DRAFT Recommended Practice for Measurements and ...

1/29/98 94 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
positioners [Cleveland and Athey, 1989] to 3-axis scanners [Stuchly, et al., 1983] and,
most recently, 6-axis robots [Balzano, et al., 1995].
A system designed for testing compliance of hand-held radio transceivers, e.g., cellular
phones, with the spatial-peak SAR safety criteria, is described in [Schmid, et al., 1996]. It
incorporates a high precision robot (working range greater than 0.9 m and a position
repeatability better than ± 0.02 mm), isotropic E-field probes with diode-loaded dipole
sensors, an optical proximity sensor for automated positioning of the probe with respect
to the phantom surface (within ± 0.2 mm) and sophisticated software for data processing
and measurement control. The useable frequency range extends from 10 MHz to at
least 3 GHz, the sensitivity is reported to be better than 1 mW/kg and the dynamic range
extends to 100 W/kg. Complex measurements, such as the spatial peak SAR value
when starting with an unknown field distribution in the body can be completed within 15
minutes. Three dimensional magnetic field probes are also available for this
commercially available system.
5.5.3 Calorimetric Determination of the Whole-Body Average SAR.
Average SAR may be measured using calorimetric methods. In the past, such methods
have been used predominantly with small animals or animal models [B38, B101; Allan
and Hurt, 1979; Blackman and Black, 1977]; recently, however, calorimetric twin-well
methods have been successfully used to measure SAR in a full-size human model
[B102]. The heart of the measurement system is the calorimeter device itself, and
gradient-layer devices are commonly used. Gradient-layer calorimeters have a
convenient voltage output signal that is proportional to the rate of heat energy flowing out
of the device (positive voltage) or the rate of heat energy flowing inward (negative
voltage). The signal generally has very low noise, and the sensitivity of a typical device is
about 1.3 J/(mV-s).
In a laboratory setting, calorimetric SAR measurement begins with the thermal
equilibration of the test object, usually a realistic animal model or a scaled human model.
It is assumed that the laboratory temperature is ostensibly constant and is the same as
that of the thermally stabilized test object and calorimeter. The test object is then
irradiated for a measured period of time after which it is immediately placed inside the
calorimeter. The calorimeter output voltage is then periodically monitored until all of the
irradiation-induced heat energy has flowed from the object, and it is again at room
temperature. This process may take hours or days depending on the size and mass of
the object. By this time, the calorimeter voltage is zero, and the area under the curve
described by the time-course of the calorimeter voltage is proportional to the energy
deposited in the object. That area is then multiplied by the calibration constant of the
device to obtain the total energy deposition in joules. Division of the energy deposition by
the irradiation time in seconds yields the rate of energy deposition (power) in watts;
average SAR is obtained by dividing the resulting power by the mass (kg) of the test
object.
If two matched calorimeters are used in conjunction with two identical test objects, these
procedures can be used in the absence of strict temperature control, such as in an
outdoor environment. However, twice the physical effort is required for outdoor SAR
measurements, and all of the apparatus should be given some sort of protection from the
effects of direct sunlight, rain, etc.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.