Lufft CHM 8k, Руководство пользователя, Страница: 11 / 72

Operating Manual CHM 8k
R1.7 / 01-2020
Technical Description 11
4
Technical Description
The
CHM 8k 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 8k uses the lidar method as a measuring principle (Lidar: light detection and ranging): short
light pulses 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 attenuated backscatter
coefficient as the first output parameter from the instrument. The different target parameters like the cloud
base layers and the aerosol layer height are calculated from these data.
The CHM 8k detection system is based on an analog detection method with a 25 nm bandwidth optical
filter in front of the detector. It is designed to suppress background light effectively, while being compatible
with the broader emission spectrum of InGaAs laser diodes. The analog detection system offers high
dynamical range without saturation of the detector.
The
CHM 8k 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 unit (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 8k
The housing of the
CHM 8k 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 8k. 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 coated 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 8k.
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.
4.2 Functional units of the inner housing
The functional units of the device are:
•
Transmitting and receiving unit (measuring unit - LOM)
•
Control board and related components