FAQ

• Pyranometers

  • How can I check if there are interferences from the cable?

    • When we calibrate the sensors there is no signal bounce other than the time that the pyranometer needs to reach its final value (time constant) if however there are electrical inferences and the shielding of the cable and data logger is not good then you can expect noise. A good way of testing this is by connecting a dummy pyranometer with the same cable (length and position) to the data logger. (Dummy pyranometer is a 1 kOhm resistor) This will show any interference coming from the cable.

  • Negative output during nighttime measurements?

    • This error is related to the zero offset type A. Normally this zero offset is present when the inner dome has a different temperature from the cold junctions of the sensor. Practically this is always the case when there is a clear sky. Because of the low effective sky temperature (<0 °C) the earth surface emits roughly 100 W/m2 longwave infrared radiation upwards. The outer glass dome of a pyranometer also has this emission and is cooling down several degrees below air temperature (the emissivity of glass for the particular wavelength region is nearly 1). The emitted heat is attracted from the body (by conduction in the dome), from the air (by wind) and from the inner dome (through infrared radiation). The inner dome is cooling down too and will attract heat from the body by conduction and from the sensor by the net infrared radiation. The latter heat flow is opposite to the heat flow from absorbed solar radiation and causes the well known zero depression at night. This negative zero offset is also present on a clear day, however, hidden in the solar radiation signal.
      
      Zero offset type A can be checked by placing a light and IR reflecting cap over the pyranometer. The response to solar radiation will decay
      with a time constant (1/e) of 1 s, but the dome temperature will go to equilibrium with a time constant of several minutes. So after half a minute the remaining signal represents mainly zero offset type A.
      
      Good ventilation of domes and body is the solution to reducing zero offsets even further. Kipp & Zonen advises the CVF 3 Ventilattion 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.

      

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

    • It is indeed possible to reach a value of 1400 W/m² or slightly higher. The maximum radiation from the sun above the atmosphere is 1367 W/m². However at high altitudes with a clear sky and some bright white cumulus clouds (not covering the sun) it is possible to get above the 1400 W/m². These clouds will act like a mirror and reflect (extra) solar radiation to the sensor and through this effect reach these high values. So it is possible, but only under these extreme conditions. Under a clear sky without clouds the radiation is definitely below the 1367 W/m².

  • What is the directional or cosine response?

    • Radiation incident on a flat horizontal surface originating from a point source with a defined zenith position will have an intensity value proportional to the cosine of the zenith angle of incidence. This is sometimes called the ‘cosinelaw’ or ‘cosine-response’ and is illustrated in figure 11. Ideally a pyranometer has a directional response which is exactly the same as the cosine-law. However, in a pyranometer the directional response is influenced by the quality, dimensions and construction of the domes. The maximum deviation from the ideal cosine-response of the pyranometer is given up to 80° angle of incidence with respect to 1000 W/m2 irradiance at normal incidence (0°).

      

  • What parameters or errors should we take into account if the source of light comes from a certain angle?

    • If the Pyranometer remains horizontal the error involved is the directional error listed in the Pyranometer brochure.

      For CMP 3 < 20 W/m²  and for CMP 22 < 5 W/m²

  • Can I use a pyranometer under water?

    • The CMP series can also be used under water, the depth is limited to 1 meter and can only be used for short measurements.

      It is advisable not to keep the Pyranometer of the CMP series under water for longer than 30 minutes.

      The SP Lite2 pyranometer and the PQS 1 PAR Quantum Sensor can be used for a longer period under water, the depth is limited to 2 meters. Please also take  “breaking of light on the water surface” in consideration.

      

  • If I use a pyranometer under water, can I connect a data logger to it ?

    • Yes, however the data logger needs to be placed on the surface (it is weather resistant, but cannot be lowered into the water).

       

  • What is the calibration frequency of a pyranometer?

    • We advise to re-calibrate the Pyranometer every two years. 

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

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

      

  • What is the WMO standard for the pyranometers?

    •  

      	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	Secondary Standard	Secondary Standard

  • What is the resolution of a pyranometer?

    • The instrument has an analog output, therefore the resolution is infinite. Every change is noticed, no matter how small it is.

  • What is the bandwidth of a pyranometer?

    • The bandwidth of most pyranometers is 285 to 2800 nm. This covers the full solar spectrum as shown below.

      There are some exceptions:

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

       

       

  • In our PV application the cable from the CMP 11 (50 meters) will go along other cables that come from the PV panels in which there is a DC voltage and around 100 Amps. Will these cables affect the measurement?

    • The disturbance on the cables on the CMP 11 is difficult to judge from a distance. A test would give the best criteria in this case.

      Simply cover the CMP 11 so it is fully dark (in box with cloth etc.) Log the data over a period that disturbance is expected, at least one day.

      If the data is zero no problem is to be expected.

       

       

  • Do you have filters that can be used to verify spectral distribution over the following wave lengths? Ultraviolet - B 280-320 Ultraviolet - A 320-360 and 360-400 Visible 400-520, 520-640 and 640-800 Infrared 800-3000nm.

    • No, we do not have filters for any of our pyranometers. The only way to do this in a correct way is to use a filter dome. Otherwise the directional response would be affected.

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

    • The AMPBOX is the best solution.

      You will need a suitable PSU and a shunt resistor of 500 Ω to convert the current output (4..20mA) to a voltage output of 2-10V , or you will need a shunt resistor of 50 Ω to convert the current to a voltage output of 0.2-1V.

      

  • What kind of pyranometer do you suggest for usage inside a greenhouse?

    • CMP 6 in combination with PQS1 PAR Quantum Sensor is advised.  CMP 6 for outside usage to measure Global solar radiation. PQS1 to measure  PAR radiation inside which is most sensitive for plants and crops.

      

  • What type of pyranometer can I use for my fixed PV panels farm?

    • For this application the CMP10 and SMP10 are advised as they have an internal drying cartridge that will last for at least 10 years.

      Please note that the pyranometer needs to be mounted in the same angle (POA) as the PV panel. 

       

      

      For users that prefer the desiccant visible Kipp & Zonen offers the CMP11 and SMP11 with visible and user changeable desiccant.

  • What type of pyranometer can I use for my solar concentrators farm?

    • None, solar concentrators are reflecting the direct solar radiation  to a concentrator and are tracking the sun. You will need a pyrheliometer on a sun tracker to measure direct solar radiation.

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

    • Yes, we do have a Pyranometer with the same spectral characteristics as a PV panel. This is the SP Lite(2) Pyranometer.

      Our SP-Lite is based on a silicon diode which has a response from 400 – 1100 nm.
      The advantage is the response time, which is as fast as any PV panel ( milli seconds).
      The disadvantage is that not all PV panels have the same spectral range. 
      A thermopile pyranometer covers the full spectral range of the sun and will give a more accurate measurement of the total (global) solar radiation.

  • Is it possible to connect the Pyranometers to a computer? That way, I could, using software (if there is any available), measures solar radiation all the time, non-stop.

    • The output from thermopile Pyranometers, such as our CMP Series, is very low – typically around 10 milli-volts on a clear sunny day. To resolve changes of 1 W/m² requires an ADC with an accuracy and resolution of around 5 micro-volts. These PC interfaces are very expensive and difficult to find in a form that is easily interfaced to the PC. This is why meteorological data loggers are normally used that can cope with the low signal levels.

      Kipp & Zonen has solutions like handheld- or fixed location data loggers.

  • I would like to know what kind of output the CMP 6 Pyranometer has (analog or digital)? What voltage range do you have?

    • The CMP 6, as with all our solar radiometers based on thermopiles has a continuous small analoge voltage output. For CMP 6 an irradiance of 1 W/^m2 generates an output signal in the region of 5 to 15 micro-volts. We have additional solutions to increase this voltage.

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

    • NIST in the USA supplies calibration services to industry – in case of light they characterise 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 accepted world standards for the calibration of radiometers for the measurement of global or direct broadband solar radiation are as 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 having a certain temperature will exchange radiation with its surroundings. The domes of upward facing radiometers will exchange radiation primarily with the relatively cold atmosphere. In general, the atmosphere will be cooler than the ambient temperature at the Earth’s surface. For example, a clear sky can have an effective temperature up to 50°C cooler, whereas an overcast sky will have roughly the same temperature as the Earth’s surface. Due to this the Pyranometer domes will ‘lose’ 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 generate a small negative output signal which is commonly called Zero Offset type A. This effect is minimized 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 3.

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

    • No, all the Pyranometers have a 180 degree field of view. When mounted horizontally, they cannot see light reflected from the ground due to its design.

  • 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. 

  • Does a Pyranometer require any power?

    • Our thermopile-based instruments, including the CMP range of pyranometers and the CH(P) 1 pyrheliometer, do not require power to operate. They generate a small voltage output in response to the solar radiation.

  • Negative output during night-time measurements?

    • This error is related to Zero Offset Type A. Normally this zero offset is present when the inner dome has a different temperature from the cold junctions of the sensor (the instrument housing). Practically this is always the case when there is a clear sky. 

      Because of the low effective sky temperature (< 0 °C) the Earth’s surface emits roughly 100 W/m2 of long-wave infrared radiation upwards. 

      The outer glass dome of a pyranometer also has this emission and is cooling down several degrees below air temperature (the emissivity of glass for the particular wavelength region is nearly 1). The emitted heat is attracted from the body by conduction in the domes, by wind and from the domes through infrared radiation. The heat flow is opposite to the heat flow when absorbing solar radiation and causes the well-known zero depression at night. This negative zero offset is also present on a clear day but is hidden in the solar radiation signal. 

      Zero Offset Type A can be checked by placing a light and IR reflecting cap over the pyranometer. The response to solar radiation will decay with the response time of the instrument, but the dome temperature will go to equilibrium with a time constant of several minutes. So, after about half a minute the remaining signal is mainly Zero Offset Type A. Good ventilation of the domes and housing minimizes zero offsets and increases stability. Using the Kipp & Zonen CVF4 ventilation unit can reduce Zero Offset Type A by about 50 %. 

  • Maximum and minimum irradiation quantities?

    • Due to reflection from certain types of clouds the global irradiance at sea level can rise above the extra-terrestrial direct irradiance (the Solar Constant) of 1367 W/m² at the top of the atmosphere (WMO 1982). Values up to 1500 W/m² have been reported. Because the clouds move, this irradiance value mostly appears as short events of a few minutes duration.

  • Is the pyranometer calibration affected by the length of the signal cable?

    • With longer cable lengths the impedance increases, however it does not affect the radiometer sensitivity for the following reason. For the SMPX the impedance of the voltage measurement device is at least 1000 times more than the impedance of the pyranometer plus cable. Therefore the current that goes through the readout cable is negligible and won’t generate an offset. 

      The digital RS-485 output can operate over cable lengths up to 1000 m, depending on the baud-rate used.