Pressure transducers convert the hydrostatic pressure of water at a given depth into an electrical signal that can be recorded. Transducers are available in many different configurations that exploit a variety of properties of different materials or devices to accomplish this conversion. Submerged pressure transducers can be suspended in a stilling well or installed in a protective pipe which is perforated to admit water. The transducer is fastened in place, submerged below the minimum expected water level. To produce an output of gage pressure (i.e., referenced to ambient atmospheric pressure), the transducer is vented to the atmosphere via a vent tube integrated into the cable carrying the electronic output signal of the transducer. The free end of
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the vent tube should terminate in the instrument enclosure, and a desiccant should be used to prevent water vapor entry into the vent tube, as this can lead to corrosion of the transducer and errors in its output. A flexible bladder can be used as a desiccant replacement, provided that expansion and contraction of the bladder does not change the pressure (i.e., the pressure inside and outside the bladder must be the same)
Advantages of pressure transducers are
The relative simplicity of installation, since a stilling well is not required, and their accuracy, which can range from ±1.0 to ±0.1 percent of the maximum range that can be measured by the transducer.
Accuracy and cost are generally proportional. Disadvantages include
The need to maintain the desiccant .pack associated with the vent tube and the requirement to protect the transducer from freezing or remove it from service during the winter.
The most significant calibration issue for pressure transducers is avoiding drift of their output at zero pressure, since this can lead to relatively large percentage errors in flow rate at minimum discharge conditions.
Fouling of the opening to the transducer can also be a problem. 4.2.2.8 Pressure Bulb
This instrument consists of a flexible bulb that is placed in a perforated metal container for protection and connected by an air tube to a mechanical pressure gage and recorder or to a pressure transducer with an electronic output. The container and flexible bulb are fixed in place below the minimum water level to be recorded. Any change in water level changes the pressure inside the system and thus is recorded.
Advantages of this recorder are
The container and bulb do not require a stilling well and the distance between the bulb and the recorder may be up to 50 m (1 75 ft).Hence, the installation of the system is simple and relatively cheap while the recorder can be placed at a suitable location.
34 The major disadvantage of the pressure bulb is
The error in the recorded water level is generally ±2% of the maximum range that can be measured by the recorder. If this range, for example, is 1.0 m, the error in recorded head is ±0.02 m for all heads. As a result, at the minimum flow condition the measured flow rate can be rather inaccurate. Also, system leaks can cause operational failure.
Despite these disadvantages, the pressure bulb is very suitable for relatively temporary installations and sites at which the greatest accuracy is not necessary. A regular calibration between the staff-gage reading and the recorded water level is required for this type of instrument to maintain sufficient accuracy.
4.2.2.9 Bubblers
This instrument consists of a tube that is usually fastened with its open end at least 0.05 m below the lowest water level to be recorded. The tube is connected to a supply of air from a cylinder of compressed air or a small compressor and to a pressure gage or a pressure transducer plus a recorder. Air flows very slowly from the open end of the tube, and the pressure required to overcome the head of water above the end of the pipe is measured and recorded. The method by which the pressure is measured and recorded may be similar to that of the pressure bulb or may involve recent electronic devices. The advantages and disadvantages are somewhat similar to the pressure transducer system already described, except that the transducer is not submerged, so it need not be removed in freezing weather and there is less scaling and fouling of the transducer. Not submerging the transducer has proven to dramatically improve the reliability of bubbler systems compared to submerged pressure sensors.
Relatively long transmitting distances can be achieved with the bubbler system. Installations of 300 m have been used. On these long lines, it is best to use two small 3-mm inside-diameter tubes for economy and accuracy. One tube carries the bubble air supply from the source to the bubble outlet at 3 to 5 bubbles per second. The second tube is attached as a branch line as near as practical to the bubble outlet, preferably within 2 to 5 m. This second tube then senses the bubble pressure at the desired distance. Because there is essentially no flow in the sensing line after stabilization, there are no appreciable friction losses. On the source line, even 3 to 5 bubbles per second cause a significant pressure drop in several hundred meters, and thus the source line cannot also serve as the sensing line at long distances. Thermal gradients and pressure change in
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the sensing line may become significant only if large vertical distances are encountered. Figure 7.3 illustrates the schematic arrangement of a remote bubble gage.
Transmission distance is limited primarily by the allowable response time needed for a change in flow to be detected. The gage becomes more sluggish with increasing sensing line length because larger volumes of air must be moved to achieve a new stable pressure reading. For 300 m of 3-mm line (inside diameter), stability is usually reached in several seconds, depending on the volume sensing requirements of the pressure sensing gage. For example, a large-bore manometer requires more volume shift than a small pressure gage, but the manometer may be more sensitive.
Figure 4.4 Schematic for a remote recording bubbler system that is not sensitive to transmission distances up to 300 m.
Self-contained bubbler systems have been developed in recent years that integrate a small pressure compressor and optional pressure tank, the transducer, and associated electronics into a low-power unit that can easily function continuously on solar power (Figure 4.5). Another variation on the bubbler concept is the double-bubbler, in which bubbles are delivered alternately through two different tubes that terminate a fixed vertical distance apart in the water column. If the same transducer is used to sense the atmospheric pressure and the pressure in each tube, one can compensate for changes in the transducer calibration, producing a more accurate
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measurement. Accuracy of commercially available bubbler systems has improved in recent years and can now be on the order of ±0.003 m (±0.001 ft).