From the sewer exit and go up the stairs on your left and enter the tower.  You will see a spiral stair-case on your left.  Go to the base of the stairs.   SAVE THE GAME.   Put your gas mask on and head up the stairs.  Do not dawdle, your DLU is on a timer, keep moving.   At the top of the stairs head towards a door, turn left and go as far into the room as possible, there is a door on your right marked with a warning sign.

Go in and your will see some cylinders on your right and a control panel further in.   Head to the control panel.   Switch the left-most control to manual and turn the brass wheel to reduce the pressure to zero.

Move to the cylinders on your right.   Use your gas cylinder and attach it to the left-most oxygen cylinder.   Turn the red knob until the pressure drops down.

Move back to the control panel.   Select manual on the four controls to the right and turn them off.   Select manual on the left-most control and turn the pressure up.   This should pump gas into the security room next door.