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Studies of Excess Heat and Convection in a Water Calorimeter

View Article: PubMed Central - PubMed

ABSTRACT

To explain a difference of 0.5 % between the absorbed-dose standards of the National Institute of Standards and Technology (NIST) and the National Research Council of Canada (NRCC), Seuntjens et al. suggest the fault lies with the NIST water calorimeter being operated at 22 °C and the method with which the measurements were made. Their calculations show that this difference is due to overprediction of temperature rises of six consecutive 60Co radiation runs at NIST. However, the consecutive runs they refer to were merely preliminary measurements to determine the procedure for the NIST beam calibration. The beam calibration was determined from only two consecutive runs followed by water circulation to re-establish temperature equilibrium. This procedure was used for measurements on 77 days, with 32 runs per day. Convection external to the glass cylindrical detector assembly performed a beneficial role. It aided (along with conduction) in increasing the rate of excess heat transported away from the thin cylindrical wall. This decreased the rate of heat conducted toward the axially located thermistors. The other sources of excess heat are the: (1) non-water materials in the temperature probe, and (2) exothermic effect of the once-distilled water external to the cylinder. Finite-element calculations were made to determine the separate and combined effects of the excess heat sources for the afterdrift. From this analysis, extrapolation of the measured afterdrifts of two consecutive runs to mid radiation leads to an estimated over-prediction of no more than about 0.1 %. Experimental measurements contradict the calculated results of Seuntjens et al. that convective motion (a plume) originates from the thermistors operated with an electrical power dissipation as low as 0.6 μW, well below the measured threshold of 50 μW. The method used for detecting a plume was sensitive enough to measure a convective plume (if it had started) down to about the 10 μW power level. Measurements also contradict the NRCC calculations in predicting the behavior of the NIST afterdrifts.

No MeSH data available.


Details of measurement probes #1 and #2 and computer-model probe #3. Dimensions are in millimeters.
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f5-j65dom: Details of measurement probes #1 and #2 and computer-model probe #3. Dimensions are in millimeters.

Mentions: Figure 5 shows details of probes #1 and #2. The optically determined dimensions shown are in millimeters. The thermistors were embedded in epoxy to provide good thermal contact with the capillaries. The mass of epoxy was kept small by limiting its length to about 1.1 mm (as shown) beyond the thermistors in the capillary. The capillary end was enclosed with a tapered glass plug ground down to a thickness of either 0.15 mm or 0.18 mm, which gives an average thickness of 0.165 mm. A computer analysis of the excess heat behavior took into account all the dimensions, and values of the physical, thermal, and radiation properties of the materials shown. The values are listed in Table 1.


Studies of Excess Heat and Convection in a Water Calorimeter
Details of measurement probes #1 and #2 and computer-model probe #3. Dimensions are in millimeters.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4862815&req=5

f5-j65dom: Details of measurement probes #1 and #2 and computer-model probe #3. Dimensions are in millimeters.
Mentions: Figure 5 shows details of probes #1 and #2. The optically determined dimensions shown are in millimeters. The thermistors were embedded in epoxy to provide good thermal contact with the capillaries. The mass of epoxy was kept small by limiting its length to about 1.1 mm (as shown) beyond the thermistors in the capillary. The capillary end was enclosed with a tapered glass plug ground down to a thickness of either 0.15 mm or 0.18 mm, which gives an average thickness of 0.165 mm. A computer analysis of the excess heat behavior took into account all the dimensions, and values of the physical, thermal, and radiation properties of the materials shown. The values are listed in Table 1.

View Article: PubMed Central - PubMed

ABSTRACT

To explain a difference of 0.5 % between the absorbed-dose standards of the National Institute of Standards and Technology (NIST) and the National Research Council of Canada (NRCC), Seuntjens et al. suggest the fault lies with the NIST water calorimeter being operated at 22 °C and the method with which the measurements were made. Their calculations show that this difference is due to overprediction of temperature rises of six consecutive 60Co radiation runs at NIST. However, the consecutive runs they refer to were merely preliminary measurements to determine the procedure for the NIST beam calibration. The beam calibration was determined from only two consecutive runs followed by water circulation to re-establish temperature equilibrium. This procedure was used for measurements on 77 days, with 32 runs per day. Convection external to the glass cylindrical detector assembly performed a beneficial role. It aided (along with conduction) in increasing the rate of excess heat transported away from the thin cylindrical wall. This decreased the rate of heat conducted toward the axially located thermistors. The other sources of excess heat are the: (1) non-water materials in the temperature probe, and (2) exothermic effect of the once-distilled water external to the cylinder. Finite-element calculations were made to determine the separate and combined effects of the excess heat sources for the afterdrift. From this analysis, extrapolation of the measured afterdrifts of two consecutive runs to mid radiation leads to an estimated over-prediction of no more than about 0.1 %. Experimental measurements contradict the calculated results of Seuntjens et al. that convective motion (a plume) originates from the thermistors operated with an electrical power dissipation as low as 0.6 μW, well below the measured threshold of 50 μW. The method used for detecting a plume was sensitive enough to measure a convective plume (if it had started) down to about the 10 μW power level. Measurements also contradict the NRCC calculations in predicting the behavior of the NIST afterdrifts.

No MeSH data available.