DRAFT Recommended Practice for Measurements and ...

1/29/98 65 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
Fig. 4.9 Equipment used for Calibrating Clamp-on Induced Current Meters
4.6.3 Techniques for Calibrating Temperature Probes
For the calibration of thermal probes used in SAR measurements, a capability to resolve
temperatures down to tenths of degrees C is required. For clinical settings with human
subjects, even higher precision is needed [CETAS ]. For comparisons to reference
thermometers, the ability to produce a stable thermal environment over the range of
approximately 10ø C to 50ø C is also required.
In earlier years, it was impractical for a small laboratory to possess a standard platinum
resistance thermometer (SPRT) along with the bridge circuitry developed by the National
Bureau of Standards. Therefore, many laboratories maintained ultra-stable standard
reference thermistors from which highly accurate comparisons were conducted using
four-lead resistor-measurement circuits with current-reversing switches and thermal
"calibration blocks" to ensure near-zero temperature gradients inside a controlled water
bath [Hochuli ]. During the past 10 years, however, self-contained and self-calibrating
platinum resistance thermometers have become commercially available such that
tedious resistance measurements that were fed into a computer-based curve-fitting
program of the thermistor equation [Steinhart & Hart ] became unnecessary.
Still required is an insulated, stirred water-bath to provide a homogeneous environment
for probe-to-probe comparisons. Sufficient measurement time is also a significant factor
in the accuracy of thermal probe calibrations. To avoid the negative effects of stray
thermal gradients between the compared probes, several minutes, at least, should
elapse before recording new thermal levels.
4.7 Calibration of SAR Instruments
4.7.1 Implantable E-field Probe Calibration Techniques.
The response of E-field probes implanted in biological tissue or tissue simulating material
is enhanced by a factor that depends on the dielectric properties of the material.
Therefore the implantable E-field probe should be calibrated in lossy dielectric media at
points where the absolute value of E is known. Calibrations have been performed in
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.