  Here, we report our studies on the continuous loading of a U-magneto-optical
trap (U-MOT) on atom-chip from background rubidium (Rb) vapor generated using a
dispenser source in ultra high vacuum (UHV) environment. Using the U-MOT
loading curves, the partial pressure due to Rb vapor and pressure due to
background gas have been estimated near the MOT cloud position. The estimated
pressure due to Rb vapor increased from $\sim \; 1.4 \times 10^{-10}$ Torr to $
\sim \; 4.1 \times 10^{-9} $ Torr as Rb-dispenser current was increased from
2.8 to 3.4 A. The increase in dispenser current also resulted in decrease in
loading as well as lifetime of the MOT cloud. This study is useful for magnetic
trapping experiments where accurate information of pressure in chamber is
important for the lifetime of the magnetic trap.
