is activated, reducing
the
mirror
tem-
perature
until
condensation of
atmo-
spheric
water
vapour
occurs.
The
tem-
perature
at
which
this
water
vapour
condenses on
the
mirror surface is
known as
the
dew-point.
Above
the
mirror
there
is a Light
Emitting
Diode (LED)
and
a light
sensitive
transistor
(receiver).
These
are
arranged
so
that
light from
the
LED
is
not
normally
detected
by
the
receiver. However, when condensation
· occurs, a film of dew coats
the
mirror.
Light from
the
LED
is
then
scattered
by
the
dew, some of
it
being
detected
by
the
receiver. An electronic circuit
regulates
the
cooling of
the
mirror so
as to
maintain
a stable film of dew.
When
the
temperature
has
stabilized
the
dew is assumed
to
be stable
and
the
mirror
temperature
is measured
by a
PtlOO
temperature
sensing ele-
ment
below
the
mirror.
To
conserve power, a dew-point
measurement
is
taken
only every
third
minute.
The
presence of bodies capa-
ble of
emitting
water
vapour
may
cause
the
dew-point to vary slightly.
When
the
measured dew-point is un-
stable, a message
to
that
effect is dis-
played.
Air
Velocity
Transducer
The
Air Velocity
Transducer
(MM 0038) is based on
the
constant
temperature
anemometer
principle.
Air velocity is
measured
as a function
of
the
heat
loss from a
heated
body, by
measuring
the
power
input
required
to
maintain
a
constant
temperature
dif-
ference between
the
two sensor ele-
ments
(Fig. 6).
The
heat
loss is, however, also a
function of
the
temperature
and
direc-
tion
of
the
airflow,
and
the
radiative
exchange with
the
surroundings.
The
errors associated with
these
effects are
eliminated
through
the
design
and
construction of
the
MM
0038 which
uses two identical
temperature
sensi-
tive ellipsoidally
shaped
bodies, one of
which is
heated
and
the
other
unheat-
ed.
The
eccentricity of
the
ellipsoids
and
the
length
of
the
heating
coils are
optimised
to
give
the
smallest possible
variation
in
directional sensitivity.
The
controlled electrical heating
maintains
a
constant
temperature
dif-
ference
of
l5°C
(27°F),
independent
of
the
ambient
temperature,
between
the
two bodies.
Inputs
The
five
different
types
of
Trans-
4
Fig.
7 Four Transducers mounted on the Delta
Arm
and Tripod
ducer
can
be simultaneously connect-
ed
to
the
Analyzer.
(Tripod
mounting
of four
of
the
Transducers
is shown in
Fig. 7).
With
the
exception of
the
Air
and
Surface
Temperature
Transduc-
ers,
the
Transducers
have
different
in-
put
plugs, which eliminates
the
possi-
bility of incorrect connections.
The
Air
and
Surface
Temperature
Trans-
ducers have identical
inputs
enabling
either
two Air or two Surface
Tem-
perature
Transducers
to
be used
at
the
same time.
A system configuration of
the
In-
door Climate Analyzer
Type
1213 is
shown in Fig.
9.
Signals from
the
2
B:
146
Btu/ft
2
h
F.TEMP.
TEMP.
21
. 0 o C
0.04
m/s
---
TEST!
HG
---
Transducers
are
fed
into
analogue cir-
cuits which provide signal condition-
ing.
The
analogue signals
are
then
converted
to
digital
data
via a twelve
bit
AID Converter.
Microcomputer
Contained
within
the
Microcomput-
er
are a real
time
digital clock, a
ROM
(Read Only Memory) for
program
and
algorithm storage
and
a RAM (Ran-
dom Access Memory) for storage
of
the
measured
data.
The
Type
1213
also incorporates a
D/
A
Converter
which provides linearized analogue
signals
to
the
analogue
outputs
on
the
rear
panel of
the
Type
1213.
47.3
° F
II
VAP.
PRES.
.921
kPa
II
0.
03
m/sl
l
II
REL.HUMIDITY
36
X II
MEASURIHG
II
COHHECT TRAHSDUCER
II
II
SELECT
RECORD-TIME
DISPLAY
DR
PLOT
?
II
ELAPSED
TIME
o.
osll
GAIH
CALIBRATIOH
~
REPLAYIHG
SELECT
REPLAY
DATA
II
SAMPLE
HO.
II
840941
Fig.
8 Examples
of
some
of
the Indoor Climate Analyzer
Type
1213 Displays
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