Thermometer | all you need to know in chemical process

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I will explain about thermometer.

Thermometers are the most familiar, fundamental, and important process instruments.

Body temperature is also important for humans.

Chemical plants involve invisible dangers, and in the worst-case scenario, they can cause major damage to nearby communities.

To avoid such a tragic outcome, we must constantly monitor the situation.

The most easily understood and important indicator is temperature.

Thermometers, which are instruments that display temperature, are very important for engineers.

The structure is simple.

Mechanical engineers who tend to avoid important instruments because they don’t understand them should start by understanding thermometers.

A thermometer is an easy-to-understand instrument.

Measurement principle

There are two things you need to know about measurement principles.

Electric type and volumetric type.

There are three types of thermometers depending on the measurement principle: electric, volumetric, and radiation, but the radiation type is almost never used at the chemical plant level.

It is sufficient to understand two types:

electric type

The plinciple of electric type uses the Seebeck effect.

Electric current flows when two different metals are brought into contact at two points and the temperature at one point is different from the other.

Bimetal has a similar idea.


Bimetal is based on the principle that different metals are brought into contact with each other on a flat surface, and when the temperature of the metals increases, they deform due to the difference in thermal expansion.

Speaking of process thermometers, this is the type!

It’s enough to think so.

Convert temperature information to electrical information. If possible, that electrical signal can be sent to a DCS and easily used for supervisory control.

The structure is also simple.

Let’s keep this in mind when it comes to thermometer knowledge!

Volume type

The volume type is sometimes called the pressure type.

This is the same principle as the Bourdon tube pressure gauge.

This method uses the fact that the volume increases as the temperature rises to obtain the indicated value.

Temperature information, displacement information

This is used as a local thermometer.

It is the same feeling as using a Bourdon tube pressure gauge as a local pressure gauge.

If you want to use it for control with DCS, you need to convert displacement information into electrical information.

Considering the time and error involved in converting displacement information once, electric type is overwhelmingly more reasonable.

Volumetric formulas are often used only when you want to see the indicated values ​​directly on-site .


I can’t find any weaknesses in the resistance thermometer.

If you’re having trouble, just use a resistance thermometer . Speaking of thermometers, we think of resistance thermometers.

This understanding is sufficient.

electrical resistance of metals

It takes advantage of the fact that the electrical resistance of metals is related to temperature.

The higher the temperature, the greater the resistance.

Consider when electricity is passed through a copper wire.When electricity is passed through it, the copper wire becomes hot.

This is because the electrons of the copper atoms move.

The higher the temperature, the more active the motion of molecules and atoms.

If the atoms are actively moving, the direction will hinder the movement of electrons.

Electrons have to pass through atoms that are difficult to move, which increases resistance.

wheatstone bridge

Wheatstone Bridge is an area studied in junior high and high school.

The Wheatstone bridge principle is used for resistance temperature detectors.

Wheatstone Bridge (thermometer)

The R part is where the temperature is actually measured.

Using a variable resistor Rv and fixed resistors R1 and R2, if you change the value of the variable resistor Rv so that the value of the galvanometer G becomes 0 when the R of the temperature measuring section changes, the temperature measuring resistor R can be changed. You can know.

If you prepare the temperature-resistance relationship data for this temperature-measuring resistor R in advance, you can find out the temperature of the temperature-measuring section from the variable resistor Rv.


Since a thermometer, other types have drawbacks.

Here are the disadvantages of thermocouples.

Disadvantages of thermocouples

  • There are two metals near the process.

Thermocouples use two metals.

This is required close to the process.


If there are two metal parts, malfunctions such as the contacts coming off may occur.

The atmosphere inside a factory is affected by the process environment, so it is more prone to corrosion and other deterioration than the general atmosphere .

Rather than using a thermocouple that has one more element of deterioration, it would be safer to use a simple resistance temperature detector.


Since a thermometer is a resistance temperature detector , other types have drawbacks.

Here are the disadvantages of bimetal.

Disadvantages of bimetal

  • Accuracy is not high
  • Due to movement, it is easy to deteriorate

Bimetals seem to be less accurate than thermocouples.

However, this is a level that I am not conscious of as a machine shop.

Bimetals are a mechanism that deforms metal, so they deteriorate more easily than thermocouples.

As an equipment shop, you can understand this as the difference between static equipment and dynamic equipment.

Static devices are considered thermocouples, and dynamic devices are bimetals.


I don’t think you even need to know about the inflatable type, but let’s list the disadvantages.

Disadvantages of inflatable

  • There is a risk of foreign matter contamination
  • Affected by capillary tube


Introducing the structure of an expansion thermometer.

The image below is said to be a liquid type.

Inflatable (thermometer)

A brief summary of the characteristics of expansion thermometers is as follows.

  • Detects temperature with thermosensor
  • The volume of the fill liquid in the capillary tube changes.
  • The angle of the Bourdon tube changes
  • The indicator scale changes

Contamination with foreign matter

The key to an expansion thermometer is the filled liquid .

Chemical plant engineers must consider the danger of mixing the filled liquid with the substances they handle. No one is thinking. Probably.

This means that instrumentation engineers do not take chemical plants that seriously.

An equipment manufacturer that easily advances chemical anchors. A chemical plant engineer who doesn’t think about anything.

This is exactly the same structure.

capillary tube

One of the limitations of expansion thermometers is the problem of capillary tubes.

This is more complicated than the problem with thermocouple thermometer protection tubes .

If the length of the capillary tube is too long, there will be a huge change in the volume of the filled liquid, which will cause errors.

If the ambient temperature of the capillary tube is high or low, the filled liquid will be affected by factors other than the temperature of the sensing part.

Due to the physical constraints of the capillary tube,

The thermometer mounting position and indicator position within the process system are fixed.

With field indicators, it is faster to see the displacement information as it is than to see it as an electrical signal, so

As an on-site indicator, an expansion thermometer is more advantageous.

However, the demand for field-type indicators whose position is fixed is steadily declining.

Of course, data can be transferred to a remote value by using a piezoelectric element that converts displacement into electricity in the indicator section.

Thermocouple thermometers have an overwhelming advantage in that they do not contain displacement information!

Thermometer protection tube and mounting position

As for the installation information regarding the thermometer, there are issues with the protection tube and the installation location.

protection tube

For thermocouple side thermometers, the metal sensing part cannot be brought into direct contact with the process liquid.

Therefore, a protection tube is required.

protection tube

As long as the protection tube is just a tube, it becomes a factor that impedes heat transfer.

If the protection tube is thick

  • Responsiveness worsens
  • The error from the true value increases

There are disadvantages such as.

The thinner the wall thickness, the better, but the issue of strength comes into play here.

In particular, process fluids are highly corrosive, so a protective tube made of high-quality material is required to ensure corrosion resistance while reducing wall thickness.

Mounting position

Let’s consider a case where a thermometer protection tube is inserted into the piping.

Depth (thermometer)

In this diagram, if the thermometer is placed at point c near the center of the piping, the error will be -2°C, and if the thermometer is placed at point d, outside of the piping, the error will be -15°C.

The error is lower if the protection tube is pointed closer to the center of the piping.

That can be said.

When inserting the protective tube as deep as possible, it acts in a direction that compresses the flow path area within the piping.

If you do this, the pipe may swell only in the part pointing to the thermometer protection tube, as shown in the diagram below.

Pipe expansion (thermometer)

There is also the effect of errors related to the mounting position.

Flow position (thermometer)

The error is smallest when the thermometer is installed in position b in the diagram .

This is set up against the flow.

Point a in the diagram has the largest error, but this error is due to the long length of the protection tube.

Comparing figure a and figure c, a has an error of -45℃ and c has an error of -2℃.

Both are set perpendicular to the flow, so the only difference is the length of the protection tube.

If you are using a tank nozzle, design the nozzle specifically for it.


Related Article

Related information


We have summarized the details of thermometers used in batch chemical plants.

There are four types: resistance temperature detectors, thermocouples, bimetals, and expansion types, but as long as you understand thermocouples, the basics are OK.

The rest can be considered as related information.

Thermometers are simple in structure and principle, but they are extremely important instruments that indicate process hazards.

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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