MULTICAL®
403
Kamstrup
A/S
∙
Technical
description
∙
5512
‐
1689_B1_GB_03.2017
47
4.4
Operating
pressure
of
MULTICAL®
403
In
connection
with
installations
it
has
proved
practical
to
work
with
minimum
the
pressure
mentioned
in
table
2
below:
Nominal
flow
q
p
[m³/h]
Recommended
back
pressure
[bar]
Max.
flow
q
s
[m³/h]
Recommended
back
pressure
[bar]
0.6…10
1
1.2…20
2
15
1.5
30
2.5
Table 2: Recommended back pressure at various flow sensor sizes
The values in the table apply to temperatures up to approx. 80 °C
The
purpose
of
recommended
back
pressure
is
to
avoid
measuring
errors
as
a
result
of
cavitation
or
air
in
the
water.
Cavitation
does
not
necessarily
happen
in
the
sensor
itself,
but
can
also
occur
as
air
bubbles
created
by
pump
cavitation
and
regulating
valves
mounted
before
the
sensor.
It
can
take
considerable
time
until
such
bubbles
have
been
dissolved
in
the
water.
Furthermore,
water
can
include
dissolved
air.
The
amount
of
air
which
can
be
dissolved
in
water
depends
on
pressure
and
temperature.
This
means
that
air
bubbles
can
be
formed
due
to
a
pressure
drop
in
the
installation,
e.g.
caused
by
a
velocity
rise
in
a
contraction
or
above
the
sensor.
The
risk
of
influence
from
the
above
is
reduced
by
maintaining
a
fair
pressure
in
the
installation.
In
relation
to
table
2,
the
steam
pressure
at
current
temperature
must
be
considered
too.
The
values
in
the
table
apply
to
temperatures
up
to
approx.
80
°C,
the
graph
in
Diagram 2
applies
to
higher
temperatures.
Furthermore,
it
must
be
taken
into
consideration
that
the
mentioned
pressure
is
the
back
pressure
at
the
flow
sensor
outlet
and
that
the
pressure
is
lower
in
a
contraction
than
before
one,
e.g.
in
case
of
cones.
This
means
that
the
pressure,
if
measured
elsewhere
in
the
installation,
may
differ
from
the
pressure
at
the
flow
sensor.
The
explanation
of
pressure
drop
due
to
velocity
increase
is
found
by
combining
the
continuity
equation
and
Bernoulli’s
equation.
The
total
energy
from
the
flow
will
be
the
same
at
any
cross
section.
It
can
be
reduced
to:
P
+
½
v
=
constant.
where: P = pressure,
=
density
,
v
=
velocity.
Dimensioning
a
flow
sensor
you
must
take
the
above
into
account,
especially
if
the
sensor
is
used
in
the
area
between
q
p
and
q
s
described
in
EN
1434
,
and
in
case
of
heavy
contractions
of
the
pipe.
Diagram 2. Steam pressure as a function of temperature
0
0,5
1
1,5
2
2,5
3
80
85
90
95
100
105
110
115
120
125
130
[b
ar]
[°C]
Steam pressure
