selection, and it is mainly a matter of whether to use on-site type or incorporate DCS.

However, it is very important for a chemical plant, so I would like to understand it, including the surrounding knowledge.

A pressure gauge is a familiar instrument that is used like a valve.

unit of pressure

First, let’s sort out the units of pressure.

Nowadays, the unit is Pa.

When I joined the company, there were quite a few people using kgf/mm2.

There are still people who use Torr on the vacuum side, myself included.

You probably want to standardize on Pa, but it will take quite a while for it to become established.


kgf/mm2 is an old unit. I hardly use it now.

However, it is very easy to understand intuitively.

Rather, the old system of units used indicators that were easy to understand.

1kgf/mm2 is the pressure when a 1kg load is applied to an area of ​​1mm2.

It’s a very reasonable unit.

The biggest point is that 1kgf/mm2 is equal to atmospheric pressure.

1kgf/mm2 ≒ 100kPa ≒ Atmospheric pressure

It’s convenient, but it can be a big problem if you use it incorrectly.

For example, if you use the expression “4 kg of…” to mean “the pump pressure is 400 kPa…”, you’ll get confused!

Is it a kilo of 4kgf/mm2 or a kilo of 4kPa ?

It looks like a joke and becomes a problem.

This can be a fatal problem, especially in vacuum systems.

water column

I still use mmAq today.

The water column is an idea that converts pressure into the height of water.

100kPa atmospheric pressure = 10m water column

That’s common.

10m×9.8m/s2 ×1,000kg/m3 = 98,000N/m2 = 98kPa

This is the relationship.

The water column is used to measure micropressure. It’s a manometer.

Since the pressure is measured at the height of the water, a difference in height of 100mm can be detected as a difference of 1kPa.

Another great advantage is that it can be measured using only a transparent tube and water!

Using this relationship, install the vacuum heat exchanger at least 10m higher than the water ring vacuum pump .

Even if the water is affected by the atmospheric pressure of the outside air in the Full Vacuum system, it will only rise up to 10 meters, so the idea is that by making the heat exchanger over 10 meters high, it will not flow back during the process.

mercury column

The mercury column is older than the water column.

There are still more opportunities to use it as Torr than as mmHg.

I think it comes up as a basic unit of pressure when studying science in school.

760mmHg = 101.3kPa

In batch systems, it is only used as a vacuum gauge, and what should be expressed in kPa units is sometimes expressed in mmHg for older people.

Recently, this has become less common, and more and more people are trying to pass in kPa units.

If you know the difference between kPa and mmHg, which is about 8 times, you can do mental calculations.

Bourdon tube pressure gauge

The Bourdon tube has a structure as shown in the diagram below.

  • bending metal into a C shape
  • One end open, other end closed

It has this structure.

When a pressurized fluid is applied to the open end, the pressurized fluid flows into the Bourdon tube.

It is then filled with fluid to the other end.

The C-shaped tube is deformed under pressure.

The Bourdon tube pressure gauge has a structure that visually checks the amount of deformation with a scale.

This is a mechanism that detects pressure by displacement.

Because metal deforms, the following is required:

  • Resistant to plastic deformation. Easily deformed elastically.
  • Less hysteresis in deformation.

As a general-purpose metal that satisfies these conditions, SUS316 and so on.

Although we are focusing only on the Bourdon tube, the link mechanism also has a certain lifespan.

Since it is made of common metal parts, the technology is well established and there is probably nothing to worry about.

If it breaks, just replace it

You can use it with such a light concept.


Fluid flows into the Bourdon tube.

If highly corrosive chemicals or slurry with a high solid content flow into the Bourdon tube, it may cause malfunction.

This is where the idea of ​​a diaphragm system comes into play.


A diaphragm membrane is inserted into the flange part shown in the figure, and the pressure of the process fluid is transmitted to the Bourdon tube side through the diaphragm membrane.

The Bourdon tube is filled with a special liquid.

Diaphragm membranes include stainless steel, tantalum, PTFE, etc.

This idea is a general-purpose method that is also used in differential pressure type liquid level gauges.


Used for high temperature fluids such as steam.

When high-temperature fluid enters the Bourdon tube, the elastic modulus of the Bourdon tube changes and errors may occur.


Filling the siphon tube with water has the effect of cooling the high-temperature steam liquid.

However, pressure can still be measured.

The siphon tube is filled with water, and when steam is added to the siphon tube, the water inside the siphon tube moves toward the Bourdon tube.

As a result, water or air is mixed into the Bourdon tube.

If you run steam for a long time, you can expect that the water in the siphon pipe will evaporate.

Since the siphon tube is in contact with the temperature of the atmosphere, it is cooled by the atmosphere, and some water remains in the siphon tube at all times.

As a result, high-temperature steam fluid does not enter the Bourdon tube.

You can think of the length of the siphon tube as being determined by the temperature of the steam used.

Electric pressure transmitter

The essence of electrical pressure transmitters is to convert force into electrical signals.

The role of a generator is to convert force energy into electrical energy , and the role of a motor is to convert electrical energy into force energy .

  • Force energy → Electrical energy generator
  • Electric energy → force energy  motor

Here, force refers to mechanical force or physical force in physics.

Nowadays, the expression power’ is often used in the sense of 〇〇power.

I feel a little reluctant to express it with just one word, power’…

Story was it.

Generators and motors target energy and are somewhat different from signals.

The required signal capacity is different.

Motors use a large amount of electricity, but transmitters require a small amount of electricity.

Is it faster to judge whether 440V is necessary or 24V is sufficient?

Diaphragm and capacitance

How can we convert force information into electrical information?

The answer is diaphragm and capacitance.


It takes advantage of the diaphragm’s ability to deform due to force.

A diaphragm membrane is placed on the high-pressure side and low-pressure side, and liquid is sealed inside it.

Encapsulant fluid is a liquid that can store electrical charge.

In many cases, the high pressure side is the reaction vessel and the low pressure side is the atmosphere.

When pressure is applied to the diaphragm membrane on the high pressure side, the diaphragm membrane deforms and the pressure of the liquid on the high pressure side increases.

This force causes the movable diaphragm attached to the center to deform.

The electrical circuit moves as shown in the diagram below.


The capacitance of a capacitor is inversely proportional to the distance between the electrodes.

It takes advantage of the fact that the electric current changes when alternating current voltage is applied by changing the capacitance of the capacitor.

This part is about electric circuits and electromagnetism, so I will omit it.

Can be widely applied

Since it is assembled using physically basic elements such as a diaphragm and capacitance, it is extremely versatile.

  • pressure as a pressure gauge
  • liquid pressure as a liquid level gauge
  • fluid pressure as a flow meter

It can be used for pressure gauges, liquid level gauges, and flow meters, so it can be used for almost all process instruments in batch chemical plants other than thermometers.

I consider this a basic signal converter.

Liquid column pressure gauge

The U-shaped manometer is a famous liquid column pressure gauge.

A U-shaped manometer is something like the figure below.


It’s a simple structure that looks like something you’d see in a physics textbook.

A liquid is contained at the bottom of the U-shape, and another liquid or gas is placed on the left and right sides of the U-shape.

The pressure difference between the left and right sides of the U-shape is expressed by the height difference of the liquid at the bottom of the U-shape.

This is how it works.

Can be used as a low pressure gauge

The U-shaped manometer can be used as a low pressure gauge.

1mAq is 10kPa, 50cmAq is 5kPa.

If you want to create a slight pressure reduction using a fan or scrubber, the target pressure is around 5kPa.

You can use a micropressure gauge here, but you can also use a U-shaped manometer.

This is especially useful when you don’t have much money and have to do something on your own.

It is still useful for improvements at the field level.

Water management is necessary

The heart of a manometer is the liquid inside. Usually water.

The problem is that we have to manage this water.

Be careful, especially when using the system for the first time.

Before I knew it, the water was gone. That makes a good deal.

  • The pressure in front and behind the manometer touched and the water was blown away.
  • Water evaporates or freezes depending on the temperature and surrounding temperature environment.

These problems occur.

laws and regulations

Pressure gauges are classic instruments, so they are subject to legal restrictions just like liquid level gauges.

Let’s take a look at fire protection, labor safety law, and high pressure gas.

fire department

Regarding fire protection, the following regulations are a prerequisite.

Cabinet Order Concerning Regulation of Dangerous Materials Article 9, 16

Equipment that pressurizes dangerous materials or equipment that is likely to increase the pressure of the dangerous materials handled shall be equipped with pressure gauges and safety devices specified by Ordinance of the Ministry of Internal Affairs and Communications.

The phrase “pressurizes hazardous materials” is included, so all process liquid pumps fall under this category.


Boilers were probably the first to be covered by the Industrial Safety and Health Act.

Boiler Construction Standards Article 66 Pressure gauges shall be installed

in the steam section of a steam boiler, water column pipes, or steam side connecting pipes leading to the water column pipes as specified in the following items.

Boilers are special equipment for batch-type chemical plants, so the pressure gauges here must be treated in a special way.

pressure vessel

In addition to boilers, the Industrial Safety and Health Act covers pressure vessels.

The pressure vessel structural standards include single pressure and double pressure.

Structural Standards for Pressure Vessels Article 68  First-class pressure vessels shall be equipped with pressure gauges

as specified in the following items .  (1) It is possible to easily know whether the cock or valve is open or closed.  (ii) The maximum index of the scale of the pressure gauge shall be one that indicates a pressure that is 1.5 times or more and less than 3 times the maximum working pressure.

Structural Standards for Pressure Vessels, Article 73, 

Part 1 (excluding the provisions of Items 2 to 4 of the Table of Article 2, Article 43, and Articles 45 to 62) The provisions shall apply mutatis mutandis to second class pressure vessels.

In batch chemical plants, there are many dual-pressure general-purpose containers, and steam exceeding 200kPa is used on a daily basis.

Therefore, attach a pressure gauge to the steam line.


Strainers are an area that is difficult for machine shops to pay attention to.

However, the Industrial Safety and Health Act clearly states this at the notification level.

Regarding devices under the proviso to Article 272, Item 2 of the Industrial Safety and Health Regulations and the proviso to Article 16, Item 2 of the Specified Chemical Substance Hazard Prevention Ordinance 1 Devices that can confirm that valves or cocks are securely

  415 The notification states that a device that can confirm that a valve or cock is securely closed” is a device that can ensure that the flow of hazardous materials is completely controlled by these valves or cocks without opening or removing strainers, etc.A device that can confirm that the strainer, etc. Something.

Batch chemical plants use strainers extensively.

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, 19 High pressure gas equipment shall be equipped with a pressure gauge
as specified by the Minister of Economy, Trade and Industry, and if the pressure within the equipment exceeds the allowable pressure, the pressure shall be immediately adjusted to the allowable pressure. A safety device shall be provided that allows the following to be returned to:

Since it is a high-pressure gas, it is natural to attach a pressure gauge.

Batch-type chemical plants do not have many high-pressure gas facilities, so this is also treated as a special case.


Related article

If you want to know more, check out the article below.

Related information

P&ID instrument

Level gauge



We compared the types and characteristics of pressure gauges used in chemical plants.

Bourdon tube pressure gauge, electric pressure transmitter, liquid column pressure gauge

When it comes to machinery, we tend to think only about Bourdon tube pressure gauges.

A pressure transmitter is used for control, and the same concept can be used for liquid level gauges, making it highly versatile.

Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

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