<s>
The	O
selection	O
criteria	O
include	O
the	O
physical	O
:	O
phase	O
(	O
liquid	O
,	O
solid	O
or	O
slurry	O
)	O
,	O
temperature	O
,	O
pressure	O
or	O
vacuum	B-General_Concept
,	O
chemistry	O
,	O
dielectric	O
constant	O
of	O
medium	O
,	O
density	O
(	O
specific	O
gravity	O
)	O
of	O
medium	O
,	O
agitation	O
(	O
action	O
)	O
,	O
acoustical	O
or	O
electrical	O
noise	O
,	O
vibration	O
,	O
mechanical	O
shock	O
,	O
tank	O
or	O
bin	O
size	O
and	O
shape	O
.	O
</s>
<s>
Also	O
important	O
are	O
the	O
application	O
constraints	O
:	O
price	O
,	O
accuracy	O
,	O
appearance	O
,	O
response	O
rate	O
,	O
ease	O
of	O
calibration	B-General_Concept
or	O
programming	O
,	O
physical	O
size	O
and	O
mounting	O
of	O
the	O
instrument	O
,	O
monitoring	O
or	O
control	O
of	O
continuous	O
or	O
discrete	O
(	O
point	O
)	O
levels	O
.	O
</s>
<s>
These	O
include	O
vibrating	O
,	O
rotating	O
paddle	O
,	O
mechanical	O
(	O
diaphragm	O
)	O
,	O
microwave	O
(	O
radar	B-Application
)	O
,	O
capacitance	O
,	O
optical	O
,	O
pulsed-ultrasonic	O
and	O
ultrasonic	B-Application
level	O
sensors	O
.	O
</s>
<s>
Typical	O
systems	O
for	O
point	O
level	O
detection	O
in	O
liquids	O
include	O
magnetic	O
and	O
mechanical	O
floats	O
,	O
pressure	O
sensors	O
,	O
electroconductive	O
sensing	O
or	O
electrostatic	O
(	O
capacitance	O
or	O
inductance	O
)	O
detectors	O
—	O
and	O
by	O
measurement	O
of	O
a	O
signal	O
's	O
time-of-flight	O
to	O
the	O
fluid	O
surface	O
,	O
through	O
electromagnetic	O
(	O
such	O
as	O
magnetostrictive	O
)	O
,	O
ultrasonic	B-Application
,	O
radar	B-Application
or	O
optical	O
sensors	O
.	O
</s>
<s>
Ultrasonic	B-Application
level	O
sensors	O
are	O
used	O
for	O
non-contact	O
level	O
sensing	O
of	O
highly	O
viscous	O
liquids	O
,	O
as	O
well	O
as	O
bulk	O
solids	O
.	O
</s>
<s>
Ultrasonic	B-Application
level	O
sensors	O
are	O
also	O
affected	O
by	O
the	O
changing	O
speed	O
of	O
sound	O
due	O
to	O
moisture	O
,	O
temperature	O
,	O
and	O
pressures	O
.	O
</s>
<s>
Turbulence	O
,	O
foam	O
,	O
steam	O
,	O
chemical	O
mists	O
(	O
vapors	O
)	O
,	O
and	O
changes	O
in	O
the	O
concentration	O
of	O
the	O
process	O
material	O
also	O
affect	O
the	O
ultrasonic	B-Algorithm
sensor	I-Algorithm
's	O
response	O
.	O
</s>
<s>
Since	O
the	O
ultrasonic	B-Algorithm
transducer	I-Algorithm
is	O
used	O
both	O
for	O
transmitting	O
and	O
receiving	O
the	O
acoustic	O
energy	O
,	O
it	O
is	O
subject	O
to	O
a	O
period	O
of	O
mechanical	O
vibration	O
known	O
as	O
"	O
ringing	O
"	O
.	O
</s>
<s>
The	O
requirement	O
for	O
electronic	O
signal	O
processing	O
circuitry	O
can	O
be	O
used	O
to	O
make	O
the	O
ultrasonic	B-Algorithm
sensor	I-Algorithm
an	O
intelligent	O
device	O
.	O
</s>
<s>
Ultrasonic	B-Algorithm
sensors	I-Algorithm
can	O
be	O
designed	O
to	O
provide	O
point	O
level	O
control	O
,	O
continuous	O
monitoring	O
or	O
both	O
.	O
</s>
<s>
Due	O
to	O
the	O
presence	O
of	O
a	O
microprocessor	O
and	O
relatively	O
low	O
power	O
consumption	O
,	O
there	O
is	O
also	O
the	O
capability	O
for	O
serial	O
communication	O
from	O
to	O
other	O
computing	O
devices	O
making	O
this	O
a	O
good	O
technique	O
for	O
adjusting	O
calibration	B-General_Concept
and	O
filtering	O
of	O
the	O
sensor	O
signal	O
,	O
remote	O
wireless	O
monitoring	O
or	O
plant	O
network	O
communications	O
.	O
</s>
<s>
The	O
ultrasonic	B-Algorithm
sensor	I-Algorithm
enjoys	O
wide	O
popularity	O
due	O
to	O
the	O
powerful	O
mix	O
of	O
low	O
price	O
and	O
high	O
functionality	O
.	O
</s>
<s>
Microwaves	O
(	O
also	O
frequently	O
described	O
as	O
radar	B-Application
)	O
,	O
will	O
penetrate	O
temperature	O
and	O
vapor	O
layers	O
that	O
may	O
cause	O
problems	O
for	O
other	O
techniques	O
,	O
such	O
as	O
ultrasonic	B-Application
.	O
</s>
<s>
Microwaves	O
are	O
electromagnetic	O
energy	O
and	O
therefore	O
do	O
not	O
require	O
air	O
molecules	O
to	O
transmit	O
the	O
energy	O
making	O
them	O
useful	O
in	O
vacuums	B-General_Concept
.	O
</s>
<s>
Two	O
basic	O
signal	O
processing	O
techniques	O
are	O
applied	O
,	O
each	O
offering	O
its	O
own	O
advantages	O
:	O
pulsed	O
or	O
time-domain	O
reflectometry	O
(	O
TDR	O
)	O
which	O
is	O
a	O
measurement	O
of	O
time	O
of	O
flight	O
divided	O
by	O
the	O
speed	O
of	O
electromagnetic	O
waves	O
in	O
the	O
medium	O
(	O
speed	O
of	O
light	O
divided	O
by	O
the	O
square	O
root	O
of	O
the	O
dielectric	O
constant	O
of	O
the	O
medium	O
)	O
,	O
similar	O
to	O
ultrasonic	B-Application
level	O
sensors	O
,	O
and	O
Doppler	O
systems	O
employing	O
FMCW	O
techniques	O
.	O
</s>
<s>
Just	O
as	O
with	O
ultrasonic	B-Application
level	O
sensors	O
,	O
microwave	O
sensors	O
are	O
executed	O
at	O
various	O
frequencies	O
,	O
from	O
1GHz	O
to	O
60GHz	O
.	O
</s>
<s>
The	O
first	O
is	O
done	O
by	O
monitoring	O
a	O
microwave	O
signal	O
that	O
is	O
transmitted	O
through	O
free	O
space	O
(	O
including	O
vacuum	B-General_Concept
)	O
and	O
reflected	O
back	O
,	O
or	O
can	O
be	O
executed	O
as	O
a	O
"	O
radar	B-Application
on	O
a	O
wire	O
"	O
technique	O
,	O
generally	O
known	O
as	O
guided	O
wave	O
radar	B-Application
or	O
guided	O
microwave	B-Application
radar	I-Application
.	O
</s>
<s>
Non	O
contact	O
microwave-based	O
radar	B-Application
sensors	O
are	O
able	O
to	O
see	O
through	O
low	O
conductivity	O
'	O
microwave-transparent	O
'	O
(	O
non-conductive	O
)	O
glass/plastic	O
windows	O
or	O
vessel	O
walls	O
through	O
which	O
the	O
microwave	O
beam	O
can	O
be	O
passed	O
and	O
measure	O
a	O
'	O
microwave	O
reflective	O
 '	O
(	O
conductive	O
)	O
liquid	O
inside	O
(	O
in	O
the	O
same	O
way	O
as	O
to	O
use	O
a	O
plastic	O
bowl	O
in	O
a	O
microwave	O
oven	O
)	O
.	O
</s>
<s>
They	O
are	O
also	O
largely	O
unaffected	O
by	O
high	O
temperature	O
,	O
pressure	O
,	O
vacuum	B-General_Concept
or	O
vibration	O
.	O
</s>
<s>
Additionally	O
,	O
they	O
solve	O
some	O
of	O
the	O
application	O
limitations	O
of	O
ultrasonics	O
:	O
operation	O
in	O
high	O
pressure	O
and	O
vacuum	B-General_Concept
,	O
high	O
temperatures	O
,	O
dust	O
,	O
temperature	O
and	O
vapor	O
layers	O
.	O
</s>
<s>
Guided	O
wave	O
radars	B-Application
can	O
measure	O
in	O
narrow	O
confined	O
spaces	O
very	O
successfully	O
,	O
as	O
the	O
guide	O
element	O
ensures	O
correct	O
transmission	O
to	O
and	O
from	O
the	O
measured	O
liquid	O
.	O
</s>
<s>
On	O
bulk	O
solids	O
and	O
powders	O
,	O
GWR	O
offers	O
a	O
great	O
alternative	O
to	O
radar	B-Application
or	O
ultrasonic	B-Algorithm
sensors	I-Algorithm
,	O
but	O
some	O
care	O
needs	O
to	O
be	O
taken	O
over	O
cable	O
wear	O
and	O
roof	O
loading	O
by	O
the	O
product	O
movement	O
.	O
</s>
<s>
One	O
perceived	O
major	O
disadvantage	O
of	O
microwave	O
or	O
radar	B-Application
techniques	O
for	O
level	O
monitoring	O
is	O
the	O
relatively	O
high	O
price	O
of	O
such	O
sensors	O
and	O
complex	O
set	O
up	O
.	O
</s>
<s>
Magnetostrictive	O
level	O
and	O
position	O
devices	O
charge	O
the	O
magnetostrictive	O
wire	O
with	O
electric	O
current	O
,	O
when	O
the	O
field	O
intersects	O
the	O
floats	O
 '	O
magnetic	O
field	O
a	O
mechanical	O
twist	O
or	O
pulse	O
is	O
generated	O
,	O
this	O
travels	O
back	O
down	O
the	O
wire	O
at	O
the	O
speed	O
of	O
sound	O
,	O
like	O
ultrasound	B-Application
or	O
radar	B-Application
the	O
distance	O
is	O
measured	O
by	O
time	O
of	O
flight	O
from	O
pulse	O
to	O
return	O
pulse	O
registry	O
.	O
</s>
<s>
Due	O
to	O
the	O
presence	O
of	O
a	O
microprocessor	O
and	O
low	O
power	O
consumption	O
,	O
there	O
is	O
also	O
capability	O
for	O
serial	O
communication	O
from	O
to	O
other	O
computing	O
devices	O
making	O
this	O
a	O
good	O
technique	O
for	O
adjusting	O
calibration	B-General_Concept
and	O
filtering	O
of	O
the	O
sensor	O
signal	O
.	O
</s>
<s>
For	O
use	O
in	O
open	O
air	O
applications	O
,	O
where	O
the	O
sensor	O
cannot	O
be	O
mounted	O
to	O
the	O
bottom	O
of	O
the	O
tank	O
or	O
pipe	O
thereof	O
,	O
a	O
special	O
version	O
of	O
the	O
hydrostatic	O
pressure	O
level	O
sensor	O
,	O
a	O
level	B-Architecture
probe	I-Architecture
,	O
can	O
be	O
suspended	O
from	O
a	O
cable	O
into	O
the	O
tank	O
to	O
the	O
bottom	O
point	O
that	O
is	O
to	O
be	O
measured	O
.	O
</s>
<s>
Pressure	O
level	B-Architecture
probes	I-Architecture
are	O
submerged	O
directly	O
into	O
the	O
liquid	O
and	O
remain	O
permanently	O
floating	O
above	O
the	O
tank	O
bottom	O
.	O
</s>
<s>
The	O
connecting	O
cable	O
of	O
level	B-Architecture
probes	I-Architecture
has	O
several	O
tasks	O
to	O
fulfil	O
.	O
</s>
<s>
The	O
cable	O
also	O
includes	O
a	O
thin	O
air	O
tube	O
that	O
directs	O
the	O
ambient	O
air	O
pressure	O
to	O
the	O
level	B-Architecture
probe	I-Architecture
.	O
</s>
<s>
Level	B-Architecture
probes	I-Architecture
are	O
therefore	O
usually	O
designed	O
as	O
relative	O
pressure	O
sensors	O
,	O
which	O
use	O
the	O
current	O
ambient	O
pressure	O
as	O
the	O
zero	O
point	O
of	O
their	O
measuring	O
range	O
.	O
</s>
<s>
Without	O
this	O
so-called	O
relative	O
pressure	O
compensation	O
,	O
level	B-Architecture
probes	I-Architecture
would	O
not	O
only	O
measure	O
the	O
hydrostatic	O
pressure	O
but	O
also	O
the	O
air	O
pressure	O
on	O
the	O
liquid	O
column	O
.	O
</s>
<s>
Filling	O
levels	O
below	O
the	O
level	B-Architecture
probe	I-Architecture
are	O
not	O
detected	O
.	O
</s>
<s>
Depending	O
on	O
the	O
requirements	O
of	O
the	O
site	O
,	O
level	B-Architecture
probes	I-Architecture
offer	O
different	O
features	O
:	O
</s>
<s>
It	O
is	O
highly	O
recommended	O
for	O
liquid	O
level	O
measurement	O
applications	O
where	O
ultrasonic	B-Application
,	O
float	O
or	O
microwave	O
techniques	O
have	O
proved	O
undependable	O
.	O
</s>
