2.5.3 Gradient 17
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2.5.3 Gradient
Probe selection would be easier if all fields were large and uniform, but most fields are
limited in volume and contain gradients (changes in magnitude). Hall effect probes
measure an average magnitude over their active area, making it necessary to under-
stand the relationship between active area and field gradients.
Severe field gradients are always experienced as the active sense element is moved
away from a permanent magnet pole, making it important to know the distance
between the active area and probe tip. The distance between probe tip and active
area is specified for axial probes, but is less easily defined for transverse probes.
D Nominal active area: HSE and HST probes have a nominal active area on the order of
1 mm (0.04 in) diameter, which is useful for all but the most stringent applica-
tions. The measured field is the average of the active area, but without severe
gradients; therefore, the measured value accurately represents the true field.
Field mapping with standard probes is also practical if a mapping resolution of
1 mm (0.04 in) or greater is acceptable.
D Small active area: HSE and HST probes with a smaller active area are also available
from Lake Shore for measurements in severe gradients, or for high resolution
mapping applications.
D UHS probes: UHS probes have a very large active length, up to 89 mm (3.5 in). They
are designed to measure very low magnitude, large volume ambient fields with
little gradient.
2.5.4 Probe Durability
All Hall effect probes are fragile. The sensor, normally located at the tip of the probe
stem, must not be bent, physically shocked, or abraded. It may be tempting to choose
a probe with the thinnest transverse stem or smallest diameter axial stem; however,
it is always best to choose the most robust probe that fits the immediate application.
For example, the HMMT-6J04-VR (aluminum stem) is less prone to damage than the
HMFT-3E03-VR (flexible stem), and the HMMA-2502-VR (6.35 mm [p in] diameter
aluminum) is more durable than the HMNA-1904-VR (4.76 mm [3/bg in] diameter
fiberglass) with its exposed Hall sensor.
Never fasten a probe stem to another object. If a probe is clamped, always apply the
clamp to the handle. Improper mounting can cause damage to the probe.
2.6 Probe
Accuracy
Considerations
The user must consider all the possible contributors to the accuracy of the reading.
Both the probe and gaussmeter have accuracy specifications that may impact the
actual reading. The probe should be zeroed before making critical measurements.
The zero probe function is used to null (cancel) the zero offset of the probe or small
magnetic fields. It is normally used in conjunction with the zero gauss chamber, but
may also be used with an open probe (registering Earth’s local magnetic field). If you
wish to cancel out large magnetic fields, use the relative mode (section 4.5.6).
2.6.1 Probe
Temperature
Probe temperature can also affect readings. Refer to the two separate temperature
coefficients listed in Chapter 1 and section 2.4.2 for an explanation of the tempera-
ture coefficients. The (HST) probes exhibit a low temperature coefficient of gain due
to the inherent thermal stability of the materials used in their construction.
2.6.2 Probe Orientation
Probe readings are dependent on the angle of the sensor (Hall sensor) in relation to
the magnetic field. Maximum output occurs when the flux vector is perpendicular to
the plane of the sensor. This is the condition that exists during calibration at
Lake Shore. The greater the deviation from orthogonality (field perpindicular to the
plane of the sensor), the larger the error of the reading. For example, a 5° variance on
any one axis causes a 0.4% error, a 10° misalignment induces a 1.5% error, etc.
(FIGURE 3-7).
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