Using the LI-870 as a standalone gas analyzer

The LI-870 CO₂/H₂O Analyzer is based on the LI-850 CO₂/H₂O analyzer, but with a more rugged case and connectors. It was designed for soil flux measurements in conjunction with the 8200-01S Smart Chamber or the LI-8250 Multiplexer. However, it can also be used as a standalone analyzer for a variety of applications. This document describes a few necessary considerations.

Useful part numbers

Part Number	Description	Notes
300-07124	Male quick-connect fitting	One included in LI-870 spares kit
300-07125	Female quick-connect fitting	One included in LI-870 spares kit
8150-250	15 meters Bev-a-line tubing	4 meters included in LI-870 spares kit
870-772	AC power adapter
Switchcraft CA761KSxxxxx	DC power cable	From online retailers
392-17655	1.2-meter USB cable	Sealed USB-B connector to unsealed USB-A
392-17784	2-meter USB cable	Sealed USB-B connector to unsealed USB-A
392-06652	2-meter USB cable	Unsealed USB-B connector to unsealed USB-A; included in LI-870 spares kit
286-04198	12 VDC, 3.5 LPM pump	From online retailers

Connectors

The air inlet and outlet are quick-connect fittings. The spares kit includes one male and one female quick-connect fitting (part numbers 300-07124 and 300-07125, respectively). The spares kit also includes four meters of Bev-a-line IV tubing. If more tubing is required, a 15-meter roll of Bev-a-line can be purchased from LI-COR as part number 8150-250.

Power

The LI-870 can run on 10 to 17 VDC, and consumes up to 2 A during warmup. After warmup, it consumes about 0.42 A at 12 VDC on average.

For indoor applications, the 870-772 power adapter (purchased separately) allows the instrument to run on AC mains power. It is not recommended for outdoor use.

For outdoor applications, the LI-870 can run on 12 VDC batteries or a solar power system. A special adapter is required, not available from LI-COR. It is made by Switchcraft, part number CA761KSxxxxx (depending on the desired cable length). Generic connectors may be available from hobby shops. For example, CA761KS07984 has a 2-meter cable with bare wire leads. The red wire connects to the positive terminal of a battery, while the black wire connects to negative.

Communication cable

The LI-870 cannot log its own data—it needs to be connected to a computer, datalogger, or microcontroller to record data. It has an external USB-B connector. This connector can be sealed to the outside elements when used with the correct cable. LI-COR offers two cables with a sealed USB-B connector (the other end is an unsealed standard USB-A connector):

• 392-17655 is a 1.2-meter cable
• 392-17784 is a 2.0-meter cable

The LI-870 can also be used with a standard USB-B to USB-A cable. The spares kit even includes a standard 2-meter cable (part number 392-06652). However, a sealed connection is recommended for long-term outdoor deployment.

Flow rate

The LI-870 has a built-in pump that runs at 0.75 LPM. However, this may not provide adequate flow from the inlet, depending on the application. For example, if you were to connect a LI-COR 8200-104 chamber to the LI-870 directly, good air mixing in the chamber requires a flow rate of 1.7 LPM or higher. We recommend a flow rate no higher than 1 LPM through the LI-870, so a secondary pump would need to be added. LI-COR offers a 12 VDC, 3.5 LPM pump as part number 286-04198. The pump can be connected to the inlet of the LI-870 as shown in Air Pumps and Accessories for the LI-830, LI-850, and LI-7000.

In addition, some applications (like small volume injections) may not need a pump at all, instead relying on a carrier gas. We recommend a flow rate no higher than 1 LPM.

Logging data

The LI-870 has an XML output. XML can be logged in its default state by a terminal program on a computer, datalogger, or microcontroller. Or the XML string can be parsed into individual readings. Documentation about the XML output can be found in the Integrator’s Guide: https://www.licor.com/documents/945g90zvrem6z4tzlf5dk2ujl3g6hp46.

On a computer, the LI-8x0 Interface software can also be used to log data. It is available for Windows or Mac here. Once the LI-870 is powered on and connected to the computer via USB, open the software and click the Connect button in the middle. Select your analyzer from the list. Data will populate the window after connecting.

Click on the Setup Logging button in the bottom right corner to configure the log file. This log file will be a text file that can be opened in a text editor or spreadsheet program.

Turning off the pump

When using a carrier gas, you will need to turn off the internal pump. Download the LI-8x0 Interface Software and connect as described above. On the bottom right corner of the main window, there is a button to turn the pump off. You should only need to turn off the pump once—the setting is saved even if the LI-870 is turned off and on again.

Data analysis

If you are measuring CO₂ flux from a closed chamber, you will need to perform the calculations yourself. The Smart Chamber and LI-8250 Multiplexer perform flux calculations, but not the LI-870 analyzer by itself. You can follow these calculations given in Deriving the flux equation for CO2. The calculation requires measurements of CO₂, H₂O, temperature, and pressure, as well as total system volume and sample area or mass. You will need to know or estimate the temperature of the air inside the chamber, instead of using the temperature reading from the LI-870 (which is always maintained at 50 °C). The volume of the LI-870 is 33.5 cm³. Multiply the length of Bev-a-line IV tubing used (in centimeters) by 0.0792 to get the tubing volume in cm³.

If you are performing small volume injections into an open-flow system, you can use the LI-Integrator software (available for Windows) to calculate the peak and area under the curve. It is described here.