@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
