I’ve always been interested in devices, gadgets, machines, and tools. Especially those of a scientific nature. I don’t know when I first learned about glass barometers (also called water barometers*). I was interested, but not sure if I wanted to pay full retail price to obtain a glass barometer and see for myself how it worked and if it was worth the money. One of the great thing about the U.S. of A. is the thrift shops. You never know what you’re going to find. One day, there is was: a glass barometer—for $5. That’s at least $20 less than the price of a new one. Sold! Of course it was used, but that makes no difference in this case. As long as it’s not broken, it will work perfectly. If anyone else is interested, here’s all I know about glass barometers:
A glass barometer is basically a vessel shaped something like a pitcher with a long spout. The body is sealed except for a connection to the spout at the bottom. The only opening is at the end of the spout at or above the top of the body. Because the spout is attached near the bottom, once the barometer is partly filled with liquid, any air above the water line inside the body is trapped (and for it to work, there has to be some air trapped inside the barometer).
When atmospheric pressure is high, the air outside the barometer pushes down on the liquid in the spout compressing the air trapped inside the barometer. This happens because water is practically incompressible, while air can easily be compressed, so the atmospheric pressure outside the bottle is transferred through the liquid to the air inside the bottle. Thus, a low level of liquid in the spout indicates high atmospheric pressure, which usually means clear weather for the next several hours.
When atmospheric pressure is low, the air trapped inside the barometer expands and pushes the liquid up into the spout. So, a high level of liquid in the spout indicates low atmospheric pressure, which often means the possibility of cloudy or stormy weather over the next several hours.
Filling a glass barometer isn’t easy because the liquid has to go through the narrow spout and up into the barometer. The best method I’ve found is to use the tube from a spray bottle. Just unscrew the trigger sprayer to remove it and pull the tube off. Put your liquid of choice (I use a mixture of water and blue Windex) in an empty bottle, hold the tube at the top and wrap a couple inches of tape around the neck of the bottle and the base of the tube to make a (mostly) watertight seal. Insert the end of the tube into the spout of the water barometer and carefully pour the liquid into the barometer. You will need to tilt the barometer several times to let bubbles of air out of the body and into the spout so they can escape and allow the liquid to go into the body.
Once filled, the body should contain be about one-third filled with liquid and two-thirds filled with air. However, the air inside must be completely trapped. The more trapped air there is inside the barometer, the more compression and expansion will occur and that will move the liquid in the spout.
Another tip: after your barometer is filled and functioning, you should calibrate it to normal atmospheric pressure. Normal atmospheric pressure is 29.92 inches (at sea level, less at higher elevations). A glass barometer doesn’t have any markings to give an exact pressure reading, but you can still give it some calibration. Find the normal barometric pressure for your location’s elevation. Then check your local weather website for the current barometric pressure reading. When the local atmospheric pressure is normal, calibrate your glass barometer by tilting it to transfer liquid between the spout and the reservoir until the level of liquid is the same in both. Once your barometer is calibrated, then you easily see that high pressure, e.g., higher than normal, is indicated whenever the level in the spout is below the level in the reservoir; low pressure is indicated whenever the level in the spout is above the level in the reservoir.
Something to watch out for: The air inside your barometer will also expand and contract due to changes in temperature in the surrounding air. If you want it to be as accurate as possible, you will need to keep your glass barometer in a room where the temperature stays constant. This would require more home cooling and heating than I want to pay for, so I just take my changes in my house’s inside temperature into account when I look at the water level.
* And also called the “Goethe barometer” (named for Johann Wolfgang Von Goethe, who helped popularize them).