= Reference-bath thermometer comparison
A motivating real procedure is NIST's ambient <thermometer> verification. Its apparatus is a deep beaker of <water>, a calibrated reference <thermometer>, a test <thermometer>, and a magnetic stirrer with any heater switched off. Let the <water> approach room <temperature>, stir gently, immerse both sensing tips well below the surface, and wait before reading. NIST specifies about $20\,\mathrm{cm}$ of <water>, $10$–$15\,\mathrm{cm}$ immersion where possible, and an initial five-minute wait. Record test then reference, repeat in reverse order, and apply the reference's certificate correction. Subtract corrected reference from test: the difference estimates the test instrument's error at this bath <temperature>. Agreement between the two reading orders checks repeatability. <Mercury Thermometer Alternatives: Verification Methods for Alternative Thermometers by National Institute of Standards and Technology>[“Comparison with another calibrated thermometer at ambient temperature”, apparatus and steps 1–10; “Measurement at the steam point”, pressure and elevation correction]{ref}
For example, suppose a test reads $24.7\,{}^\circ\mathrm C$, a reference reads $24.4\,{}^\circ\mathrm C$, and its certificate says to add $0.1\,{}^\circ\mathrm C$. The corrected reference is $24.5\,{}^\circ\mathrm C$, so the test error is $+0.2\,{}^\circ\mathrm C$: it reads high. These are illustrative readings. This verification tests a previously calibrated instrument at one point; it does not establish the two-point response used in <two-point thermometer calibration example>.
Additional controls follow from the <thermal equilibrium> premise. Keep tips away from walls and bottom, interchange probe positions to test a <temperature gradient>, and check that longer waits and modest immersion changes leave agreement unchanged within the required tolerance. A flat trace alone can be a <steady state> maintained by competing <heat transfer> paths. Five minutes is a procedural starting point, not a universal equilibration time. Excessive stirring can supply <work> and warm the bath, while probe stems can exchange <heat> with the room. These controls test whether both probes sample the same state; neither one-point agreement nor this experiment proves the <zeroth law of thermodynamics> for every material and condition.
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