Description: A shaft encoder is a contact sensor that is driven by a float with a shaft and pulley, enclosed in a stilling well. Historically, a shaft encoder and stilling well was one of the most common methods of measuring water level. The components include a stilling well, inlet pipes from the water, float, tape, wheel and shaft encoder. Water comes into the stilling well through inlet pipes from the water source. As water level increases and decreases, a float moves up and down with the water level. A tape attached to the float turns a wheel which is connected to the shaft on the encoder. As the wheel turns so does the shaft. The shaft encoder will then convert the shaft rotations into an electronic signal (either analog or digital), which will be recorded by a data logger.
The shaft encoder and components are relatively inexpensive though the civil works for this type of station is among the most expensive of all civil works related to water level measurement. As with all water level sensors, maintenance is important, though the stilling well is slightly more labor intensive than other surface water sensors due the need to keep the inlets and well free of sediment and debris. The sensor itself does not typically require calibration, though it should be checked routinely and possibly reset to the outside staff gauge level. An ideal location for a stilling well utilizing a shaft encoder is a stable river channel, especially where the stilling well is installed. Stilling well/shaft encoder solutions work quite well on canals also
Costs USD Lower: $800
Costs USD Upper: $1,500
Accessories: float, tape, wheel, shaft, stilling well, staff gauge
Advantages: Shaft encoders are relatively inexpensive. The sensor does not require calibration. This application works well in reservoirs and channels that have a stable fixed channel.
Disadvantages: Civil works is relatively expensive. If not properly installed the counterweights and float can interfere with each other. At times, the tape that travels around a pulley can ""jump"" the pulley. This can occur during very rapid changes in water level. Not suitable for channels with heavy sedimentaiton or channel morphology
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