Lake Shore Model 421, Руководство пользователя

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Lake Shore Model 421 Gaussmeter User’s Manual
5-10 Accessories and Probes
5.4 REFERENCE MAGNETS
Magnetic reference standards containing highly stable permanent magnets have been in use for many
years. The highest quality units are usually shielded from external magnetic effects and use Alnico V or
VI magnets for long-term stability. They are supplied in both transverse (flat) and axial configurations.
Typical flat reference magnets are usually stabilized for use at ambient temperatures between
0 – 50 °C and have nominal temperature coefficients of about –0.02%/°C. Because the temperature
coefficient is negative, the field strength will be reduced as the temperature rises. Since these
references are temperature cycled during manufacture, their change with temperature is predictable
and retraceable; they will always return to a known value at any specific ambient temperature.
The high-permeability shell which surrounds the reference magnet serves two function: (1) it shields the
magnet from external field, and (2) serves as the flux return path. Physical damage to the outer shell
can cause a permanent change in the gap flux density. Reference magnets should not be dropped or
physically abused. Magnets of this type can have magnetic reference values ranging from 100 G to
20 kG, but the most widely-used value is 1 kG. Reference magnets accuracy is typically ±0.5%, except
for magnets of 200 G or less; for these magnets, the limit of error is generally ±1%. The reference
magnet gap is nominally 0.060 inch but may range from 0.040 to 0.250 inch for special units. The
usable "plateau" in the reference gap generally encompasses an area of about 0.5 square inches.
In reference magnets used for axial field probes, Alnico V or VI is the usual magnet material, charged to
saturation and stabilized down to a particular value. The same temperature coefficients hold true as in
the transverse probe and the same care in handling must be observed. This assembly uses concentric
mu-metal shield cans to protect the magnet from the effects of external magnetic field. Axial reference
magnets are available in values up to 1 kG, with 500 G being the most widely-used value.
When a probe is inserted completely through the access guide, three distinct magnetic peaks will be
observed on the gaussmeter. One peak occurs as the probe enters the magnet, a second (and greater)
peak is observed as the midpoint is reached, and a third (smaller) peak is read as the probe leaves the
magnet. The calibration point is the largest reading in the midpoint area. Its amplitude will be
approximately twice that of the readings that occur where the probe enters or leaves the magnet.
P-421-5-11.bmp
Figure 5-11. Lake Shore Reference Magnets