TM 5-852-4/AFM 88-19, Chap. 4
measure the thermistor resistances and the calibration
sharpened to -in.-diameter steel rod can be pushed
55
down to as much as 15 feet with the help of a pair of
curve used to convert to temperature.
Thermistor
clamp-grip pliers as illustrated in figure 7-1a. By making
systems are complex to fabricate and cost more than
a hole part way by some method, hand probing can be
thermocouples but careful field fabrication, installation
extended further. In slightly stiffer soils a sledgehammer
and observation techniques can produce results
may be needed. Hand augering or test pitting may also
repeatable to better than 0.01 C (0.02 F) and accurate
be used.
For soils in which manual methods of
to better than 0.05 C (0.09 F). Where slight
penetration are difficult or impossible, or when a frozen
temperature changes are critical their extra expense may
layer must be penetrated, a drill rod or other rod can be
be justified. However, in foundation work the added
driven by a pneumatic drill. Regular exploration, auger
precision often may not be usable to real advantage.
or other power-operated drill rigs may also be used. The
Experience has shown that uncalibrated thermistors with
interface between a frozen layer and an underlying
direct reading instruments are seldom accurate to 1 IF.
unfrozen layer will usually be detectable by the decrease
An assembly intended for underground installation must
in resistance to penetration.
However, if there is
be very carefully protected against moisture, because
question, the scheme shown in figure 7-lb may be tried,
variations of moisture content within the system would
raising the L-shaped rod in contact with the wall of the
change resistances in both the bead and the insulation
bore hole until the interface is felt.
and hence the apparent temperatures, Thermistors of
b. Soil electrical resistance. When a soil
the type contained within a shock resistant glass bead
protective cover should be used and the connecting
freezes, a large increase in its electrical resistance
system of wires and insulations should be assembled
occurs. This resistance can be measured between
with elaborate precautions against moisture penetration.
copper or conductive epoxy electrodes on the outside
Because thermistors are also pressure sensitive, the
surface of a nonconductive pipe which is buried vertically
thermistor beads must be surrounded by protective metal
in the soil. The variation of soil resistance with depth
sleeves or other enclosures within the assembly when
determined with this device delineates frozen and
pressures such as from freezing of soil or water may
unfrozen strata. A schematic of a device developed by
occur. Possible errors caused by resistances of leads
Aitken at CRREL'6 together with typical electrical
and contacts in the circuits must be avoided. The
resistance data are shown in figure 7-2. The CRREL
thermistors must be stabilized by an accelerated aging
tests were accomplished successfully using a low level
process at temperatures well above the operating range
direct current system. In some soil conditions it might be
and thermal stress adjusted by cycling to temperatures
necessary to make an a-c resistance measurement to
below those expected in application.
avoid polarization at the electrodes.
c. Seismic. The rigidity and consequently the
(2) Thermistors eliminate the ice bath
problems of thermocouples and the problem of thermal
seismic velocity of soil increases when it is frozen.
gradients across the signal lead connector, making field
Consequently, conventional refraction seismic surveys
observational techniques less complex. However, a
can be used to determine the depth to frozen ground
heated shock-mounted enclosure is still required to
below unfrozen material in the same fashion that
house the Wheatstone bridge, in the same manner as
bedrock is delineated. For relatively shallow annual thaw
for the potentiometer for thermocouples, for reliable field
zone depths, special seismic equipment capable of
results. Care must be taken to control the amount of
handling extremely short refraction time intervals must
current put through the thermistor during the Wheatstone
be used. However, the thickness of a frozen layer
bridge measurements so as not to change its resistance
(annual frost or permafrost) cannot be determined by
by more than the accuracy of measurement required. In
refraction seismic methods. Reflection type equipment
precise work, errors due to thermocouples in the system
is currently being tested for use in permafrost
84,85,124
must be avoided by proper observational techniques.
.
areas
d. Color change of material within a tube
7-5. Other systems for freeze and thaw monitoring.
inserted into the ground. If a sealed tube filled with moist
Several systems other than thermocouples or
sand is inserted into the ground inside of a fixed outer
thermistors are possible for locating the interface
tube, sealed at the bottom, positions of frozen and
between frozen and unfrozen materials and may under
unfrozen zones in the ground can be determined at any
some conditions be used in their place. Some of these
desired time by withdrawal and inspection of the tube.
do not require the assumption of a freezing temperature
Dyes such as methyl blue or fluorescein are used to
but determine the interface directly.
a. Probing. In summer, a steel rod can be
intensify the color difference. Devices of this type have
been investigated or used in Alaska by the Permafrost
driven through unfrozen soil in the annual thaw zone until
Division
it encounters underlying permafrost. Several methods of
driving may be used. In soft, fine-grained soils a
7-3
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