Ground Water

Ground water level measurements can be done with an absolute pressure logger or an external differential pressure logger. 

absolute pressure logger and external differential pressure logger

Absolute Pressure Logger.
An absolute pressure logger can be installed in a test well or bore hole by attaching the logger to a wire (of non corroding material) and placing the logger at the appropriate depth, which is below the expected minimum water level. Additionally an barometric reference logger is needed to measure the air pressure. For short pumping tests under stable barometer conditions it is possible to work without the barometric reference logger. Because the logger is only 4.7 cm in diameter is easy to install it in a test well or bore hole. If the diameter of the bore hole is smaller than 4.7 cm you can use an external differential pressure logger of which the sensor is only 17.5 mm in diameter.



External Differential Pressure Logger.
The Tirtaharapan pressure logger equipped with an external differential sensor is typically used to measure inside small bore holes. Due to its very small size the sensor will fit in a 1" (2.54 cm) bore hole. The sensor is connected to the logger electronics through a vented cable, the logger can so be placed at the widened top of bore hole. In this configuration the data in the logger can be read without disturbing the installation, this can be important if the data must read very often.

The length of the vented cable must be specified when ordering the logger. The length can not be changed afterwards. The air pressure side of the sensor must be sealed to avoid water entering the vented cable, this can seriously damage the sensor or at least diminish the reliability of the measurement. The seal covering the air pressure side of the sensor must be flexible to make sure that when the atmospheric pressure is changing the pressure in the vented cable will change accordingly, if not this will make the measurements worthless.

The seal can overcome a pressure change of about 50mBar (larger deviations are not common in the atmosphere) however moving the logger from 0 meter to 1000 meter altitude will give a pressure difference of approximately 100 mBar. The logger should not be moved to a different altitude after the seal has been applied to the logger.