Kipp&Zonen LAS MkII Scintillometer - Manual (Page 17)

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Please follow the instructions in this section carefully for the mechanical and electrical installation of the LAS MkII scintillometer.
Do not turn on power to the transmitter or receiver until instructed to do so.
Ensure that fixings and mountings are securely tightened when instructed to do so.
2.1 Included with the product
Check the contents of the shipment for completeness (see below) and note whether any damage has occurred during transport. If
there is damage, a claim should be filed with the carrier immediately. In the case of damage and/or the contents are incomplete,
contact your local Kipp & Zonen representative or e-mail the Kipp & Zonen customer and product support department at:
[email protected]
Note The LAS MkII is rugged, but it contains sensitive optical and electronic parts. Please keep the original packaging
to safely transport the scintillometer to measurement sites or for other shipments.
The following items are included with the LAS MkII scintillometer:
LAS MkII transmitter with pan and tilt adjuster and baseplate
LAS MkII receiver with pan and tilt adjuster and baseplate
2 × alignment telescope with detachable mounting, adjusted for each transmitter and receiver
2 × sun shield with two fixing screws
2 × 100 mm diameter aperture restrictor with fixing kit, for transmitter and receiver
2 × 10 m cable with 4-pin plug for transmitter signal output and receiver analogue connections
1 x 10 m cable with 8-pin plug for receiver digital communication connections
2 x 10 m cable with 4-pin connector for 12 VDC power input
2 x 3 mm hexagonal Allen keys, for fitting the sun shields
2 x 4 mm hexagonal Allen keys, for fitting the telescopes
1 x CD-ROM containing EVATION software and a pdf file of this LAS MkII instruction manual
8 x spare desiccant packs
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Throughout this manual the following symbols are used to indicate to the user important information.
General warning about conditions, other than those caused by high voltage electricity, which may result in physical
injury and/or damage to the equipment or cause the equipment to not operate correctly.
Note Useful information for the user
1.1 Product overview
The LAS MkII Large Aperture Scintillometer is an optical instrument with a 150 mm diameter beam that is designed for measuring
the path-averaged structure parameter of the refractive index of air (C
n
2 ) over horizontal path lengths from 250 m to 4.5 km 1 . When
the supplied 100 mm diameter aperture restrictors are fitted, the path length can be from 100 m to 1 km 1 .
LAS MkII uses a transmitter and receiver horizontally separated by several kilometres to measure intensity fluctuations in the
air known as scintillations. This is the same effect, but of much smaller amplitude, as the ‘shimmering’ of air over very hot or
cold surfaces that causes a mirage.
The scintillations seen by the instrument can be expressed as the structure parameter of the refractive index of air (C
n
2 ). The
light source of the LAS MkII transmitter operates at a near-infrared wavelength of 850 nm. At this wavelength the observed
scintillations are primarily caused by turbulent temperature fluctuations.
Therefore, C
n
2 measurements obtained with the LAS MkII can be combined with temporally and spatially coherent meteorological
observations of air temperature, wind speed and air pressure to derive the free convection sensible heat flux (H
free
). An accessory
meteorological sensor kit is available for this purpose, which connects to the LAS MKII receiver.
The LAS MkII is self-contained. The system can be locally configured at the receiver with a display and menu keys and has
internal digital signal and data processing and data storage. When the data is exported to the included EVATION software
package running on a computer the surface sensible heat flux (H) can be calculated.
Compared to traditional point measurement systems, the LAS MkII operates at spatial scales comparable to the grid box size of
numerical models and the pixel size of satellite images used in meteorology, hydrology and water management studies. The LAS MkII
has important applications in energy balance and water balance studies, because the surface flux of sensible heat is linked to latent
heat flux (L
v
E ) and evapotranspiration (ET). For these measurements a complete LAS MkII ET System is available.
This manual provides information related to the installation, maintenance, calibration, product specifications and applications
of the scintillometer.
If any questions should remain, please contact your local Kipp & Zonen representative or e-mail the Kipp & Zonen customer and
product support department at:
[email protected]
Please go to www.kippzonen.com for information about other Kipp & Zonen products, or to check for any updates to this manual
or software.
1 The maximum usable path length depends upon the atmospheric conditions, the path lengths given are for ‘clear’ conditions (visibility 10-20 km). In general, it is
best to use the scintillometer at full aperture at path lengths down to 250 m, and the aperture restrictors for shorter paths. However, for field-scale measurements
at a range of distances up to 1 km it may be convenient to leave the restrictors fitted.
17
Instruction Manual - LAS MkII Scintillometer
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2.1.1 The transmitter
The transmitter housing contains a very efficient, eye-safe LED operating at 850 nm wavelength in the near-infrared region. The
LED mounting is axially adjustable to position it at the focus of a Fresnel collimating lens. The near-parallel beam is output
through a glass window with an aperture diameter of 150 mm. There is a self-regulating heater for the window that can disperse
rain, dew, frost and snow. Electronics pulse the LED at 6.5 - 7 kHz and the drive signals can be monitored on the signal output
connector.
There is a baseplate with rugged and easy to use pan and tilt adjustment. A mounting rail on the top of the housing is used to
fit the alignment telescope or the white heat shield. There are two drying cartridges containing desiccant to keep the transmitter
dry internally.
The transmitter is powered by 12 Volts DC.
1.2 Key parts of the LAS MkII Scintillometer
1.2.1 Transmitter and receiver
The drawing shows the key common parts of the LAS MkII transmitter and receiver:
Sun shield fitted to the mounting for the alignment telescope
Tilt (vertical) adjustment screws
Pan (horizontal) adjustment screws
Transmitter window, with heater and Fresnel lens behind
Drying cartridges
Baseplate
1.2.2 Transmitter rear panel
The drawing shows the key parts of the LAS MkII transmitter rear panel:
Power indicator (red)
Transmitter power adjustment knob (remove screw-on cover)
Signal output connector (4-pin)
Power input connector (12 VDC)
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1.2.3 Receiver rear panel
The drawing shows the key parts of the LAS MkII receiver rear panel:
Display
Power and status indicator (green)
Menu navigation keys
Meteorological sensor kit connector (8-pin)
Analogue signal connector (4-pin)
Power input connector (12 VDC)
Digital interface connector (8-pin)
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2.1.2 The receiver
The beam from the transmitter enters the receiver through a glass window with an aperture diameter of 150 mm. There is a
self-regulating heater for the window that can disperse rain, dew, frost and snow. A Fresnel lens focusses the 6.5 - 7 kHz pulsed
radiation onto a very sensitive large-area photodiode detector with a thin-film optical filter that only transmits radiation in a
waveband around 850 nm, blocking ambient light from reaching the detector. The detector and filter assembly are axially
adjustable to position the detector at the focus of the lens.
Analogue electronics are tuned to the 6.5 - 7 kHz carrier wave and to the scintillation frequency band of 0.2 Hz to 400 Hz. These
signals are rectified and are available at the analogue signal connector. All the remaining electronics are digital. The system can
be locally configured at the receiver with a display and menu navigation keys and has internal signal and data processing and
storage. It can calculate and log Cn2 measurements and, with the accessory meteorological sensor kit connected, the sensible
heat flux (H) can be calculated and stored. External communication is via the digital interface connection.
There is a baseplate with rugged and easy to use pan and tilt adjustment. A mounting rail on the top of the housing is used to
fit the alignment telescope or the white heat shield. There are two drying cartridges containing desiccant to keep the receiver
dry internally.
The receiver is powered by 12 Volts DC.
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2.1.3 Alignment telescopes
Each transmitter and receiver has a telescope, individually adjusted to align with its optical axis, and each telescope is labelled
accordingly. These telescopes attach by clamps to mounting rails on the tops of the transmitter and receiver housings and enable
alignment of the transmitter and receiver at long path lengths. They are not completely weatherproof and should be removed after
alignment and replaced by the sun shields. A 4 mm hexagonal Allen key is supplied for the mounts of each telescope.
2.1.4 Sun shields
After alignment of the transmitter and receiver the telescopes should be removed and replaced by the sun shields. These are each
attached by two screws to mounting rails on the tops of the transmitter and receiver housings. A 3 mm hexagonal Allen key is
supplied for the fixing screws of each sun shield.
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2.1.5 Aperture restrictors
The full beam aperture of 150 mm enables operation over path lengths from 250m to 4.5 km, depending upon the atmospheric
conditions. For shorter path lengths, from 100m to 1 km, restrictors with apertures of 100 mm are fitted in front of the windows of
the transmitter and receiver.
2.1.6 Power cables
Two 10 m long cables with 4-pin waterproof plugs are provided for the transmitter and receiver 12 Volt DC power inputs.
2.1.7 Signal cables with 4-pin plugs
Two 10 m long cables with 4-pin waterproof connectors are provided. One for the transmitter signal outputs, the other for the
receiver analogue signal connection.
2.1.8 Signal cable with 8-pin plug
One 10 m long cable with an 8-pin waterproof plug is provided for the receiver digital interface connection.
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2.1.9 Allen keys
Two 3 mm hexagonal Allen keys are supplied for fitting the transmitter and receiver telescopes and two 4 mm hexagonal Allen keys
for fitting the sun shields.
2.1.10 CD-ROM
The supplied CD-ROM contains the EVATION software package and this manual as a pdf file.
2.1.11 Desiccant packs
Eight spare packs of self-indicating silica gel desiccant are supplied for the drying cartridges of the transmitter and receiver.
2.2 tools required
In addition to the items supplied with the LAS MkII scintillometer, the following equipment is required for performing the installation:
• At least two people
• Stable mounting bases for the transmitter and receiver or tripods
• Printed copy of the manual, from the supplied CD-ROM
• Tape measure for determining the installation height
• 2 x radios or mobile ‘phones for communication between transmitter and receiver operators
• 2 x sets of mounting bolts and suitable wrenches
• 2 x sources of 12 Volts DC power
2.3 Location and support base
When choosing the location for scintillometer measurements care has to be taken that certain requirements are met.
2.3.1 Path orientation and avoiding direct sunlight
Avoid locating the transmitter and receiver where direct sunlight may be in their views. The Fresnel lenses will focus the light onto
the transmitting LED and the receiving optical filter and photodiode and there is a risk of damage due to overheating.
It is recommended to select a path that is approximately parallel to the Earth’s surface (i.e. horizontal) and has a north-south
orientation to avoid any problems caused at low sun angles. If this is not possible, pick an orientation where some obstruction in
the background (buildings, trees) blocks the direct sun.
Direct sunlight close to the optical axes of the transmitter and receiver may permanently damage optical parts.
In order to maximise the footprint of the measurement, it is recommended to select a path between the transmitter and receiver
which is perpendicular to the predominant wind direction.
Ensure that the optical path between the transmitter and receiver is free of any obstructions.
2.3.2 Minimum installation height to prevent saturation
When the scintillation intensity rises above a certain limit the theory, on which the scintillation measurement method is based,
is no longer valid. When this occurs, the relationship between the measured amount of scintillations (σ
lnI
2 ) and the structure
parameter of the refractive index of air (C
n
2 ) fails. This phenomenon is known as saturation.
In order to prevent saturation, C
n
2 must stay below a certain saturation criterion (S
max
), i.e. the scintillometer can measure well
only under weakly scintillating conditions. The dependence of C
n
2 on the optical wavelength ( λ), the aperture diameter (D), the
measurement height (z
LAS
) and the path length (L) can be written as follows:
C
n
2 ( λ, D, z
LAS
, L ) S
max
The path length and the measurement height are the only variables that can be adjusted in order to keep C
n
2 below the saturation
criterion. A scintillometer installed at a height close the Earth’s surface; will see more scintillations than a scintillometer
installed high above the surface. As the path length increases more scintillation will be observed.
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This means that over long distances (several kilometres) the scintillometer must be placed high above the surface in order to
prevent saturation. Over shorter distances (several hundred meters) the scintillometer can be installed closer to the surface.
Over dry areas the surface sensible heat flux is large, resulting in higher C
n
2 values than over wet surfaces where the sensible
heat flux is small.
The graphs below show the minimum height of the LAS MkII for different surface conditions as a function of the path length. The
area above the curves in the figure is the so-called non-saturation zone. Below the curves saturation will occur. Based on the
user’s preferred path length, and the surface conditions of the area of interest, the user must install the LAS MkII scintillometer
at a height that lies in the non-saturation zone.
Full 150 mm aperture
Restricted 100 mm aperture
.
For example, a LAS MkII installed over a relatively wet area (h ~ 100 W/m²) and a path length of 3 km must be installed at a
height of at least 12 m.
Ensure that the LAS MkII is operating in the ‘non-saturation’ zone.
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2.3.3 Effective beam height
Determine the effective height of beam of the LAS MkII (z
LAS
) along the path precisely. The surface sensible heat flux (H) derived
from the structure parameter data is very sensitive to the height (see Appendix A). When the area is relatively flat and the beam
is parallel to the surface the effective height is easy to determine (z
transmitter
= z
receiver
= z
LAS
).
The path-weighting function is symmetrical and bell-shaped having a centre maximum and tapering to zero at the transmitter
and receiver end. This means that the LAS MkII is most sensitive in the middle of its path. For situations where the area is not
flat, or for slanted paths, it is recommend to measure the height of the beam at several points along the path.
The figure below shows how the weighting function must be used in order to estimate the precise height of the beam above the
surface for non-flat areas. The effective height calculator in the EVATION software package can be used to find the effective
height, as described in Appendix E. For more information see Hartogensis et al, 2003
1 .
The graphs above illustrate a LAS MkII path (beam) over a non-flat area. Based on the elevation map (surface) and the LAS
path-weighting function, an effective beam height of 46 m was calculated.
1 Hartogensis, O.K., Watts, C.J., Rodriguez, J-C. and De Bruin, H.A.R.: 2003, ‘Derivation of an Effective Height for Scintillometers: La Poza Experiment in Northwest Mexico’, J. Hydro-Meteorol. 4, 915-928.
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2.3.4 Operation in the constant flux layer
In order to derive the surface fluxes of sensible heat from the scintillometer measurements (C
n
2 ) we use the Monin-Obukhov Similarity
Theory (MOST) (see Appendix A). MOST is widely used in meteorology and is usually applied to the Surface Layer (SL) and hence is
sometimes called the Surface Layer Similarity. The SL is roughly the lowest 10% of the Planetary Boundary Layer (PBL).
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The PBL is directly influenced by the earth’s surface and its depth varies between roughly 100m and 2km. In general the PBL
increases during the day, when the Earth’s surface is heated by the sun, and decreases again during the night. Within the SL the
variation of fluxes (such as the sensible heat flux H and latent heat flux L
v
E ) is negligible with respect to the magnitude of their
value at the surface.
Therefore, fluxes measured at a certain elevation in the SL can be considered as being representative of the exchange processes
occurring between the Earth’s surface and the atmosphere above. The SL can be further divided into the Roughness Sub-layer
(RS), influenced by the structure of the roughness elements (e.g. plants, trees, buildings etc.), and the Constant Flux Layer
where fluxes are assumed to be horizontally and vertically constant (due to turbulent mixing). This means that measurement
techniques that apply MOST for estimating surface fluxes can be applied only in the Constant Flux Layer.
Therefore the LAS MkII must be installed at a height such that it is located above the Roughness Sub-layer and is measuring
within the Constant Flux Layer.
The depth of the SL typically varies between 20 m and 100 m. The upper level strongly depends on the diurnal cycle of surface
heating and cooling and the presence of clouds. Like the PBL, the SL increases during the day as the surface is heated by the sun
and is maximum at sunset (~100 m), before it decreases again due to cooling of the surface at night (~20 m).
The height of the Roughness Sub-layer, and thus the lower level of the Constant Flux Layer, depends strongly upon the size, form
and distribution of the roughness elements. Usually, over tall vegetation, the height of the Roughness Sub-layer is taken to be
equal to three times the obstacle height (or roughness elements h).
In case the estimated height of the Roughness Sub-layer is below the minimum height to avoid saturation, use a height that
ensures the LAS MkII is in the non-saturation zone.
Ensure that the LAS MkII is measuring in the Constant Flux Layer and in the non-saturation zone.
Note More detailed information about the theory of the scintillation technique can be found in Appendix A.
Note A list of symbols and abbreviations can be found in Appendix B.
2.4 Mounting
The LAS MkII can only function properly when the transmitter and receiver are precisely optically aligned. By mounting the
scintillometer on a stable support, signal loss and regular re-alignment procedures will be avoided. Vibrations in the mounting
structure must be prevented, which can lead to overestimated C
n
2 values. This particularly applies to masts or towers that can
bend and vibrate in the wind.
Mount the LAS MkII transmitter and receiver on stable and vibration-free supports.
Adjustable tripods are only suitable for short-term measurements. Where they are not on hard surfaces, a board should be used
to prevent the tripod legs sinking in and affecting the beam alignment.
If tripods are used, ensure that they cannot easily fall over and possibly damage the transmitter or receiver.
The pan and tilt adjusters of the transmitter and receiver have a baseplate. This has a central M16 thread to fit the mounting
bolts of industrial tripods, such as the accessory adjustable heavy-duty tripod package for the LAS MkII.
The baseplate has 4 slots for 10 mm bolts, on 184.2 mm diameter, which can be used for fixing the transmitter and receiver to a
customer-supplied support or to the accessory Kipp & Zonen tripod floor stand and/or height extension tube. The bottom view
of the baseplate is shown below.
2.5 Electrical and data connections
There are two waterproof connectors located on the rear panel of the transmitter and four on the rear panel of the receiver.
2.5.1 Power connector
The transmitter and receiver are each provided with a 4-pin waterproof plug fitted to a 10 m long cable that is terminated in
tinned wires, for 12 Volts DC (nominal) power to the instrument and heater.
Transmitter and receiver power connector and cable
The LAS MKII transmitter and receiver must be grounded through the shield of the power connector (also connected
to pin 4) for protection against lightning and electrical interference.
The LAS MKII transmitter and receiver power inputs should be protected by fuses.
Instrument power +, 1 A normal or slow-blow type. Heater power +, 4 A slow-blow type.
2.5.2 Transmitter signal output connector
The transmitter is provided with a 4-pin plug for signal outputs, fitted to a 10 m long yellow cable that is terminated in tinned wires.
Transmitter signal connector and cable