Page 80 of 82 OM0408/49
The magnetic state of a specimen is generally described by the following equation:
B=µ
0
(H+M) … (1)
where:
B is the flux density of the specimen in T (Tesla). (B = µH)
µ
0
is the permeability of free space in N A
-2
. This is a constant (4πx10
-7
)
H is the applied field strength in Am
-1
.
M is the magnetisation of the specimen in Am
-1
. (M = c
vol
H)
Dividing through by H we get:
µ= µ
0
+
µ
0
c
vol
… (2)
where:
µ is the permeability of the specimen (in N A
-2
)
k is the volume magnetic susceptibility of the specimen (dimensionless)
Rewriting, we get:
µ
0
c
vol
= µ - µ
0
… (2)
The MS2/3 magnetic susceptibility system relies on the principle that any changes in the
permeability of a core will cause a change to the inductance of a wound inductor.
The sensors operate on the principle of AC induction. Power is supplied to the oscillator circuit
within the sensor, generating a low intensity alternating magnetic field.
The frequency of oscillation is determined by the inductance of the system. When the inductor
contains only air, the permeabilty µ
0
determines the inductance. When a sample is introduced
inside the inductor, the change in permeability also leads to a change in inductance.
The meter reads the frequency values for µ
0
and µ, and uses them to calculate the change in
inductance, and thus the magnetic permeability. The magnetic susceptibility is then calculated
using equation (2).
The value of µ
0
is constant but the variable of interest is relatively small. Therefore any thermally
induced sensor drift needs to be eliminated by occasionally obtaining a new ‘air’ value, to re-
establish the µ
0
reference.
To Next Page
Loading...