TM 5-852-4/AFM 88-19, Chap. 4
thermistors may be used. The choice is based chiefly on
the positions of the 3-F isotherm. To take maximum
92
the degree of precision required. Sohlberg presents a
advantage of this situation, differential values of emf
comprehensive analysis of the basic systems and their
between sensor thermocouples may be measured
sub-features.
Thermohms, which are wound wire
directly. Since experience indicates that the accuracy
resistors that experience resistance change with
level of portable potentiometer field work is about + 0.1
temperature, have been used extensively in the past but
to 0.2 F ( 0.1 C), differential temperature
are now little used because of their expense and
measurements may approach this degree of accuracy on
because their substantial size, their volumetric heat
an individual assembly if all other sources of error,
capacity, and their heat conductance capacity in both
including thermocouple wire characteristics, influence the
probe and cable present substantial potentials for error in
apparent emf's from all thermocouples equally. To
temperature measurements.
enhance the probability of the latter situation, it is
a. Thermocouples.
desirable that all the thermocouples in a given assembly
(1) In essence, a thermocouple is a very
be made from the same run of thermocouple wire. Thus,
weak battery created simply by joining wires of two
in actual frozen ground measurements, thermocouples
dissimilar metals. The strength of this "battery" changes
may often be used to greater advantage than their
with changing temperature and by measuring the small
capacity to yield true temperatures would suggest, so
voltage produced the temperature at the bimetallic
long as temperature gradients are reasonably
junction can be determined. When an electrical circuit
pronounced.
consisting of two dissimilar wires is closed at both ends,
(2) Thermocouples are relatively simple
two thermocouples are created. They are in electrically
and cheap compared to other electrical ground
opposite directions and if the two junctions are at the
temperature measurement systems. They give good
same temperature, the voltage produced by one offsets
results when temperature gradients are large enough so
that produced by the other. If the two junctions are not at
that very high precision in individual readings is not
the same temperature, a net voltage difference is
needed. Their disadvantages include the problems of
produced. Consequently, it is not the temperature at a
making and maintaining ice baths and of using and
single thermoelectric junction that is measured but rather
protecting standard cells under winter conditions. In
the difference in temperature between two junctions.
order to obtain reliable temperature measurements, any
Thermocouple circuits are generally arranged so that
potentiometer should be maintained at a temperature
one of the junctions (the sensor) is placed at the point
above 0 F and the ice bath container kept above 3' 'F.
where an unknown temperature is to be measured and
This is best accomplished in the field in winter by placing
the other (the reference) is placed in a mixture of ice and
the potentiometer in a shock-mounted heated (electric or
water which maintains itself at 0 C (32 F). The
exhaust gas) enclosure in an oversnow or other vehicle
and making provision for the observer to carry the ice
difference in emf is usually measured by millivolt
bath container and the potentiometer batteries and
potentiometer
together
with
an
unsaturated
standard cell beneath his parka connected to the
mercurycadmium standard cell voltage reference. An
potentiometer by extension leads. The leads from the
electronic null detector should be used with the
thermocouple cable can then be fitted with a suitable
potentiometer. The two dissimilar wires commonly used
2
connector and attached directly to a mating connector on
are copper and constantan. Tables are available for
the potentiometer enclosure. The thermal gradient
converting voltage readings obtained on a precision
across the signal load connector will cause a zero offset
potentiometer for these wires when the reference
but the amount of this offset can be easily determined by
junction is maintained at 0C (32 F).
If field
connecting the input terminals on the potentiometer
measurements on a top quality installation are obtained
together with a short length of wire and reading the offset
carefully, with first rate equipment, the temperatures can
voltage.
be obtained within about 0.4 C (3/4 F) (+) of the true
b. Thermistors. When greater precision is
values. If the measurements on a given thermocouple
needed than is obtainable with thermocouples,
assembly are all made in a single operation over a
thermistors of a select type, stabilized and properly
reasonably short period of time, so that temperatures do
calibrated, should be used.
not change significantly, the relative temperatures
(1) Thermistors
are
semiconductors
indicated between the thermocouples can be correct
consisting of compounds of various metal oxides which
within much closer limits.
Often differences in
exhibit extremely large changes in resistance with
temperature between various points are much more
temperature change.
Resistance variation with
important and useful than actual temperature levels.
temperature is approximately exponential and a fairly
Since the nucleation temperature is usually displaced
92
elaborate calibration procedure is required. Sohlberg
below 32 F in finegrained soils, the positions of
has described such a procedure. A Wheatstone bridge
interfaces between frozen and unfrozen materials are
is used to
usually more accurately determined by changes in slope
of the curve of ground temperatures vs. depth than by
7-2
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