= Regulated calibration bath
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.<A Third Generation Water Bath Based Blackbody Source by Joel B. Fowler>[A Third Generation Water Bath Based Blackbody Source, §§2–3, printed pp. 592–593: bath construction, controller and setpoint procedure]{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.<A Third Generation Water Bath Based Blackbody Source by Joel B. Fowler>[A Third Generation Water Bath Based Blackbody Source, §4.1 and Tables 1–2, printed pp. 594–595: fixed and movable probes, immersion control, measured nonuniformity and uncertainty components]{ref}
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