LI-7000 Description

The LI-7000 CO₂/ H₂O Gas Analyzer was designed to expand on the capabilities of the LI-6262 CO₂/ H₂O Gas Analyzer.  It is the result of customer input over years of development, designed to meet the exacting standards our customers have come to expect from LI-COR. 

The design features that contribute to the LI-7000 performance are:

A long life (>20,000 hours), vacuum sealed tungsten filament source.  This source has low power consumption, and is highly stable. Nickel-plated optical cells to minimize water sorption. 

The optical cells can be removed and cleaned with compressed air or water (a cleaning kit is included with the LI-7000), and reassembled without requiring factory recalibration. CO₂ and H₂O detectors simultaneously measure CO₂ and H₂O vapor in the same air stream. 

A dichroic beam splitter at the end of the optical path provides radiation to two separate detectors, one filtered to detect radiation absorption of CO₂ and the other to detect absorption by H₂O. 

A spinning chopper shutter disk allows for radiation to alternately pass through each gas sampling cell.  The rotation of the disk is precisely controlled at a high chopping frequency, which reduces sensitivity of the detector to low frequency vibrations.  This results in lower analyzer noise, fast response, and increased sensitivity.

Digital Signal Processing (DSP) immediately digitizes the signal from the detectors.  All further signal processing is in the digital domain to avoid analog component drifts.

Modular components in user accessible areas of the analyzer provide a simple method of cleaning the gas sampling cells and changing internal chemicals. This minimizes the need for factory service, saving you down time and money.

Making Measurements

The LI-7000 CO₂/ H₂O Gas Analyzer is a differential analyzer, in which a known concentration (which can be zero) gas is put in the reference cell, and an unknown gas is put in the sample cell.  The instrument software provides continuous output of the absolute concentration in the sample cell as well as the differential concentration.

What is Reference Estimation Mode? In Reference Estimation Mode (REM), the reference cell gas concentration is estimated, based on previous calibration with a known concentration of gas in the reference cell. In REM, independent measurements of gas concentration in both cells are made; the differential is then calculated. An accurate differential measurement is obtained even when both air streams are changing. To use REM you simply choose a different operational mode. In the diagram at left, a simple switch allows for periodic sampling of a reference gas in the reference cell; the switch then allows sampling of the second (unknown) air stream.

Changing the mode of operation does not require any plumbing modifications; simply enter the reference gas concentration (if known), or choose REM operation.

The LI-7000 has two auxiliary input channels that can be used to input an analog signal from an external sensor.  CO₂ and H₂O references may be input through these channels, which is useful for automated calibration schemes.