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Model
5526A
V.O.L.
Compensation Factor
The tables in this
section
indicate that the
correction value
to be applied for these
"standard"
conditions, for metric
or
English
units,
is
"728.6",
which
is the
"defau
lt"
value
bui lt into the
Measurement Disp lay unit for
"V.D.l.".
Df
course, this va lue
is
not likely to be
the correct one
for
your
particular
conditions;
the
remainder of
this
User's Guide
section provides detai
ls
about how to
make
a
more
exact
determination
of
the co
mpensation
number.
NOTE
If
you
do not
use
an Air Sensor
,a
nd do not manually
enter
a
V.D.L.
value,
"728.8"
(the
value
for "standa rd
"
conditions)
is
automati-
ca
lly
used.
HOW DO YOU DETERMINE THE V.O.L. COMPENSATION FACTOR?
1.
SENSOR
CONSIDERATIONS
To determine
a V.D.l.
compensation
value, you
must first determine
ambient
temperature,
pressure,
and relati
ve humidity.
You
should place
your
sensing
devices
so
that they measure the
environment close
to the measured
path
.
You shou ld
also
be sure to
allow
enough time for
the
se
nsors to
stabilize. The accuracy
with
which you
determine your
compensation
number
(and,
thus,
the
accuracy
o
f
your
Di stan
ce
measurement
)
will
be no
beller
than the
accuracy
with which
you measure your
ambient
conditions,
so
your
sensors
should
be
accurate.
As
a
guide, an error of one part-per-million wi
ll
occur in
the
distance
measurement
for
each error
of
-
•
1-degree Celsiu
s
(2
degrees
Farenheit
)
in
air
temperature,
or
-
•
2.5-
mm
(0.1
inch)
of mercury
in
pressure,
or
-
•
80% of
the
relative humidily.
2. " ABSOLUTE"
PR ESSURE VERSUS
" ADJUSTED"
PR ESSURE
The
ambient pressure
used
in
determining
your compensation number
must
be
"a bsolute"
pressure, no
t "adjusted
"
press
ure
(w
hi ch
is usua lly
absolute
pressu
re
that
has bce
n "corrccted"
10
sea level").
"
Adjusted pressure",
as
defined
here, is
the absolute pressure
that you
wou
ld measure at a
given
locat
ion
,
if
that location
was at
sea
level
and the weather
conditions
were
the
sa
me
.
Suppose,
for
example,
that you
were
in Denver
(Colorado,
U.S.A.)
at a time when
the weather report
sa
id the
pressure was
"762
mm
"
(30.00 inchesl
of
mercu
ry.
If
this
were an "adjusted
pressure,"
and since
Denver's
altitude is
about
1.5
km
(5000
feCI
),
Ihe absolute air
pressure
there
and
then
would
be
closer to
635
mm
(25
inches)
of
mercury.
To measure
pressure,
you
need an absolute
pressure
indica
tor
,
which
is
equivalent
to
a
barometer
that
has
not been correoed
to sea
level. Wh en
such
a
pressure
indica
tor
is
not readily avai
lable, you
can
make a
reasonable
approximation to
absolute
pressure
by
reducing
the adjusted
press
ure
obtained
from
the
nearest
weather
stat
ion
(at
the local
airport,
for
instance)
by
2.5
mm
(0.1
inch
)
of
mercury
for ea
ch
30
metres
(100
feel)
of
altitude.
ThaI
is
-
PABS
""
PADJ
-
altitude
x
2.5
PA
SS
=
PADJ
-
altitude
X
0.1
'"
100
(for
metric
units)
(fo
r En gli sh
units)
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