Lake Shore 475, Руководство пользователя

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Lake Shore Model 475 Gaussmeter User’s Manual
Probes and Accessories 7-11
7.7 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 transverse 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 functions: (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 magnet
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 2 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.
Reference_Magnets.bmp
Figure 7-16. Lake Shore Reference Magnets