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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.


Radiation run showing linearity of response and initial and post radiation drifts.
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f2-j65dom: Radiation run showing linearity of response and initial and post radiation drifts.

Mentions: The effectiveness of the cylinder as a barrier to external convection is shown by the recording in Fig. 2. The thermistor powers were approximately 100 μW. The temperature rise is linear during the 160 s of radiation, and the initial part of the afterdrift appears to be the same as the initial drift. The latter part of the afterdrift begins to show a slight decrease in slope at about 40 s after beam turnoff. The beginning of this cooling trend appears to be the result of conduction within the cylinder caused by external convection that transports cooler water to the vicinity of the cylinder (also discussed in Sec. 8.2). In the absence of external convection, the cooling trend would have been noticeable at a much later time (see Fig. 1 of [7]) as a result of conduction starting mainly in the beam penumbra. The behavior of the afterdrift shown in Fig. 2 contradicts the calculated predictions of Seuntjens et al. [8]. Their Fig. 20 and Table 4 predict a rapid rate of decrease in the afterdrift at the 100 μW level for a radiation time of only 70 s.


Studies of Excess Heat and Convection in a Water Calorimeter
Radiation run showing linearity of response and initial and post radiation drifts.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2-j65dom: Radiation run showing linearity of response and initial and post radiation drifts.
Mentions: The effectiveness of the cylinder as a barrier to external convection is shown by the recording in Fig. 2. The thermistor powers were approximately 100 μW. The temperature rise is linear during the 160 s of radiation, and the initial part of the afterdrift appears to be the same as the initial drift. The latter part of the afterdrift begins to show a slight decrease in slope at about 40 s after beam turnoff. The beginning of this cooling trend appears to be the result of conduction within the cylinder caused by external convection that transports cooler water to the vicinity of the cylinder (also discussed in Sec. 8.2). In the absence of external convection, the cooling trend would have been noticeable at a much later time (see Fig. 1 of [7]) as a result of conduction starting mainly in the beam penumbra. The behavior of the afterdrift shown in Fig. 2 contradicts the calculated predictions of Seuntjens et al. [8]. Their Fig. 20 and Table 4 predict a rapid rate of decrease in the afterdrift at the 100 μW level for a radiation time of only 70 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.