SAM-III Construction Manual
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Environment
Temperature
The SAM-III sensor is very sensitive to temperature variations. The temperature coefficient is
approximately –100 to –150 nT/°C (the minus sign indicates that the amplitude increases as the
temperature decreases). To minimize temperature effects, the sensors must be insulated from
temperature changes in their environment. However, it has been reported that the sensor can overheat
if insulation is applied directly to it. Therefore, when an insulated enclosure is used, there should be
space around the sensor to allow convection and radiation cooling.
Burial in earth to reduce sensor temperature variations
Placing the sensors in a watertight fixture and burying the fixture 0.5 to 1.5 m (or more) below the soil
surface is a viable method to eliminate or greatly reduce diurnal temperature variations at the sensor.
The watertight sensor enclosure can be placed in a small foam picnic cooler with additional thermal
insulating foam fitted around the fixture and the entire assembly then buried in the earth. The cooler
should have at least one hole in the bottom to allow drainage.
Burying the sensors at a reasonable depth will eliminate diurnal temperature effects but will not
eliminate seasonal temperature effects. It is relatively simple to calculate the soil temperature at depth
by making assumptions about the soil thermal characteristics. Of course, these assumptions are made at
the expense of accuracy but they provide a good starting point. Burial depth calculations are described
in
Burial Depth of SAM-III Magnetometer Sensors
.
When the sensors are buried, in soil their outputs will drift upwards as they become accustomed to the
cooler environment. It may take more than a week for the temperature of the sensors to stabilize
depending on the fixture insulation and their depth.
When the sensors are buried in soil, they must be sealed to prevent moisture intrusion. A sensor
enclosure buried 1 m below the surface will be subjected to a hydrostatic pressure of 1 m head of water
during rain and when the ground is water saturated. This pressure is equivalent to 1.4 pounds/sq. inch
or about 10 kilopascals, a nominally small amount but enough to force water through even the smallest
pinholes or defects in the enclosure seals. A drawing of a sensor enclosure using commonly available
materials (in the United States) is shown at the end of this section.
Because copper wires are very good thermal conductors, the cables feeding the sensor installation also
must be buried at the same depth as the sensors for a distance of at least 1 m and preferably more from
the sensor fixture. Do not place the cables horizontally to above the sensor location and then drop
directly down to the sensors.
Location
Place the sensors in a magnetically quiet location where they will experience as little local magnetic
disturbance as possible, usually in an open field or yard away from any buildings and automobile traffic.
