Medford Clock & Barometer

Wheel barometer tube replacement

Replacing the mercury tube in a wheel or banjo type barometer requires only a few simple hand tools and some basic skills using them. The steps involved are the removal of the old tube, installation of the new and then calibration.

First of all, remove any rings or jewelry. If mercury comes into contact with them it will ruin them.

Get into the habit of holding the barometer at a 50 degree angle. If these instruments are not hanging on a wall this is the position they want to be in.

If the existing tube is broken or if there are any mercury droplets in the back if the instrument, perform all of this work outside. Even if there is no mercury present working outside is a very good idea.

Removal

Begin by turning the instrument over to gain access to the back door. You can put the bottom of the barometer on the seat of a chair and lean the top on the back of the chair. This is a good angle to work. Open the back door to expose the tube compartment. The main tube should be going up the left hand side of the compartment. The "j" bend will be at the bottom towards the middle of the compartment and there should be a 6" -7" glass guide tube on the right. Hanging by threads from the pulley are two glass wiughts. These go in the "j" tube and guide tube. They look like the tops of thermometers and should be 1" - 2" in length. The weights are two different sizes with the heavier on riding on the mercury column.
( Click photo for another image )

Pull the weights from their tubes and let hang or tape out of the way. Looking at the long tube there should be 2 or three pieces of fine wire twisted around the tube holding it in place. Gently untwist these wires. They will not be very tight. There should also be one at he bottom on the short leg of the tube. As you have noticed, there isn't much holding these tubes in place.

Installation

Unpack the new tube if you have not done so already. You will see that there is a special plug installed which keeps the mercury sealed in the top chamber of the tube and a cork which protects against spillage. To remove this, support the "j" part of the tube while you gently slide the cork up and then "unscrew" the wire rod by the handle on the top while using a slight upward pressure. Once removed slide the cork off the wire and reinsert it into the top of the tube. This will prevent any dirt or wire pieces from falling into the tube. Handle the tube carefully. If tilted too quickly there is a chance the impact from the mercury hitting the top could break it.

Place the tube in the barometer and wrap the holding wires around it. If the wires break replace with any fine soft wire that you have. (Twist ties with the paper stripped off work okay.)

The new guide tube and weights shipped witht the tube can be used or discarded. If the new weights are used the thread may need to be replaced or extended. This gets a little more complicated so I recommend using the old ones if possible.

Insert the glass weights in the mercury and guide tubes. The heavier, or larger weight rests on the mercury and the lighter one goes into the guide tube. By pulling down on the string as far as it will go on one side and then the other side it should properly align itself in the grooves of the pulley.

Calibration

Once the tube is in place the instrument must be calibrated. To perform this operation you must first obtain an accurate current pressure. This is available from the weather channel, the local airport or another reliable barometer. Turn the instrument over and remove the two screws holding the brass bezel and glass to the barometer. Remove the bezel and set aside. Remember your right and left, or top and bottom for replacement. A piece of tape at the 12:00 position is an easy way to do this.
While holding the pully from the back pull the indicating hand off with an unscrewing motion. It shouldn't be that tight but some are. Place the barometer in a perfect vertical position. Check the back to make sure the threads for the weight and counterweight are in the correct grooves in the pully, and that they move freely. Tap the instrument 4 - 5 times to stabilize the mercury in the tube. Place the hand on the shaft so it is pointing to the correct pressure. Do not press tight. With the hand on the shaft, tilt the instrument and observe movement of the hand. Tap 4 - 5 times to see where the hand settles. If it needs a slight correction, hold the pulley in the back and turn the hand. Once it settles in the correct position after the tilting and tapping, press it on the shaft a little harder while supporting the pulley in the back with your other hand.. Make sure the hand does not rub against the dial.
Before replacing the bezel, make sure there is clearance between the indicator hand and the index hand and that they do not interfere with each other. Replace the bezel and make sure the hand does not rub against the glass.

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