Model
2000
Series
DewPrime
Dew
Point
Hygrometers
Theory of Operation
4.1 SYSTEM OPERATION
4.1.1 Main Control Loop
be EdgeTech
Model2000 SeriesDewPrime
Dew
:
_I
-
Point Hygrometer accurately measures the
dew
-
point of a
gas
by
cooling
a
su:rf3ce
over
which
the
gas
flows until a layer of dew forms on the
surface,
then
maintaining
the temperature of that surface such
that the layer of dew remains uniform. This control
loop
operates according to the method shown in the
block
diagram, Figure
4-1
.
The
surface is a
mirro
r
,
with
an
LED light source shining on it and reflecting to
an
optical
sensor. Another LED light source shines
directly
on a second optical sensor.The system is set up so
that
when the miiror is dry, a positive voltage is output
from
amplifier
Al.
This
positive voltage results in
positive
current from amplifier A2, a voltage-to-current
con-
verte
r
,
into the thermoelectric heater/cooler to cool
the
mirror .surface. When the dew point
temperatme
is
reached, a layer of dew
begins
to form on the
mirror,
resulting
in
decreased reflectance of the LED light and
a
positive signal at the input snmming point of
amplifier
Al.
Since Al is an inverting amplifier, the voltage
output
of A1 will reduce (possibly become negative) and
result
in
a
decrease
in cooling current (or
poSSioly
a
reversal,
causing heating to
occur)
.
The
temperature of the
mirror
surface is
maintained
closely in this manner, and
the
temperalUre of the mirror is monitored by means of
a
Platinum Resistance Thermometer (PRT),
embedded
beneath the miiror
surface.
4.1.2
Temperature
The PRT is part of a bridge, and the bridge
output,
proportional to the temperature of the mirror smface,
is
amplified by A3 and converted into a digital
value
approximately once per second, which is used for
display
of the dew point, analog output (via a
digital-to-analog
converter), monitoring of
alarm
.
levels,
and
RS-232
digital
outputs
.
4.1.3 Automatic Balance Control (ABC)
At programmable
intervals
,
the Model 2000 Series can
b
e
forced into an Automatic Balance Control (ABC)
cycle
t
o
compensate for the buildup of
contaminants
on
the
mirror
surface, which may change the optical
balance
of the
system.When
an ABC is begun, the control
loop
is intem.Jpted,
the
mirror is heated so that the dew
is
evaporated,
and
a new balance point is found
that
compensate
s
for the contaminants that may
have
accumulated
and caused a change in the reflectivity
of
the
mirror.The
control loop is then
reconnected,
and,
after some time, normal control
operatio
n
is
attained.
4
.
1.4
Microprocessor
Control
All timing fimctions, such as when and how often
ABCs
occur, parameter setting
such
as alarm
levels, and
outputs
,
both analog and
232,
are under
micropro-
cessor
control
.
All
parameters can be viewed
and/or
changed via the front panel keypad. All parameters
can
also be
viewed/changed
via the RS-232 port.
The
keyboard
menu system is arranged with the user
in
mind
.
All functions can be
acusse
d
by at most four
key
presses, some by
tw
o
.
When a key is
pressed,
the
microprocessor
stops its normal routine of
temperature
measurement
to service the user.No:rmal operation
is
resmned when the user is finished with the task,
or
when a time-out of about 10
seconds
between
key
presses
occurs.When
using the RS-232 port to
commu-
nicate with the
Model2000
Series, normal operation
is
interrupted.
4.2 CIRCUIT DESCRIPTIONS
4.2
.
1
Thermoelectric
Dew
Point
Tempenture
Control
Circuits
The
thennoelectric
dew point temperature
control
circuit
heats and cools the mirror
surface
of the
dew
point
Sensor
to the temperature necessary to have
a
layer of dew on the mirror
that
is in equilibrium
with
the
moisture
in
the
sample gas in the
Sensor
.
It
also
maintains
that equilibrium even though the
temperature
necessary to do so may vary.To accomplish this,
an
LED light source shining on the
mirror
surtace
of
the
Sensor is driven from a constant current
source.
This
circuit maintains the LED current constant regardless
of
changes in cable resistance, cable length,
temperature,
etc
.
The
light reflected from the
miiror
surfac
e
in
the
Sensor is detected by a
direct
phototransistor
.
A
sepa-
rate LED and phototransistor are also located in
the
Sensor and serve as bias controls on the effects
of
temperature
changes on the LED output intensity
and
4-1
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