National Physical Laboratory

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.

SAR probes 1 

SAR probes 2 

SAR probes 3 

The energy absorped per unit volume (SARV) in the liquid is calculated using:

SAR probes 4 

The systems are traceable to Primary National Standards and meet the calibration requirements for companies seeking accreditation for their SAR measurement services.

SAR probes 5Table 1: SAR standards at NPL 

Applications NPL's SAR standards

1.      Compliance testing of products, such as mobile phones

SAR probes 6 

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 programme: Mobile Telecommunications and Health Research (MTHR).

SAR probes 7Measured SAR distribution in the Petri dish 

SAR probes 8Measurement of SAR from head coil in 3T MRI (at PRB) 

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.

Click here to download flyer (Adobe pdf file 5.9 MB)


Customer Service tel: +44 20 8943 8538

Last Updated: 25 Mar 2010
Created: 24 Jun 2009


Please note that the information will not be divulged to third parties, or used without your permission