Modei2DDD
Serie
s
DewPrime
Dew
Point
Hygrometers
----------
Maintenance and
Calibration----------
read near center scale when
th
e
ABC LED is
steadily
illuminated. It performs the same
fimctions
as the
ABC
circuitry
and
determines a starting point for the
ABC.
It
is
particularly useful to adjust this
BALANCE
control
when a new Sensor is attached to the
Model2000 Series
Co ol
Unit
After the
Model2000
Series Control Unit
has
been
on
for
more
than
one hour, depress the manual
ABC
(MABC)
.key
on the front
panel
This causes the
ABC
indicator to light steadily. Jf there is a dew layer on
the
Sensor miiror, the 10-LED
bargraph
CONTROL
indica-
tor will show the imbalance by being
completely
unlit
or
by having one LED
lit
As the dew
evaporates,
more
LEDs
will light, moving the
bargraph
toward the
center.
'When
the bars stop
illuminatin
g
and the ABC light is
on
in a steady condition, BALANCE
potentiometer
R52
can
be adjusted to
ma.ke
the CONTROL bars
indicate
as
close to the center of the scale
as
poSSiole (4-6
LEDs
lit).
It is preferable to make this
adjustment
after the
Sensor
mirror surface has
been
cleaned
(subsection
5.3).
Once
set, and as contaminants build up on the Sensor
mirror
surface, the CONTROL barn begin to
indicate
more
to
the left of center whenever the
instrument
is
in
its
automatic
self-standardization
(balance)
mode,
and
whenever
th
e
ABC
lamp
is on
steadily
and the
Sensor
mirror is dry.If the
IJli:lror
surface
is
knovm to be
clean,
and the CON1ROL bar LEDs
indicate
to the far left
or
right
with a dry miiTor
and
a
steady ABC
light,
then
R52
should be adjusted. After R52 is
adjusted,
and
contami-
nant buildup causes the
CONTROL
bars to
indicate to
the far
left
1Dlder
the
same
conditions
of dry
miiror
ABC
lamp
is on
steady,
the mirror should be
cleane
d
in
accordance with the standard
practice
outlined in
subsec-
tion5.3.
5.6.2
Balance
for
Sl Sensor
Move the selector switch
Sl
on the
main
board
away
from the
"660"
position.
Follo
w
the
directions
in
subsec-
tion 5.6.1 using
Rlll
(Bal2)
instead
ofR52.
5.6.3
Thickness
TI:UCKNES
S
control
potentiometer
R64 varies
the
thickness
of the dew layer on
the
Sensor mixror
surface
when the dew layer on the miiTor
surface
is
in
equili'b-
rium.
with
the
moisture in the air
sample
passing over
the
mirror. The
1HICKNESS
comrol has only a
minimal
effect on the dew point temperature.Any setting of
this
control represents
a
compromise between
fast
dynamic
response of the Sensor and insensitivity to
contaminants
on the miiror
surface.The Model2000
Series, as
shipped
,
has
been set for optimmn response with
mirror
cleaning
intervals in excess
of90
days for nonnal gas
sample
environments
.
(
The
time between mirror clean-
ing periods is determined largely by the quality of
the
filtering of particulate matter from the
gas
sample.)
If
a
faster response to changes in dew point
temperatures
is
required,
the
dew
layer thickness can be reduced.
Conversely, iflonger periods between mirror
cleanings
is required for the
operating
conditions
being
experi-
enced, the TID:CKNESS comrol can be
increased..
TinCKNESS
control
potentiometer
R64is a single-tum
device
.
Before
changing this control, always record
its
factoxy-set position for ease in returning to the
original
position at a later date.Use
Test
Points
1P12
and
1PDGND
for this
measurement.
If the
TI:UCKNESS
control is set too
thin,
i.e., few
bars
illuminate on the CONTROL indicator, the
Sensor
mirror temperature
will
not depress to the
dew
point
temperature
.
If
set
too thick, the Sensor will not
control
on the
dew
point
temperature,
but will continue to
cool
UDtil
a dew point
temperature,
approaching
the
maxi-
mum Sensor miiror
depression
from ambient tempera-
ture capability,is
reached.
Whenever the
TinCKNESS
comrol
is
adjusted,
always
depress the PABC key on the front panel to
perform
an
ABC cycle before returning to nonnal operation.
5.6.4
Gain
GAIN control
potentiometer
R57 is a
s:ingle-tum
control
similar to the
TillCKNESS
control
.
The GAIN
control
adjusts the overall gain of the
Model2000
Serie
s
control
loop that controls the proper dew point
temperature.
The
overall gain of the control loop is dependent on
the
electric gain of the control loop
circu:itry,
and
the gain
of
the condensation
phenomenon
on the dew point
Sensor
mirror
surface.The
gain of the condensation
phenom-
enon is related to the mobility of the condemate,
which
is both a ftmction of the absolute temperature and
the
state of the
condensate
(water or ice), being high
for
5-3
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