An electrochemical sensor is provided that includes a housing defining a cavity for a reference electrolyte and defining an opening to the cavity configured to be proximate to a target fluid. The sensor further includes a junction plug comprising a porous material and a cross member impermeable to a target fluid positioned between the junction plug and the cavity. The cross member includes a planar portion disposed against the junction plug that defines an aperture to enable electrochemical communication between the target fluid and the reference electrolyte.