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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.


Related in: MedlinePlus

Sketch of computer geometry used for calculating conductive heat flow from the cylinder wall. Dimensions are in millimeters.
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f9-j65dom: Sketch of computer geometry used for calculating conductive heat flow from the cylinder wall. Dimensions are in millimeters.

Mentions: Figure 9 shows the geometry selected for the computer calculations. The glass wall was assumed to be uniform in thickness (0.3 mm) and infinite in extent so that the calculated flow of heat is perpendicular to the glass wall. Therefore, the distance PP′ along the cylinder and probe axis (YY′) could be chosen to be any convenient length. An impervious heat boundary was chosen to be at 120 mm, large enough that the annular water ring, radii of 16.5 mm to 120 mm, would act as an infinite heat sink for excess heat conducted from the cylinder wall such that no excess heat would arrive at the boundary 30 min after 70 s of radiation. Figure 10 (curve a) shows the calculated excess thermistor temperature as a result of excess heat arriving from the glass wall. The excess heat, along the axis, is at a maximum at 498 s after radiation. Table 4 shows the percent excess values from 0 s to 100 s.


Studies of Excess Heat and Convection in a Water Calorimeter
Sketch of computer geometry used for calculating conductive heat flow from the cylinder wall. Dimensions are in millimeters.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f9-j65dom: Sketch of computer geometry used for calculating conductive heat flow from the cylinder wall. Dimensions are in millimeters.
Mentions: Figure 9 shows the geometry selected for the computer calculations. The glass wall was assumed to be uniform in thickness (0.3 mm) and infinite in extent so that the calculated flow of heat is perpendicular to the glass wall. Therefore, the distance PP′ along the cylinder and probe axis (YY′) could be chosen to be any convenient length. An impervious heat boundary was chosen to be at 120 mm, large enough that the annular water ring, radii of 16.5 mm to 120 mm, would act as an infinite heat sink for excess heat conducted from the cylinder wall such that no excess heat would arrive at the boundary 30 min after 70 s of radiation. Figure 10 (curve a) shows the calculated excess thermistor temperature as a result of excess heat arriving from the glass wall. The excess heat, along the axis, is at a maximum at 498 s after radiation. Table 4 shows the percent excess values from 0 s to 100 s.

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.


Related in: MedlinePlus