A laser-based water level sensor and robust insulated and heated ...

A laser-based water level sensor and robust insulated and 

heated multi-well stilling well system for ice-prone temperate 

and sub-arctic sub-arctic environments. 

Steve Forbes (presenter) (Director, Canadian Hydrographic Service Atlantic) 

Phillip MacAulay (Head, Tides and Currents CHS Atlantic) macaulayp@mar.dfo-mpo.gc.ca 

Chris Coolen (Multi-disciplinary Hydrographer CHS Atlantic) 

Fred Carmichael (Tidal ( Technician CHS Atlantic) ) 

Sub-arctic, , ice-prone p 

environment 

Nain tide station, Labrador 

(on a nice day) 

Insulated, heated, 

multi-well stilling well 

Water level measurement 

systems/sensors 

4 

3 

2 

1 

0 

data quality is only as good as 

collection systems 

R R 

A

Introduction 

• Tide gauges measure water levels (the obvious) 

• Uses of tide gauge data 

- Basic sounding reduction 

Hd Hydrography h [

CHS Atlantic’s Permanent Water Level Network 

(many are ice-prone) 

16 tide gauges, 000’s kms of coastline 

1430 

Long Term Sea Level (GLOSS) 

Multi-well Laser systems - 08 

New Multi-well Laser Sites - 09 

2145 

0065 

0365 

2000 

1805 

0490 

1700 

0491 

1680 

0612 

0665 

0755 

0990 

0905 

0835

Water level sensors (no magic solution) 

Total error estimates ignoring stilling well effects 

(Perspective) Wharf thermal expansion, expansion order +/- +/ 5mm 5 mm 

$.2K $.5K $1K 

+/- ~5-10 cm +/- ~2-3 mm +/- ~2-3 cm 

1) Tide Board 

4 

2) Tape Drop: standard t d d 

device used to set relative 

sensors, (sensors with offsets) 

CHS sensors 

$5K $.8K $1.5K $5K $5K 

+/- 1% +/- 1% +/- ~2-3 mm +/- ~1-2 cm 

+/- 1% 

~2-3 cm ~2-3 cm 

~2-3 cm 

3) Float and PPulley lle 4) Bubbler 

5) SSubmersible bmersible pressure press re 6) Laser sensor: 7) Radar sensor: 

CHS Legacy sensor Sutron continuous fast 

response bubbler 

sensor Spectre Sensor Dimetix DLS-B15 

pump 

P atm 

8) Acoustic sensor: 

Single 3 manual 

Sound speed 

measurement Mech chain and Water density Relatively untested 

variation 

2 

weight errors 

variation stilling well required 

C(t,h, …) 

1 

0 

Stilling gwell required q – R 

Stilling well Optional – O 

Relative Measurement – R (must be set relative to a known) 

Absolute Measurement – A (direct measurement) 

h 

o 

o 

o 

o 

P= wgh 

o 0 0 0 

P=wgh R, A R, R 

O, R O, R R, A 

Surface integration 

measurement 

bias 

O, A 

O, A 

Require stilling well in the 

presence of ice

The stilling well (a necessary evil) 

• Provides sensor protection and act as an analog mechanical 

filter via small dia entrance to large dia pipe (by area ~1/100 to 1/150) 

- Frequency dependent dependent filter filter, stills high high frequency frequency water motions 

• Filter parameters change with fouling/plugging example 

• Can be expensive to install and replace 

• Requires significant significant vertical vertical infrastructure infrastructure for for support 

support 

• To achieve true independence each sensor requires its own well 

• Known potential sources of error 

- water density inside well may may not not be/stay the same as outside resulting 

in different water surface level inside vs outside the well example 

- Inlet/exit flow non-linearity can introduce level offset under active 

wave conditions (in-out ( flow resistance hysteresis y effect) ) 

• Cold weather issues 

- direct damage of well by ice (crushing) Nain well failures in 02 and 08 

- Ice formation in water in well (freezing) ( g) 

- ice/frost buildup above water along inside of well (not a frost-free appliance) 

- heating costs for un-insulated systems 

All persistent Nain problems

Stilling Well Filtering (a relative example) 

Data from three sensors, , laser, , bubbler, , submersible pressure p sensor, , 

in three wells, same location, one well experiencing brief period of minor plugging 

pressure 

sensor 

bubbler 

laser 

phase lag 

amplitude 

reduction 

Well developing minor plugging (dirty harbor): amplitude 

Well developing minor plugging (dirty harbor): amplitude 

reduction and phase lag develop for 20 min period wave energy, 

energy at longer tidal periods uneffected

Stilling Well Entrainment of Fresh Water 

Example, extreme rain event late in winter in the presence of significant pack 

ice leading leading to to deep deep penetration penetration of of fresh fresh water and infiltration into stilling well 

According to float and pulley sensor 

water level inside well rises 

All three sensor 

read the same 

Fresh water 

infiltration 

WL According to bubbler and pressure 

sensor reading pressure at depth 

Water surface level potentially not equivalent 

inside and outside the well 

(error persistent until well is physically flushed)

Heated Multi-well stilling well 

Thermostat 

Heat tape 

line water 

6) 

1) 14” 14 HDPE tough tough protective outer outer casing casing ( (~1 1 inch thick) 

2) 6” PVC Main well for laser float or float and pulley 

Thermocouple 


3) Three 2” satellite wells, 

3) 

– t two with ith stilling tilli orifices ifi for f a backup b k bubbler b bbl 

sensor and gnomen tape drop 

– one free flowing for a fast response pressure 

5) 

4) 


1) 

4) Two 1-1/4” heater tubes filled with environmentally 

2) 

friendly anti-freeze 

5) Insulation, marine grade slow expansion polyurethane 

2lb/ft foam foam from top of well to below low low water 

water 

6) 120 v self-regulating (heat output drops as temperature 

rises) submersible heat tapes (10 W/ft) in heater tubes, 

with additional thermostatic control 

(shut off at ~ 6 C) 

1) ) 

4) 

Pros: 

• Heating distributed along full length of well 

prevents both ice and frost frost formation 

• Insulation reduces heating costs 

• Multi-well design requires only one installation 

for multiple truly independent sensors 

5) 

3) 

2) 3) 

3) 

4) 

Low w

Well Construction Overview 

1) Assemble and align pipe bundle bundle using 

316 SS hose clamps 

2) Construct and fit foam spacers at predetermined 

(~ 28 inch) intervals along 

pipe bundle bundle, secure in place with 

polyurethane spray foam 

3) Drill 2 inch holes in outer HDPE pipe 

to correspond with mid points 

b between f foam spacers 

4) Slide pipe bundle into outer casing 

5) Spray in slow-expansion polyurethane 

foam thru 2 inch access holes starting 

starting 

at one end of pipe and working 

sequentially to other end. Use lots, 

bleeds past spacers filling all voids, 

excess expands expands out access holes 

Fully assembled 25 ft multi-well 

- Cost ~ $ 2.5-3 K 

- Weight ~ 3 - 400 lbs, easily 

t transported t d on l long trailer, t il possible ibl to t 

carefully man-handle into place 

without heavy machinery 

2) 2) 

) 

1) 

4) 

3) 

5)

Laser Sensor 

• Dimetix DLS-B15 Laser (+/- 1.5 mm accuracy) 

• Adjustable laser leveling mount and leveling-in 

reference plate (laser 0 ref is to front face): laser 

is leveled leveled in in from from bench bench-marks marks to ref plate 

• Sutron Xpert logger using standard RS232 

input/decoder software block (6 second sample 

repeat period, 1 minute average) 

• Laser Float: 1” PVC foam board, 5” diameter, 

rounded edges (spec grav ~ .6) floats ~8mm 

above water surface 

- Edges grooved to improve water shedding 

- Embedded rare-earth magnets for float 

placement and retrieval (see float tool) 

- Top p surface p painted to p prevent laser p penetration 

into PVC and to thus provide clear reflection 

• Potential Improvements 

- Thicker float (1-1/2 or 2 inch) greater above 

water ater height ~12-16 12 16 mm 

- Electrostatic paint application for smoother top 

surface (surface must be smooth over length 

scale of laser dot for data-spike-free operation) 

Ref plate 

Float placement tool 

steel disc with eye bolt

Weight g enters water 

F+P Float sinks (data offset low) 

Weight enters 

water 

Laser vs Float and Pulley 

(weight entering water error) 

Laser 

Fl Float t and d pulley ll (F+P) 

Chain and weight wet 

F+P Float time lag (data offset high) 

Weight exits 

water

Nain Installation (piling mount) 

Installing 

Stilling well 

Laser spot 

Laser Float 

(laser on) 

Heaters and 

heater control 

Heater tube 


Bubbler 


Tape 

drop 

Tape 

drop 

Stilling 

well 

Pressure 


Heater tube

St John’s Installation (wharf mount) 

heater 

Tape 

Bubbler drop 


hheater t 


Pressure 


Tape 

drop

Nain Sensor Comparison 

Each sensor independent, in own stilling well 

Laser checked against g tape p drop p( (in own stilling g well) ) time-averaged g test ggood 

to +/- 1 to 3 mm 

Bubbler and pressure sensor least-squares calibrated to laser for slope (water density) and offset 

Errors between sensors explainable based on stilling well behavior: pressure sensor in well with little 

stilling, bubbler exhibits minor spiking at ~15 min interval (software bug in auto-zero process, 

manufacturer to provide p fix )

Summary 

• Two example insulated and heated multi-well installations operational 

for 8 months: only minor teething issues experienced 

• Following multi-well installation heating costs at Nain Labrador 

significantly reduced 

reduced 

• Multi-well system relatively simple, inexpensive, and easy to construct 

and install: system robustness, longevity and system maintenance 

issues still to be determined 

• Laser-float sensor accuracy appears stable at ~ 2-3 mm, further 

confirmation required but clearly better than a standard float and 

pulley system: minor data-spiking (occasionally a few data spikes per 

day, easily cleaned) suspected source, float surface imperfections. 

Suggested solution, thicker float, better painted surface QC 

• Stilling well errors still a factor: a possible solution to counteract 

inside inside-outside outside water water density density generated generated errors errors is under consideration 

consideration

A laser-based water level sensor and robust insulated and heated ...

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