LI-COR Li-7500 Gas Analyser

LI-COR LI-7500 Gas Analyser

Purpose of instrument:-

The LI-COR LI-7500 Gas Analyser is used to make rapid response measurements of water vapour and carbon dioxide concentration in the atmosphere. When combined with simultaneous measurements from a co-located sonic anemometer, humidity and CO₂ fluxes can be calculated, using the eddy-covariance method.

LI-7500 Sensor head

Principle of operation:-

The LI-COR LI-7500 Gas Analyser operates by measuring the absorption of infrared radiation at different wavelengths. The picture above shows the sensor head of the instrument. Sapphire windows protect the infrared source and detector. The optical path length is 12.5cm.

For each gas, the power recieved from the infrared source is measured at two wavelengths - one at a wavelength that is absorbed by the gas, and the other at a non-absorbing reference wavelength. The absorbance is then approximated by:-

   a_i = 1 - A_i/A_io

where A_i is the power received at the absorbing wavelength and A_io is the power received at the reference wavelength. This measurement is carried out by the LI-7500 at a rate of 152Hz. A value of a is thus obtained for both H₂O and CO₂ (after making a correction for the cross-sensitivity between the two gases), and this is used by the LI-7500 to produce a calibrated output of the number densities (in mmol/m³) of the gases (or, alternatively mass density, which is obtained by simply multipying the molar concentration by the molecular weight of each of the gases.)

There is a small pressure dependency in the conversion of the absorbances of the gases to number density (and for CO₂, also a temperature dependency), which the LI-COR accounts for using sensors built into the electronics enclosure. The accuracy of these measurements does not have to be very high.

In order to convert the output of the LI-7500 into more useful quantities, however, (such as converting H₂O number density into a specific humidity [g/kg], or CO₂ number density into molar fraction [ppm]), accurate pressure and temperature measurements of the air are required. Post-processing of the LI-COR data uses pressure and temperature sensors located on the 10m mast to perform these calculations.

Further information on this instrument can be obtained from the manufacturer's website: http://www.licor.com

Operational notes:-

Problems have been found to occur with this instrument in the presence of precipitation. In fact, all data from the instrument, when there is rain, snow, fog or ice on the windows, should be treated with caution.