The PMO6-cc is an electrical substitution radiometer, and is a self-contained system to measure the direct normal incidence solar irradiance. The system consists of two parts, a sensor head (absolute cavity pyrheliometer) and a control unit. The heart of the sensor head is a blackened cavity in which the solar radiation is absorbed. The PMO6-cc was developed at PMOD/WRC as a successor of the earlier types from the PMO6 instrument series. The PMO6-cc (“cc” = “computer controlled”) control unit allows the system to be either manually or remotely controlled from an external computer. The control unit contains a PC and voltmeters to conduct measurements and communicate with the user using its display and touch-panel or its RS232 port.
Figure 1. PMO6-cc radiometers being calibrated next to the WSG.
Principle of operation
Electrical substitution radiometers are based on an electrically calibrated heat flux transducer and a cavity as receiver. The cavity ensures high absorption over the spectral range of interest. The principle of the measurement relies on the substitution of the radiant power by electric power. During operation of the instrument, an electronic circuit controls the electric power dissipated in the cavity in order to maintain a constant heat flux differential with respect to the so-called compensating cavity. The compensating cavity is identical to the senor cavity is never illuminated but follows ambient temperature changes. Therefore the differential heat flux is independent of ambient temperature.
Traceability to the World Radiometeric Reference (WRR)
Each PMO6-cc is calibrated against the World Standard Group to provide direct traceability to the World Radiometric Reference.
Operation
The standard mode of operation of the PMO6-cc provides one irradiance reading every 120 to 180 seconds. The number of measurement cycles and the integration time are free parameters. The PMO6-cc can be programmed to autonomously start the measurements at a predefined time. After every open/closed cycle, the measurement result (solar irrdiance) is shown on the display, stored in the on-board non-volatile memory and also written to the RS232 port. After completion of the set number of measurement cycles, all irradiance values can be displayed or read from the non-volatile memory via the RS232 port.
Functionality check and configuration
The touch interface on the front panel of the control unit can be used to check the functionality and to set instrument and measurement parameters, as well as to initiate the measurements manually.
Specifications: PMO6-cc radiometer
| Accuracy (Absolute) Accuracy (Trac. to WRR) |
0.15% < 0.1% |
| Precision | 50 ppm |
| Range | up to 1400 W.m-2 (or custom design available) |
| Operating temperature range | -10°C to +40°C |
| Field-of-view (full angle) | 5° |
| Slope angle | 1° |
| Dimensions | 80 x 80 x 230 mm |
| Mass | 2.04 kg |
Specifications: PMO6-cc control unit
| Accuracy | Absolute measurement uncertainty is less than 50 ppm of the radiometer full-scale range |
| Outputs | Analog: ± 10 V Digital: RS232, 9600Bd, 8 Bit, non Parity, 1 Stop |
| Operating temperature range | 0°C to +40°C |
| Dimensions: | 280 x 270 x 80 mm |
| Mass | 3.04 kg |
| Power requirements | 8.5 W |
PMO6-cc discontinued. New model PMO8 available from Davos Instruments
The PMO6-cc has recently been discontinued, and replaced by the PMO8 model, now manufactured by Davos Instruments. Calibration of the PMO6-cc is still carried out by PMOD/WRC while service and repair are also now conducted by Davos Instruments.