DRAFT Recommended Practice for Measurements and ...

1/29/98 139 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
E2. Calibration Techniques
Two approaches may be used in performing these calibrations. First, absolute
calibrations can be performed at regular time intervals at several temperature points over
the range of interest. This is accomplished by comparing the temperature probe being
calibrated with a standard thermometer, usually a platinum or thermistor type. The
platinum resistance thermometer has good long-term stability, but the thermistor type
has a smaller sensing element and thus has a smaller thermal mass. This smaller
thermal mass minimizes the creation of temperature gradients in the calibrating water
bath and thermal lag problems caused by use of a large metallic temperature probe with
its inherently large thermal mass and long time constant. If a dynamic temperature
calibration technique is used, a well-stirred water bath should be used. Also, the rate of
rise of the temperature of the water must be adjusted so that it is slower than the time
constants of either of the probes. The unknown and the reference probes should be
placed in intimate contact at their sensor-areas (usually the first few cm near the tip). It
is best to use an automated data acquisition system to ensure reliable, instantaneous,
and simultaneous readings from all temperature probes. If absolute calibrations of the
unknown thermometer can be performed with accuracy approaching the resolution of the
instrument being calibrated, this type of calibration is preferred. In some cases an
instrument under calibration will not accurately indicate absolute temperature but will
correctly measure a change in temperature ∆T. In this case, the ratio (∆T-absolute)/(∆Tmeasured)
is determined over the range of interest. If this value is constant over the
range of temperatures to be studied and over a reasonable period of time, e.g., from
week to week, accurate SAR data can be obtained even when absolute temperature can
not be precisely defined.
When using thermometers to measure SAR in electromagnetic fields, one must be
aware of the possibility of RF interference in the thermometer's sensor, leads or
electronics. Several methods can be used to determine the magnitude of RFI. One is to
note changes at the instant that the RF power is switched on and off. If large, immediate
changes are found, the probes should only be used to measure temperature prior to and
just after RF exposure. Artifacts may also occur due to the interaction of the RF field with
the electrical leads attached to the temperature sensing element of the probe. This
interaction can excite a thermally induced voltage at the junction of two dissimilar
materials (thermoelectric effect), including high resistance (carbon-loaded Teflon) leads
joined to metallic wires. Since this phenomenon is due to the heating of a junction,
shielding of these areas should be used to minimize this source of measurement error.
Metal foil or RF absorber can be used to shield these junctions. Details of these
problems and additional solutions may be found in Hochuli [E1].
With either the dynamic or static calibration technique, it is useful to perform two
calibrations of ∆T. The first was described above; a temperature rise in a water bath.
The second calibration should be performed using a temperature decrease in a water
bath. If a difference in the calibration factors exists for the unknown probe when
determined by the two methods, the thermal time constant of one or both of the probes
may have been longer than the time period that elapsed between the thermometer
readings taken during one or both of the calibrations. To correct this, the calibration
process should be repeated, using a slower rate of temperature increase or decrease in
the bath.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.