What is A Precision Blackbody?
Entrance aperture of a windowless graphite tube blackbody furnace at 2000 °C.
A Precision Blackbody is a source of thermal radiation that is more than adequate to perform the calibrations and calibration testing that you plan to perform; nothing more, nothing less.
You certainly wouldn't want one that was less than adequate! Unfortunately, some people don't understand what adequate precision entails and get less than they need.
Adequate precision means it has sufficient temperature uncertainty to minimize transferred errors to the calibrated device. There are always transferred errors! Uncertainty starts with a traceable calibration certificate that has an adequate value.
Also, the furnace emissivity has to high enough to contribute only a minimum additional error. That depends on both temperature level and the measuring waveband of the unit being tested.
It is recommended that the furnace entrance aperture to be at least twice the diameter of the spot known as the "measuring spot diameter" or minimum resolvable spot size, to help minimize size-of-source effect errors. First, you have to know what they are (and like temperature uncertainty, they are always there). A sufficient aperture diameter aids in characterizing the size-of-source error ,if your equipment supplier has failed to tell you the value (and most do!).
We'd be happy to help you understand this error source and the need for a larger aperture feature better, but realize, please, that ASTM's Standard Test Practices for Radiation Thermometers E-1256 spells out what it is and how one can test for it in the case of a single waveband thermometer. The Standard does a good job and doesn't cost very much.
All the individual errors or uncertainties need to be combined using the approved method of ISO Guide 25 (see NIST's Uncertainty Guide) and the net result must meet your needs.
A laboratory blackbody approximates 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. It is easy to get it wrong. Don't be afraid to ask advice.
Thermo Gauge Instruments, Inc. supports and services Thermogage's renown graphite tube blackbody furnaces.
Copyright 2003 Thermo Gauge Instruments, Inc. , All rights reserved