Model 2000 Series DewPrime
Dew Point Hygrometers
-----------
Theory of Operation
phototransistor gain. The combined outputs of the
direct
and bias
phototransistors
are used to drive the
Control
Amplifier
circuit.
If
the
mirror surface is dry,
this
Control
Amplifier circuit instructs the Cool/Heat
Power
Amplifier to cool the
mirror
surface.
When
too
much
dew
forms on the mirror
surface,the
Cool/Heat
Ampli-
fier reduces the cooling level to heat the minor
surface,
i
f
necessary.
This
circuitry,
i.e
.
,
the
phototransistor
sensing the
reflectivity
of the Sensor mirror surface,
the
Control Amplifier circuit, and the Cool/Heat
Amplifier,
is
connected to form a
servo-controlle
d
loop
.
When
operating, it adjusts itself
automatically
to control
the
Sensor
mirror surface
al
the
temperature
required
to
maintain
a
layer of dew on the mirror
surface
in equilib-
rium with the gas sample
aroun
d
it.
Whe
n
the Model 2000 Series is in the
Automatic
Balance Control (ABC) cycle mode, the servo loop
described above is interrupted, and the
Cool/Heat
Amplifier
is forced to heat the mirror
surD.ce
to
evapo-
rate any dew or frost present so that the circuitry may
automatically
compensate
for changes in mirror reflec-
tivity, should they occur. Once
this
compensatio
n
has
been
achieved, the loop is
reconnected
and allowed
to
control on the dew point temperature once
more.
4.2.2 Sensor PRT Resistance-to
-
Voltage
Converter
The temperature of the
mirror
surface
is measured
by
means
of a
Platinum
Resistance Thermometer
(PRn
embedded beneath the
mirror surface.The
resistance
of
the PRT device varies
almost
linearly with changes
in
temperature. The
circuitry
in this section interfaces
with
the PRT with
adjustments
for ZERO, SPAN, and
LINEARITY.
4.2.3 Automatic Balance Control
Circuits
1
The
Model2000
Series
automatically
verifies its
own
performance on a timed
sequential
basis by means of
an
automatic
self-standardization
circuit.This
circuit
adjusts
for changes in the
condition
of the
mirror
surface,
or for changes in any of the circuitry
associated
with the Sensor optical
system
and control
loop.The
Automatic Balance
Control
(ABC) cycle is
initiated
'Patented.
upon instrument tum-on. In addition, the ABC cycle
can
be initiated automatically at programmable
intervals.
An ABC cycle may also be initiated
manually
at
any
tiJne by depressing the MABC (Manual
Automatic
Balance Control) key on the Control Unit front
panel
keypad, or remotely via the RS-232
port.
Associated with the ABC cycle is the tiJne
required
to
complete the cycle. The bulk of the cycle tiJne is
made
up
of a period of time when
the
dew point Sensor
mirror
sur:&ce
is
being
heated above the ambient
tempeiature to
remove any condensate on it, either dew or
frost.
1bis
heat time is preprogrammed in
the
Model
2000.At low
temperatures, greater time is needed to evaporate
the
condensate.J
f
the temperature aithe beginning of
the
ABC cycle is greater
than
0°C, the heat time is
1.0
minute
.
If
the
temperature
is
0°C
and -25°C,
the
heat time is 2.0
minutes
.
At
temperatures below
-25°C,
the heat time is 4.0
minutes.
At
the end of the
selected
tiJne period, the circuit
automatically
balances
the
optical
bridge and control loop and returns to
normal
operation.
4.2.4 Alarm Set and Protection
Circuits
With the Model 2000 Series Control Unit, it is
possible
to establish set points for operation of each of two
alann
relays. Once the set point has been established,
between
-50
and+
1
00°C,
the
alarm
relay remains
de-energized
as
long as the
actual
dew point
does
not satisfy the
alann
set
point, and energizes as soon as the actual dew
point
satisfies the alarm
set
point. The outputs of each
relay-
a
single pole, double throw (SPOT)
contact-are
aU
brought to the rear panel connector for use in
customer-
provided
alarm
indicato
r
ciJ:cuits.
Protection
circuits
have been incorporated into
the
Model2000
Series electronics to prevent damage to
the
Sensor from overheating caused by normal
circuit
operations
.
The
first of these protection circuits is
a
thermostat installed in the base of the Sensor to
shut
down Sensor thermoelectric current in the event that
the
Sensor base temperature exceeds
+
1
05°C
.
Since
the
Sensor is designed to operate in ambient
temperatures
up
to
+
1
00°C
,
it is possible,
if
attempting to read vezy
low
dew points under these conditions, that the
energy
4-3
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