The invention provides a pH value detection biological reaction system. The system comprises a bioreactor vessel (51) having a plastic wall (50) and a pH sensor (40, 60, 80, 100, 120) attached to said plastic wall (50). The pH sensor (40, 60, 80, 100, 120) includes a sensor body (82) having a flange (76) sealingly attached to the plastic wall (50). The sensor main body (82) has a reference electrolyte (142) and a first detection element (152) arranged in the reference electrolyte (142). The first detection element (152) is configured to contact both the reference electrolyte (142) and the sample solution (52) within the bioreactor vessel. A second detection element (62, 92, 148) is positionable inside the bioreactor vessel (51). The pH sensor (40, 60, 80, 100, 120) has multiple configurations, including a protective configuration in which the at least one detection element is isolated from the interior of the bioreactor container (51), and the at least one detection element is fluid ground connected to the internal working configuration of the bioreactor vessel (51).