Temperature Non-uniformity Uncertainty
When the AERI Blackbody is not operated at ambient temperature, thermal gradients will develop in the cavity wall that are proportional to the temperature difference between blackbody set point and ambient . Typically, the AERI Hot BB is run at 60°C at with an ambient temp. of 20°C; gradient numbers presented below assume this typical Blackbody operation point.
Three types of gradient are accounted for in the AERI Blackbodies:
- Thermistor to cavity inner wall temperature gradient. This gradient in the aluminum is calculated using the AERI Blackbody thermal model and is found 0.01 C for the apex thermistor and 0.02 C for the thermistors on the cylinder diameter. The gradient in the paint due to free convection and radiation must be added to these values. Results of testing indicate that these values are less than 0.01C.
- Longitudinal gradients along cavity inner wall. With the AERI Blackbody thermal model and the AERI Optical Design model, Temperature Weighting Functions are calculated using the available thermistors located in the cavity apex (1) and on the cylinder diameter (3).
- Azumuthal variations around the cavity inner wall due to convection. When the Blackbody is tipped toward one side, these gradients tend to be symmetric about the "top" side and "bottom" side. The top and bottom cylinder diameter thermistors are averaged to account for this gradient which is on the order of (0.07 C).
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