What is the big difference between CMP 11 and CMP 21?
The CMP 11 uses a default temperature compensation setting and the dependency is ±1% from -10 to +40°C. The CMP 21 is individually tested and the temperature compensation is optimised. It is ±1% from -20 to +50°C. However, from -10 to +40°C it is within ± 0.5%, typically ± 0.3%. In addition a temperature sensor is fitted and the temperature response curve is supplied. Each CMP 21 has the directional (cosine) response tested, and this is also supplied. This means that for the serious scientist the irradiance values can be corrected for temperature and solar elevation – increasing the accuracy. This is not possible with the CMP 11. BSRN requirements state that the solar radiometers must be fitted with an internal temperature sensor and the data recorded, so CMP 21 is compliant to this, but CMP 11 is not.
