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Joel B. Fowler is an electrical engineer whose work at NIST included the design and characterization of a water-bath blackbody source for radiometry. His 1995 paper describes a cavity whose temperature and effective emissivity could be controlled and measured accurately.^[ref]

The apparatus placed a shaped copper cavity in a stirred, temperature-controlled water bath. The cavity geometry increases the number of internal specular reflections, raising its effective emissivity toward one, while the bath reduces temperature variation. The paper reports operation from 278 K to 353 K and calculated emissivity 0.9997, increased to 0.99997 with a restricted aperture. Geometry, thermal control and measurement uncertainty accounting therefore work together to create a useful measurement calibration source.^[ref]

Identity and discipline: upload.wikimedia.org/wikipedia/commons/7/74/A_Third_Generation_Water_Bath_Based_Blackbody_Source_(IA_jresv100n5p591).pdf

A regulated bath holds its temperature near a chosen setting through feedback control, while stirring reduces internal temperature gradients. Its finite heat capacity buffers disturbances; external heating and cooling replace the ideal heat bath's unlimited capacity. This is useful for thermometer calibration and for maintaining a blackbody calibration source at a known temperature.

Joel B. Fowler's A Third Generation Water Bath Based Blackbody Source reports a NIST test of a stirred water bath containing a copper blackbody cavity. A bottom plate supplied heating and cooling; a double stirrer circulated water. Electronic feedback control regulated heating. An electronic thermometer with a thermistor probe provided the readout. The preparation was to approach a selected setting, wait for stabilization, adjust against the measured temperature and then log continuously.^[ref]

To test spatial uniformity, one thermistor stayed near the cavity tip while another visited ten locations at three heights. The simultaneous reference distinguished spatial differences from temporal drift. An insulated dome reduced thermal conduction along probe casings. Table 1 reports nonzero departures at several settings: circulation did not make every location identical. The measurement uncertainty analysis separately included uniformity, stability, setting, immersion and readout contributions. These tests supported a bounded temperature assignment around the cavity, rather than infinite heat capacity or perfect thermal equilibrium throughout the apparatus.^[ref]