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DM330 Series - Magnetostrictive Level System User's Manual
1.3
Technology
In a magnetostrictive level sensor a current pulse is sent down a
wave guide made of a special nickel alloy wire designed to enhance
magnetostrictive properties. A permanent magnet within a float
is used to indicate the position or level being measured. The
interaction of the current pulse with the magnetic field created by a
float (with magnet) produces a torsional strain pulse that travels at
approximately the speed of sound along the wire. A small induction
pickup coil senses the strain pulse. The position of the float is
determined with high precision by measuring the time between the
launching of the current pulse and the arrival of the torsional strain
pulse.
The magnetostrictive wire is linearized during manufacture and
the speed of the torsional pulse is determined for the specific sensor.
Inherently, magnetostrictive sensors have very high resolution and
repeatability.
Magnetostrictive technology is excellent as well for applications where
the dielectric constant is very low or is changing. The technology has
been used quite successfully for the detection of leaks in underground
storage tanks, for example. The measurement of a 0.1 gallon leak
out of a 10,000 gallon tank over a period of one hour is the standard
for EPA mandated leak detection.
A Current Interrogation Pulse creates an
electromagnetic field along the Magnetostrictive wire.
Magnetostrictive wire within tube...
pulse
Strikes field of permanent magnet & creates a torsional strain pulse
or wave guide twist.
...causing two torsional strain pulses to travel in each direction along
wire (the wave guide) at approximately the speed of sound.
Magnetostrictive wire within tube...
t
-t
1
2
Time between initial pulse t
is used to determine float position & level.
2
Permanent Magnet in Float
t
t
1
2
reaction pulses
Permanent Magnet in Float
t
1
t
2
and second t
1
Figure 1-1
Magnetostrictive Theory
Reflector
Reflector
t
2
reflected return pulse
2
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