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DB1 Liquid Level Measurement Sensor
1. Overview
The double bubbler is a self-calibrating system for liquid level measurement.
This manual focuses on water level measurement in stilling wells or open
channels, but the information applies to other liquids as well. The system
detects water level by measuring the pressure required to force a nitrogen gas
bubble from a submerged tube. The nitrogen pressure in the tube is equal to
the hydrostatic pressure created by the exiting bubble. Before each stage
measurement, the pressure transducer is calibrated by measuring the
hydrostatic pressure difference between two tubes installed a fixed vertical
distance apart in the liquid to be measured. These values are used to calculate
the multiplier value for the liquid level measurement. This self-calibration
minimizes the effects of temperature and long-term drift on the pressure
transducer, allowing a single pressure transducer to achieve a much higher
measurement accuracy (±.05%). The self-calibration theory is discussed in
detail by Dedrick and Clemmens (1984, 1986).
A single pressure transducer is used to measure three pressure lines (h1, h2, or
offset). The line to be measured is controlled by the three 2-way solenoid
valves on the manifold. The "bubble rate" is controlled by the two needle
valves on the manifold assembly labeled #1 and #2. The needle valve labeled
#1 controls the bubble rate for the pressure line used to measure h1. The
needle valve labeled #2 controls the bubble rate for pressure line #2 used to
measure h2. The needle valve is turned clockwise to decrease and counter-
clockwise to increase the bubble rate. The suggested bubble rate is 1 to 3
bubbles/sec. The third solenoid valve is used to measure atmospheric pressure,
which will be used as the offset value.
The manifold assembly requires considerably more power compared to a
normal sensor. For this reason, an SP10 10-watt solar panel or AC power in
conjunction with a sealed rechargeable battery is recommended for the DB1
system.
References
Dedrick, A.R. and A.J. Clemmens, Double-bubblers coupled with pressure
transducers for water level sensing, Transactions of the ASAE, Vol. 27, No. 3,
pp. 779-783, 1984.
Dedrick, A.R. and A.J. Clemmens, Instrumentation for monitoring water
levels, Proc. of Agri- Mation Conference and Exposition, ASAE, March 3-5,
Chicago, IL, pp. 148-152, 1986.
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