Pyrometer

Pyrometer (by ancient Greek. Πῦρ / pyr / "Fire" ), also called radiation thermometer used for non-contact temperature measurement. Temperatures between -50 ° C and 4000 ° C can be measured with such equipment.

Inventor

The actual inventor of the pyrometer is difficult to determine. Pieter van Musschenbroek and Josiah Wedgwood probably have both invented something called her and other scientists of their time pyrometer, but these devices are not comparable with modern pyrometers. The Encyclopædia Britannica calls William Chandler Roberts - Austen as the inventor of the pyrometer.

Basics

Each object having a temperature greater than 0 Kelvin emit thermal radiation, the intensity and location of the emission maximum is dependent on its temperature. This radiation is detected by the pyrometer, and evaluated. If the test object is colder than the pyrometer, the radiation flux is negative, ie the pyrometer is heat radiation to the measurement object ( which is due to the second law of thermodynamics ), which can be also evaluated.

Basis is the Stefan- Boltzmann law, according to which the total radiation power P for an ideal black body of the absolute temperature T (in K) and the area A depends ( in m² ). Using the Stefan- Boltzmann constant = 5.6704 · 10-8 W m- 2 K -4 is:

Real bodies, also called gray body radiation is lower by a factor of intensity:

For a non-contact temperature measurement is necessary to set the emissivity, so know the heat radiation of the measured object.

Pyrometer types

In a two-color pyrometer (also Verhältnispyrometer or 2-color pyrometer called ) the intensity ( energy quantity ) is not only measured in a wavelength range, but it is the ratio of intensities at two different " colors " are formed. This means that the temperature is not determined on the basis of the brightness, but due to the color of the radiation. In this method, the emissivity ( shortening of the division ) play in forming the ratio of the measurement, no matter if he is not strongly dependent on wavelength for the relevant material to be measured.

Pyrometer sometimes only evaluate a restricted through a filter to a specific wavelength range small part of the radiation spectrum. They are called Schmalbandpyrometer - the signal processing easier, since the spectral sensitivity curve of the sensor plays a negligible role.

If the wavelength range wider, one speaks of a band radiation pyrometer.

Under a total radiation pyrometer is meant a device which detects the radiation of a measurement surface across the entire spectral range. However, since the pyrometer associated lenses, windows and radiation receiver work for only a limited wavelength range, there is not strictly a total radiation pyrometer, but only band radiation pyrometer. However, it has established itself as an agreement, even to speak of total radiation pyrometers, if at least 90 % of the potential at a certain temperature radiation be evaluated.

For glowing objects there is a visual process in which the incandescent light from a tungsten ribbon lamp ( incandescent lamp with a tungsten band instead of a - helix ) is associated with the object to be measured to cover. You can now change the current of the lamp until her image disappears before the measurement object - then the strip temperature is equal to the measured object. The adjustment of the lamp current has to read the temperature, a temperature scale. A measuring apparatus is called a filament pyrometer and belongs to the group of comparison pyrometer.

Measuring wavelength

Which area is optimum for the desired measurement, depends on the material to be measured and its temperature.

Wavelengths ( MIR) for temperatures around room temperature are in the Middle infrared in question. There are thermal and pyroelectric sensors.

Temperatures above about 350 ° C can be determined in the near infrared with IR photodiodes. Thus, a germanium photodiode, for example, a maximum reception wavelength of about 1.9 microns. The more suitable material InGaAs can be produced from 1.9 to 2.6 microns depending on the composition for maximum reception wavelengths.

Temperatures from about 700 ° C can ( maximum reception wavelength from about 0.9 to 1.1 microns ) or comparison processes in the visible spectral range are measured with silicon photodiodes.

At the maximum reception wavelength of silicon photodiodes ( 1.1 microns ) has a body having a temperature of 3000 K to be radiation maximum, silicon photodiodes, however, all temperatures are measured above about 700 ° C. Generally the temperature range of a pyrometer is much easier to expand upward as downward as increasing with increasing temperature, the radiant power of all wavelengths.

Most of the receiving wavelength range of high-temperature pyrometers by the photo receiver used is determined.

Emissivity correction

The emissivity of the material has to be known for a measurement by means of a pyrometer. This generally depends not only on the material of the measurement object, but also of the wavelength ( the reception wavelength of the pyrometer used) and therefore the temperature of the object.

While most organic materials (wood, plastic, paper, paint) as well as glass and ceramics have very high emissivities ( 0.95 ) in the middle (MIR ) and far infrared ( FIR), bare metals emit better at short wavelengths ( violet end the visible spectral range ) and have in the near (NIR ) and mid infrared (MIR ) significantly lower and therefore less favorable for the measurement of emission levels ( burnished gold in the MIR range, for example, only about 0.02).

If on the other hand, for example, anodized metal ( aluminum) or highly oxidized, it has a distinct higher emissivity of 0.9 in the MIR. Even with painted metals ( any color! ) The significantly higher emissivity of the paint for the temperature measurement is relevant.

Pyrometer therefore often have a possibility of correction for the emissivity, such as a knob ( potentiometer) with a scale of 0 .. 1 Some hand-held pyrometer ( " Infrared Thermometer " ) and an additional measurement input to a contact temperature sensor (eg a thermocouple ). For calibration of the pyrometer for an unknown material, ie for emissivity determination, the temperature can be measured first with this additional sensor; the setting for the emissivity of the pyrometer is then adjusted until the non-contact measurement yields the same result as that with the contact sensor.

Detectors

As detectors for thermal pyrometer (eg bolometers, pyroelectric sensors or thermopile from thermocouples) or photoelectric detectors ( cooled or uncooled photodiodes ) are used. The lens or window for devices in the near infrared region is made of glass or quartz glass. In the mid and far- IR devices are windowless, or the lenses or windows are made of crystals such as germanium, CaF2, ZnS, ZnSe, KRS5 or polyethylene (PE ) or polypropylene (PP).

Advantages of non-contact temperature measurement

• Very rapid measurement (<1 s to less than 1 microseconds, depending on the device )
• Very long, continuous measurement ranges are possible (eg 350 ... 3500 ° C)
• No wear
• No temperature influence on the measurement object, or fault of poor thermal contact
• No mechanical damage to sensitive objects such as foil or paper
• No problem with moving test objects
• Possibility of measuring at high voltages, electromagnetic fields or aggressive materials

Disadvantages of contactless temperature measurement

• The emissivity must be known for material, wavelength and temperature.
• In particular, for metals complicate strong emissivity variations for precise measurement (eg copper: 0.012 (polished, 327 ° C), 0.78 (strongly oxidized, 25 ° C), 0.91 (strongly oxidized, 527 ° C)).