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 (C_n²), 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 C_n² 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 C_n². 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.

In case the path length has been set incorrectly during the installation of an LAS scintillometer, WINLAS can correct the C_n² 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 C_n² 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 C_n² values calculated by the LAS using the incorrect path length setting.

                                (Wang et al., 1978)

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

And finally re-calculate C_n² with the correct path length:

End of FAQ.

A scintillometer measures the path weighted structure parameter of air, C_n², using an optical transmitter and receiver.

In certain cases of relatively high C_n² values the signal can become saturated depending on the diameter of the lens, wavelength and path length.

The so called saturation limit for C_n² can be derived using the following formula (Ochs and Hill 1982)

C_n² < 0.18^.D^5/3.L^-8/3.λ^2/6

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

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:

The LAS and X-LAS scintillometer can be connected to the CR10x and CR1000 data loggers from Campbell Scientific.

Configuration examples for these data loggers can be viewed on this page, here:

CR10x

CR1000

Campbell CR10x Data Logger

A (X)LAS scintillometer can be connected to a Campbell CR10x data logger using a 2:1 voltage divider like VDIV2.1 from Campbell. The reason for this is the fact that the LAS has an output of 0..-5V and the standard input range of the CR10x is ± 2.5V. 

The procedure is as follows:

1. Upload the example CR10x configuration file (or your own) to the data logger. Please note that this will erase pre-existing data! The example configuration file can be downloaded here: LAS_CR10x.CSI.
2. Connect the LAS to the CR10x using the following wiring diagram:

Collect data using the data logger and check for normal operation of the Scintillometer data collection.

Campbell CR1000 Data logger

The connection procedure for the CR1000 is similar to the CR10x. The same connection to the terminals for differential measurement of the LAS signals can be used. However, the input range of this data logger is ± 5V so a voltage divider is not required.

A example configuration for the CR1000 in LoggerNet format can be found here: LAS_CR1000.CR1

End of FAQ.