Abstract: An assembly for insertion in a bath of molten material to form therewith an electrochemical cell for determining the concentration of a constituent of the bath. The assembly is of the type including a body structure of refractory material from which protrudes a first electrically conductive element providing means for establishing an electrical connection with the bath and a reference electrode structure separated from the bath by means of a rigid electrolyte material. Reference electrode structures of this general type may be comprised of a closed end tube of electrolyte material or a tube of electrically non-conductive refractory material having a pellet of electrolyte material closing the immersion end thereof to separate the material of the bath from the means establishing a reference potential.