this invention comprises a minimal elastance underwater breathing apparatusuba ) in which the elastance component of uba impedance is substantially eliminated by forcing the breathing bag to expand and contract in a horizontal plane with small vertical displacements and hence minimal elastance . the invention further includes a servo - mechanism operating with a microprocessor , 3 - axis position sensor and rotatable joints which act to keep the breathing bag oriented horizontally no matter what the diver &# 39 ; s position . the principles of the present invention may be adapted to existing uba with redesign and additions .