The marine science community requires accurate, cost-effective, and reliable pH sensors capable of long-term, stable operations <em>in-situ</em> from coastal to deep-sea environments. Spectrophotometric pH sensors based on lab-on-chip (LOC) technology have been shown to offer long-term accuracy that can sample every 10 minutes. However, for applications where higher-frequency measurements are important, this maximum sample rate may be limiting, in addition to the power requirements needed to operate the sensor.</p> <p>In contrast, commercially available pH optodes (PyroScience GmbH) are relatively inexpensive, consume little power and have a small form factor, but with intense use the pH sensitive membrane can photo-oxidise, causing signal drift. The combination of LOC and optode technologies, however, can be used to provide long-term, high-frequency and high-stability <em>in-situ </em>pH data, but protocols to correct for sensor drift need to be developed and evaluated.</p> <p>To examine sensor drift and develop protocols to account for it, we suspended two LOC pH sensors with two pH optodes at 0.5 m depth from a floating pontoon within a harbour in Southampton, UK for six months (June–December 2023). This is a highly dynamic tidal environment with substantial biofouling. The optode (AquapHOx-L-pH, PyroScience GmbH) and an independent pH sensor (Deep SeapHOx V2, Sea-Bird Scientific) measured at a high frequency (e.g., ≤5 min) alongside a LOC pH sensor measuring at a lower frequency (e.g., ≤2 hr). Triplicate lab validated co-samples were collected each week, in addition to dedicated sensors monitoring the temperature, salinity, dissolved oxygen and tidal height. We find good agreement i.e., mean ∆pH = -0.022 ± 0.023 (3,182 data points in common) pH units between the SeapHOx and LOC sensors, in addition to individual field accuracies of <0.020 pH units. As expected, we found significant signal drift (e.g., generally ≤0.012 pH units per day) and offsets (e.g., 0.1–0.2 pH units) with the pH optodes after intensive use in a high biofouling environment. However, by coupling accurate LOC pH data to high frequency optode data, we corrected the optode signal drift/offset and achieved a similar field accuracy (<0.02 pH units) to the SeapHOx sensor even when using ultra-low LOC pH sensor measurement frequencies (e.g., several days to weeks). Overall, this work provides the oceanographic community with guidelines on how to achieve accurate, rapid and long-term pH measurements, while also balancing power requirements, by combining two complementary pH sensing technologies.