AlanG, Thanks for the complete explanation. I guess the most obvious follow-on is that since the "Williamson two-frequency pyrometer, recommended to Bob Greenyer by industry experts as the most accurate technique available" (the one which has been used successfully in this older testing)- then why not also include it now in the new test?
Is it not available? Assuming that the Williamson is not available where the present test is to take place, can we assume that even so --- with the longer wavelength IR testing, since the coating does have a smaller effect (less than when a pyrometer is used) it will be applied in the spirit of achieving the best possible result ? Even if the coating is only a PR gesture, the numerous comments from experts on the lack of a black coating would make one think that an uncoated tube will be not acceptable. -----Original Message----- From: AlanG @Jones According to Aremco, their spec was based only on "anecdotal information". We proposed to do formal testing for them and they provided a sample for that purpose. Our testing was done with a Williamson two-frequency pyrometer, recommended to Bob Greenyer by industry experts as the most accurate technique available. We leased the instrument specifically for this test. Our data can be seen at: https://drive.google.com/open?id=0BxxJkjesxe4kSkVlZEhRS251Sk0 Note that the measured emissivity values are for the pyrometer sensing range 2.5-5 um and are somewhat lower than the corresponding values for the 6-12 um range of IR cameras. The emissivity values are thus relative to the wavelengths measured. Nevertheless, the measurements show a clear decline of emissivity from 0.73 at 920°C to 0.62 at 1128°C (thermocouple temperatures), the range over which the color change was observed. Of further interest is the increase in measured emissivity of the bare high-alumina tube surface. We attributed this to the shorter wavelength of emitted IR as the temperature increased. We did further measurements with the alumina tube in a furnace, to eliminate possible error from heater coil IR transmitted through the alumina, and got temperature readings very close to the furnace set point. We concluded that the shorter wavelength measurements used by the pyrometer would benefit from the coating by eliminating near-visible IR for which the alumina is partially transparent. This phenomenon has a smaller effect on measurements made typical IR instruments in the 6-12 um range where alumina is opaque. On 7/29/2015 11:46 AM, Jones Beene wrote: > This image appears to shows a bleaching of the coating, but it is not > clear if this actually changes the emissivity - and that situation > would be strongly at odds with the information on the suppliers > website. Is it possible you chose the wrong product (# 840-C is for > one for ceramic surfaces)? At any rate, it would be a huge mistake not > to apply a high emissivity coating prior to testing. The worst that > could happen with such a coating is highly preferable to doing nothing > at all. Jones
