What is an optical pyrometer and what is it used for?

The optical pyrometer is an instrument used to measure the temperature of an object remotely and without physical contact with it, taking advantage of the physical principles of the spectrum of thermal radiation emitted by any hot body.

This instrument has various industrial uses, mainly in the iron and steel industry, foundries, boilermaking and blacksmithing. It should not be confused with optical thermometers, which use a similar working principle, but which have greater precision although they are limited to working with low and medium temperatures.

Optical pyrometer: a bit of history

The optical pyrometer It has little more than a century of history, and is considered a classical instrument, although it still has use in the industry, despite the fact that there are more precise digital measuring instruments.

First Morse pyrometer

The principle of operation is similar to those that are still manufactured today: the brightness emitted by a surface is compared with that of a lamp —regulated by a rheostat—; when the brightness coincides, the approximate temperature is searched on a dial. In the first models this brightness was compared through a principle called evanescent filament: the light of the object to be measured was superimposed on that emitted by an incandescent filament; By varying a rheostat, its brightness increased or decreased, with which it was possible to make both coincide, with which the filament seemed to disappear.

Operation

In the pyrometers Today, light is also compared using different optical filters that absorb unnecessary wavelengths – and reducing the intensity of the light, in order to make its operation safe – allowing only wavelengths close to red to pass. A lamp, powered by a battery, provides a reference light. Focus is placed on the filament, and the dial begins to turn; There are three possible options in this operation:

• The filament is darker: means that the filament is colder than the material to be measured.
• The filament is brighter: the filament is hotter than the material to be measured.
• The filament is not visible: the filament is at the same temperature as the material to be measured.

Optical pyrometer – Schematic

What uses does it have?

All industry where it is necessary to measure the temperature of hot objects (above 700 ° C), the pyrometer will be a necessary instrument.

It is used, for example, in blast furnaces to determine the temperature of the pig iron inside the crucible, or after bleeding from the furnace. It also finds use in foundries of iron, lead, aluminum, copper and other metals, to know, at a glance, if the metal reached the melting temperature.

Optical Pyrometer – Application

Other uses can be applied in glassware, where it is necessary to know precisely the temperature of the molten glass; in blacksmiths, to determine if a metal part is ready to receive a heat treatment or to be subjected to plastic deformations on the forging; in general, to control the temperature of furnaces intended for melting or heating different materials.

Advantages and disadvantages of the optical pyrometer

Like all direct measuring instruments, the optical pyrometer it has advantages and disadvantages derived from its design. Let’s see what they are.

Optical pyrometer

Advantages

• It allows a direct and almost immediate measurement; It is only necessary to focus on the objective (if it is not already there) and turn the rheostat until the brightness of the material and the filament are equal.
• It is only necessary to have a charged battery to operate the lamp and the multimeter that determines the relationship between the electric current and the calibrated temperature at the time of manufacture.
• The precision of the instrument is very high in relation to the temperature range that it allows to measure: ± 5 ° C.
• It is a very simple instrument, easy to assemble, disassemble, clean and maintain.
• The instrument also makes it possible to directly observe the part, regardless of whether it is red-hot or close to the melting point.
• The measurement is carried out remotely, without the need to have physical contact with the element to be measured.

Disadvantages

• To effectively measure the temperature of an object, it must be at least 700 ° C.
• It is not possible to obtain a continuous measurement, with variable values ​​over time.

Comparison with other optical temperature measurement instruments

At present the optical pyrometer it is being replaced by more modern and versatile instruments, such as the radiation pyrometer or the infrared thermometer. The latter operates by means of a laser, which is aimed at the surface to be measured; on a digital display you can instantly read the temperature of that surface. This instrument, in addition to being simpler to operate, is more versatile, since it allows measuring a much greater range of temperatures — and, in the more sophisticated models, it even allows measurements to be downloaded via Bluetooth or USB.

The radiation pyrometer is another classic instrument, used for even higher temperatures (such as those that titanium could give off when it reaches its melting temperature, for example).

Optical radiation pyrometer

There is a simpler instrument, which could also be classified as optical pyrometer, which simply consists of a scale with a gradient that goes from dark red to white; By comparing the color of the hot object with that scale, it is possible to obtain a constant that is then searched for in a table, tabulated according to different materials.

It is also necessary to take into account thermal imaging cameras, which combine the advantages of the optical pyrometer with the simplicity and immediacy of the measurement of infrared thermometers: the user can see on the screen, in false colors, not only the temperature of an object but also superficial variations, in an immediate way.