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Specific Absorption Rate
(SAR) Measurement Service
Daniel Bownds and Benjamin Loader
National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK
SAR is the specific absorption rate of radio frequency energy in a material and has
the unit W kg-1. It forms part of the restriction limiting exposure of humans to radiofrequency fields over the frequency range 100 kHz to 10 GHz. SAR is related to the
electric field in a dielectric by where E is the electric field in the liquid, s its conductivity
and r its density. The sensitivity of the probe is dependent on the electrical properties of
the liquid in which it is immersed, so it is necessary to calibrate in a liquid with the same
electrical properties. Probes are calibrated using matched-waveguide systems,
as recommended by IEEE and CENELC Standards
Applications of NPL’s SAR standards
1. Compliance testing of products, such as mobile phones
NPL calibrates the probes used for SAR testing products
Related standards:
IEEE Std 1528 “Recommended Practice for Determining the Peak Spatial-Averaged
Specific Absorption rate (SAR) in the Human Body Due to Wireless Communication
Devices: Experimental Techniques”
BS EN50361 “Basic Standard for the measurement of specific absorption rate related
to human exposure to electromagnetic fields from mobile phones (300 MHz to 3 GHz)”
FCC OET65C “Evaluating compliance with FCC Guidelines for Human Exposure to
Radio frequency Electromagnetic Fields”.
IEC 62209-1,2 “Human exposure to radio frequency fields from hand-held and bodymounted wireless communication devices - Human models, instrumentation, and
procedures”
2. Dosimetry for research into health effects
of electromagnetic fields
NPL provides metrological expertise to support the Department of Health research
program “Mobile Telecommunications and Health Research” (MTHR) programme
Exposure system
The energy absorped per unit volume (SARV) in the liquid is calculated using
Measured SAR distribution in the petri dish
3. Human exposure assessment
The systems are traceable to Primary National Standards and meet the calibration
requirements for companies seeking accreditation for their SAR measurement services.
Table 1: SAR standards at NPL.
SAR is the basic restriction for human exposure to EMF given by for ICNIRP 10 MHz
(100kHz) to 10 GHz . The EU Directive 2004/40/EC will make these restrictions mandatory
for workers. The exposure to EMFs during MRI is of particular interest.
Measurement of SAR from head coil in 3T MRI (at PTB)