November 2022 Volume 4

EQUIPMENT & TECHNOLOGY

Figure 6: Basic pyrometer functional diagram

• Single wavelength A very common pyrometer is the single wavelength or ‘single color’ type. The detectors are available in short, mid and longwave variations and among the most economically priced because of the simplicity of design. A single wavelength To improve upon a good thing, what are termed two-color pyrometers or ratio pyrometers are available. The principal difference is, instead of a single detector sensitive at one wavelength, the two-color design incorporates two discrete detectors, each sensitive at a different wavelength, for dual wavelength temperature measurements. In this configuration, IR energy upon passing through the focusing optics is directed at the two internal detectors. From here the signal value output from one detector is then divided by the signal value of the other detector resulting in a value that is a ratio of the two detectors. This ratio is converted by the pyrometer processor into a temperature dependent signal. The two-color pyrometer has several advantages of the single color pyrometer, namely that it can maintain a high level of temperature accuracy despite obstructions resulting from smoke and lens fouling. It is also able to more accurately measure the temperature of low-emissivity materials such as aluminum, stainless steel, and copper alloys. pyrometer is the commonly used in forging. • Dual wavelength (Two-Color infrared)

Figure 5: Table of common materials and their emissivity Understanding now that materials radiate IR, what is a pyrometer? A pyrometer is an optical device with a focusing lens to direct infrared radiation emitted by a target (e.g., a hot billet) onto an internal thermal detector which absorbs the IR radiation. The detector then converts the absorbed radiation into an electrical signal which is amplified and converted into a temperature-dependent output signal [4]. Pyrometers have the advantage over contact measuring devices as they accurately measure object temperatures at distances more than 80 inches [2]. They can also measure very high temperatures, exceeding 3500°F, which is greater than most thermocouples. Another advantage is rapid temperature measurement. The acquisition time for pyrometer is very low, 10-15 milliseconds. This means a pyrometer can make between 60 and 100 temperature measurements per-second. [5] Pyrometer types When selecting a pyrometer for forging applications there are several parameters which must be specified when ordering, the foremost of these is wavelength. The term wavelength applied to IR pyrometer specification refers to the particular of wavelength or frequency at which the detector is sensitive at. Just as the visible light portion of the electromagnetic radiation is a spectrum with colors ranging from red to violet (long wavelength to short), the IR portion is also spectral with an IR ‘colors’ range from long IR (8-14um) to short IR (0.6-3um) [6]. Of course, IR radiation is invisible to the naked eye and does not have ‘color’ as we are familiar with. However, specifying the detector wavelength is important and depends on application temperature and material characteristics.

FIA MAGAZINE | NOVEMBER 2022 10