Introduction
Solar radiation at Earth's surface is defined as total radiation across a wavelength range of 280 to 4000 nm (shortwave radiation). Total solar radiation, direct beam and diffuse, incident on a horizontal surface is defined as global shortwave radiation, or shortwave irradiance (incident radiant flux), expressed in Watts per square meter (W m⁻², equal to Joules per second per square meter).
Pyranometers measure global shortwave radiation. Apogee MP series solar radiation meters use silicon-cell pyranometers, sensitive to approximately 350-1100 nm of the solar spectrum (about 80% of total shortwave radiation). These pyranometers are calibrated to estimate total shortwave radiation across the entire solar spectrum. Their specifications compare favorably to World Meteorological Organization (WMO) moderate and good quality classifications, and International Organization of Standardization (ISO) second and first class classifications, though they do not meet spectral specifications for WMO or ISO certification due to limited spectral sensitivity.
Typical applications include measuring incoming shortwave radiation in agricultural, ecological, hydrological weather networks, and solar panel arrays.
Apogee Instruments MP series meters consist of a handheld meter and a dedicated pyranometer. The pyranometer is integrated into the top of the meter housing for the MP-100, or connected by cable to an anodized aluminum housing for the MP-200. Both integrated and separate sensors contain a cast acrylic diffuser (filter), a photodiode, and are potted solid with no internal air space. MP series meters display real-time irradiance readings on an LCD, indicating radiation incident on a planar surface from any angle of a hemisphere. They include manual and automatic data logging features for spot-check measurements or calculating daily solar insolation.
Sensor Models
Apogee MP series solar radiation meters are self-contained, coming complete with a handheld meter and sensor.
MP-100: Features an integrated sensor on the meter housing.
MP-200: Features a separate sensor connected by cable to the meter housing.
The sensor model number and serial number are located on a label on the backside of the handheld meter.
Specifications
| Specification | MP-100 | MP-200 |
|---|---|---|
| ISO 9060:2018 | Class C (previously known as second class) | Class C (previously known as second class) |
| Calibration Uncertainty | ± 5 % (see Calibration Traceability) | ± 5 % (see Calibration Traceability) |
| Measurement Repeatability | Less than 1 % | Less than 1 % |
| Long-term Drift (Non-stability) | Less than 2 % per year | Less than 2 % per year |
| Non-linearity | Less than 1 % (up to 1750 W m⁻²) | Less than 1 % (up to 1750 W m⁻²) |
| Response Time | Less than 1 ms | Less than 1 ms |
| Field of View | 180° | 180° |
| Spectral Range | 360 to 1120 nm (wavelengths where response is 10 % of maximum) | 360 to 1120 nm (wavelengths where response is 10 % of maximum) |
| Directional (Cosine) Response | ± 5 % at 75° zenith angle | ± 5 % at 75° zenith angle |
| Temperature Response | 0.04 ± 0.04 % per °C | 0.04 ± 0.04 % per °C |
| Operating Environment | 0 to 50 °C; < 90 % non-condensing relative humidity up to 30 °C; < 70 % non-condensing relative humidity from 30 to 50 °C; sensors can be submerged up to 30 m depth. | 0 to 50 °C; < 90 % non-condensing relative humidity up to 30 °C; < 70 % non-condensing relative humidity from 30 to 50 °C; sensors can be submerged up to 30 m depth. |
| Meter Dimensions | 126 mm length; 70 mm width; 24 mm height | 126 mm length; 70 mm width; 24 mm height |
| Sensor Dimensions | 24 mm diameter; 33 mm height | 24 mm diameter; 33 mm height |
| Mass | 150 g | 180 g |
| Cable | 2 m of two conductor, shielded, twisted-pair wire; TPR jacket (high water resistance, high UV stability, flexibility in cold conditions) | 2 m of two conductor, shielded, twisted-pair wire; TPR jacket (high water resistance, high UV stability, flexibility in cold conditions) |
Calibration Traceability: Apogee Instruments MP series solar radiation meters are calibrated via side-by-side comparison to the mean of four Apogee model SP-110 transfer standard pyranometers under high intensity discharge metal halide lamps. These transfer standards are calibrated against at least two ISO-classified reference pyranometers under sunlight in Logan, Utah. The ISO-classified reference pyranometers are recalibrated biennially at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, whose standards are traceable to the World Radiometric Reference (WRR) in Davos, Switzerland.
Spectral Response
The spectral response graph illustrates the weighting factors across different wavelengths for Apogee silicon-cell pyranometers. The spectral response is estimated by multiplying the spectral response of the photodiode, diffuser, and adhesive. Measurements were made using a spectrometer for the diffuser and adhesive, and manufacturer data for the photodiode. The graph shows that the sensor is most sensitive in the 350-1100 nm range.
Temperature Response
The temperature response graph shows the mean temperature response of four Apogee silicon-cell pyranometers. Measurements were taken at approximately 10 °C intervals across a temperature range of -10 to 50 °C under sunlight. Each pyranometer uses an internal thermistor to measure temperature. A reference blackbody pyranometer was used to measure solar intensity at each temperature set point. The graph indicates a slight positive correlation between temperature and the difference from reference readings.
Cosine Response
Cosine response defines the measurement error at a specific angle of radiation incidence. For Apogee silicon-cell pyranometers, the error is approximately ± 2% at 45° solar zenith angle and ± 5% at 75° solar zenith angle. The diagram illustrates this concept, showing the sensor head and the angles of incidence.
The cosine response graph displays the mean response of eleven Apogee silicon-cell pyranometers, with error bars representing two standard deviations. Measurements were made during outdoor radiometer calibrations at NREL. Cosine response is calculated as the relative difference in sensitivity at each solar zenith angle compared to sensitivity at 45° solar zenith angle. Blue symbols represent AM measurements, and red symbols represent PM measurements. The graph shows minimal error up to about 70° zenith angle, with a sharp increase thereafter.
Deployment and Installation
Apogee MP series solar radiation meters are designed for spot-check measurements and daily insolation calculations using their built-in logging feature. For accurate measurements of global shortwave radiation on a horizontal surface, the sensor must be level. Each MP model includes a different mounting option for achieving a level plane.
The AL-210 leveling plate is recommended for the MP-100.
The AL-100 leveling plate is recommended for the MP-200. The AL-120 mounting bracket is recommended to facilitate mounting to a cross arm.
Sensors should be mounted to avoid obstructions (e.g., weather station tripod/tower, other instrumentation) that could shade the sensor.
Operation and Measurement
MP series solar radiation meters feature a user-friendly interface for quick and easy measurements.
Powering the Meter: Insert the included CR2320 battery into the battery holder, ensuring the positive side (+) faces out. The battery door is located on the meter's back panel.
Activating the Display: Press the power button [⏻] to activate the LCD. The meter enters sleep mode after two minutes of inactivity to conserve battery life, shutting off the display.
Main Menu: Press the 'mode' button to access the main menu for selecting logging mode (manual or automatic) and resetting the meter.
Logging a Reading: Press the 'sample' button to log a reading during manual measurements.
Navigation: Use the 'up' and 'down' buttons to make selections in the main menu and to scroll through logged measurements on the LCD display.
LCD Display: The display shows the total number of logged measurements (upper right corner), the real-time irradiance value (center), and selected menu options (bottom).
Logging Modes:
- Manual (SMPL): Press 'mode' once, then use 'up'/'down' to select 'SMPL'. Press 'mode' twice to confirm. Press 'sample' to record up to 99 manual measurements. A counter shows saved measurements.
- Automatic (LOG): Press 'mode' once, then use 'up'/'down' to select 'LOG'. Press 'mode' twice to confirm. The meter measures every 30 seconds. Every 30 minutes, it averages these measurements and stores the average. It can store up to 99 averages. Every 48 averaged measurements (24 hours), it stores an integrated daily total in mega joules per square meter per day (MJ m⁻² d⁻¹).
Reset and Data Download
Resetting the Meter: In SMPL or LOG mode, press 'mode' twice (RUN should blink), hold 'down', and press 'mode' once. This erases saved measurements for the selected mode only.
Review/Download Data: Logged measurements can be reviewed on the LCD using the 'up'/'down' buttons. Press 'sample' to return to real-time readings. Daily totals are only accessible by downloading to a computer.
Downloading Data: Requires the AC-100 communication cable and software (sold separately). The meter uses the UART protocol; the AC-100 converts it to USB. Standard USB cables will not work. Setup instructions and software are available from the Apogee website: http://www.apogeeinstruments.com/ac-100-communcation-cable/.
Data logging modes:
- (SMPL) 99 Sample Measurements: Viewable on meter LCD & downloadable.
- (LOG) 99 Log Measurements: Viewable on meter LCD & downloadable.
- (LOG) 99 Daily Total Measurements: Downloadable Only.
Spectral Errors for Measurements with Silicon-cell Pyranometers
Apogee MP series meters are calibrated in a lab under electric lamps, simulating clear sky conditions at a 45° solar zenith angle. Spectral errors can occur when measurements are made in conditions differing from calibration (e.g., cloudy vs. clear sky), due to the limited spectral sensitivity of silicon-cell pyranometers compared to the full solar radiation spectrum.
The spectral response graph compares the solar radiation spectrum at Earth's surface with the SP Series Pyranometer Spectral Response. Silicon-cell pyranometers are sensitive to the 350-1100 nm range. When the spectral content of solar radiation differs significantly from what the pyranometers were calibrated for, spectral errors result.
Silicon-cell pyranometers can measure shortwave radiation in conditions other than clear sky, but spectral errors may occur. The graphs below show spectral error estimates based on varying solar zenith angles and atmospheric air mass. The diffuser is designed to minimize directional errors, as shown in the cosine response graph in the Specifications section. The table provides spectral error estimates for various shortwave radiation sources relative to clear sky solar radiation.
Spectral error vs. Solar Zenith Angle: This graph shows spectral error as a function of solar zenith angle, assuming calibration at a 45° zenith angle. Error is generally low up to around 60°, then increases.
Spectral error vs. Atmospheric Air Mass: This graph shows spectral error as a function of atmospheric air mass, assuming calibration at an air mass of 1.5. Error increases with increasing air mass.
| Radiation Source (Error Calculated Relative to Sun, Clear Sky) | Error [%] |
|---|---|
| Sun (Clear Sky) | 0.0 |
| Sun (Cloudy Sky) | 9.6 |
| Reflected from Grass Canopy | 14.6 |
| Reflected from Deciduous Canopy | 16.0 |
| Reflected from Conifer Canopy | 19.2 |
| Reflected from Agricultural Soil | -12.1 |
| Reflected from Forest Soil | -4.1 |
| Reflected from Desert Soil | 3.0 |
| Reflected from Water | 6.6 |
| Reflected from Ice | 0.3 |
| Reflected from Snow | 13.7 |
Maintenance and Recalibration
Moisture or debris on the diffuser can cause low readings. While the domed diffuser and housing aid self-cleaning from rainfall, dust or salt deposits can accumulate and block the optical path. Dust or organic deposits should be cleaned with water or window cleaner and a soft cloth or cotton swab. Salt deposits should be dissolved with vinegar and removed similarly. Never use abrasive materials or cleaners on the diffuser.
The Clear Sky Calculator can help determine the need for recalibration. It calculates total shortwave radiation incident on a horizontal surface for any location and time. It is most accurate near solar noon in spring/summer months, with an estimated accuracy of ± 4% over multiple clear days. For best accuracy, the sky must be completely clear, as cloud reflection can increase incoming radiation above predicted values.
To check for recalibration needs, input site conditions into the calculator and compare measured total shortwave radiation to calculated clear sky values. If sensor measurements consistently differ from calculated values by more than 6% over multiple days near solar noon, the sensor should be cleaned and re-leveled. If discrepancies persist after a second test, contact calibration@apogeeinstruments.com.
Steps to Replace a Handheld Meter Battery:
- Use a Phillips head screwdriver to remove the battery cover screw.
- Lift and slide the outer edge of the battery cover away from the meter.
- Use your thumb to slide the battery out of the holder. If difficult, turn the meter on its side so the opening faces down and tap it gently against an open palm to dislodge the battery.
- Slide the new battery into the holder with the flat side facing up.
Troubleshooting and Customer Support
Verify Functionality: Pressing the power button should activate the LCD and show a real-time irradiance reading. Direct the sensor to a light source and verify the reading responds proportionally to changes in distance. Blocking all radiation should result in a zero reading.
Battery Life: A CR2320 battery typically lasts many months with continuous use. A low battery indicator appears when voltage drops below 2.8 V DC. The meter may function for a short time, but pushbuttons will eventually stop responding, and logged data may be lost. Pressing the power button puts the meter in sleep mode, which maintains logged measurements but still draws slight current. For long-term storage, remove the battery to preserve its life.
Low-Battery Error after Battery Replacement: A master reset usually corrects this error. If the indicator persists, double-check the new battery voltage is above 2.8 V.
Master Reset: If the meter is unresponsive or shows anomalies (like a persistent low battery indicator), a master reset can correct the problem. Note that this erases all logged measurements.
- Press the power button to activate the LCD.
- Slide the battery out of the holder; the LCD will fade.
- After a few seconds, slide the battery back in.
The LCD will flash segments and show a revision number (e.g., “R1.0”), indicating the master reset was performed and the display has returned to normal.
Error Codes and Fixes: Error codes appear on the LCD and flash until corrected. Contact Apogee if fixes do not resolve the issue.
- Err 1: Battery voltage out of range. Fix: Replace CR2320 battery and perform master reset.
- Err 2: Sensor voltage out of range. Fix: Perform master reset.
- Err 3: Not calibrated. Fix: Perform master reset.
- Err 4: CPU voltage below minimum. Fix: Replace CR2320 battery and perform master reset.
Modifying Cable Length: Additional cable can be spliced to the MP-200 sensor. The cable wires are soldered directly to the meter's circuit board. Care must be taken when accessing the board to splice the cable. Alternatively, two splices can be made between the meter and sensor head. For details on extending sensor cable length, visit: http://www.apogeeinstruments.com/how-to-make-a-weatherproof-cable-splice/.
