Temperature Rise Measurements
22.1
Background
The SAR in tissue simulating liquid can be assessed by means of temperature rise or by measure-ments of induced electric field.
SAR=c∆T dt t=0 (22.1) SAR=σ|E| 2 ρ (22.2)
For SAR evaluation of low-power wireless devices, the required dynamic range of an E-field probe should range from 2.5 V/m to 400 V/m, which corresponds to a range in temperature rise from 3µK/s to 40 mK/s. This requirement limits the usage of temperature probes for compliance test-ings of wireless devices. Besides the low sensitivity, temperature measurements have disadvantage of being extremely time consuming and tedious.
However, temperature rise measurements are still widely used in bioelectromagnetic research be-cause the sufficient spatial resolution (<1 mm2) can be developed without any difficulties, as well as, for transfer probe calibrations.
In this Chapter, basic guidelines for conducting SAR assessments using temperature-rise method are described.
22.2
Temperature Probe Design and Characterization
Temperature probe is based on the NTC sensor. The probe tip and body are made of PEEK material with Silicon seals. The probe can be used in water, sugar-water solutions, nutrient solutions and glycol solutions. Maximum storage and usage temperature is from -20◦C to 80◦ (Note: The DAE unit will produce a AD overflow at temperature below zero.)
WARNING: The probe tip is very fragile so that the probe has to be handled with utmost care. Do not flex or bend the flexible sensor tip. Always leave the protection cover on the probe, except when making measurements.
The protective cover can be used as a position reference for accurate probe positioning, e.g., in a robot positioning system. The following protocol explains the usage and the role of the protective cover:
1. Make sure that the cover is correctly mounted. 2. Move to the surface with the cover in place. 3. Store the actual position.
4. Move the probe backwards and remove the cover. 5. Go back to the stored position.
6. Move the probe 10.0 mm forward. The probe tip is now just touching the surface. The active element is 1.0 - 1.5 mm from the surface (for the exact distance seeMechanicalpage of the probe specifications).
IMPORTANT: Always clean the probe with warm water after use in liquid. When the probe has been used in sugar solutions, remove the cover and clean it separately. The cover might become stuck if the sugar crystallizes in its thread.
22.3
DASY Software Settings
In theGeneralpage the information on the probe type and serial number is stored. For temperature probesSurface Detection is disabled for both detection principles.
Mechanical properties page
In the Mechanical page the informa-tion on the probe dimensions is stored (probe protective cover is not in-cluded!). These are then used in the software to establish the transformation from the robot’s tool flange to the probe tip and to align the probe in the light beam unit. These dimensions are spec-ified byManufacturerand can not be modified by the user.
Sensitivity page
The sensitivity factors (a, b, c and d) correspond to the data on the probe cal-ibration sheet and cannot be modified in User or Administrator mode. The
Temperature Range indicated the valid-ity of the calibration parameters.
The data acquisition electronics read the voltages over the NTC (Uy) and the shunt resistances (Ux, Uz). The temperature is evaluated as follows:
T[◦C] =a·log x+d 1−b·x +c with x= 2·Uy Ux+Uz (22.3) a, b, c, d = DASY software parameters.
Note: As with all other SPEAG probes, the probe parameters and specifications depend on the data acquisition (DAE) circuitry.
22.4
Applications
The SAR in lossy media can be assessed by measuring the local temperature rise in the media during exposure:
SAR[W/kg]≈c∆T
∆t with c = heat capacity of the media in J/(kg K) (22.4)
• Make sure that the media is in thermal equilibrium before exposure, e.g., that there is no temperature gradient before exposure.
• After each exposure restore the media to thermal equilibrium (e.g., through stirring, etc.).
• Check the results for disturbing heat equalizing processes by repeating the measurements with different power levels and/or different exposure times. Convection processes have a non-linear behavior.
Note: The optimal exposure time depends on the viscosity of the media. For water-salt solutions or dibutylglycol-solutions the measurement time should not exceed 5 seconds. For water-sugar solutions the measurement time can be up to 20 seconds. In jelly-like media even higher exposure times are possible.
The figure above shows a typical temperature-rise curve. The overall measurement time has been set to 30 s, whereby the RF exposure was turned ON in the 5th second and OFF in the 22nd second (non-linearities in the curve during ON and OFF of the RF power are clearly visible). For the post-measurement evaluation two intervals needs to be defined: time interval for the evaluation of temperature rise (Exposure ON rate) and the initial temperature gradient interval before exposure (Exposure OFF rate). The final result,Net rateis the difference between the two intervals.
Note: It is important to verify that the temperature is constant before exposure for thermal equilibrium. The temperature rise during exposure should be a straight line. Any deviation is due to non-liner heat equalizing processes and may lead to the wrong measurement results.
