A high-resolution in situ sensing system and method for providing continuous measurements of at least one dissolved gas including a sample liquid processing cell having at least a first conduit defining a first passage with at least one gas-permeable wall capable of passing the at least one pre-selected dissolved gas from the sample liquid into a reagent fluid. The at least one gas-permeable wall substantially resists flow of the sample liquid therethrough. Reagent fluid is directed through the first conduit while moving the sample liquid and the reagent fluid relative to each other in one of a concurrent and a countercurrent flow relationship to achieve either partial or full equilibration between the sample liquid and reagent fluid to generate at least partially equilibrated reagent fluid in a substantially continuous manner.