The Fortin barometer measures the relative barometric pressure at a specific location. The unit consists of a metal housing that holds a glass tube filled with mercury. At the base of the barometer is an adjustable well, called the cistern, which holds a volume of mercury. A vacuum within the glass tube draws the mercury up from the cistern, to varying heights, depending on changes in barometric pressure. As the mercury in the barometer oxidises over time, the cistern of a Fortin barometer will need cleaning. This procedure requires specialised tools and handling.
Place the barometer in the clamp stand with three clamps. Position one clamp at the top of the housing, and one at its middle. Place the third clamp around the cistern, at the bottom of the barometer.
Loosen the three bolts holding the metal housing to the cistern. Loosen the clamps holding the metal housing. Raise the housing up, until it clears the centre clamp.
Tighten the centre clamp around the glass tube. Remove the metal housing. Tighten the top clamp around the glass tube.
Put on rubber gloves. Loosen the glass ring around the top of the cistern from the bushing on the glass tube. Draw out some of the mercury in the cistern with the syringe.
Turn the adjustment screw at the bottom of the cistern to lower mercury from the glass tube. Continue to remove mercury with the syringe and lower the adjustment screw, alternately, until all the mercury is removed
Place the mercury in a clean glass bottle. Withdraw the mercury with another syringe and long needle from the bottom of the bottle, leaving the mercury oxides behind.
Remove the cistern from the clamp. Remove the glass ring and leather bag from the cistern. Wipe all metal and glass parts with tissue paper dampened with white spirit. Allow the parts to dry completely.
Invert the leather bag. Wipe any dust or oxides from the bag with a soft cloth. Return the bag to its original position.
Place a long Teflon tube with a stop cock mounted on it, leading to a vacuum pump. Insert the Teflon tube into the glass tube, until the tip of the Teflon tube reaches the top of the glass tube.
Replace the leather bag and the glass ring to the cistern. Put the reassembled cistern into the clamp on the stand. Leave space between the bushing and the glass ring for the Teflon tube.
Add some cleaned mercury to the cistern. Turn on the vacuum pump. Open the stop cock to remove air and draw mercury into the glass tube. Continue to add more mercury to the cistern, as needed.
Place the metal housing next to the glass tube, as a measure of the mercury level. Apply the vacuum until mercury reaches the level that equals the current approximate barometric pressure.
Remove the Teflon tubing. Reposition the glass ring and cistern to fit onto the upper bushing.
Open the top clamp on the glass tube. Slide the metal housing onto the glass tube. Open the bottom clamp and slide the housing down onto the bushing. Insert the three bolts through the flange on the housing and thread them in to hold the cistern in place.
Tighten the upper clamp. Adjust the screw on the bottom of the cistern, to set the level of the mercury in the glass tube to the precise barometric pressure.
Mercury is considered a toxic substance and should be handled with care. Avoid spilling mercury and dispose of all cleaning materials appropriately.