We present a device which makes possible accurate determination of the absorption length of saturated metal vapors inside a long cylindrical absorption cell functioning in the heat-pipe regime. The device is simple to construct and to operate. The absorption length is determined by the spacing between two movable hot windows welded to inner pipes which are open ended in the cold part of the cell. In this configuration there is no differential pressure on the hot windows and thus can be used also in high-pressure studies. Moreover, thin windows can be used to minimize their effect on absorption measurements. A continuous independent adjustment of the absorption length and the vapor density can be achieved by simple adjustment of the hot windows spacing. This is accomplished by moving the inner pipes from outside the cell without opening it. A movable inner thermocouple is also added to control and measure the temperature and its distribution along the absorption region.