A method of making a test device comprised of a container divided into two chambers, each holding a volume of a clear test liquid, the chambers separated by a porous barrier. The liquid is introduced into the lower chamber by adding a liquid to the upper chamber through the open top of the container and then applying a vacuum to completely evacuate air from the lower chamber by drawing the air through the barrier and liquid in the upper chamber. Restoring air pressure forces liquid in the upper chamber into the lower chamber, to completely fill the same while leaving a volume of liquid in the upper chamber. The application of a vacuum is preferably carried out by placing the container with liquid in a receptacle containing a liquid to be boiled, heating the liquid to boil the same and thereafter condensing the vapor with the receptacle sealed to develop a very high vacuum which draws the air out from the lower chamber of the container. The boiling can be done with the receptacle sealed to develop a higher temperature and later vented prior to being resealed for cooling and condensing of the vaporized liquid to further enhance the level of vacuum achieved.