Changes in elevation and the onset of storms cause changes in barometric pressure. Stormy weather can produce pressure differences around 25mb (millibar) through a day, as different parts of the passing storm produce differing barometric pressure disruptions. One mb is equivalent to about one centimeter of water, this means a storm can cause almost 10 inches of water level error, a significant error over an area of, say, 30 feet.
It’s important to remember these pressure differences spread across the storm in varying degrees as well. If you are monitoring a large area with multiple water level monitors, some areas of the site may be under different air pressure loads during the same time period. If your sample site uses a series of absolute water level monitors without barometric compensation, the data you collect may be irretrievably corrupted.
Elevation also matters
Elevation also plays a role in barometric pressure compensation. When an absolute transducer is moved to a new elevation, corrections must be made internally because of the significant changes in barometric pressure.
- Barometric pressure grows by about 35mb for every thousand feet of elevation
Tell me about absolute water level monitors
Absolute water level sensors don’t use vented cables to automatically compensate for changes in barometric pressure. These often have one side of the sensing element exposed to the water and one side sealed. These systems require the purchase of an external barometric pressure monitor.
We don’t recommend passing up this additional tool, even if you’re working in an area with historically stable barometric pressure. A variance of even 5mb can corrupt your data, and pressure at sea level has been known to vary around 80mb through the year.
You will need to determine the elevation of the measurement site as well.
You can learn about all our groundwater data loggers and find the right one for your job on our website. Questions? Call us and we’ll get you sorted out.