BS EN 50413:2009 pdf free download – Basic standard on measurement and calculation procedures for human exposure to electric, magnetic and electromagnetic fields (0 Hz- 300 GHz).
The E-field probe should have an isotropic response (within ± 1 dB). Because of the short wavelength in tissue and since the field may have large spatial gradients, the probe size should be as small as possible (for example: dipole length 2 mm to 4 mm). The probe should affect the field as little as possible. Care has to be taken to avoid significant influence on SAR measurements by any reflection from the environment (such as floor, positioner, etc.) or from unknown sources.
NOTE To obtain 3-dimensional SAR distributions in exposed human phantoms by the E-field-probe technique, it is preferable to use an automatic probe positioning system, for example an industrial robot.
The measurement equipment shall be calibrated according to 5.6 as a complete system. Sensitivity, linearity and isotropy of the probe system shall be determined in the tissue equivalent liquid. Uncertainties should be addressed as specified in 5.5. Measurements in close vicinity to media interfaces result in errors due to boundary effects. These effects depend on the probe size and can be quantified as a function of distance from the surface using a waveguide calibration set-up. After quantification these effects can be compensated in order to minimize errors.
5.4.2 Internal temperature measurements
Assessment of SAR via temperature measurement is performed using a temperature probe that is positioned in a liquid-filled phantom model of the human body or a part of it (for example: the head), which is exposed to an electromagnetic field. From the measured temperature increase, the local SAR distribution can be calculated using the formula:
The temperature-rise measurement has to start at a thermal balance. This equation is applicable under the condition that the heat diffusion effect can be disregarded. If the diffusion effect cannot be disregarded, an integral equation including the heat-diffusion factor has to be employed.
Common types of available equipment for temperature measurements in exposed body models use probes consisting of a high-resistance thermistor or optical probes. Temperature probes have very small tips allowing high spatial resolution. The temperature resolution of these probes is typically 0,005 K to 0,1 K, which limits the sensitivity of SAR to about 0,03 W/kg. In order that the probe does not perturb the electromagnetic field, it is constructed using high-resistance thermistors connected to high-resistance leads or by using fibre optics.
NOTE To determine the three-dimensional SAR distribution or local peak SAR in a phantom model, the temperature probe has to be moved inside the phantom using a similar positioning system as for E-field probes.
Temperature rise in a phantom model (or even at the surface of a real human being) can also be assessed by infrared imaging devices. Hence, a thermographic camera may be used to determine the 2-dimensional temperature-rise and SAR distribution in solid phantom cross-sections or at surfaces. First the phantom has to reach uniform temperature. Directly after the exposure of a few seconds the phantom is placed in front of the thermographic camera and a thermographic image is immediately taken to map the temperature-rise profile. Temperature profiles inside the phantom may be taken by separating the phantom at specially prepared cuts.
The sensitivity of temperature sensors is relatively low in comparison to E-field probes. In order to achieve a reasonable sensitivity in SAR assessment via temperature measurement, in general, high-power exposure sources have to be applied to get a sufficient temperature increase in a short time interval. Low-power devices (e. g. hand-held mobile phone) have to be fed in the test with an additional (external) source.
Calibration of temperature-measurement equipment includes, beside the general probe calibration, a careful evaluation of heat diffusion processes.
5.4.3 Calorimetric measurements of heat transfer
Calorimeters allow the measurement of the whole-body averaged or partial-body averaged SAR for human body models exposed to electromagnetic fields. Averaged SAR is derived according to the equation below by measuring the total energy absorbed in a body with the mass during the exposure time: