To achieve highly accurate controlled depositions a specially designed vapour source, for metals with low to medium melting temperature, was developed. A quartz crystal microbalance is fitted inside the evaporation chamber to provide real-time rate and thickness monitoring. A carefully planned geometry allows maintaining a small deposition area centred on the sample without compromising flow rate measurements. Dosing rates as low as 0.02 ML/min are easily achieved, therefore providing true sub-monolayer control. This source was tested and calibrated for Ag and is being successfully used to study the growth of Ag clusters on TiO2.