10 Manual Barometric Compensation

This section describes how to perform manual barometric compensation on Levelogger data files when a Barologger was not dedicated as a barometric recorder. This section describes available sources of barometric record, conversion factors for common barometric units to water column equivalent and how to perform the compensation in spreadsheet formats using available barometric information. Finally, this section describes the most accurate and representative methods of obtaining barometric data and performing barometric compensation on submerged Levelogger data.

For short term tests during which the barometric pressure varies insignificantly, the collection of continuous barometric data may be unnecessary. In this event, take a reading from an open air exposed Levelogger prior to running the short term test and record this level. This level represents the barometric pressure. Similarly, at the end of your test, take another barometric reading and record this measurement. After the submerged Levelogger data has been exported to a spreadsheet program compensate your submerged Levelogger data files for barometric pressure. If no appreciable change in barometric reading occurred, you may write in the first cell of a new column a simple calculation that subtracts the water column equivalent of your barometric reading from the submerged data file, then copy and paste this calculation to all the cells in that new column. The new column will represent the barometrically compensated liquid level.

Barometric data can be collected on site using a recording barometer or from a local weather station. If setting up the barometer, set the recording interval to that of the Levelogger sampling interval or some multiple of the Levelogger interval. To compensate submerged Levelogger data using barometric data collected from an on site barometric logger or a nearby weather station several steps must be taken:

Export both the Levelogger data file and the barometric file to a spreadsheet.

In the spreadsheet, convert the barometric data column from its barometric measurement units (typically atm, mm Hg, psi, mb or kPa) to feet or meters of water column equivalent using the conversion factors in Table 10-1.

Then write another calculation in another column to subtract the Levelogger’s pressure zero point offset value from the converted barometric data. The Levelogger’s zero point offset is 31.17 ft (9.5 m) of water column less an altitude correction. 31.17 ft or (9.5 m) is the lowest expected barometric pressure at mean sea level. As elevation increases above mean sea level, the lowest expected barometric pressure decreases at a rate of approximately 1.21/1000 ft or m (altitude/826) of altitude. Therefore the Leveloggers barometric pressure offset at 1500 ft elevation will be:

(Zero Point Offset at Ø MSL) –
(Altitude Correction)
= (Elevation-Corrected Zero Point Offset)
(31.17) – (1500/826) = 29.35 ft

The Levelogger Gold automatically adjusts to this elevation-corrected zero point offset when the altitude is input into the programming setup accurately. The altitude used to calculate the altitude correction MUST be the altitude input in the altitude field of the Levelogger setup. Therefore, if the altitude at time of the above Levelogger setup was uncertain and an altitude value of 1200 ft was input in the altitude field, then 1200 ft must be used to calculate the elevation-corrected zero point offset. The elevation-corrected zero point offset is subtracted from the barometric pressure expressed in water column equivalent to determine the amount of barometric pressure the Levelogger is recording above its elevation-corrected barometric pressure zero offset.


For previous Levelogger versions use an altitude correction of 1:1000.

This net barometric pressure expressed as water column equivalent is the amount of barometric pressure the Levelogger is currently sensing. This value is then subtracted from the Levelogger submerged data file. Example 10.1 provide a case study of how manual barometric compensation is performed.

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When analyzing barometric data it is important to keep in mind that storm events commonly reduce total atmospheric pressure by about 1.7% from pre-existing high pressure conditions. 1.7% converts to approximately 0.6 ft or 0.2 m of water level equivalent barometric fluctuation.

The most accurate and reliable method of obtaining representative barometric compensation of level readings is to employ an accurate surface barometric recorder (ie. the Barologger). The traditional convention has been to use a vent tube reaching from the surface to the transducer. This vent tube terminates behind the transducer diaphragm, theoretically filtering barometric effects on level readings from total pressure. After much experience with erroneous barometric compensation due to crimped, damp, wet and cut tubing, unacceptably slow response to small barometric change and many situations where barometric compensation was not required, we decided vented tubing did not offer the level of integrity in barometric compensation our clients deserved. A recording barometer does and will provide the data necessary to access barometric compensation requirements.

Vented tubing has been used in the past for several reasons. One it was a cheap, low tech method of balancing out the effect of barometric pressure on a monitored water surface. Second, when the tubing is cared for, inspected and tested for failure regularly, it responds reasonably well to steep barometric gradients such as when a large scale atmospheric front moves across the surface. The seldom discussed problem is that the response time to the much more common incremental barometric change is unacceptable. The vented tubing often leaked or its connection to the pressure probe leaked, causing the transducer to fail and experience irreparable damage. Some manufacturers, recognizing the inherent inferiority of passive venting, have adopted a method of automatic vent tube testing whereby a small vacuum pump at the surface constantly clears the vent of water and in some cases provides a warning when the integrity of the tubing has been compromised. This elaborate method of making a cumbersome technology work, requires significant battery power to operate. The use of a Barologger as an on-site barometer or the use of local weather station barometric data is less expensive and provides more reliable results than the real hidden costs associated with the use of vented cable technology to provide barometric compensation.