The filters on the intake assemblies should keep the IRGA sample cell clean for a long time. A long-term test of an AP200 outside Logan, UT, showed that the sample cell was still clean after a year of operation. However, if it becomes difficult to span the LI-840A, if there is excessive noise, or if there is excessive drift in either the CO2 or H2O zero, then it may be necessary to clean the sample cell. Refer to the instructions in the LI-840A user manual.