Selection of secondary devices for the continuous measurements necessary to convert from stage to discharge is an important facet of developing an automated monitoring program. Important criteria for these secondary devices include:
• Wide measurement range
• Accuracy and precision over the entire range
• Minimal calibration loss with time
• Insensitivity to suspended solids in flow
• Capacity to internally convert stage to discharge
• Capacity to trigger an associated sampler
• Unattended operations
Secondary devices are divided into four categories: float-operated devices; ultrasonic devices; bubbler devices (manometers and transducers); and combination bubbler-magnetic devices.
In the simplest of designs, a float is connected to a strip chart or digital recorder via flexible steel tape. In most applications, float-type devices require a stilling well to damp out surges and rapid fluctuations in water surface elevation. In addition, most float-operated devices do not provide an internal stage-to-discharge conversion.
These secondary devices rely upon the travel time of an ultrasonic signal from a transponder to the water surface and back. This type of meter functions in a noncontact mode, and is therefore free from clogging and freezing. However, ultrasonics are sometimes subject to spurious signals from floating matter and foam. Some devices have internally programmable read-only memories (PROMs) and microprocessor circuitry to provide stage-to-discharge conversion using the unique relationships of the primary device.
In bubbler devices, gas is forced through a fixed orifice, oriented to assure that only static head is measured. The static pressure required to maintain a given bubble rate is proportional to the height of the water column above the
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