Precision

Blackbodies

The Dual Cavity Feature
And Its Benefits

Cutaway side view of windowless graphite tube furnace dual cavities

Perhaps one of the least appreciated features of the graphite tube furnace design are the standard dual cylindrical cavities that are built in to every unit.

Above is a cutaway view of the cavity geometry, There is a web of graphite between the two cavities. The entire unit is fabricated from a single piece of graphite. There are no joints or seams.

Both cavities have the same length to diameter ratio of 5:1. They are identical and heat and cool identically. They have the same temperature and temperature distributions. One is used for the control pyrometer, the other for the test pyrometer. It doesn't matter which unit aims in which end since there is true symmetry present. It is easy to check by swapping ends.

The benefit that this one feature provides is surprising, beside the obvious ability of being able to read or record both the test and reference temperatures simultaneously.

The cavity spectral emissivity is very high resulting in precision calibration. From the geometry alone one would calculate it to be 0.995 or possibly a bit higher in the 1 to 3 micrometer wavelength region. (We've heard from some users with test estimates that suggest it is closer to 0.998). For calibrations in the 1 to 3 micrometer region it doesn't matter very much for some basic reasons.

The error introduced by a 0.995 emissivity cavity in the reading of 1 and 3 micrometer radiance temperatures from true, are approximately -0.5 °C and -1.7 °C, respectively, at 1000°C. A 0.998 cavity results in -0.2 °C and -0.7 °C error for the same conditions. A very small difference. (For 2000°C, the errors are about three times larger).

In all cases, unless one is performing very precise metrology, the differences are much less, about 1/4th to 1/10th the magnitude, of the calibration uncertainty on the best laboratory calibration certificates for infrared thermometers and radiation pyrometers at these wavelengths.

The big benefit is precision of results since both sensors are not significantly affected by the cavity spectral emissivity.

Precision Blackbodies

Laboratory blackbodies approximate Max Planck's ideal. We strive to provide the closest approximation that a reasonable budget and the needs for precise sources of Blackbody thermal radiation can provide.