Table 1.
Instruments and sensors for measurement and their calibration
| What to measure | Units | Measured by | Precision of instrumentb | Accuracy of readingb | Calibrated by and when |
|---|---|---|---|---|---|
| Radiation (PARa) | µmol m−2 s−1 | Quantum sensor | ±1 % | ±10 % | Comparison with a reference sensor or against a standard quartz-halogen lamp traceable to a (inter)national standard (e.g. US NIST). Once per annum |
| Radiation (net) | W m−2 | Net radiometer | ±2 % | ±5 % | Comparison with a reference meter or in a temperature controlled calibration chamber capable of evaluating short and long wavebands. Once per annum |
| Radiation (spectral) | µmol m−2 s−1 nm−1 | Spectroradiometer | ±1 % | ±5 % | Comparison with a reference meter or against a standard source of radiation (e.g. ASTM G138-06). Once per annum |
| Irradiance (solar) | W m−2 | Pyranometer | ±1 % | ±5 % | Comparison with a reference meter or against a standard source (traceable to the World Radiometric Reference) in an integrating sphere. Once per annum |
| Radiation (integral) | MJ m−2 d−1 or mol m−2 d−1 | Calculated from accumulated radiation data | – | – | – |
| Air temperature | °C | RTD, thermocouple or thermistor (shaded and aspirated in air with speed ≥3 m s−1) | ±0.1 °C | ±0.2 °C | Comparison with a reference thermometer (e.g. traceable against US NIST) by placing in melting crushed ice and boiling distilled water. Once per annum |
| Substrate temperature | °C | RTD, thermocouple or thermistor (ensure good contact with substrate) | ±0.1 °C | ±0.2 °C | As above |
| Surface temperature | °C | Infrared temperature sensor, fine wire thermocouple | ±0.1 °C | ±0.2 °C | Infrared sensor: Comparison with a reference surface thermometer mounted on the same surface within the field of view of the infrared sensor Thermocouple: as above |
| Atmospheric moisture: relative humidity or vapour pressure deficit (VPD) | % or kPa | Capacitance, dewpoint sensor, psychrometer, or IRGA (infrared gas analyser) | Relative humidity: ±2 % Dewpoint temp.: ±0.1 °C VPD: ±0.3 kPa |
±5 % ±0.5 °C ±0.5 kPa |
Humidity generator; unsaturated salt solution calibration standards (35 and 80% RH). Once per annum |
| Air speed | m s−1 | Anemometer (range 0.1–15.0 m s−1) | ±2 % | ±5 % | Wind tunnel. Once per annum |
| pH | – | pH probe (range 3–10) | ±0.1 pH | ±0.1 pH | Standard solutions. Before every measurement, or weekly in continuous measurement applications |
| Electrical conductivity (EC) | S m−1 | Electrical conductivity meter | ±3 % | ±5 % | As above |
| Dissolved oxygen | mg L−1 | Dissolved oxygen meter (maintain adequate solution flow rate and ensure temperature compensation) | ±3 % | ±5 % | As above |
| Atmospheric CO2 concentration | µmol mol−1 | Silicon based NDIR (non-dispersive infrared) sensor as part of an IRGA (infrared gas analyser) | ±1 % | ±3 % | Certified calibration gases for low and high end of the measurement range, and/or precision gas mixing instrument. Once per week |
aReferred to as photosynthetically active radiation (PAR: 400–700 nm) for general usage and described as photosynthetic photon flux density (PPFD) by many journals, professional societies and manufacturers of quantum sensors.
bPrecision is how close the measured values are to each other. Accuracy is how close a measured value is to the actual (true) value.