HP 5890 SERIES II Service Manual page 250

Often confused with baseline upsets, an offset is a shift in the baseline that does not return quickly to
the original level. Baseline offsets may be caused by air leaks but more commonly are due to a change
in gas purity or flow rate in the detector. Poor carrier gas or improperly conditioned filters and traps
should be suspected whenever offsets occur.
Extraneous Peaks
Air peaks are sometimes seen in a chromatogram when leakage occurs because the valve rotor does
not seal properly. These leaks may not be detectable by using the soap- bubble method.
If a leak is suspected but cannot be located with soap bubbles, a pressure check will determine defi-
nitely if a leak exists. Extraneous peaks can occur sometimes due to contamination or improper condi-
tioning of the valve. If leaks are not apparent, clean or condition the valve.
Obviously other causes, totally unrelated to the valve, may exhibit similar symptoms. Impure carrier gas
(i.e., containing water) can cause extraneous peaks.
Locating Leaks
Leak- checking the plumbing involved in a valve configured system must be done carefully and me-
thodically. Several methods may be used, and the best choice will depend upon expediency, accessi-
bility, and the magnitude of the leak.
Pressure Check
The pressure check method will indicate, but sometimes not isolate, a leak in the flow path. Since this
method does not necessarily isolate the leak, other leak check methods may be needed to locate the
leak specifically.
Each valve in a system has two flow paths, ON and OFF. A leak sometimes occurs in
only one of these two positions. Check both.
1. Disconnect the detector from the valve system.
2. Cap the valve system at its outlet and pressurize to 689 kPa (100 psi). Allow 2 to 5 minutes for
pressure to equilibrate. If a flow sensor exists, it should read zero flow.
3. Turn the knob on the regulator counterclockwise until it turns freely. The regulator is now
turned off and the gauge is indicating pressure within the valve system.
4. Generally, the pressure will drop quickly for approximately 30- 60 seconds; then stabilize. After
this initial drop, the gauge should not indicate more than a 6.89 to 13.78 kPa (1 to 2 psi) drop
during a 10 minute period
5. If no leak is indicated, actuate all valves and repeat steps 2 through 4.
6. If a leak does show up, try to pinpoint the source using a soap bubble meter. Do not assume
that the leak exists only at the valve. Often plumbing connections such as unions or bulkhead
fittings are at fault. See " Valve Box Top Assembly Removal"in this section should it become
necessary to expose the valve system.
7. If the leak cannot be found easily, divide the system in half and repeat the pressure check.
Continue dividing by halves, and pressure check until the leak is isolated.
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SVC 7- 14