DRAFT Recommended Practice for Measurements and ...

More documents

Recommendations

Info

1/29/98 138 C95.3-1991 Revision — 2 nd Draft 10/98 Draft Appendix E Calibration of SAR-Measuring Temperature Probes E1. General For accurate determination of SAR with non-perturbing temperature probes, the precision with which small temperature changes can be measured must be optimized. Calibrations to assure maximum precision must be performed at regular time intervals, the length of which is dependent on the type of temperature probe to be calibrated. Some must be calibrated daily (especially certain fiber optic probes) whereas some can be calibrated monthly with minimal loss in accuracy. Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft, subject to change.
1/29/98 139 C95.3-1991 Revision — 2 nd Draft 10/98 Draft E2. Calibration Techniques Two approaches may be used in performing these calibrations. First, absolute calibrations can be performed at regular time intervals at several temperature points over the range of interest. This is accomplished by comparing the temperature probe being calibrated with a standard thermometer, usually a platinum or thermistor type. The platinum resistance thermometer has good long-term stability, but the thermistor type has a smaller sensing element and thus has a smaller thermal mass. This smaller thermal mass minimizes the creation of temperature gradients in the calibrating water bath and thermal lag problems caused by use of a large metallic temperature probe with its inherently large thermal mass and long time constant. If a dynamic temperature calibration technique is used, a well-stirred water bath should be used. Also, the rate of rise of the temperature of the water must be adjusted so that it is slower than the time constants of either of the probes. The unknown and the reference probes should be placed in intimate contact at their sensor-areas (usually the first few cm near the tip). It is best to use an automated data acquisition system to ensure reliable, instantaneous, and simultaneous readings from all temperature probes. If absolute calibrations of the unknown thermometer can be performed with accuracy approaching the resolution of the instrument being calibrated, this type of calibration is preferred. In some cases an instrument under calibration will not accurately indicate absolute temperature but will correctly measure a change in temperature ∆T. In this case, the ratio (∆T-absolute)/(∆Tmeasured) is determined over the range of interest. If this value is constant over the range of temperatures to be studied and over a reasonable period of time, e.g., from week to week, accurate SAR data can be obtained even when absolute temperature can not be precisely defined. When using thermometers to measure SAR in electromagnetic fields, one must be aware of the possibility of RF interference in the thermometer's sensor, leads or electronics. Several methods can be used to determine the magnitude of RFI. One is to note changes at the instant that the RF power is switched on and off. If large, immediate changes are found, the probes should only be used to measure temperature prior to and just after RF exposure. Artifacts may also occur due to the interaction of the RF field with the electrical leads attached to the temperature sensing element of the probe. This interaction can excite a thermally induced voltage at the junction of two dissimilar materials (thermoelectric effect), including high resistance (carbon-loaded Teflon) leads joined to metallic wires. Since this phenomenon is due to the heating of a junction, shielding of these areas should be used to minimize this source of measurement error. Metal foil or RF absorber can be used to shield these junctions. Details of these problems and additional solutions may be found in Hochuli [E1]. With either the dynamic or static calibration technique, it is useful to perform two calibrations of ∆T. The first was described above; a temperature rise in a water bath. The second calibration should be performed using a temperature decrease in a water bath. If a difference in the calibration factors exists for the unknown probe when determined by the two methods, the thermal time constant of one or both of the probes may have been longer than the time period that elapsed between the thermometer readings taken during one or both of the calibrations. To correct this, the calibration process should be repeated, using a slower rate of temperature increase or decrease in the bath. Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft, subject to change.
Page 1 and 2:
1/29/98 C95.3-1991 Revision—3rd D
Page 3 and 4:
1/29/98 3 C95.3-1991 Revision — 2
Page 5 and 6:
1/29/98 5 C95.3-1991 Revision — 2
Page 7 and 8:
1/29/98 7 C95.3-1991 Revision — 2
Page 9 and 10:
1/29/98 9 C95.3-1991 Revision — 2
Page 11 and 12:
1/29/98 11 C95.3-1991 Revision —
Page 13 and 14:
1/29/98 13 C95.3-1991 Revision —
Page 15 and 16:
1/29/98 15 C95.3-1991 Revision —
Page 17 and 18:
1/29/98 17 C95.3-1991 Revision —
Page 19 and 20:
1/29/98 19 C95.3-1991 Revision —
Page 21 and 22:
1/29/98 21 C95.3-1991 Revision —
Page 23 and 24:
1/29/98 23 C95.3-1991 Revision —
Page 25 and 26:
1/29/98 25 C95.3-1991 Revision —
Page 27 and 28:
1/29/98 27 C95.3-1991 Revision —
Page 29 and 30:
1/29/98 29 C95.3-1991 Revision —
Page 31 and 32:
1/29/98 31 C95.3-1991 Revision —
Page 33 and 34:
1/29/98 33 C95.3-1991 Revision —
Page 35 and 36:
1/29/98 35 C95.3-1991 Revision —
Page 37 and 38:
1/29/98 37 C95.3-1991 Revision —
Page 39 and 40:
1/29/98 39 C95.3-1991 Revision —
Page 41 and 42:
1/29/98 41 C95.3-1991 Revision —
Page 43 and 44:
1/29/98 43 C95.3-1991 Revision —
Page 45 and 46:
1/29/98 45 C95.3-1991 Revision —
Page 47 and 48:
1/29/98 47 C95.3-1991 Revision —
Page 49 and 50:
1/29/98 49 C95.3-1991 Revision —
Page 51 and 52:
1/29/98 51 C95.3-1991 Revision —
Page 53 and 54:
1/29/98 53 C95.3-1991 Revision —
Page 55 and 56:
1/29/98 55 C95.3-1991 Revision —
Page 57 and 58:
1/29/98 57 C95.3-1991 Revision —
Page 59 and 60:
1/29/98 59 C95.3-1991 Revision —
Page 61 and 62:
1/29/98 61 C95.3-1991 Revision —
Page 63 and 64:
1/29/98 63 C95.3-1991 Revision —
Page 65 and 66:
1/29/98 65 C95.3-1991 Revision —
Page 67 and 68:
1/29/98 67 C95.3-1991 Revision —
Page 69 and 70:
1/29/98 69 C95.3-1991 Revision —
Page 71 and 72:
1/29/98 71 C95.3-1991 Revision —
Page 73 and 74:
1/29/98 73 C95.3-1991 Revision —
Page 75 and 76:
1/29/98 75 C95.3-1991 Revision —
Page 77 and 78:
1/29/98 77 C95.3-1991 Revision —
Page 79 and 80:
1/29/98 79 C95.3-1991 Revision —
Page 81 and 82:
1/29/98 81 C95.3-1991 Revision —
Page 83 and 84:
1/29/98 83 C95.3-1991 Revision —
Page 85 and 86:
1/29/98 85 C95.3-1991 Revision —
Page 87 and 88: 1/29/98 87 C95.3-1991 Revision —
Page 137: 1/29/98 137 C95.3-1991 Revision —
show all

DRAFT Recommended Practice for Measurements and ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?