Years of data depend on
fractions of a second

Eddy covariance is a micrometeorological method for measuring the vertical exchange of a quantity such as carbon dioxide, water vapour or heat between a surface and the atmosphere. It works by measuring the vertical wind component and the quantity of interest at the same point, many times a second, and taking the covariance of the two over an averaging period, typically 30 minutes.

Because the transport happens in short-lived turbulent eddies rather than as a steady flow, the instrumentation has to resolve those eddies as they pass. That is what sets the requirement for fast 3-axis wind measurement rather than a conventional weather station.

The three-dimensional wind vector, at a rate fast enough to resolve the turbulent eddies carrying the flux, with a known and stable response. The vertical component is the one the flux calculation depends on, and it also needs sonic temperature from the same measurement so that the sensible heat flux and the density corrections can be derived from a single instrument.

Head geometry and calibration matter as much as the headline accuracy figure, because the sensor structure itself sits in the flow it is measuring.

High enough to resolve the turbulent structures carrying the flux, which for most surfaces means tens of hertz. The R3-50 and HS-50 output at 50 Hz and the R3-100 and HS-100 at 100 Hz.

Because the flux is a vertical transport, and the vertical wind component is the term the calculation is built on. A 2-axis anemometer measures horizontal speed and direction and simply does not produce the quantity the method needs.

Measuring all three axes at the same point also allows the coordinate rotation or planar fit that flux processing applies to correct for tilt and terrain, which cannot be done from horizontal wind alone.

Symmetrical for all-direction work where the footprint moves with the wind rose. Horizontal where accurate vertical-flow analysis matters and the sensor geometry itself would otherwise disturb the flow being measured.

The head is a physical object standing in the flow it measures, so its geometry decides which wind directions are disturbed by the structure and how much of the vertical component reaches the measurement path cleanly.

A symmetrical head accepts wind from all directions on equal terms, which suits sites where the footprint moves around the wind rose. A horizontal head opens up the vertical path, which suits work where vertical-flow analysis is the point and structural disturbance of that component has to be minimised. Neither is universally correct: the choice follows the site and the science.

0 to 45 m/s across R3-50, R3-100, HS-50 and HS-100, with 0 to 359 degrees on direction. These are accuracy and data-rate instruments rather than storm-survival sensors.

The Research range runs from 9 to 30 V DC, drawing less than 150 mA at 24 V DC and under 4 W in operation. MetConnect runs from 5 to 30 V DC at less than 70 mA at 12 V DC.

RS422 is native on the Research range. RS232 and analogue outputs come through the PCI or PCIA interface, which needs to be specified alongside the instrument.

Match the output format and the calibration traceability, and where possible run the old and new instruments in parallel for a season so the relationship between them is measured rather than assumed.

Because a flux dataset is used years after it is collected, often by people who were not there when it was collected, and its value depends on being able to say what the instrument was doing at the time. Calibration is what turns a plausible number into a defensible one.

The Research range is supplied with individual calibration as standard rather than a type figure, and Labcal is an independent UKAS-accredited laboratory, Lab 0625, operating to ISO/IEC 17025:2017. Recalibration on a stated interval gives the series a documented history rather than an assumption.

Set by the group and by what the data is used to argue. Where results are published or feed a network protocol, an annual traceable calibration is common.

Labcal is UKAS-accredited to ISO/IEC 17025:2017 and calibrates air velocity, temperature, humidity, pressure, and gas and liquid flow.

Radiation and soil heat terms alongside the turbulent fluxes. MetConnect takes a four-wire Pt100 to IEC 60751 and two analogue inputs, with 5 V available on the sensor connectors, so soil probes and a compatible radiometer can feed the same record.

Yes. MetConnect accepts a four-wire Pt100, two analogue inputs at 0 to 5 V or 4 to 20 mA, and a contact closure for a tipping-bucket rain input.

Both measure three axes. The Research range goes to 50 or 100 Hz with individual calibration as standard, built for measurement quality. WindMaster tops out at 32 Hz and covers general 3-axis work and additional profile levels.

Start with the rate the science needs, then the head geometry, then the site temperature. The R3-50 and HS-50 output at 50 Hz, the R3-100 and HS-100 at 100 Hz. R3 heads are symmetrical, HS heads are horizontal and carry a built-in inclinometer for levelling on a tower.

Operating temperature is the specification that most often decides it. The R3-50, R3-100 and HS-50 operate from -40 to +60 degrees C. The HS-100 is 0 to +50 degrees C, so it is not the instrument for a cold-climate or high-altitude deployment.