CONTINUOUS MONITORING OF DISSOLVED-OXYGEN AND TOTAL DISSOLVED-GAS PRESSURE-BASED ON HEADSPACE PARTIAL PRESSURES

We describe an instrument prototype that measures total dissolved gas pressure (TGP) and dissolved oxygen (DO) without need for standard wet ted probe membranes or gas permeable tubing. Measurements were based i nstead on gas-phase partial pressures that develop within the head-spa ce of a pressure vessel. and on Henry's Law. Performance trials establ ished that differences between standard instrumentation and test instr ument Delta P readings average -1.6-mm Hg (SD = 2.19, range -4.8 to -5 .3) within the Delta P range of -72 to 123mm Hg. Additional trials dem onstrated the ability of the instrument to closely follow positive and negative changes in TGP at rates approaching 30-mm Hg/h. Here, the di fference between test and standard instrumentation derived TGP average d 3.4-mm Hg (range 0-41), Differences in Delta P measured by test inst rument 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 ins trument reading was highest for low water throughput; equilibrium time s 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-instru mentation 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 indistingui shable from Winkler titration at 24.2 C; at 11.1 C there was a tendenc y for test instrument readings to be lower than Winkler titration at D O < 14 mg/L and higher at DO > 14 mg/L. Trials conducted under biologi cal fouling conditions demonstrated the test instrument's ability to o perate with 30% of the maintenance required by standard instrumentatio n.