Table 1 Instruments and sensors for measurement and their calibration
From: Guidelines for measuring and reporting environmental parameters for experiments in greenhouses
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 |