Calibrating reference-grade Platinum Resistance Thermometers (PRTs) and Standard PRTs (SPRTs) demands extreme stability and uniformity—typically only achievable in metrology labs. Until now. Modern portable liquid baths, with stability better than ±0.01°C and uniformity within ±0.02°C, are revolutionizing field-based reference calibration. This is critical for national standards labs, aerospace contractors, and research institutions that need to maintain a chain of traceability outside the lab. We explore the technical requirements, from bath fluid selection to sensor immersion depth, for achieving accredited-level calibrations in the field.