Abstract: An apparatus and a method for simultaneously testing the integrity of the welds in the plate pairs of a heat exchanger of the type made up of a stack of pairs of annular plates. The plates of each pair are substantially identical, having corresponding sets of first and second gas ports arranged alternately about each plate. The plates of each pair are joined together by continuous welds formed about each of their corresponding and aligned first and second gas ports. Those portions of the plates located between adjacent first and second gas ports are spaced from each other to permit passage of gas therebetween. The test apparatus comprises a fixed base member and top shiftable vertically toward and away from the base member between open and closed positions. The base member and the top member support, respectively, upper and lower annular seal assemblies.