IR Instruments Tube Emissivity Considerations
Emissivities for ferritic alloy steel furnace tubes (carbon steel – 9 Chrome) usually fall into a fairly narrow range of values, about 0.85 – 0.95. High emissivity and low variance permits a more accurate and repeatable IR. On the other hand, stainless steel furnace tubes can have a wide variation in emissivity <0.5 for areas that have been ground or polished to >0.8 for highly oxidized or sooty surfaces. Low emissivity and wide variation make it nearly impossible to obtain reliable and accurate IR readings. Sandblasting a stainless steel tube surface to a dull finish will provide a consistent emissivity that is sufficiently high to permit more accurate and repeatable IR readings of stainless steel tubes.
Scale, soot, or ash on tube surfaces can interfere with the infrared temperature measurement. We must usually rely on the experience of the infrared technicians to assess the impact of these factors.