LAS MkII ET System

• Complete solution

• Field-scale observations

• Easy to install

• Low maintenance

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LAS MkII ET System

The LAS MkII Evapo-Transpiration (ET) system is a complete solution for monitoring the energy balance within the boundary layer. The system comprises a LAS MkII scintillometer, net radiation sensor, meteorological sensors, data acquisition, and has several telemetry and power supply options.

It is specifically designed for field scale observations of the path average energy fluxes using the scintillometry technique.

Our EVATION® software suite is included as standard to view real-time data numerically and graphically as well as advanced data post-processing. Evation reads the ET system measurement data, but can also use files from from other types of data acquisition systems.

All the instruments and equipment of the LAS MkII ET system are selected and designed to offer easy installation and low maintenance operation.

For a full overview of the available configurations and options, please contact us using the Request Information button.

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FAQ

  • How can I calculate the Cn2 Saturation limit for my LAS MkI Scintillometer?
    • A scintillometer measures the path weighted structure parameter of air, Cn2, using an optical transmitter and receiver.

      In certain cases of relatively high Cn2 values the signal can become saturated depending on the diameter of the lens, wavelength and path length.

      The so called saturation limit for Cn2 can be derived using the following formula (Ochs and Hill 1982)

      Cn2 < 0.18.D5/3.L-8/3.λ2/6

      Where:

      D

      the diameter of the scintillometer

      [0.15m or 0.3 m]

      L

      the path length

      [m]

       

      λ

      the emitted wavelength

      [880 nm]

      In the calculation tool below, you can calculate the saturation limit for the LAS and X-LAS scintillometer as a function of Path length.

      Path length [m]
      Saturation Limit LAS (10cm) in m-2/3

       

      Saturation Limit LAS (15cm) in m-2/3

       

      Saturation Limit X-LAS (30 cm) in m-2/3

       

       

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  • How does WINLAS correct for an incorrect potentiometer setting?
    • In case the path length has been set incorrectly during the installation of an LAS scintillometer, WINLAS can correct the Cn2 data.

      In order to do so please use the following procedure.

      Step 1: 

      After entering the relevant info in the parameters section, enter the path length setting set with the potentiometer of the LAS receiver in meters and enter the correct path length in the input field below.

       

       Select OK

       Step 2:

       Select ‘Run…’  in the WINLAS file menu.

       Result:

      WINLAS will now process the Scintillometer Cn2 data using a correction algorithm for the actual path length.

       Theoretical background

       WINLAS corrects the path length in the following way:

      Using the following equation to derive the intensity fluctuation data from the recorded Cn2 values calculated by the LAS using the incorrect path length setting.

                                      (Wang et al., 1978)

       

      Where 

      D

      Aperture diameter    ~ 15cm



      L    

      Initial path length

      σlnI

      Variance of log intensity

       The equation is re-written to yield the variance of the intensity fluctuations:

                      

      And finally re-calculate Cn2 with the correct path length:

                      

      End of FAQ.

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  • Can I use an LAS Scintillometer over open water?
    • If a body of water takes up for a major portion of the measurement path, using a scintillometer is not recommended.

      A scintillometer is an instrument that can measure the ‘amount’ of scintillations by emitting a beam of light over a horizontal path. The scintillations ‘seen’ by the instrument can be expressed as the structure parameter of the refractive index of air (Cn2), which is a representation of the ‘turbulent strength’ of the atmosphere.

      The scintillations are mostly a result of temperature and water vapour fluxes from the earth surface. Scintillometers use an infra red light source and are designed to be primarily sensitive to scintillations from temperature fluxes.

      Over most surfaces over land, the temperature related scintillations are most dominant and the those from water vapour are less significant. This means that Scintillometer will yield reliable Cn2 values. In addition, if the so called Bowen ratio is known, additional correction can be applied to improve the measurement even further.

      However, over open water the scintillations from water vapour are dominant over temperature fluctuations. As a result, a scintillometer operating at infra-red wavelengths will underestimate values for Cn2. Although in principle the measurement could also be corrected using the fore mentioned Bowen ratio, this ratio is in many cases not accurately known over open water.

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  • How do I set the path length using the Potentiometer on the LAS MkI receiver?
    • Once the LAS has been installed and properly aligned the Path Length dial knob at the receiver control panel must be set for the correct distance between the transmitter and the receiver. The Path Length dial knob has 10 turns maximum with a vernier counter and a locking mechanism.

      These graduations are NOT in units of distance! The precise path length must first be converted to a dial knob setting (Pot) using the following relationship for the LAS. The equations below can be used to find the correct Potentiometer setting as a function of pathlength for the LAS and X-LAS.

      In addition you can use the calculation tool below to calculate the correct potentiometer setting for the (X)LAS.

      LAS:

                  

      X-LAS:

                 

       

      Path length [m]
      Potentiometer Setting [LAS]

       

      Potentiometer Setting [X-LAS]

       

       

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