water level measurement hydrography skills set training course no. 27743 june 2012
TRANSCRIPT
WATER LEVEL MEASUREMENT
Hydrography Skills Set Training
Course No. 27743June 2012
AN EARLY HISTORYOriginated in Ancient Egypt (3000 year s ago)• - 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
• - 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
Options for Water Level Measurement
Manual Measurement
Float Operation
Submersible ‘Wet’ Pressure Sensor
Gas Purge Operation
Options for Water Level Measurement
Manual Measurement
Manual Measurement
Staff Gauge
Contact Gauges
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’
Manual Measurement
Cantilevered Weight Gauge
(Clear Ck at Golden, Colorado)
Manual Measurement
Maximum Height Indicator (Skokomish River, Washington).
Options for Water Level Measurement
Float Operation
Float Operation
Typical Shaft Encoders
Options for Water Level Measurement
Submersible ‘Wet’ Pressure Sensor
Pressure Transducers – ‘Wet’
‘Gauge’ Type(stainless steel)
‘Gauge’ Type(delrin/brass) ‘Absolute’ Type
Options for Water Level Measurement
Gas Purge Operation
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
Pressure Transducers – ‘Dry’
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
Bubble Unit Typical Installation
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)
Compressors – ‘Bubbler'
Gas Purge (Open) System ‘Bubbler’
Single Orifice / Single Line
Gas Purge (Open) System‘Bubbler’
Single Orifice / Dual Line
Gas Purge (Open) System ‘Bubbler’
Dual Orifice
Old Style (Closed) Gas System
‘Hydrostatic’ (Closed) Gas System
Compressors – ‘Hydrostatic’
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
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
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
Laser Systems • Changes in water level based characterised by intensities of
angular reflectance of light
Comparison of Methods Method Advantages Disadvantages
Gauge •Low cost optionEasily installed
•Need to engage a gauge readerManual data managementOngoing datum checks
Weight Gauge Alternative to staff gauges Ongoing maintenanceHigh installation cost
Dipping •Easy to set up •Inaccurate in some casesCost of electric dip tapesOngoing datum checks
Float Minimal ongoing maintenance ReliableAccurate
High establishment costsEnvironmental issues during installation phase‘Time of Lag’OH&S issues (working at heights, confined spaces)
Pressure - ‘Closed’Less expensive than float well installationMinimal maintenance
Siltation effectsLeakages not easily identified
Pressure – ‘Open’Less expensive than float well installationMinimal maintenanceProven and reliable systemWidely used
Siltation effectsMedium to high maintenanceMoisture and oil ingressLeakageGas cylinder issues (safety consideration)
Comparison of Methods Method Advantages Disadvantages
Compressor •Eliminates gas bottle transportation and storage (safety consideration)Minimal ongoing maintenance
•High establishment costsNeed to replace desiccant (in some types only)
Ultrasonic •AccuracyReliabilityWidely usedEasily installed
•Must be in a vertical position above waterDead ZoneEffects of foamEffects of air movement
Doppler •Positioned on surface or bed of streamProven reliabilityWidely used
•Bed mounted systems in accessible
Radar •Easily installed •Range < 35 metresMust be in a vertical position above waterDead ZoneFalse echoesNot suited for turbulent streamsMore suited to wide streamsAffected by electro-magnetic interference
Laser •Range > 150 metresAccurate in turbulent waterVertical alignment above stream NOT requiredNo false echoesWorks well on narrow and wide streamsUn-affected by electro-magnetic interference
•Requires clear line of sight•Affected by fog
Questions ?