There are four common causes for bad readings on LI-8xxx instruments:
- Incorrect zero and span calibration parameters.
- IR light source that is failing or has failed.
- Contamination (dirt, corrosion, liquids) in the optical path.
- 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.
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.