Attention is drawn to the different surface cleanliness which can exist in different systems under uhv conditions at the same pressure level. Experimental data are presented on gas evolution processes, due to gas evolution from internal surfaces and to those originating from the bulk of the solid, of relevance, towards the attainment of clean surfaces. The critical temperatures are determined for the desorption peak maxima to occur, on multi-gas component, low surface coverage, copper surfaces at sub-zero temperatures, for the species argon, hydrogen, carbon monoxide and methane. Multiple desorption peaks indicate desorption from different surface states, in agreement with high temperature experiments. The superior performance obtained with highly pre-cleaned titanium sublimation pumps generates conditions below the working region of most mass spectrometers. For measurement in this region and to monitor surface conditions a simple field electron emission microscope based technique is developed. Changes in surface coverage are analysed by colour superposition techniques and in later development by computerised techniques.