Advances of Dual Wavelength in Thermometry - WILLIAMSON - #1

While the concept of ratioing radiometry has been around since the early 1950's, recent design and hardware changes are yielding higher performance, low-temperature capability, and reduced price.

W.R. Barron, Williamson Corp,

Modern material processing plants require extremely accurate knowledge and control of product and process temperature. Many continuous processes, as well as such basic batching operations as vacuum chambers, do not lend themselves to contact thermometry. The problem can be either the operating environment or a product that should not be touched. Furthermore, operations that are adopting statistical process control pro­cedures need even more precise and realistic methods of obtaining accurate and usable temperature data.

Infrared thermometry is used through­out industry for product temperature measurement in the - 40 to 2500°C (- 40 to 4500°F) range. A variety of designs are available to meet specific application re­quirements. For less demanding jobs, a single-wavelength instrument can be used to measure the radiant energy from the target surface.

For more sophisticated applications, where absolute accuracy is critical and the product is undergoing a physical or chem­ical change, dual- and multi-wavelength radiometry should be considered. The con­cept of ratioing radiometry has been around since the early 1950s, but recent design and hardware changes are yielding higher performance, low-temperature ca­pability, and reduced price.

Let us briefly look at the environmen­tal elements and basic system components of noncontact temperature measurement as shown in Figure 1. The target or the sur­face to be measured is of prime concern here. Target size, temperature limits, emissivity, and process dynamics must be taken into account when selecting the in-

cable with lens, an optical front end that includes filtering and a detector, and signal conditioning. In the design shown here, a filter wheel is used with a single detector to offer drift-free operation and long-term reproducibility.

Let us now take a closer look at the emissivity factor. In some cases, the target does not approximate a blackbody radiator, and the target environment, consisting of its surroundings and the atmosphere in the sensor's line of sight, is not so well con­trolled as it is during the calibration pro­cess. The radiant energy seen by the sen­sor combines three elements: emission from the non-blackbody target with a spec­tral emissivity of less than unity; reflection of any irradiation from the surroundings; and a participating atmosphere involving absorption or emission by atmospheric gases in the sight path, e.g., the emission from flames can increase the signal, while the absorption caused by airborne partic­ulate can reduce it.

The radiation thermometer output in a real-world application will be a signal cor-

— SURROUNDINGS, T_sur

RADIATION THERMOMETER

S.Tx

TARGET, ATMOSPHERE

Tc e\ EMISSION AND

ABSORPTION

Figure 1. When selecting noncontact temperature meas­urement instruments, it is necessary to take into account not only the target and its emissivity, but also the sur­roundings and intervening atmosphere.

strument, as related to field of view, spec­tral response, and response time. It is also essential to characterize the surroundings (e.g., flames, infrared heaters, induction coil), and the atmosphere (dust, dirty win­dow, flames, excessive heat, etc.) in order to select the optimum instrument for the application.

The typical radiation thermometer (see Figure 2), is an electro-optical device in­corporating either a lens or a fiber-optic

a SIGNAL

Figure 2. In this design of a dual-wavelength radiation thermometer, a filter wheel is used with a single detector to provide drift-free operation, long-term reproducibility, and a temperature range of 1S0-2400°C (300-4400°F).