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Operation Manual 87
Columns
The column flow mode determines whether pressure or flows are available as setpoints for
the GC inlets. When all columns in the flow path are defined, you can enter pressures or flows.
If any column in the flow path is not defined, then the settings for the inlet may be restricted
based on the inlet type and whether the column is in flow or pressure mode.
There are two different flow modes that control the mass flow rate through the column:
Constant flow and Ramped flow. The flow rates are corrected to NTP (normal temperature
and pressure, 25 °C and 1 atmosphere).
There are two different pressure modes that control the pressure at the head of the column;
Constant pressure and Ramped pressure. These pressures are gauge pressures—the
difference between the absolute pressure and the local atmospheric pressure.
Constant flow Maintains a constant mass flow rate of carrier gas in the column throughout
the run. If the column resistance changes due to a temperature program, the column head
pressure is adjusted to keep the flow rate constant. This can shorten runs significantly.
Ramped flow Increases the mass flow rate in the column during the run according to a
program you enter. A column flow profile can have up to three ramps, each consisting of a
programmed increase followed by a hold period.
The pressure modes are not available if the column is not defined or the inlet’s mode is set to
Flow control. Pressures are gauge pressures—the difference between the absolute pressure
and the local atmospheric pressure.
Because most detectors present little resistance to the column flow, the gauge pressure at the
column head is usually the same as the pressure difference between column inlet and exit.
The mass selective detector and the atomic emission detector are the exceptions.
• Constant pressure— Maintains a constant gauge pressure at the head of the column
throughout the run. If the column resistance and gas density changes during a run, the
gauge pressure does not change but the mass flow rate does.
• Ramped pressure— Increases the column head gauge pressure during the run according
to a program you enter. A column pressure profile can have up to three ramps, each
consisting of a programmed increase followed by a hold period.
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