The Open Path Solution

Designed for Eddy Covariance

The open path design of the LI-7700 provides elegant solutions to many of the challenges of measuring atmospheric CH4 flux over various ecosystems, while providing excellent co-spectra.

Quality Data

Eliminating pumps and tubing mean there are no pressure drops, water sorption effects, or time delays to confound the measurement.

 

Low Power Operation

Low power requirements for the entire system enable the LI-7700 to be powered with solar panels in remote locations (~ 8W during normal operation).

Low Maintenance

Self-cleaning optics mean low maintenance requirements and less data loss.

Data Output

High Speed

The LI-7700 features direct Ethernet data output at up to 40 Hz. Data can be output directly to a computer or routed through a network connection or modem for remote access. Seven analog input channels are available for collecting data from a sonic anemometer and thermocouples.

Flexible Inputs and Outputs

The optional LI-7550 Analyzer Interface Unit provides for a variety of additional data output options, including SDM, RS-232, and high speed DACs for connection to data logging devices. The LI-7550 also provides four additional auxiliary input channels for connecting other sensors.

Onboard Data Logging

The LI-7550 Analyzer Interface Unit logs data to an internal removable USB storage device. High-speed data logging is easily configured to record methane measurements and data from numerous auxiliary sensors (e.g. sonic anemometer, LI-7500 Open Path CO2/H2O Analyzer) through the user interface software. Simply remove the USB storage device and plug it into a computer to view your data.

Innovative Technology

How the LI-7700 Works

Recent advances in laser technology and advanced engineering enabled LI-COR to develop of the LI-7700 Open Path CH4Analyzer. The LI-7700 uses a single-mode tunable near infrared laser source, operating at ambient temperature, which greatly reduces power consumption and eliminates the need for cryogenic cooling systems.

Using Wavelength Modulation Spectroscopy (WMS), the laser scans across a single feature in the 2v₃ absorption band of methane near 1.6 microns with high resolution and at a high repetition rate. The wavelength is modulated at sub-MHz frequency, virtually eliminating 1/f noise of the laser source and allowing detection of fractional absorption (smaller than 10-5). The LI-7700 demodulates the resulting signal at twice the modulation frequency, and then compares it to a reference signal shape to determine CH4 concentration. Pressure and temperature induced changes in line shape and population distribution, as well as changes in laser power and mirror reflectivity are compensated using computational fitting algorithms. This ensures that measurements remain accurate over a wide range of pressure and temperature conditions.

This combination of innovative technologies allows for an open path measurement with the speed and resolution required for eddy covariance measurements of methane.

Advantages of WMS

Wavelength Modulation Spectroscopy has distinct advantages over other spectroscopic techniques used for trace gas measurements. It provides a strong signal with very low noise. This enables a shorter optical path-length with fewer reflections and reduces sensitivity to contamination of mirrors.

A multipass Herriott cell with off axis alignment allows for a short physical path length (0.5 m) but a 30 m total optical path length. Typical trace gas analyzers require optical paths up to several kilometers in length to make a comparable measurement.

The 0.5 m optical cell is optimal because it enables the methane density measurement with relatively few reflections. With fewer reflections, the mirrors in the optical path are less vulnerable to contamination than mirrors used in other spectroscopic instruments. Instruments that use Cavity Ring-Down Spectroscopy (CRDS) and Integrated Cavity Output Spectroscopy (ICOS) require mirrors with 99.99% or higher reflectivity.

The LI-7700 makes in-situ methane density measurements at ambient pressure. CRDS and ICOS rely upon vacuum pumps to draw down pressure in the optical cell which sharpens absorption lines.

LI-COR has licensed certain aspects of the LI-7700 that were developed with expertise from Southwest Sciences, Inc. (Santa Fe, NM).
This project was funded in part by the US Department of Energy through a Small Business Innovation Research grant (DE-FG02-05ER84283).

Preliminary Specifications*

LI-7700 Open Path CH4Analyzer. Includes power and Ethernet cables, calibration fixture, washer assembly, mounting hardware, instruction manual, spares kit, carrying case, and software CD.

LI-7700 Specifications

• Resolution (RMS noise): 5 ppb @ 10 Hz and 2000 ppb CH4
• Measurement Range: 0 to 25 ppm @ -25 °C, 0 to 40 ppm @ 25 °C
• Data Communication: Ethernet (up to 40 Hz)
• Detection method: Wavelength Modulation Spectroscopy 2f detection
• Linearity: Within 1% of reading
• Operating Pressure Range: 50 to 110 kPa
• Output Bandwidth: up to 20 Hz
• Operating Temperature Range: -25 to 50 °C
• Power Requirements: 10.5 to 30 VDC
• Power Consumption: 8 W nominal, 16 W during cleaning cycle
• Dimensions:
• Sensor: 14.33 cm dia (5.64 in), 82.8 cm height (32.6 in.)
• Optical Path: 0.5 m physical path (1.65 ft), 30 m measurement path (98.4 ft)
• Weight: 5.2 kg (11.5 lbs)
• User Interface: Windows®based software supports all setup, configuration, and calibration functions through Ethernet connection

LI-7550 Specifications

• Data Storage: Removable USB Storage Device. 4 Gigabyte Provided (expandable with user supplied Industrial Grade USB Flash Drive)
• Data Communication:
  • Ethernet: up to 40 Hz
  • Synchronous Devices for Measurement (SDM): up to 33.3 Hz
  • RS-232: 57,600 baud, 20 records per second max.
  • DAC: 0-5 V, 40 Hz
• Bandwidth: Up to 20 Hz
• Inputs: Ethernet, 4 analog inputs, differential ±5 V, 16 bit
• Operating Temperature Range: -25° to 50° C
• Power Requirements: 10.5 to 30 VDC
• Power Consumption: 10 W nominal
• Dimensions: 35 cm × 30 cm × 15 cm (13.8" x 12" x 6") external dimensions
• Weight: 4.4 kg (10 lbs)

*Specifications subject to change without notice.

The LI-COR Advantage

Reliability

LI-COR knows that field instruments require durability that bench-top instruments often lack. With that in mind, we designed the LI-7700 to make high-quality measurements through the extremes of the environment. Its lightweight design, extremely low power requirements, and low maintenance operation make it an ideal instrument for demanding field deployments.

Innovation

The scientific and engineering staff who lead the Environmental product line have collectively published and presented hundreds of posters, papers and platform talks at scientific conferences and in scientific journals over the last 38 years. LI-COR scientists are actively involved with the scientific community and regularly collaborate with leading scientists around the world.

Support

For over 38 years, LI-COR has developed innovative instruments and provided scientists with new tools for cutting edge research – and we will continue to do so. LI-COR operates with sound, enduring business principles, and you can count on us to support your LI-COR instruments for years to come.