LI-6400XT Sensor Head


CO2/H2O Analyzers in the Sensor Head

The LI-6400XT leaf chamber is within the gas analyzer sample volume, providing real-time measurements of water vapor and CO2 exchange at the leaf surface. This results in a highly responsive system with automated control of chamber conditions that can rapidly detect and respond to changes in chamber CO2 concentration.

The absence of time delays allows fast, automatic control of chamber humidity at user-defined set points, even when the transpiration rate is changing. The absence of return tubing to the analyzers eliminates equilibration times due to water vapor sorption on the tubing walls.

Analyzer Operation

The LI-6400XT sensor head has two non-dispersive infrared gas analyzers, both of which measure absolute concentrations of CO2 and water vapor. Infrared radiation (IR) from the IR source passes into the leaf chamber mixing volume and is reflected twice 90° by gold-plated mirrors. After passing through the leaf chamber mixing volume, the radiation passes through optical filters and onto the IR detector. The band-pass filters limit the IR so the detector is alternately exposed to radiation in the sample and reference wavelengths for both CO2 and water vapor.

The detector housing is sealed and continuously purged of CO2 and water vapor to prevent interference from ambient CO2 and water vapor. The sample and reference analyzers can be electronically matched at any time without altering environmental conditions in the leaf chamber.

Auxiliary Input and Output Channels for Custom Applications

For applications that use additional electronic components (e.g., mixing fans in custom chambers, temperature sensors), the LI-6400XT provides user-programmable analog, digital, and pulse counting inputs; analog and digital outputs; and a regulated power supply.

Input channels:

  • Four differential analog
  • Two digital
  • One pulse counting

Output channels:

  • Eight digital open drain
  • Seven digital-to-analog (8-bit)
  • One digital-to-analog (12-bit)
  • Uncalibrated CO2 and H2O reference and sample analyzer outputs (100 ma)
  • 5V regulated power supply (100 ma)
  • Battery voltage (fused, 200 ma)

LI-6400XT Sensor Head - Side View

Mixing Fan

Leaf Chamber

CO2 & H2O Sample Optical Path

Mixing Volume

Chopper Housing with sample (upper) and reference (lower) detectors

CO2 & H2O Reference Optical Path

Thermocouple Assembly


LI-6400XT Sensor Head - Top View

Sample Analyzer Detector - solid-state, thermoelectrically cooled to minimize drift and noise

Peltier Cooler - for chamber temperature control

Focusing Lens - increases analyzer sensitivity by maximizing optical energy throughput

Gold Mirrors - enhance IR reflection and provide long-term stability

Mixing Fan - circulates air within sample analyzer and leaf chamber volume

Photodiode - feedback control for stable source output

IR Source (for both analyzers) - long life, high stability

Chopper Filter Wheel - tunes the detector and rejects IR radiation outside wavelengths of interest

Soda Lime/Desiccant - purges housing of CO2 and water vapor to prevent interference

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