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HALO Users Manual
Revision G
30
4.3 Modes of Operation
The HALO has several modes of operation. Each mode has specific purpose.
Figure 4-7 Operating Mode Select Window
SERVICE Mode
The SERVICE mode is used to put the unit in standby to perform maintenance, or to make manual
adjustments. While in
SERVICE
mode no measurements are taking place.
TUNE Mode
The TUNE mode is used to determine the off-peak ring-down time of the system. This base line is used
in calculating the final concentration. The laser will be set to a wavelength away from the absorption
peak and several ring-downs are measured to provide a Tau Zero measurement.
Tune Cycle
The off-peak ring-down time is measured by performing a Tune cycle and is required upon initial
placement of the instrument. During shipment of the unit, it is possible that shock and vibration may have
caused slight changes in the alignment of the cavity. Running a Tune Cycle will automatically
compensate for any such changes.
We recommend running a Tune Cycle (to ensure precision of your measurements):
1. Upon installation.
2.
Whenever the sample gas is changed.
3.
Once per month even if the gas type does not change.
You should ALWAYS run a Tune Cycle when switching between an inert gas and a non-inert specialty
gas (e.g., HCl, HBr, NH3, etc.). Also when changing from one of the specialty gases to an inert.
You should run a Tune cycle if the contaminant level in the gas drops by more than 100 times (for
example going from 1000 ppb to 10 ppb).
Whenever possible, perform the Tau Zero measurement on the driest gas available.
To start a Tune Cycle, press
TUNE
on the Operating Mode Select Window. When the TUNE Cycle is
complete the unit will revert back to the main window.