LI-6400XT Performance: Technology
CO2/H2O Analyzers in the Sensor Head
The LI-6400 was the first photosynthesis measurement system to put the gas analyzers in the sensor head - minimizing measurement time delays. These analyzers feature a novel, open-path design with the optical bench of the sample analyzer open directly to the leaf chamber mixing volume.
Leaf dynamics are measured in real-time because the return tubing between the leaf chamber and the console has been eliminated. There are no time delays to confound correlations between gas exchange and changes in environmental driving variables such as light, CO2 mole fraction, etc.
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 also eliminates equilibration times due to water vapor sorption on the tubing walls.
The LI-6400XT sensor head has two complete, dual-path, non-dispersive infrared analyzers, which both measure absolute concentrations of CO2 and H2O.
Analyzer Operation
Infrared radiation from the sample analyzer source passes into the leaf chamber mixing volume and is twice reflected 90° by mirrors. The mirrors are gold-plated to enhance IR reflection and provide long-term stability.
After being reflected through the leaf chamber mixing volume, IR passes through a chopping filter wheel and into the sample analyzer detector. Absorption and optical reference wavelengths for CO2 and H2O are filtered by 1 of 4 narrow-pass filters on the chopping wheel.
The reference analyzer measures incoming gas concentrations and is located directly below the sample analyzer. The sample and reference analyzers can be matched at any time, either manually or automatically, without altering environmental conditions in the leaf chamber.
The sample analyzer detector, reference analyzer detector, and chopping filter wheel are sealed in a housing that is continuously purged of CO2 and water vapor to prevent interference.
Through years of experience, LI-6400XT systems have been proven to be robust and reliable, even in the most rigorous field conditions.
Spare Input and Output Channels
User-programmable analog and digital I/O channels are available to support external sensors. Input channels include five differential analog, two digital, and one pulse counting. Digital outputs: 8 open drain. Analog outputs: 7 D/A 8-bit, 1 D/A 12-bit, uncalibrated CO2 and H2O reference and sample analyzer outputs, +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
A fan in the mixing volume of the optical bench pushes air through inlet ports into the upper and lower sections of the leaf chamber. The fan draws air from the leaf chamber through a central flow path. The entire flow path is plated to minimize water sorption and can be disassembled and cleaned in the field without affecting factory calibration.
LI-6400XT Console
The LI-6400XT system console combines a data acquisition system with a high speed computer for dedicated data logging and computations. High speed analog to digital converters support fast response applications. The backlit graphical display allows any 12 experimental variables or 3 real-time graphs to be displayed at once. All computed variables are calculated and displayed in real-time at 2 Hz.
The 66 key ASCII keyboard is membrane sealed and designed to be used under harsh field conditions.
Battery Operation
The LI-6400XT System, including optional accessories like the LED light source, is powered by 12VDC batteries stored in the console. Four batteries and a battery charger are included with the system, providing 4 to 8 hours of operation. The optional 6400-70 AC Adapter fits in the battery compartment.