Hm, I just found this probe: Orion™ Economy Series pH Combination Electrodes
The diameter is 6mm, which I believe would fit into the vial cap A’s port, and the price is reasonable (I know I know). It has a BNC connector, and can integrate with the EZO pH system¹:
I’m currently writing some software to incorporate the latter into the Pioreactor: [wip] Integrating pH / DO probes (software only) · Issue #483 · Pioreactor/pioreactor · GitHub
This came from this paper: https://www.biorxiv.org/content/10.1101/2024.03.03.582641v1.full.pdf
¹I’m kinda confused about the connector though. The probe has a BNC connector, while the EZO has an SMA connector. I think you need a BNC to SMA connector.