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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.


Typical preliminary set of consecutive runs. Time increases from right to left. The heating is caused by the thermistor response to absorbed radiation. The spikes are caused by the heating and manual adjustments of the Wheatstone bridge balancing arm to keep the recorder trace on scale.
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f4-j65dom: Typical preliminary set of consecutive runs. Time increases from right to left. The heating is caused by the thermistor response to absorbed radiation. The spikes are caused by the heating and manual adjustments of the Wheatstone bridge balancing arm to keep the recorder trace on scale.

Mentions: Figure 4 shows the temperature traces of a typical series of five consecutive runs. Time increases from right to left. The spikes shown are caused by radiation heating and manual adjustments of the Wheatstone-bridge balancing arm to keep the recorder trace on scale. The duration of the runs was varied around 70 s to produce a temperature rise of 0.5 mK. Observation of these preliminary runs was helpful in determining the measurement procedure for the beam calibration. It was observed that the drifts of the first two runs are essentially constant, while the afterdrift of the third run shows a small change as a result of a cooling effect that increased, as indicated, in subsequent drifts. This was caused by the external convective cooling effect that eventually reached the thermistor by internal conduction sometime during the third radiation run. Therefore, the measurement procedure was to make only two consecutive runs followed by circulation of the once-distilled water. This pattern of runs was made on 77 days in which 32 runs were made on each day. It required about 6 h to make these runs, including the time to fill the water container to an exact level each day, circulate the water, and reduce the drift to an acceptably small value. Although the radiation runs were about 70 s, calculated studies of excess heat caused by the probes are presented in Sec. 6.1 for radiation runs from 30 s to 180 s.


Studies of Excess Heat and Convection in a Water Calorimeter
Typical preliminary set of consecutive runs. Time increases from right to left. The heating is caused by the thermistor response to absorbed radiation. The spikes are caused by the heating and manual adjustments of the Wheatstone bridge balancing arm to keep the recorder trace on scale.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4-j65dom: Typical preliminary set of consecutive runs. Time increases from right to left. The heating is caused by the thermistor response to absorbed radiation. The spikes are caused by the heating and manual adjustments of the Wheatstone bridge balancing arm to keep the recorder trace on scale.
Mentions: Figure 4 shows the temperature traces of a typical series of five consecutive runs. Time increases from right to left. The spikes shown are caused by radiation heating and manual adjustments of the Wheatstone-bridge balancing arm to keep the recorder trace on scale. The duration of the runs was varied around 70 s to produce a temperature rise of 0.5 mK. Observation of these preliminary runs was helpful in determining the measurement procedure for the beam calibration. It was observed that the drifts of the first two runs are essentially constant, while the afterdrift of the third run shows a small change as a result of a cooling effect that increased, as indicated, in subsequent drifts. This was caused by the external convective cooling effect that eventually reached the thermistor by internal conduction sometime during the third radiation run. Therefore, the measurement procedure was to make only two consecutive runs followed by circulation of the once-distilled water. This pattern of runs was made on 77 days in which 32 runs were made on each day. It required about 6 h to make these runs, including the time to fill the water container to an exact level each day, circulate the water, and reduce the drift to an acceptably small value. Although the radiation runs were about 70 s, calculated studies of excess heat caused by the probes are presented in Sec. 6.1 for radiation runs from 30 s to 180 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.