I know folks have gone the “cheaper” route with atlas sensors (https://www.atlas-scientific.com/ph.html) but i’ve worked more with Hach environmental/YSI as more field water quality industry standard. I also check In Situ (multi-sensor platforms) or Fondriest (distributor of various sensor vendors) every now and then.
As far as I know, pH probes are still prone to a lot of calibration drift, so i’d check calibrations before going out to the field and be careful with glass electrode systems (fragility).
Post-fire, i would anticipate changes almost all major water quality parameters to change: specific conductance (similar vendors as mentioned previously), turbidity from loose sediment delivery, biochemical properties (carbon, nitrogen, phosphorous), etc. Time-wise, i’d anticipate one short pulse close in time of fire event from near shore charred sediment, gradual pulse from atmospheric deposition shortly after fire, and short pulses following from any subsequent rain events- note, mostly educated guesses here in time-series stuff. ‘Luck and be safe! -h