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Monitoring using the SR-18

SR-18 UVB Scanning Radiometer

Instrument Description

The radiometer sensor head unit is a weatherproof housing with cosine-corrected diffuser for light sampling, a filter wheel with 18 2nm-nominal band pass interference filters mounted in the periphery, a collimator, and a solar blind R-1657 photomultiplier tube (PMT).

All of the components within the sensor head unit are environmentally housed for extended year-round field operation. All components of the sensor head, except the PMT, are designed to operate without significant temperature effects over a -25° to +55° C ambient temperature range. The PMT housing is temperature regulated with a thermostated thermoelectric system. A dessicant canister allows filled with indicating silica gel maintains low humidity inside the sensor head. A viewing port in the canister allows the operator to determine when to replace the silica gel.

The filter wheel turns at 15 RPM and has 2nm half-power-band-width light filters with nominal center wavelengths from 290nm to 324nm. Every four seconds, a new scan begins.  The PMT current output (which is proportional to the radiation intensity through each filter) is converted to a voltage and measured by a precision 20-bit A/D converter.

Data from all light filter channels, and the two dark readings are averaged for one minute intervals, and along with internal temperature and control information, transmitted over three wire RS-232 data cable to the data acquisition and control unit.

The second unit is the external power supply for the sensor head system. This power supply provides electrical power both to the head electronics and to the thermoelectric PMT temperature regulator. This unit is contained in a weatherproof housing.

The third unit, the data acquisition/storage unit, consists of an off-the-shelf PC running in an office environment with a hard drive, a 3.5 inch floppy disk drive, serial port and a modem/network connector running under a Windows 9x environment. Data would routinely be downloaded from the hard drive to the removable floppy, or transmitted via modem, network or Internet to the data management facility.

The complete SR-18 User Manual can be found at: SR-18 MANUAL.pdf