DRAFT Recommended Practice for Measurements and ...

1/29/98 95 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
Although the calorimetric determination of the energy stored in the model once inside the
calorimeter is quite accurate per se, the overall system accuracy in terms of SAR is
limited by how closely the test object models the actual object and by the amount of
irradiation-induced heat that escapes from the object without being measured. This
method, moreover, requires sufficient time for the thermal equilibration processes and
requires sufficient energy deposition in the test object to produce a calorimeter output
signal that is enough above baseline to be measurable. However, the Dewar-flask
method of calorimetry is a relatively simple, straight-forward way of determining the
whole-body average SAR of small-bodied animals [Padilla and Bixby, 1986]. The
calorimetric technique of determining a whole-body average temperature requires that
the cadaver be immersed in a Dewar-flask containing a medium, such as water, at a
known temperature; then the temperature of the cadaver, following irradiation, can be
determined by noting the final temperature of the cadaver/medium mixture.
5.6 Hazard Assessment; Estimation of Internal SAR from External-Exposure Field
Measurement Data
5.6.1 General.
Contemporary MPEs are based on whole-body-averaged SAR thresholds for biological
effects in RF-irradiated animals. The extrapolation of plane-wave exposures to wholebody-averaged
SAR is supported by the many dosimetric studies, both experimental and
mathematical, reported in the literature [B42]. For human exposure to plane-wave fields,
the MPEs are often the maximum permissible plane wave exposure limit that ensures
that a given whole body SAR will not be exceeded. The significance of exposure in close
proximity to a near-field source, however, is difficult to estimate, especially when only
external field strength data (obtained from radiation hazard survey meters) are available.
The relationship between the spatial maximum field strength and the SAR is very
complex varying considerably as the orientation and spatial distribution of the fields
change with respect to the exposed object. Consequently, induced SAR in an object or a
person near a radiator or passive reradiator is extremely difficult to estimate from
measured data in the form of external field strengths. These measured data can only
provide the basis for crude estimates of the maximum spatial SAR useful for establishing
temporary personnel protection guides corresponding to a measured exposure situation
(see also 5.6.2.4).
5.6.2 Enhanced or Reduced SAR in Personnel in Close Proximity to Passive
Reradiators or Active Radiators.
Absorption in personnel exposed to reactive near fields may be either enhanced or
reduced when compared with exposure in plane-wave fields. The reactive near-field
region, for both active and passive radiators, i.e., where the reactive fields dominate over
radiating fields, is limited to distances much smaller than a wavelength. Most MPEs are
expressed in terms of those values of E 2 , H 2 or W for plane-wave exposures that ensure
that the whole-body averaged SAR is below some nominal value. In many cases, where
or how these parameters are to be measured, e.g., spatial averaging, is not specified. In
some cases, a maximum spatially localized SAR may be specified. This local SAR
defines the maximum SAR that is allowed to exist in any small volume of tissue, e.g., one
gram or one cm 3 . During RF hazard surveys, the exposure field strengths are
measured, not the internal SAR. When a surveyor measures the field strength in a
region that may be occupied by a person, and the field strength exceeds the applicable
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.