There are four common causes for bad readings on LI-8xxx instruments:

1. Incorrect zero and span calibration parameters.
2. IR light source that is failing or has failed.
3. Contamination (dirt, corrosion, liquids) in the optical path.
4. Failed detector.

1)  Incorrect zero and span calibration parameters:

Check these values using the LI-COR interface software for each instrument by going to:

For LI-820 and LI-840, LI-840A: View – Calibration – Advanced

For the LI-8100, LI-8100A: Utilities – Calibration – Manual

The typical values for these parameters are as follows:

Solution: If the values are outside this range

a)  Reset them back to factory calibration values, which can be found on our website at licor.com/env/support

b)  Reset them to ‘1’ and perform a zero and span for each gas you adjust.

2)  IR light source that is failing or has failed.

The light source used in these instruments has a typical life time of 18,000 hours (2 years continuous use). If the unit has a failed light source it will typically report readings that oscillate between -50 ppm and some large value (2000 ppm or millions) for CO₂.

Check:

For LI-820, LI-840, and LI-840A: Remove the cover and inspect the IR light source assembly (located on the left side of the optical bench). A working light source will blink twice every second.

For LI-8100 and LI-8100A: Using the LI-COR interface software go to View – Instrument Status and review the field ‘Operating Hours’. If it indicates more than 18,000 hours the light source most likely needs replacement.

Solution – For all instrument the light source is a user replaceable component. The part number is 800-902 for LI-820, LI-840 and LI-840A. For LI-8100 and LI-8100A the part number is 8100-902. A user calibration (set zero and span for both CO₂ and H₂O) should be performed after the light source replacement.

3)  Contamination (dirt, corrosion, liquids) in the optical path

Check – Check the optical path for each instrument by following the instructions given in the respective manual.

Solution – Cleaning the optical path by following the instructions given in the respective manual.

4)  Failed Detector

Check – A failed detector will be unresponsive to changes in concentration. After all the above three items have been checked, run a zero and span (ambient) gas through the instrument; if there is no response then most likely the detector has failed.

Solution – For all instruments the detector is a user replaceable component. The part number is 800-903 for the LI-820, and 800-906 for the LI-840 and LI-840A, and for the LI-8100 and LI-8100A.