Specific Absorption Rate Probes
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 radio frequency 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-wave guide systems, as recommended by IEEE and CENELC Standards.
The energy absorped per unit volume (SARV) in the liquid is calculated using:
The systems are traceable to Primary National Standards and meet the calibration requirements for companies seeking accreditation for their SAR measurement services.
Applications NPL's SAR standards
1. Compliance testing of products, such as mobile phones
NPL calibrates the probes used for SAR testing products.
- 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 programme: Mobile Telecommunications and Health Research (MTHR).
3. Human exposure assessment
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.
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