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How do ISO 9060 Class A, B and C pyranometers differ?
ISO 9060 Class A, B and C pyranometers differ in the performance limits they must satisfy. Class A has the most demanding requirements of the three, followed by Class B and Class C. The classification covers several sources of measurement error, rather than assigning a single accuracy percentage to an instrument.
For buyers comparing solar radiation sensors, the practical question is how these characteristics affect the measurements a project needs. This guide explains the class differences and uses RIKA SENSOR's rk200-03 V5.0 specifications to illustrate the product parameters to compare.
Quick answer
- Class A: The most stringent ISO 9060 pyranometer class, with individual testing requirements for temperature and directional response.
- Class B: Intermediate performance limits for projects whose requirements allow this class.
- Class C: The broadest limits of the three; compare the actual product specifications with the intended measurement task.
- For procurement: Check the standard edition, complete class designation, performance data, test documents and calibration certificate together.
1. What Do ISO 9060 Classes A, B and C Mean?
ISO 9060:2018 establishes specifications and classifications for instruments measuring hemispherical solar and direct solar radiation. For pyranometers, the three classes provide a common framework for comparing instrument performance.
A class declaration concerns the applicable classification criteria as a whole. A particularly good result for one parameter does not establish the instrument's complete class.
| Comparison | Class A | Class B | Class C |
|---|---|---|---|
| Relative class requirements | Most stringent of the three | Intermediate limits | Broadest limits of the three |
| Familiar terminology from ISO 9060:1990 | Secondary Standard | First Class | Second Class |
| Annual non-stability acceptance interval | ±0.8% | ±1.5% | ±3% |
| Non-linearity acceptance interval | ±0.5% | ±1% | ±3% |
| Directional-response acceptance interval | ±10 W/m² | ±20 W/m² | ±30 W/m² |
The numerical rows summarize selected acceptance intervals, rather than the complete classification requirements. Non-linearity is assessed across 100-1000 W/m²; directional response uses the standard's defined beam and angle conditions. Consult the standard for the complete criteria and conformity-assessment rules. Classification table and parameter definitions
The older names help readers interpret legacy documents, but the 2018 revision also changed classification requirements. Check the standard edition stated in the current documentation.
2. Which Performance Characteristics Make the Difference?
The class label becomes more useful when you understand the errors and operating effects behind it. Key characteristics include response time, zero offsets, stability, non-linearity, directional response, spectral error, temperature response and other criteria.
Response time
Response time describes how quickly the reading approaches its final value after irradiance changes. Compare values measured at the same response level, such as 95% response.
A shorter response time helps resolve rapid changes in sunlight. The data logger's sampling and averaging settings must also support the intended measurement.
Zero offsets
Thermal effects can produce an output even without incoming shortwave radiation. ISO terminology distinguishes the offset associated with thermal radiation from the offset associated with changing ambient temperature. Kipp & Zonen's technical FAQ
Check which effect a datasheet describes. A zero-drift figure specified at a 5 K/h ambient-temperature change addresses a different test condition from a thermal-radiation offset.
Annual stability
Annual stability describes changes in the instrument's sensitivity over time. Lower drift helps maintain consistent measurements between calibrations. It remains a separate characteristic from calibration uncertainty.
Directional and temperature response
Directional response concerns errors associated with the angle at which sunlight reaches the instrument. This matters as solar position changes throughout the day.
Temperature response concerns changes in sensitivity as ambient temperature changes. Read its specified temperature interval: an instrument's allowable operating range and the interval used to specify temperature response can be different.
Non-linearity and spectral response
Non-linearity describes how sensitivity changes with irradiance. A non-linearity specification applies to its stated test range and does not represent the instrument's total field accuracy.
Spectral response describes how sensitivity varies with wavelength. Comparing the wavelength range alone does not establish how uniformly two instruments respond within that range. Campbell Scientific's pyranometer guide
3. What Do Spectrally Flat and Fast Response Mean?
ISO 9060:2018 introduced additional descriptors that can accompany a class designation. They describe different performance properties and deserve separate checks. Kipp & Zonen's explanation of the revision
Spectrally flat identifies instruments meeting the additional spectral-response requirement. A broad wavelength range in a datasheet, by itself, does not establish this designation.
Fast response is a defined designation associated with a 95% response time below 0.5 seconds. A response time expressed in several seconds should not be presented as evidence of this designation. EKO classification guide
When a tender requests a complete designation such as "Spectrally Flat Class A", request documents supporting that complete wording. The class letter and the additional descriptors should be read together.
4. Comparing RIKA RK200-03 Product Specifications
The RK200-03 is a thermopile pyranometer series. Its absorbing surface converts incoming radiation into heat, and the thermopile produces an electrical signal associated with the resulting temperature difference.
RIKA's V5.0 datasheet lists Class A, B and C versions. The following table reproduces the published product specifications. These are RIKA product values, rather than ISO classification acceptance limits. Assessment of a specified ISO classification uses the corresponding test documentation.
| Parameter | RK200-03 Class A | RK200-03 Class B | RK200-03 Class C |
|---|---|---|---|
| Spectral range | 280-3000 nm | 280-3000 nm | 300-3200 nm |
| Response time to 95% | ≤5 s | ≤10 s | ≤60 s |
| Non-linearity | <±0.2%, at 100-1000 W/m² | ±1%, at 100-1000 W/m² | <±3% |
| Zero drift at 5 K/h ambient-temperature change | <±2 W/m² | <±4 W/m² | ±8 W/m² |
| Annual stability, stated limit | <±0.8%/year | <±1.5%/year | <±3%/year |
| Directional response | ±10 W/m² | ±20 W/m² | ≤±30 W/m² |
| Temperature effect, from -10°C to +40°C | <1% | <2% | <8% |
| Operating temperature | -40°C to +80°C | -40°C to +80°C | -40°C to +80°C |
| Recalibration interval | 2 years | 2 years | 2 years |
Source: RIKA SENSOR, RK200-03 V5.0 technical specifications, revision dated 31 March 2025. The listed non-linearity test range is stated for the Class A and B versions.
Within this family, the published Class A version has tighter non-linearity, annual-stability and temperature-effect figures than the other versions. These are useful comparisons when deciding which configuration to evaluate for a project's measurement requirements.
The series also provides different interface options. Class A and B list unamplified mV, 4-20 mA and RS485 options. Class C additionally lists 0-5 V and SDI-12. Select the output and any required transmission module for the ordered configuration.
An interface makes the instrument compatible with a monitoring system; it does not determine its ISO class.
5. Which Pyranometer Class Should You Choose?
Start with the required measurement quality and the consequences of measurement error. A project using irradiance data for contractual performance assessment may need a different specification from a project observing general environmental trends.
Projects with demanding measurement requirements
Review Class A instruments when the project specifies tight performance limits. For the RK200-03 family, begin by checking the published Class A specifications, then request the class-related test documentation and instrument-specific calibration information required by the project.
Projects with broader measurement requirements
Evaluate Class B or Class C where their documented performance satisfies the monitoring plan. Compare the actual parameters, installation conditions and service requirements alongside the class designation.
An instrument can perform better than its class requires for an individual parameter. This is why the actual datasheet remains important even when two instruments carry the same class label.
Solar PV monitoring projects
Distinguish the instrument classification from the monitoring-system classification. IEC 61724-1:2021 addresses PV performance monitoring systems, while ISO 9060 addresses the radiation instrument.
For PV procurement, identify whether the measurement is GHI, front-side POA, rear-side irradiance or reflected radiation. Then check the required instrument characteristics, sensor locations, calibration and maintenance arrangements for that measurement.
6. What Documents Should You Request Before Ordering?
Ask the supplier to provide:
- The exact model and configuration: Class designation, output, supply, cable and accessories.
- The applicable standard edition: Including any required spectrally flat or fast-response descriptor.
- Performance test information: Covering the criteria relevant to the declared class and project specification.
- The calibration certificate: Instrument identity, sensitivity or calibration result, uncertainty, method and traceability.
- Installation and maintenance instructions: Alignment, cleaning, desiccant inspection and any site-specific environmental provisions.
- Recalibration and service arrangements: Recommended interval, service process and replacement planning.
RIKA's RK200-03 V5.0 specifications list ISO 9847 as the calibration basis and recommend recalibration every two years. ISO 9847:2023 describes calibration by comparison with a reference pyranometer.
A calibration certificate supports the reported calibration result. Class-related performance tests address a wider set of characteristics, so both documents have a role in technical review.
Frequently Asked Questions
Is Class A the highest pyranometer class in ISO 9060:2018?
Yes. Class A is the most stringent of the three pyranometer classes, followed by B and C. Additional descriptors such as spectrally flat and fast response identify further characteristics.
Does Class A mean a fixed accuracy such as ±1%?
No. A class includes several performance criteria. Measurement uncertainty also depends on calibration, operating conditions, installation and the measurement system. An individual percentage specification should be read with its parameter and test conditions.
Are Secondary Standard and Class A exactly the same?
They are corresponding terms encountered in the 1990 and 2018 classification systems. The revision changed requirements as well as terminology, so confirm the standard edition and supporting documents for the instrument being purchased.
Can a silicon photodiode sensor be assigned a class?
The classification considers instrument performance rather than relying solely on detector type. Request the documented classification for the exact model. RIKA's RK200-04 V5.0 sheet describes a 300-1100 nm photodiode sensor but does not state an ISO 9060 class.
Does an ISO Class A pyranometer automatically make a PV monitoring system Class A?
No. The radiation instrument is one component. The applicable PV monitoring requirements also address the measurement arrangement and other system-level provisions.
Discuss Your Pyranometer Requirements with RIKA SENSOR
Send RIKA SENSOR your measurement objective, required standard and class designation, installation conditions, preferred output and calibration-document requirements.
The RK200-03 series provides several configurations to evaluate against those requirements. Review the selected instrument's specifications and supporting documents together before finalizing the monitoring design.
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