FAQ

  • Atmospheric Science Instruments

      • Scintillometers

        • 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.

        • 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.

        • 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

             

             

        • 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]

             

             

        • How do I connect an (X)LAS MkI to a CR10x or CR1000 data logger?
          • 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.

        • Brewer Spectrophotometer

          • When and how should I clean the Brewer’s azimuth tracker drive?
            • The Brewer azimuth tracker has a driving mechanism based on friction between the drive shaft and the drive plate. These items will get dirty over time and the azimuth tracker is likely to slip. This can be noticed by a tracker discrepancy after AZ or SR tests.

              The drive mechanism can be cleaned by using a clean lint-free cloth with alcohol or with “garage” soap (soap with grains of sand in it). Switch the tracker power off! Remove the rear tracker cover (the cover on the side opposite to the power switch). Use the cloth with alcohol or soap to rub the dirt off the drive plate and the drive shaft. Rotate the tracker to clean the entire drive plate. Be careful not to break the wire of the safety switch. After cleaning the entire drive plate and shaft, rub them once more with a dry piece of clean lint-free cloth to remove any remaining residue of soap/alcohol.

              When this is done, rotate the tracker to aim the Brewer approximately at the sun . Put the tracker cover back on and switch on the tracker power. Now the Brewer needs to perform some tracker resets. In the Brewer software, type “PD AZ SR 10” to perform these resets. Watch the data to check that the tracker resets without discrepancies and then put the Brewer software back into its normal schedule.

          • How does the Brewer compensate Ozone measurements for temperature?
            • The Brewer instrument is capable of operating in different conditions from the tropics to the Antarctic. As the Brewer is used outside the whole year round, its Ozone measurements should not have any temperature dependency.

              During the factory testing of the Brewer it undergoes a test in the temperature chamber from 0°C to +45 °C. Standard Lamp measurements are taken throughout this entire temperature range. This is a simulated Ozone measurement based on the halogen lamp inside the Brewer. Although the intensity of the lamp does change with temperature, the wavelength shift is negligible.

              After the temperature test, the data of the SL measurements is analysed. During the analysis, the temperature correction coefficients are created. These coefficients compensate for the change in spectral response of the Brewer at the Ozone wavelengths. With the coefficients installed in the Brewer software, the Ozone measurements will not be affected by the temperature of the instrument.

          • The Brewer software says “LAMP NOT ON, HG TEST TERMINATED”. Should I replace the lamp?
            • The Brewer software will give this error message when it tries to make a HG measurement but cannot see the light of the Mercury lamp. There are several causes why the Brewer could give this error message. One of the motors could be in an incorrect position, so that the Brewer does not see the light. The PMT could not be measuring correctly or the lamp could need replacement.

              The first step in troubleshooting is doing a full reset (RE command) in the Brewer software. Then try to perform the HG test again.

              If the HG test still returns the error message one should find out if this also occurs for tests with the standard lamp. Type SL<enter> in the Brewer software.

              If Both the HG test and the SL test fail, then either a motor is not moving correctly or there is a problem with the PMT/photon counting circuitry. Use the maintenance manual for further troubleshooting.

              If the SL works but the HG fails, then there might be a problem with your lamp. For single board Brewers: Type AP to get the voltage of the Mercury Lamp (HG lamp). The voltage should be around 10 V. If the Voltage is off by 2 Volts, one should inspect the lamp.

              The HG or mercury lamp is the lowest lamp in the lamp housing. Usually, if the lamp needs replacement, the glass will have black spots or the filament will be broken.

              If the lamp needs to be replaced, do not touch the quartz envelope with your hands. Use a tissue or a piece of cloth. The lamp should be tightened firmly. Also, from the top, both filaments should be visible.

          • How often should my Brewer be calibrated?
            • For the Brewer spectrophotometer, regular recalibration is necessary for the reliability of the Brewer’s Ozone measurements. The World Meteorological Organisation (WMO) recommends that each Brewer is calibrated at least once every two years. The Brewer is a stable instrument and drifts in the instruments can be monitored and corrected because of the diagnostic tests such as Standard Lamp and Dead Time measurements.

              Some Brewer users prefer to have their Brewers calibrated every year. By doing this, they assure their Brewer data is of the highest quality. Drifts in the instrument are corrected sooner and the regular check with a reference Brewer increases the reliability of the data.

              If you would like to discuss calibration of your Brewer at the factory or at your location please contact us.