Lufft CHM 15k, User Manual

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Operating Manual CHM 15k R13 / 07-2019
Technical Description 11
4
Technical Description
The CHM 15k ceilometer is mainly used to measure cloud bases, cloud penetration depths, cloud cover
statistics, the vertical visibility and the aerosol layer. The calculated data are transferred by remote
transmission via standard digital interfaces.
The CHM 15k uses the lidar method as a measuring principle (Lidar: light detection and ranging): short
light pulses generated by a microchip solid state laser are emitted into the atmosphere where they are
scattered by aerosols, droplets and air molecules. The portion of the light that is scattered back to the
ceilometer is further analyzed. The propagation time of the laser pulses is measured and used to calculate
the distance of the scattering event.
The height profile of the backscattered signal is analyzed to calculate the backscatter intensity
β
raw
as the
first output parameter from the instrument. From
β
raw
the attenuated backscatter coefficient
β
att
can be
calculated using a calibration constant valid for the CHM 15k instruments. The different target parameters
like the cloud base layers and the aerosol layer height are calculated from these data.
The CHM 15k Nimbus detection system is based on a photon counting method. It can only be used in
combination with a laser like it is integrated in the CHM 15k. The narrow laser bandwidth enables the
usage of a 1 nm (or less) bandwidth filter in front of the detector that is required to suppress the
background light efficiently and be able to average data over several minutes. Averaging signals to reach
a certain signal to noise level is key for lidar measurements generating aerosol profiles. Compared to
analog measurement techniques, the benefits of this method are very high detection accuracy and
sensitiveness. The technique is also robust against noise.
The CHM 15k ceilometer is:
•
A compact device including heater and window blower
•
Capable of operating under the ambient conditions specified in the technical data (see 3)
•
Based on modules, e. g. the laser optical module (LOM) inside the device can be replaced
(refreshed) at the end of the laser diode lifetime
•
A device designed for 24/7 continuous operation
•
4.1 Construction of the CHM 15k
The enclosure of the CHM 15k is made of corrosion-resistant aluminum in double-shell construction.
The function of the outer shell is to dampen the external influences of:
•
Solar radiation
•
Wind
•
Precipitation
•
Mechanical shock
on the inner shell, which carries the measuring unit. The chimney effect between the outer shell and the
inner housing supports this process and avoids high temperature in the inner housing.
The housing cover protects the inner housing against dust and precipitation and mechanical damage on
the windows. The housing cover contains the opening for laser exit and laser entry. The partition in the
cover decouples the transmission area from the sensitive receiving area. An air baffle inside the cover
directs the airflow from both fans directly onto the glass windows of the inner housing.
The inner housing incorporates all the equipment needed to operate the CHM 15k. The cable feed-
throughs for data line, power supply and connection of the external fans are realized via cable glands.
For pressure equalization, the inner housing has a pressurization element with a GoreTex
® membrane.
The upper end of the inner housing forms a two-part viewing window made of float glass. The windows
are inclined at the Brewster angle. This ensures low-loss laser light transmission and optimal self-cleaning
of the windows. The fan located at the rear of the device supports the cleaning of the windows: the fans
are switched on an hourly interval and in rain / snow. The fans are also used to remove heat from the
inner housing. The fans are serviced by a service technician via the removable rear panel of the CHM 15k.
The outer door allows access to the inner housing and the glass windows, e.g. for cleaning purposes.
Access to the interior of the inner housing is possible via the inner door. The outer and inner doors are
secured with different locking mechanisms. In this way, access to the interior of the inner housing by
cleaning personnel who have not been instructed in safety-related matters is excluded. The inner door
may only be opened by the service staff of G. Lufft GmbH or by the customer’s authorized and trained
personnel.