All Your Questions on Pyranometers Answered

A pyranometer is an instrument to measure solar irradiance. Technically speaking, a pyranometer measures the solar radiation flux density (in W/m²), that is the incoming solar power reaching the Earth’s surface.

Back in 1924, Kipp & Zonen became the first manufacturer of pyranometers. Today, Kipp & Zonen is one of OTT HydroMet's strong brands for professional environmental monitoring solutions. In this article, OTT HydroMet's engineers and product experts answer frequently asked questions on pyranometers.

How can cable interferences be checked?

When calibrating the sensors, there is no signal bounce other than the time required for the pyranometer to reach its final value (time constant). However, if there are electrical inferences and the shielding of the cable and data logger is not sufficient, then you can expect noise.

A practical way of testing this is by taking a dummy pyranometer and connecting it to the data logger using the same cable (length and position). (Dummy pyranometer is a 1 kOhm resistor) This will indicate any interference coming from the cable.

Negative output during night-time measurements?

This error is relative to the zero offset type A. Typically, the presence of this zero offset is a result of the inner dome having a different temperature from the cold junctions of the sensor. This is practically always the case when the sky is clear.

Due to the low effective sky temperature (<0 °C), the surface of the Earth roughly emits 100 W/m2 longwave infrared radiation upwards. The pyranometer’s outer glass dome also has this emission and cools down several degrees below air temperature (the emissivity of glass for the specific wavelength region is nearly 1).

The heat emitted is drawn from the body (through conduction in the dome), attracted from the air (by wind), and from the inner dome (via infrared radiation). The inner dome also experiences cooling and will draw heat down from the body by conduction and from the sensor by the net infrared radiation.

The subsequent heat flow opposes the heat flow from absorbed solar radiation and results in what is known as zero depression at night. This negative zero offset can also be observed on a clear day, however, concealed in the solar radiation signal.

Zero offset type A can be checked by placing a light and IR reflecting cap over the pyranometer. The solar radiation response will decay with a time constant (1/e) of 1 second, but the dome temperature will move towards equilibrium with a time constant of several minutes. So after half a minute, any signal that remains represents mainly zero offset type A.

Appropriately ventilating the domes and body is the solution to further limiting the potential of zero offsets. Kipp & Zonen suggests using the CVF 3 Ventilation Unit for optimal ventilation and suppression of zero offset type A. Using the CVF 3, zero offset type A will be less than 3 W/m2.

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

Solar radiation at the site was greater than 1400 W/m². Is this reasonable?

Indeed, it is completely possible to attain a value of 1400 W/m² or slightly higher. The sun’s maximum radiation above the atmosphere is 1367 W/m². However, at higher altitudes with a clear sky and the presence of bright white cumulus clouds (not obstructing the sun), it is possible to achieve above 1400 W/m².

These clouds will reflect (extra) solar radiation to the sensor and, through this mirror effect, reach high values. So it is feasible, but only under extreme conditions. When a clear sky without clouds presents itself, the radiation will more than likely fall below 1367 W/m².

What is the directional or cosine response?

Radiation incidents on a flat horizontal surface that originate from a point source with a determined zenith position will have an intensity value relative to the cosine of the zenith angle of incidence. This is sometimes referred to as the ‘cosine law’ or ‘cosine-response’ and is displayed in figure 11.

Preferably, the pyranometer will have a directional response that is equal to the cosine-law.

However, in a pyranometer, the directional response is impacted by the quality, dimensions and construction of the domes. The maximum deviation from the optimal cosine-response of the pyranometer is given up to 80° angle of incidence in relation to 1000 W/m2 irradiance at normal incidence (0°).

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

What parameters or errors should be taken into account if the source of light comes from a certain angle?

The error involved is the directional error listed in the Pyranometer brochure, if the Pyranometer remains horizontal.

i.e. for CMP 3 < 20 W/m2  and for CMP 22 < 5 W/m2

Can a pyranometer be used underwater?

The CMP series can also be employed for use underwater; however, the depth is restricted to 1 meter and can only be used for short measurements.

The Pyranometer of the CMP series should not be kept underwater for longer than 30 minutes as per guidance.

The SP Lite2 pyranometer and the PQS 1 PAR Quantum Sensor can both be used for longer periods of time underwater, the depth limitation is around 2 meters. Please consider  “breaking of light on the water surface” parameters.

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

If a pyranometer is used underwater, can a data logger be connected to it?

Yes, however, the position of the data logger needs to be on or above the surface (it is weather resistant but cannot be submerged in the water).

What is the calibration frequency of a pyranometer?

Re-calibration of the Pyranometer should be conducted every two years. 

What does spectral range of 310 – 2800 nm (50% points) mean?

The 50 % points represent the wavelengths where the output of the instrument is reduced to 50 % with 100 % input.

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

What is the WMO standard for the pyranometers?

Table 1. Source: OTT HydroMet - Meteorology

  CMP 3 CMP 6 CMP 11 CMP 21 CMP 22
WMO Moderate quality Good quality High quality High quality High quality
ISO Spectrally Flat Class C Spectrally Flat Class B Spectrally Flat Class A Spectrally Flat Class A Spectrally Flat Class A

 

What is the resolution of a pyranometer?

The resolution is infinite owing to the analog output. Every change can be detected, no matter how insignificant it appears to be.

What is the bandwidth of a pyranometer?

The majority of pyranometers have a bandwidth between 285 to 2800 nm. This covers the entire solar spectrum, as shown below.

There are some notable exceptions:

  • CMP22 has a bandwidth of 200-3600 nm (Quartz glass )
  • SP Lite  has a bandwidth of 400-1100 nm (silicon photodiode)
  • CMP3 has a bandwidth of 300-2800 nm

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

Is there a standard product that converts the pyranometer output signal to 0-5 V or 0-2 V?

The AMPBOX offers the best solution. A suitable PSU is required, as well as a shunt resistor of 500 Ω to convert the current output (4.20 mA) to a voltage output of 2-10 V or a shunt resistor of 50 Ω to convert the current to a voltage output of 0.2-1 V will be needed.

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

What kind of pyranometer should be used inside a greenhouse?

CMP 6 paired with the PQS1 PAR Quantum Sensor is the best solution. CMP 6 for measuring the global solar radiation outside. PQS1 to measure PAR radiation inside, which is most sensitive for plants and crops.

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

What type of pyranometer should be used at a fixed PV panels farm?

For this application, Kipp & Zonen recommends the CMP10 and SMP10. They have a long-lasting internal drying cartridge that will offer at least 10 years of service.

Please note that the pyranometer must be mounted at the same angle (POA) as the PV panel.

The new SMP12 should be considered, too. It comes with an integrated tilt angle sensor that helps to maintain measurement quality, especially in tilted installations.

All Your Questions on Pyranometers Answered

Image Credit: OTT HydroMet

What type of pyranometer should be used on a solar concentrators farm?

None, as solar concentrators directly reflect the solar radiation towards a concentrator and are tracking the sun. You will need a pyrheliometer on a sun tracker for taking direct solar radiation measurements.

Is there a pyranometer available that has the same spectral characteristics as a PV panel?

Yes, Kipp & Zonen offer a Pyranometer with equivalent spectral characteristics as a PV panel. This is the SP Lite2 Pyranometer. The SP Lite2 is based on a silicon diode which has a response from 400 – 1100 nm.

The benefit of the SP Lite2 is the response time, which is just as fast as any PV panel (milliseconds).

The drawback is that not all PV panels possess the same spectral range. Thus, a thermopile pyranometer spans the entire spectral range of the sun and will offer a more precise measurement of the total (global) solar radiation. 

Is it possible to connect the pyranometers to a computer? That way, would it be possible to, using software (if there is any available), measure solar radiation all the time, non-stop?

The thermopile Pyranometers output, such as the CMP Series, is extremely low – generally around 10 milli-volts on a clear sunny day. 

To resolve changes of 1 W/m2, ADC with an accuracy and resolution of around 5 micro-volts is required. These PC interfaces are costly and difficult to source in a form that is easily interfaced to the PC. This is why meteorological data loggers that can cope with the low signal levels are typically employed.

Kipp & Zonen offer such solutions, such as handheld- or fixed location data loggers.

What kind of output does the CMP 6 Pyranometer have (analog or digital)? What voltage range do you have?

The CMP 6, as with all other solar radiometers based on thermopiles offered by Kipp& Zonen has a continuous small analog voltage output. For CMP 6, an irradiance of 1 W/m2 produces an output signal between 5 to 15 micro-volts. There are additional solutions available if there is the desire to increase this voltage. 

Do the pyranometers come with a calibration certificate, NIST traceable?

NIST in the USA offers comprehensive calibration services to industry – in the case of light, they are able to help characterize sensors, detectors and lamps for use in manufacturing and for luminance measurement (LUX).

They are not set up for the calibration of sensors for solar radiation and they are not a traceable reference. 

The only acceptable calibration standards of radiometers for the measurement of global or direct broadband solar radiation are listed below:

  • ISO 9059 Calibration of Field Pyrheliometers by Comparison to a Reference Pyrheliometer
  • ISO 9060 Specification and Classification of Instruments for Measuring Hemispherical Solar and Direct Solar Radiation
  • ISO 9846 Calibration of a Pyranometer Using a Pyrheliometer Guide to Meteorological Instruments and Methods of Observation, Fifth ed., WMO-No. 8 

What does Zero Offset A mean?

By physical laws, any object with a certain temperature will engage in a radiation exchange with its surroundings. The domes of radiometers that are upward-facing will primarily exchange radiation with the relatively cold atmosphere.

Generally speaking, the atmosphere will be cooler than the Earth’s ambient surface temperature.

For instance, a clear sky may have a temperature up to 50 °C cooler, whereas an overcast sky will roughly have the same temperature as the Earth’s surface. Due to these factors, the pyranometer domes experience a loss of energy to the colder atmosphere.

This causes the dome to become cooler than the rest of the instrument. This temperature difference between the detector and the instrument housing will result in a small negative output signal which is frequently referred to as Zero Offset type A.

This effect can be limited by using an inner dome. This inner dome acts as a ‘radiation buffer.’

The Zero Offset A can also be reduced by using a Ventilation Unit CVF 4.

Are there any accessories needed with the pyranometer to avoid reflected radiation from the surface? 

No, Kipp & Zonen pyranometers are equipped with a 180-degree field of view. When mounted horizontally, they are not able to detect light reflected from the ground due to their design.

What is the big difference between CMP 11 and CMP 21?

The CMP 11 utilizes a default temperature compensation setting and the dependency is ±1% from -10 to +40 °C.

The CMP 21 is tested on an individual basis, and the temperature compensation is fully optimized.  It is ±1% from -20 to +50 °C. However, from -10 to +40 °C it is within ± 0.5%, generally around ± 0.3%.

Additionally a temperature sensor is installed, and the temperature response curve is provided. Each CMP 21 has the directional (cosine) response tested and equipped.

This means that for the scientist working in serious circumstances, the irradiance values can be readily corrected for temperature and solar elevation – improving the accuracy. This is not possible with the CMP 11.

BSRN requirements state that the solar radiometers must be equipped with an internal temperature sensor and the data recorded, so while CMP 21 complies with this, the CMP 11 does not.  

Does a Pyranometer require any power?

Power is not a requirement when using Kipp & Zonen’s thermopile-based instruments, such as the CMP range of pyranometers and the CHP 1 pyrheliometer. All instruments generate a small voltage output in response to solar radiation.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.

For more information on this source, please visit OTT HydroMet.

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