Cyclic Short Term Drift; Continuous Upward Or Downward Drift - ABB PGC2000 Operation And Service Manual

8. A continuous presence of sample at the detector due to sample valve leakage. Perform Test
Procedure 3 (Sample Valve Leaking/Column Flooding).
9. Filament failure. Perform Test Procedure 6 (Filament TCD Test).

Cyclic Short Term Drift

Symptom: Baseline (detector signal) is experiencing a cyclic short term drift (see Figure 6-2)
indicated by detector signal, graphics display, or strip chart recorder (if used). If cyclic behavior is
only present on strip chart recorder, verify recorder is functioning properly and that the recorder
connecting wires are properly shielded. Ensure shield is connected to ground at only one end.
BASELINE
At the extreme, the signal may indicate a cyclic behavior with the period of each cycle ranging from a
few seconds to several minutes.
Causes:
1. Short term drift, especially cyclic operation, could be due to temperature out of control at the TC
detector or isothermal oven, "dirty" carrier, or defective flow controller. Perform Test Procedure
30 (Checking Temperature Table Values), steps 1 to 3, 8 and 9; Test Procedure 8 (Baseline
Cyclic Temperature Controller (TCD)); Test Procedure 9 (Baseline Cyclic Carrier Regulator); or
Test Procedure 10 (Baseline Cyclic Flow Controller), as applicable.
2. Another possible cause is cell vent back pressure changing. Disconnect vents to check. Perform
Test Procedure 34 (Vent Back Pressure).

Continuous Upward or Downward Drift

Symptom: Baseline (detector signal) is slowly drifting upward or downward (see Figure 6-3) as
indicated by the detector signal, graphics display, or strip chart recorder (if used).
BASELINE
Cause: This drift characteristic is generally related to detector response. Most common causes are
temperature zone temperature drift, column flooding or contamination by a sample containing heavy
hydrocarbons, leaking sample valve. Temperature zone drift normally occurs during temperature
zone warm up. Column bleed is less pronounced in capillary columns than packed columns.
6-2
TIME
NORMAL
Figure 6-2. CYCLIC SHORT TERM DRIFT
TIME
NORMAL
Figure 6-3. CONTINUOUS UPWARD DRIFT
BASELINE
CYCLIC DRIFT
BASELINE
UPWARD DRIFT
TIME
TIME
2000-OSM, F1