Flow and Pressure Control Part 1. Electronic Pneumatic Control (EPC)
Select a column mode
52
Select a column mode
The flow modes
Flow rates are corrected to NTP (normal temperature and pressure, 25°C and
1 atmosphere. For more detail, see pages 45 and 79.
• 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
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 changes, 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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