Adjustable insertion assembly for modular electrochemical potential measurement sensors, the assembly comprising: an elongated electrode holder, configured to removably receive therein an electrochemical sensor having measurement and reference half cells, the electrochemical sensor configured for electrochemically responding to a process analyte upon contact with a process fluid; a receptacle configured to slidably receive the electrode holder therein, wherein the electrode holder is configured for slidable movement within a range of motion extending from inserted to retracted positions; the receptacle having a distal end at an open distal end portion which is configured for extension through a wall of a process fluid vessel, wherein the electrode holder has a distal end at a distal end portion thereof, and one or more apertures disposed in the distal end portion of the electrode holder, to permit process fluid to flow through said aperture(s) into contact with the sensor when the electrode holder is disposed in inserted positions; a leverage member coupled to the receptacle, and configured for releasable movement relative to the receptacle; an extension captured by the leverage member and configured for being moved by the leverage member relative to the receptacle; the receptacle having an abutment configured to engage the extension in a sliding, interference fit, so that movement of the leverage member in opposite directions alternately clamps and releases the electrode holder relative to the receptacle to substantially prevent and permit movement at substantially any point within the range of movement, wherein the assembly further comprises a flange disposed at the distal end of the electrode holder, the flange configured to engage the distal end of the receptacle to close the aperture(s) to the process fluid and provide a mechanical stop for the electrode holder when the electrode holder is in its retracted position. an elongated electrode holder, configured to removably receive therein an electrochemical sensor having measurement and reference half cells, the electrochemical sensor configured for electrochemically responding to a process analyte upon contact with a process fluid; a receptacle configured to slidably receive the electrode holder therein, wherein the electrode holder is configured for slidable movement within a range of motion extending from inserted to retracted positions; the receptacle having a distal end at an open distal end portion which is configured for extension through a wall of a process fluid vessel, wherein the electrode holder has a distal end at a distal end portion thereof, and one or more apertures disposed in the distal end portion of the electrode holder, to permit process fluid to flow through said aperture(s) into contact with the sensor when the electrode holder is disposed in inserted positions; a leverage member coupled to the receptacle, and configured for releasable movement relative to the receptacle; an extension captured by the leverage member and configured for being moved by the leverage member relative to the receptacle; the receptacle having an abutment configured to engage the extension in a sliding, interference fit, so that movement of the leverage member in opposite directions alternately clamps and releases the electrode holder relative to the receptacle to substantially prevent and permit movement at substantially any point within the range of movement, wherein the assembly further comprises a flange disposed at the distal end of the electrode holder, the flange configured to engage the distal end of the receptacle to close the aperture(s) to the process fluid and provide a mechanical stop for the electrode holder when the electrode holder is in its retracted position.