
GDS-68XP Operation & Maintenance Manual, Revision 3.5
Page 12
4
OPERATION
OVERVIEW
The
GDS
‐
68XP
uses
low
cost,
reliable
electrochemical
sensors
to
detect
hydrogen
sulfide,
mercaptan,
THT
and
other
trace
gases
in
process
streams
that
contain
little
or
no
oxygen.
As
electrochemical
sensors
require
oxygen
to
operate,
the
GDS
‐
68XP
alternatively
exposes
the
sensor
to
the
process
flow
and
then
purges
the
sensor
with
ambient
air
to
maintain
the
sensor
in
a
sensitive,
oxygenated
state.
Figure 4-1: GDS-68XP Flow Diagram (Pressure)
At
the
beginning
of
each
sequence,
the
GDS
‐
68XP
performs
Conditions
the
Sensor
by
allowing
a
small
sample
of
gas
to
enter
the
sensor
flow
cell,
followed
by
a
fixed
5
minute
delay.
The
68XP
then
enters
Sample
Charge
during
which
sample
gas
flows
into
the
sensor
flow
cell
for
two
minutes,
or
until
sensor
output
exceeds
25%
of
scale.
Once
sample
pre
‐
charge
is
complete,
the
sequencer
enters
Read
mode
where
the
sequencer
processor
attempts
to
identify
a
stable
peak
value.
Once
the
sample
value
is
Purge
Air
Pump
Flow
Switch
Cal
Gas
Inlet
Sample
Inlet
Flow
Meter
Sample
Outlet
Sensor
Flow
Cell
Run
/
Cal
Valve
3
‐
Way
Valve
Pressure
Regulator
Purge
Air
Inlet
Filter
RAW
SENSOR
(Ch
1)
PPM
OUTPUT
(Ch
2)