Abstract: A temperature-stable portable device includes equipment enclosed in a portable case that seals the equipment from its surroundings, and a heat exchanger distinct from the case and attached to the exterior of the case by removable fasteners. The heat exchanger extends across at least most of a major surface of the case and is positioned to receive heat from the major surface by conduction. A sheet of material having good thermal conductivity is interposed between, and in contact with, the heat exchanger and the major surface of the case. The heat exchanger defines a chamber that is open to the atmosphere and has parallel flow passages and an arrangement for moving a heat exchange fluid through the chamber and out to the atmosphere in order to remove heat from the heat exchanger by convection.