LI-6251 CO2 Analyzers with Drifting Problems or Unable to Zero or Span

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LI-6251 CO2 Analyzers may begin 'drifting' during the course of measurements. This drift occurs as a result from particulates, film, debris, water, chemicals, etc that collect in the Optical Cells of the Analyzer. This is usually more common with instruments that sample outside air. The Optical Paths (Reference and Sample Cells) are where the infrared light travels between the Source and the Detector and subsequently where the gas concentrations are measured. Infrared light from the Source is scattered and bounced along the length of the Cell before it reaches the Detector at the opposite end. Any film or debris that collects along the walls of the Cells will affect the light's transmission and cause inaccuracies in the CO2. In addition, any material in the Cells can absorb and desorb CO2 molecules, which in turn affect the stability of their measured concentrations.

Occasionally, this drift may become large enough that when attempting to 'Span' or 'Zero' the instrument, the potentiometer knobs (on the IRGA's front panels) are "maxed out" in either direction (either all the way down to 0.0 or all the way up to 10.0...very early models may not have numbered potentiometers). The knobs cannot be turned any further to eliminate a positive or negative offset when 'zeroing'. Similarly, the knobs may not be able to turn any further to reach a Span gas concentration.

Normally, when an Analyzer leaves LI-COR after calibration, the zero and span potentiometer knobs are set close to 5.0. The analyzer is designed to allow adjustment of these knobs accordingly during calibrations to continue using the instrument to collect accurate data, even when the Cells are slightly dirty. However, once those knobs reach either end of the scale (0.0 or 10.0) and adjustments are no longer possible, the analyzer needs to be returned to the factory for calibrations. However, there are a few things to try in the field or laboratory that might alleviate this situation.

First, set up the instrument so that it is plumbed as follows. Flow CO2-free (Nitrogen) gas through tubing that is split three ways between the ports: "Sample Cell IN", "Reference Cell IN", and "TO Chopper". Use a flow rate so that between 0.5 and 1.0 lpm flows through each of the three tubes. Does the Analyzer report 'zero' values when plumbed in this fashion? If not, record the values (offsets) that are being displayed...and if the potentiometers are maxed out, in which direction (0.0 or 10.0). Typically, if the values are negative, there is a problem with the Reference Cell. Alternatively, if the values are positive, there is a problem with the Sample Cell. This will help with troubleshooting if there is a decision to contact LI-COR. If the instrument can be ‘zeroed’ and calibrated, then the problem might have been due to leaky tubing, bad fitting connections, or saturated external chemicals.

Here are a couple things to check if the instrument shows 'zero' values in the test above...

  1. Check all the tubing for leaks.
  2. This includes the tubing on the back-side of the Analyzer connecting the Reference Cell, Chopper Housing, and Chemical Scrubbing Tube if in Absolute Mode...or just the Chopper Housing & Chemical Scrubbing Tube if in Differential Mode. Sometimes, leaks can develop inside the fittings, where they are not easily seen unless the fitting is unscrewed. Tubing can also split where it connects to the hose barbs of the Chemical Scrubbing Tube. Also, on older tubing, cracks can develop where sharp bends are made.
  3. Be sure that the External Chemicals are fresh.
  4. Chemicals should be fresh, not only in the external desiccant tube, but also in the supply bottles of Soda Lime (or Ascarite II). These have been known to age and saturate even in a sealed bottle. (If using an LI-6262, be sure the Magnesium Perchlorate is fresh too).

If the instrument was showing significant offsets in the set-up described above and the Analyzer still cannot be ‘zeroed’ or calibrated, follow the suggestions listed below...

  1. Calibration coefficients might be wrong.
  2. Check that the Calibration Coefficients being used during post- processing of the data are all correct and that none have been accidentally deleted.
  3. There may be some contamination in the instrument's Optical Path (in the Reference Cell and/or Sample Cell).
  4. It may be dust, water marks, moisture, debris, or chemicals from the scrubbing column. If there is dust/dirt/etc inside the cells, this may be able to be blown clean with some compressed air. Typically if the offset is negative, then the Reference Cell is contaminated. If the offset is positive, then the Sample Cell is contaminated. To do this, remove any external tubing from the OUTLET port of the Reference Cell. Connect the Reference Cell's INLET port to a Cylinder of compressed gas (with some tubing). Turn on the cylinder and send bursts of gas down through the Reference. Do not exceed 50 psi when doing this. If there are any loose particles in the Cell, this may blow them out. Do this for a few minutes, turning the gas on-and-off a few times. Do NOT Repeat this procedure with the "Sample Cell" INLET if the Analyzer has a Pressure Transducer. Very, very few LI-6251 Analyzers have a Pressure Transducer installed, however, if the Analyzer does have one, this will damage the Pressure Transducer, which is expensive to replace. If there is not a Pressure Transducer, the Sample Cell can be blown out as described above. If there is a Pressure Transducer installed, this can be disconnected by opening the Analyzer to access the internal components. Once opened, a clear tube can be found running between the top of the Sample Cell, over to the Pressure Transducer. The Pressure Transducer is attached inside the back panel of the analyzer, encased in Styrofoam with a brass bracket. Slide the clear tubing off of the silver post on the Pressure Transducer. Once dismantled, high- pressure air can be blown through the Sample Cell too. (Do not try to blow compressed gas through the Chopper Housing).
  5. Hopefully, this purging of the Cells will have blown out any particles that were causing an offset. However, it may also only partially fix the problem...depending on what was blown out and what still might remain inside. If an Offset still remains, it may be that there is some film or water markings inside that can't be blown out. Depending on the size of the offset, the Analyzer may be able to be zeroed, calibrated, and used. If it is still too large, then the analyzer would need to be sent back to LI-COR so the Cells can be opened and either cleaned or replaced.
  6. Change the Internal Chemicals if this has not been done within the past year.
  7. See Section 7 in the manual for instructions on how to do this.
  8. Chopper wheel issues. Another possible cause of this offset might not be debris inside the cell.
  9. Sometimes, on older instruments, the chopper wheel may begin to rub against the aperture inside. There is usually a very small gap (a couple sheets of paper thick). If they begin to touch and rub together slightly, this will cause the paint to flake off in a powdery form. This paint material can then deposit on the Reference Cell window, and thus create an artificial CO2 reading which would create an offset. If this is the case, the instrument will need to be sent to LI-COR for service.

In summary, check the results of plumbing the three Ports with zero gas. Next, follow through the steps listed to try and resolve the offset. If the problem has been solved, re-plumb the analyzer with the Chemical Scrubbing Tube back in line and calibrate the instrument.

If there was dirt and dust in the Cells inspect the external Air Filters that are filtering the sample air coming into the LI- 6251 Analyzer. We recommend that Gelman filters are used (LI-COR part number 9967-008). These have a 1 micron pore size and do a good job of keeping particles from entering the Analyzer. Maybe the current filters are too dirty or have pore sizes that are too large. Also check the Internal Air Filters as shown in the Manual in Section 8.5. In addition, blowing some gas and/or water through the sampling tubes (disconnected from the Analyzer and experiment) to make sure they are also clean.

These suggestions may prevent the LI-6251's return to LI-COR for servicing. If the above suggestions still cannot resolve the issue, then contact LI-COR to arrange an RMA number for shipping the instrument back.