Kipp&Zonen CHP 1 Pyrheliometer - Manual (Page 13)

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6 MEASUREMENT ACCURACY
When a pyrheliometer is in operation, its performance is correlated to a number of parameters, such as
temperature, level of irradiance etc. Normally, the supplied sensitivity figure is used to calculate the irradiances. If
the conditions differ significantly from calibration conditions, uncertainty in the calculated irradiances must be
expected.
For a first class pyrheliometer the WMO expects maximum errors in the hourly radiation totals of 3 %. In the daily
total an error of 2 % is expected, because some response variations cancel each other out if the integration period is
long. Kipp & Zonen expects maximum uncertainty of 2 % for hourly totals and 1% for daily totals for the CHP 1
pyrheliometer.
For the CHP 1 the effect of each parameter on the sensitivity can be shown separately.
6.1 Non linearity
The non-linearity error, the sensitivity variation with irradiance is shown in Figure 5 for a range from 0 to
1000
a W/m 2 referred to the calibration at 500 W/m 2 .
Figure 5: Non-linearity sensitivity variation of a CHP 1
6.2 Temperature dependence
The temperature dependence of the sensitivity is a function of the individual CHP 1. For a given instrument the
response lies in the region between the curved lines in Figure 6. The temperature dependence of each pyrheliometer
is characterized and supplied with the instrument. Each CHP 1 has built-in temperature sensors to allow corrections
to be applied if required.
Figure 6: Typical temperature dependency of a CHP 1
6.3 Zero offset B
Proportionally to the ambient temperature the instrument temperature varies and causes heat currents inside the
instrument. This will cause an offset commonly called Zero Offset type B. It is quantified as the response in W/m
2 to
a 5 K/hr change in ambient temperature.