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WATER LEVEL MEASUREMENT Hydrography Skills Set Training Course No. 27743 June 2012.

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Presentation on theme: "WATER LEVEL MEASUREMENT Hydrography Skills Set Training Course No. 27743 June 2012."— Presentation transcript:

1 WATER LEVEL MEASUREMENT Hydrography Skills Set Training Course No June 2012

2 AN EARLY HISTORY Originated in Ancient Egypt (3000 year s ago) - monitoring of Nile River water level for flood prediction - monitoring of Nile River water level for flood prediction - approximately 20 recording stations (Nilometers) were located - Nilometer attached to temple walls, quays and the inside walls of temple annexes - approximately 20 recording stations (Nilometers) were located - Nilometer attached to temple walls, quays and the inside walls of temple annexes - units of measurement were cubits (the measurement from the elbow to the tip of the middle finger, approximately 450mm to 530mm) - units of measurement were cubits (the measurement from the elbow to the tip of the middle finger, approximately 450mm to 530mm) - modern day methods of measurement have evolved from this primitive method of water level measurement - modern day methods of measurement have evolved from this primitive method of water level measurement

3 Options for Water Level Measurement Manual Measurement Float Operation Submersible Wet Pressure Sensor Gas Purge Operation

4 Options for Water Level Measurement Manual Measurement

5 Staff Gauge Contact Gauges

6 Manual Measurement ZERO DIP DEPTH The following example shows how depth of flow can be calculated by this method in a sewer. 1.From a fixed point a measurement is taken to water level (ie the dip) 2.This reading is subtracted from a known measurement (ie the Zero) 3.Depth = Zero – Dip

7 Manual Measurement Cantilevered Weight Gauge (Clear Ck at Golden, Colorado)

8 Manual Measurement Maximum Height Indicator (Skokomish River, Washington).

9 Options for Water Level Measurement Float Operation

10

11 Typical Shaft Encoders

12 Options for Water Level Measurement Submersible Wet Pressure Sensor

13 Pressure Transducers – Wet Gauge Type (stainless steel) Gauge Type (delrin/brass) Absolute Type

14 Options for Water Level Measurement Gas Purge Operation

15 Gas Purge (Open) System Bubbler Basic Operating Principle dry nitrogen (or air) bubbled into the stream via a small diameter tube bubbles escape from the end of the tube (orifice) pressure transducer monitors changes in pressure within the tubing available as single or dual orifice/riverline options System pressure is proportional to water level

16 Pressure Transducers – Dry

17 Bubble Unit Operating Principle Uses dry nitrogen cylinder Provides a constant differential of 3-5 PSI (21-35 kPa) above the pressure head at the orifice Differential maintains a constant stream of bubbles at the orifice HS23 bubble rate pre-set at factory Eliminates risk of silicon oil entering riverline Quick Connect fitting available for check of bubble rate

18 Bubble Unit Typical Installation

19 Compressors Alternative to gas cylinders (Safety Issues) Effective air drying system essential - moisture ingress - aquatic growth Some limitations – riverline length < 200 metres - maximum head of 30 metres Low powered types available (12V: 38aH with solar)

20 Compressors – Bubbler'

21 Gas Purge (Open) System Bubbler Single Orifice / Single Line

22 Gas Purge (Open) System Bubbler Single Orifice / Dual Line

23 Gas Purge (Open) System Bubbler Dual Orifice

24 Old Style (Closed) Gas System

25 Hydrostatic (Closed) Gas System

26 Compressors – Hydrostatic

27 Ultrasonic Systems Ultrasonic pulses emitted by the transducer Pulses are reflected by the water surface and reflected back to the transducer Time from emission to receipt of the signals is proportional to the level in the vessel Mounted vertically above stream

28 Doppler Systems If the distance between the transducer and the reflecting object is decreasing, frequency increases If the distance between the transducer and the reflecting object is increasing, frequency decreases Water level measured by vertical acoustic beam Systems also capable of flow measurement

29 Radar Systems Extremely short microwave impulses are emitted by the antennae system to the water surface These impulses are reflected by the water surface and received again by the antenna system Time from emission to reception of the signals is proportional to water level in the stream

30 Laser Systems Changes in water level based characterised by intensities of angular reflectance of light

31 Comparison of Methods MethodAdvantagesDisadvantages Gauge Low cost optionLow cost option Easily installed Easily installed Need to engage a gauge readerNeed to engage a gauge reader Manual data management Manual data management Ongoing datum checks Ongoing datum checks Weight Gauge Alternative to staff gauges Alternative to staff gauges Ongoing maintenance High installation cost High installation cost Dipping Easy to set upEasy to set up Inaccurate in some casesInaccurate in some cases Cost of electric dip tapes Cost of electric dip tapes Ongoing datum checks Ongoing datum checks Float Minimal ongoing maintenance Minimal ongoing maintenance Reliable Reliable Accurate Accurate High establishment costs High establishment costs Environmental issues during installation phase Environmental issues during installation phase Time of Lag Time of Lag OH&S issues (working at heights, confined spaces) OH&S issues (working at heights, confined spaces) Pressure - Closed Pressure - Closed Less expensive than float well installation Less expensive than float well installation Minimal maintenance Minimal maintenance Siltation effects Siltation effects Leakages not easily identified Leakages not easily identified Pressure – Open Pressure – Open Less expensive than float well installation Less expensive than float well installation Minimal maintenance Minimal maintenance Proven and reliable system Proven and reliable system Widely used Widely used Siltation effects Siltation effects Medium to high maintenance Medium to high maintenance Moisture and oil ingress Moisture and oil ingress Leakage Leakage Gas cylinder issues (safety consideration) Gas cylinder issues (safety consideration)

32 Comparison of Methods MethodAdvantagesDisadvantages Compressor Eliminates gas bottle transportation and storage (safety consideration)Eliminates gas bottle transportation and storage (safety consideration) Minimal ongoing maintenance Minimal ongoing maintenance High establishment costsHigh establishment costs Need to replace desiccant (in some types only) Need to replace desiccant (in some types only) Ultrasonic AccuracyAccuracy Reliability Reliability Widely used Widely used Easily installed Easily installed Must be in a vertical position above waterMust be in a vertical position above water Dead Zone Dead Zone Effects of foam Effects of foam Effects of air movement Effects of air movement Doppler Positioned on surface or bed of streamPositioned on surface or bed of stream Proven reliability Proven reliability Widely used Widely used Bed mounted systems in accessibleBed mounted systems in accessible Radar Easily installedEasily installed Range < 35 metresRange < 35 metres Must be in a vertical position above water Must be in a vertical position above water Dead Zone Dead Zone False echoes False echoes Not suited for turbulent streams Not suited for turbulent streams More suited to wide streams More suited to wide streams Affected by electro-magnetic interference Affected by electro-magnetic interference Laser Range > 150 metresRange > 150 metres Accurate in turbulent water Accurate in turbulent water Vertical alignment above stream NOT required Vertical alignment above stream NOT required No false echoes No false echoes Works well on narrow and wide streams Works well on narrow and wide streams Un-affected by electro-magnetic interference Un-affected by electro-magnetic interference Requires clear line of sightRequires clear line of sight Affected by fogAffected by fog

33 Questions? Questions ?


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