An Open-Source Dual-Channel RTD Thermal Control Platform for Research-Scale PEM Water Electrolysis Balance-of-Plant Integration

Inter-laboratory variation in proton exchange membrane water electrolysis (PEMWE) test bench balance-of-plant systems has contributed to significant disparity in published single-cell polarisation performance data, limiting experimental reproducibility and comparison between laboratories. Harmonisation efforts identify precise low-contamination thermal control of process fluid and cell hardware as a benchmarking requirement. This paper presents the Thermal Stability Regulatory Control Tool (TSRCT-PCB-01), an open-source dual-channel MAX31865-based resistance temperature detector (RTD) thermal control platform for PEMWE research, with independent dual-loop sensing and actuation, data logging, and configurable overtemperature shutdown logic. Single-step open-loop characterisation of an isolated endplate, without process fluid flow or an operating electrolysis cell, is used to model a lag-dominant first-order-plus-dead-time plant (R2=0.9996; RMSE=0.247∘C; MAE=0.198∘C; τp/θ≈136), informing analytical gain derivation and subsequent experimental comparison of Skogestad SIMC PI and iSIMC PID tunings. Across all analysed post-settling intervals, absolute mean indicated offsets were below 0.004∘C and standard deviations were below 0.030∘C. Measured peak-to-peak spans ranged from 0.0687∘C to 0.3429∘C. Compatibility with fluorinated ethylene propylene (FEP) sensor encapsulation supports an all-fluoropolymer wetted path. TSRCT-PCB-01 is the sensing, control, and actuation module of the broader TSRCT-600 high-purity inline thermal control system. Hardware, firmware, and tuning tools are openly available through GitHub and archived in Zenodo to support reproducible electrochemical testing.