A barometer measures air pressure and can forecast the weather within a 12 to 24 hour period. Del Milan barometers measure in millimeters of mercury (mmHg) and hectopascals (mBar). These are worldwide standards. While some older barometers in United States use inches of mercury, this has been replaced and is no longer a standard measurement.
In order to tell whether air pressure is rising or falling, you must calibrate the barometer correctly. When you purchase a barometer, it must be adjusted before you can use it to accurately measure atmospheric pressure.
Setting the Barometer
Del Milan Italy Barometers are aneroid barometers. The aneroid barometer doesn’t use any liquids. It uses a small box made from beryllium and copper and spring that expands or contracts based on pressure changes. These motions cause mechanical hands to move pointing towards the current air pressure.
Obtain a local reading of barometric pressure. You will need calibrate the barometer to your location. Listen to a local weather forecast or search online to find the current barometer pressure for your location. Make sure the reading is correct for your location. Even a few km/miles can affect a barometer reading.
- Setting your barometer to your location will take into account differences in pressure caused by the altitude of your location.
- The factory setting for aneroid barometers is at sea level, but if you don’t live at sea level, you will need to calibrate it.
- Make sure you are using the correct measurement (hPa/mBar or mmHg). If you are in an area which reports the pressure in inches instead of mm, make sure to use an online converter to get the right reading. inHg is an old standard and should no longer be used.
- Set the indicator hand on your barometer. Locate the small adjusting screw on the back of your barometer. With a small screwdriver, turn the adjusting screw to move the hand to your location's current pressure. Watch the face of the dial and stop turning the screwdriver when the hand reaches the appropriate reading.
- Hang the barometer in a location that works for you. It makes no difference whether the barometer is hung on an inside or an outside wall. The pressure will be the same no matter where it hangs. Avoid locations that have frequent temperature changes such as near a bathroom or heater.
- Well-sealed and air-conditioned rooms are not as affected by changes in air pressure, so avoid these rooms if possible.
- Avoid a location that is exposed to direct sunlight as the temperature changes can affect the readings.
- Hang the barometer away from drafty locations, like near a door or a window. Air pressure is too variable in these locations.
Using the Barometer
- Set the manual hand to the current reading. Turn the barometer's center knob so that the arrow rests directly above the indicator arrow (this is the current barometric pressure for your location). You can identify the set hand by its notched arrow about halfway down the hand.
- The set hand will serve as a reference that allows you to easily tell if the pressure is steady, rising or falling.
- Check the barometer an hour later. Predicting weather using a barometer is all about changes in air pressure. You want to check the reading every few hours to determine if the pressure is changing or staying the same.
- Gently tap the face of the barometer to release any pressure changes stored in the mechanisms. Take the reading after the needle or mercury has stopped moving.
- Move the set hand if the pressure has changed so the next time you check it will be obvious what direction the air pressure is going.
- Chart the changes in pressure. Keep a journal of all the readings you take with your barometer. Sketch a small graph for the changes in a day to help with your forecasting. Is the pressure rising? Falling? Staying the same? This is all important information for predicting the weather.
- Do not expect large changes in the movement of needle. Daily changes are usually between 0.02 and 0.10 of an inch using the barometer scale. Larger variations may occur in winter and are dependent upon location and altitude.
- Take frequent readings (every few hours) and plot them on your graph.
Forecasting the Weather
Predict rain if the air pressure is falling. Generally, if the pressure is falling, the weather is taking a turn towards storms and rain. The starting point of the reading is also important in the forecast. Higher readings indicate better weather even if the pressure is falling.
Forecast improving weather when air pressure rises. As the air pressure rises, the weather tends to improve as the high pressure systems moves through your location.
- Forecast fine weather when air pressure is steady. Steady air pressure indicates long periods of nice weather and suggests that you will be experiencing more of the same. If it’s sunny and the pressure is holding, expect more sunshine! Higher pressures indicate warmer weather, while lower pressures indicate cooler weather.
Troubleshooting and questions
Why does the barometer measure in mmHg, hPa and mBar?
Del Milan barometers measure in millimeters of mercury (mmHg) and hectopascals (mBar or hPa). These are worldwide standards. While some older barometers in United States use inches of mercury, this has been replaced and is no longer a standard measurement.
Why isn't the needle moving?
Often, the needle won't move. This is very normal and is not cause for alarm. There are a number of reasons that a barometer needle won't move.
- Del Milan Barometers and other high end barometers are set to be "stiff". You will need to tap the glass to get the needle to move. This is because the direction of change is more important than the reading itself. (Read the "using the barometer" instructions above for more info)
- Heating and cooling affects the barometer. If you are in a room that is air conditioned or heated, the pressure is not changing much, and the needle won't move. Check the setup instructions for how to place your barometer.
- Air pressure can change very slowly - some areas have very slow pressure changes that may not register
- Small air pressure changes may accompany large weather changes - some areas the pressure does not change very much even if the weather is changing a lot
- Your altitude - if you are more than 3000' above sea level, your barometer may sometimes be out of range, and the needle will point low
- The needle can be stuck and needs loosening. Sometimes during transit, the mechanism can get quite tight. In this case, use a small screw driver and adjust the needle by turning the screw 1 and ½ turns to the left and then the right, do this repeatedly 4 or 5 times to help free up the spring.
- All aneroid barometers have a +-5% accuracy rating. If you are conducting scientific experiments, aneroid barometers are not accurate enough for you.
Why are my readings different to my local weather station?
Barometric pressure can be very different, even for locations that are quite near each other. Things like altitude, terrain, weather, and other factors can change the air pressure. You may pick up one reading in your house, and a different reading in your neighbours house. This is quite normal, so don't be alarmed if your pressure is not the same as your local weather station.
Here are some other resources for using your barometer
I think my barometer is faulty, how can I swap it for a different one?
Barometers are extremely complex and a variety of factors can affect it's accuracy. Additionally, Del Milan Italy barometers are made to the highest standards in the world, and have a 1 in 10,000 fault rate. We find that very often, if you think the barometer is faulty, usually there is something else that is causing a problem.
To help us troubleshoot your problem, please contact us with answers to the questions below and we can help diagnose your problem, or if we find that the unit is faulty, we can swap it over for you.
a) What is your location?
b) What is your altitude?
c) Please send us a photo of your barometer showing the reading and the reading from your local weather station at the exact same time. Please also send us a link to the source where you got your reading
d) Please send us a photo of where your barometer is situated in your house?
e) How far away is the barometer from doors or windows?
f) How far away is the barometer from heaters or air conditioners?
g) How many hours in the day do you have an air conditioner or heater going?
h) Is your house situated in a hill or valley?
i) Have you tried loosening the needle by quickly screwing the screw back and forth?
j) What level of accuracy are you expecting from the barometer?
k) Does the needle move when you tap the glass?