Absolute Vs. Vented Pressure


The Solinst Levelogger is based on an absolute pressure transducer. This means the Levelogger senses and presents all the pressure it is under, both water pressure (hydraulic head) and air pressure (barometric or atmospheric pressure). However, most users are interested in just the water level or hydraulic head, so Solinst offers a separate logger, the Barologger, to sense and log just barometric pressure and use its data to barometrically compensate the submerged Levelogger readings. The barometric compensation is performed in a simple Data Compensation Wizard in the Levelogger software. As a rule of thumb, one Barologger can be used to compensate all the Leveloggers in a 20 mile (30km) radius, so often a project will need just one Barologger to compensate all the deployed Leveloggers.

The main alternative method of barometric compensation is the use of vented cabling where a very small vent tube runs down the length of cable from the surface and terminates behind the pressure transducer. The vent tube acts as a conduit for barometric pressure change at the surface and allows the barometric pressure on the water column to be cancelled out by the pressure transmitted in the tube.

Solinst and its staff have considerable experience in the water level monitoring field with both absolute and vented technologies and chose the absolute method for the Levelogger because:

solinst levelogger barologger

  • It enables a less costly suspension cable or direct read cable deployment than that required for a vented communication cable.
  • The use of a Barologger produces very accurate water level data.
  • The barometric data can be used to determine barometric efficiency, which can be very significant, especially in confined aquifers. Vented pressure sensors assume 100% barometric efficiency and provide no data to investigate otherwise.
  • The absolute method is practically maintenance and trouble free, whereas the use of vented cable requires vent tube maintenance and assessment, and usually the purchase of hydrophobic membranes and desiccants. It requires vigilance by the user to ensure the integrity of the barometric compensation method.
  • Vent tubing can crimp, become flooded or blocked by moisture, debris or provide a conduit for water to enter the instrument. It creates uncertainty as to whether the barometric compensation system is functioning correctly.
  • The absolute method is one reason why the Levelogger’s factory calibration is expected to last the lifetime of the instrument. Vented transducers may require recalibration.

However, when the vented tubing is cared for, inspected, and tested regularly, it responds quite well and can provide very accurate readings, especially in shallow applications. Real-time data that is automatically barometrically compensated is an advantage of vented pressure transducers. There is minimal post collection data processing, which can save time.

Ultimately, due to the simplicity and economy of use, the added data tools provided, the long-term maintenance savings, lifetime calibration expectancy and equipment peace-of-mind inherent in the absolute method, Solinst will continue to use this method of barometric compensation for Leveloggers.