A barometer measures atmospheric pressure. This measurement provides valuable data for short-term weather forecasting, as pressure changes often precede shifts in weather conditions. To be useful, the instrument must provide readings that accurately reflect the current environment. Ensuring the device’s mechanism is precisely tuned to the local conditions is the first step in obtaining reliable meteorological data.
Understanding Why Barometers Need Adjustment
Most consumer barometers are aneroid types, relying on a sealed metal capsule to expand and contract with air pressure changes. Over time, the mechanical linkages can experience “drift,” a slow, gradual change that causes readings to become consistently inaccurate. Physical shocks, such as dropping the instrument or moving it to a new location, can also cause the mechanism to settle slightly out of calibration. Temperature fluctuations introduce stress on the metal components, contributing to the need for periodic adjustment. Traditional aneroid barometers require occasional mechanical correction to maintain accuracy, especially if readings seem consistently higher or lower than expected, or after the barometer has been moved to a new altitude.
Sourcing Accurate Local Reference Pressure
Before any physical adjustment can be made, obtain a precise, current reference reading for your location. The value needed is “Station Pressure,” which is the actual, uncorrected atmospheric pressure at the elevation of the reporting site. This is distinct from the “sea-level pressure” reported in general weather forecasts. The most reliable sources for this data are the National Weather Service (NWS) website or local airport weather reports (METARs). These professional sources provide real-time atmospheric data taken with highly accurate, standardized instruments. Ensure the reference pressure is current and the reporting station is geographically close to your barometer’s location for the highest precision. Using the station pressure ensures you are calibrating the barometer to the air pressure physically pressing down on the device at its specific altitude. The reference pressure must be in the same units as your barometer, typically inches of mercury (inHg) or hectopascals (hPa).
Step-by-Step Mechanical Adjustment
Once the accurate station pressure has been sourced, the physical calibration can begin. The adjustment mechanism is typically a small brass screw located on the back or underside of the instrument housing. This screw directly controls the tension on the aneroid capsule mechanism and the position of the indicator needle.
Use a small, correctly sized screwdriver to engage the adjustment screw. Movements must be extremely slight; turning the screw by more than a quarter-turn at a time can easily overshoot the required setting and stress the delicate internal components. The goal is to move the main indicator needle to match the reference station pressure value.
After making a small turn, gently tap the glass face of the barometer two or three times with your fingertip. This action allows the mechanical linkages and pivots inside the instrument to settle into their new position and overcome internal friction. Without this tapping, the reading may appear inaccurate.
Check the position of the main needle against the reference pressure. If the needle still does not align, repeat the process of making a very small adjustment to the screw, followed immediately by lightly tapping the glass face. Continue this iterative process until the needle rests precisely on the target station pressure value.
The second needle, often called the “set” or “memory” hand, is manually positioned by the user via the front glass knob to mark the last pressure reading, allowing for easy observation of pressure trends over time. It plays no role in the mechanical calibration process.
Distinguishing Calibration from Sea Level Correction
Users often confuse the mechanical calibration process with sea level correction. The adjustment performed in the prior steps makes the barometer accurate for the specific air pressure at its current physical location, which is the definition of station pressure. This value is only useful for observing local pressure changes.
Sea Level Pressure (SLP) is a calculated value where the station pressure is mathematically corrected to reflect what the pressure would be if the instrument were at sea level. Meteorologists use SLP because it eliminates the altitude variable, making it possible to compare pressure readings across vast geographical areas and create standardized weather maps.
Many high-end barometers include a separate mechanism for applying a fixed altitude correction to show SLP, but this is a separate calculation and adjustment. The primary mechanical calibration ensures the instrument is reporting the correct station pressure, and the SLP adjustment, if available, converts that accurate reading into a standardized format for forecasting comparison.