We describe an instrument prototype that measures total dissolved gas pressure (TGP) and dissolved oxygen (DO) without need for standard wetted probe membranes or gas permeable tubing. Measurements were based instead on gas-phase partial pressures that develop within the head-space of a pressure vessel, and on Henry's Law. Performance trials established that differences between standard instrumentation and test instrument OP readings average -1.6-mm Hg (SD = 2.19, range -4.8 to -5.3) within the ΔP range of -72 to 123-mm Hg. Additional trials demonstrated the ability of the instrument to closely follow positive and negative changes in TGP at rates approaching 30-mm Hg/h. Here, the difference between test and standard instrumentation derived TGP averaged 3.4-mm Hg (range 0–41). Differences in ΔP measured by test instrument and gasometer increased with water throughput; mean differences were -3.9, -4.3, and -6.0 for water throughput of 4.6, 6.7, and 7.8 L/min, respectively. However, time to reach 90% of the steady-state instrument reading was highest for low water throughput; equilibrium times averaged 90.9, 44.4, and 44.7 min for water throughput of 4.6, 6.7, 7.8 L/min, respectively. Differences between titration and test-instrumentation based DO measurements were acceptable, averaging -0.11 mg/L (SD = 0.41, range -0.85 to 0.85) over DO from 3.3 to 19.2 mg/L and for 11.1 and 24.2 C. Whereas test instrument DO readings were indistinguishable from Winkler titration at 24.2 C; at 11.1 C there was a tendency for test instrument readings to be lower than Winkler titration at DO < 14 mg/L and higher at DO > 14 mg/L. Trials conducted under biological fouling conditions demonstrated the test instrument's ability to operate with 30% of the maintenance required by standard instrumentation.