Glass, like honey, changes viscosity dramatically with a small change in temperature. Glass that is too cold will flow more slowly and will solidify too soon. Glass that is too hot will flow too quickly and will solidify more slowly. Effective control of glass stream flow and glass gob weight is therefore highly sensitive to glass temperature, and the glass industry typically wishes to control glass stream and gob temperature within a fraction of a degree.
A short-wavelength single-wavelength sensor is not appropriate for most glass applications because the glass is highly transparent at short wavelengths. The sensor reading will vary with changes in glass color, size and composition, as these factors will determine the transparency of the glass. A long-wavelength single-wavelength sensor is not appropriate for most glass applications, even though the glass is opaque at these wavelengths, because these sensors measure only the surface of the glass and are very sensitive to optical obstruction and misalignment. Varying with air currents, the temperature of the surface of the glass is significantly cooler than the bulk glass temperature and a single fingerprint on the lens of a long-wavelength sensor will result in an error of tens of degrees. For a reliable and accurate measure of glass stream or gob temperature, a dual-wavelength sensor is usually required because only a dual-wavelength sensor, properly selected, will automatically compensate for changes in glass color, size and composition. Because glass products are usually produced from recycled materials, the composition of the glass can vary even when only a single product type is produced.
The transparency of glass changes dramatically with wavelength. For a dual-wavelength sensor to produce an accurate and reliable reading, it is essential that the glass be equally transparent at both measured wavelengths. For this reason, wavelength selection is a critical criterion.
The Williamson dual-wavelength model 82-40 & the fiber-optic dual-wavelength model 92-40 are filtered at wavelengths uniquely appropriate for the measurement of glass stream and gob temperature. At the wavelengths of operation, most types of glass are equally transparent regardless of size, color or composition. Two-color sensors offered by others are filtered at wavelengths that do not view equally well through most types of glasses and are therefore not appropriate for measurement of most streams or gobs. For this reason, the Williamson dual-wavelength sensors are uniquely qualified for use in these challenging applications.