Mercury barometers consist of mercury tubes ("canes") that rely on a weight ("float") situated on top of the mercury that is connected to the barometer dial. Barometers provide atmospheric pressure readings by balancing the weight of mercury in the tube versus atmospheric pressure. If the weight of mercury is less than that of atmospheric pressure, mercury levels rise and the dial turns. Likewise, if the weight of mercury is greater than that of atmospheric pressure, mercury levels fall. Mercury barometers get damaged when the instrument is carried at 0 degrees to the horizontal, or allowed to lay flat, thus disturbing the mercury and preventing proper readings.
Empty the cane of mercury by tapping the bottom to the bottom of a jar. Alternatively, and more efficiently, insert an appropriately sized catheter, connected to a syringe, into the cane to suck out the mercury.
Clean the tube. Use another catheter to insert a solution of 50 per cent nitric acid and deionised water into the tube. Then dry the tube by using a third catheter to inject ethanol.
Clean the mercury by injecting it through a clean cloth into a jar.
Reinject mercury into the cane, while holding the cane at 30 degrees to the horizontal. Connect a catheter to a syringe and use the syringe to take up the mercury. Connect the syringe to the cane and squirt out the mercury, being sure not to make air bubbles. If you see air bubbles, withdraw the mercury and reinject it.
Move the cane to a vertical position. If you see bubbles moving through the cane, remove your mercury and start over with a clean cane.
Size a small piece of cork to plug the end of the mercury tube. Insert the cork into the end of the tube.
When reinjecting your mercury, consider using wire clamps on a laboratory ring stand to hold your tube at the appropriate angle, thus making your life a little easier. Be sure to reinject over a container from which you can easily recollect mercury, should a little dribble out of the tube.