A sonic anemometer determines instantaneous wind speed and direction (turbulence) by measuring how much sound waves travelling between a pair of transducers are sped up or slowed down by the effect of the wind. It has many applications such as studying atmospheric turbulence, monitoring air pollution, measuring evaporation and even indoor ventilation problems.

Short pulses of ultrasonic sound are exchanged on three different directions by couples of sound probes which are used alternately as transmitting and receiving units. Combining the sound velocities of different propagation directions, the 3-dimensional wind vector can be determined. Furthermore, the sound velocity in a motionless atmosphere is derived which corresponds to a measurement of the virtual temperature. The sound velocity depends not only on the temperature, but also on humidity.Therefore, the measured temperature represents the virtual temperature which is needed for most investigations of atmospheric stratification.

The sonic offers several major advantages over cup anemometers and bivane systems. Since there are no moving parts to come into dynamic equilibrium with the airflow, the sonic responds rapidly to wind velocity fluctuations. It responds linearly to wind velocity and is free from contamination from other velocity components as well as pressure, temperature, and relative humidity. The calibration of the sensor is established by its design parameters and, therefore, can be used as an absolute instrument.

Infrared Gas Analyzers (IRGA) - Licor LI-6262 and NOAA-ATDD

General principle - CO2 and H2O vapor absorb specific wavelengths of infrared radiation. By sending a tuned infrared beam through a sample area and measuring the signal attenuation, the density and concentrations can be calculated. There are two basic IRGA designs: closed-path and open-path.
Closed Path - The Licor LI-6262 is a non-dispersive, infrared gas analyzer. Concentrations of CO2 and H2O vapor are measured as the difference in absorption of infrared radiation passing through two enclosed gas sampling cells (a reference and a sample). The reference is used for a gas with known concentrations of CO2 and H2O vapor and the sample is used for the gas of interest. Infrared radiation is passed through both cells alternately with a chopper wheel and differences in light transmittance are translated to gas concentrations. The LI-6262 uses two detectors to provide separate outputs for CO2 and H2O. The reference gas typically used is dry N2 gas. When this gas is used in the reference cell of the analyzer, the reference for both CO2 and H2O vapor concentrations are zero. Infrared signal absorption is proportional to the concentration of CO2 and H2O vapor present in the sample cell. Closed-path gas analyzers require an external pump to draw the gas sample into the analyzer. It is considered that a flow rate of 10 liters min-1 is best used for eddy covariance measurements.

Open Path - The NOAA-ATDD open path gas analyzer is a non-dispersive, infrared gas analyzer. The open-path differs from the closed-path IRGA in that the infrared beam is not enclosed in any chamber and the gas being measured is allowed to flow over the sensor, in situ. A focused beam of infrared radiation is passed through the free-air and "folded" by mirrors until the beam reaches the signal detector. The amount of absorbed radiation is proportional to the density of CO2 and H2O vapor present in the air. Three absorption bands are measured: CO2, H2O, and a reference. The reference band normalizes the signal for changes in source intensity and absorption by dust and dirty optics. Since there is no reference gas, frequent and specific calibrations must be done that relate the signal output with known densities of CO2 and water vapor. Since the instrument is an open design and measures the air in situ, the detector is very fast response and eliminates specific issues associated with a pump and intake tubing.

Relative Humidity - Vaisala HMP 35C

Relative Humidity is determined by the use of a semicondutor that varies capacitance with the amount of water vapor in the air. A thin polymer film either absorbs or exudes water vapour as the relative humidity of the ambient air rises or drops. The dielectric properties of the polymer film depend on the amount of water contained in it: as the relative humidity changes, the dielectric properties of the film change and so the capacitance of the sensor changes. The electronics of the instrument measure the capacitance of the sensor and convert it into a humidity reading.

Precipitation - Texas Electronics TE525

Tipping bucket gauge is a popular instrument to measure precipitation. It consists of a collector, a triangular-shaped bucket, and electric transmitter. It generally sits titled to one side until there is a certain weight of rainfall caught (usually 0.01" or 0.1 mm). Once the preset weight of rainfall has accumulated, the bucket tips, empties, and exposed the other side to the rain. The tip takes 0.1s. The bucket makes an electrical contact during the tip that can be recorded to a datalogger. The tipping bucket gauge continuously measure the rain-fall with a ± 0.5% accuracy at 0.5 mm h-1 to ± 3.0% full scale between 25 mm h-1 and 100 mm h-1.

Temperature - Type-T Thermocouples

Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1¡C can be difficult to achieve. In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients.

The selection of the optimum thermocouple type (metals used in their construction) is based on application temperature, atmosphere, required length of service, accuracy and cost. When a replacement thermocouple is required, it is of the utmost importance that the type of thermocouple used in the replacement matches that of the measuring instrument. Different thermocouple types have very different voltage output curves. It is also required that thermocouple or thermocouple extension wire, of the proper type, be used all the way from the sensing element to the measuring element. Large errors can develop if this practice is not followed.

Temperature - NTC Thermistors

Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. Manufactured from the oxides of the transition metals - manganese, cobalt, copper and nickel, NTC thermistors are temperature dependant semiconductor resistors. Operating over a range of -200¡C to + 1000¡C, they are supplied in glass bead, disc, chips and probe formats. NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity.

PTC thermistors are temperature dependent resistors manufactured from barium titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level.

PAR Sensor - Licor LI190SB

The Licor PAR sensor measures solar radiation with a silicon photovoltaic detector mounted in a cosine-corrected head. The sensor accurately measures Photosynthetic Photon Flux Density (PPFD). PPFD is the number of photons in the 400 to 700 nm waveband incident per unit time on a unit surface. Because PPFD describes photosynthetic activity, the LI190SB is ideal for growth chambers and greenhouses. A shunt resistor in the sensor's cable converts the signal from uA to mV allowing the sensor to be measured directly by a Campbell Scientific datalogger .

Net Radiometer - REBS Q-7.1

A Net Radiometer measures the difference between the total incoming and outgoing radiation (all wavelengths). The Q-7.1 net radiometer is a high-output thermopile sensor that generates a millivolt signal proportional to the net radiation level. The sensor is mounted in a glass-rein-forced plastic frame with a built-in level. A ball joint is supplied on the stem to facilitate leveling. The sensor surface and surrounding surfaces are painted flat black to reduce reflections within the instrument and to achieve uniform performance over reflective and non-reflective surfaces. The net radiometer is subject to some degree of error caused by convective cooling as air moves past the sensors. Sensor surfaces are protected from excessive convective cooling by hemispherical polyethylene wind-shields. Wind correction is needed according to wind speed. Condensation on net radiometer windshields causes incorrect measurements. Polyethylene is used for the windshield material because it is transparent to both long and shortwave energy. The windshields are open to the atmosphere through a desiccated breather tube to prevent the domes from collapsing at night.

Soil Heat Flux - REBS HFT-3

Soil heat flux plates directly measure the sensible heat transfer across the soil surface. These sensors are typically two metal plates separated by a material of known conductivity. The temperature difference is measured by a thermopile which then outputs a millivoltage signal that is read by a data acquisition system. Thermopile detectors recognize thermal radiation. When there is a difference in temperature between the contact points of two different materials, electric voltage is produced. Typically, the output of this sensor is reported as W m-2, and usually referenced as G. Sensor placement is usually 1-2 cm below the soil surface.