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August 2009 Peltier Cooler Testing Background Further investigation was needed into the behaviour of the Peltier Cooler arrangement with an adjustment of provided power. In order to stabilize the temperature of the 0-200V voltage supply, two aluminium slabs would be shaped and used to provide some shielding from the ambient temperature interferences. The purpose of this procedure was to observe if the aluminium blocks had a significant effect in providing some temperature stabilization. The Peltier would be situated on the outer side of the bottom aluminium block, with the voltage control circuit carefully sandwiched between the two blocks. Therefore, to deduce whether or not the blocks were effective in providing some form of stabilization for the voltage tuning control, the thermal effect of the aluminium block had to be compared with what was assumed to be less effective shielding. This was an aluminium box, which was used in the first tests carried out on the Peltier cooler, where a relatively large temperature sweep was evident. Preliminaries 1) The masses of the following apparatus were taken: Two large aluminium blocks (which would be shaped for the purposes of containing the voltage supply in the final system). The box that was used to house the Peltier cooler (in the preliminary stages of testing where the saturation occurred). The mass balance used has an uncertainty of 50g. Each of the aluminium blocks therefore had masses of 5.5(5) kg. The aluminium box has a mass of 0.5(5) kg. 2) The resistance of the 25W resistor was measured as 4.7Ω, so this provided a basis measurement for the maximum current that’d be allowed through the resistor before exceeding its power rating. It was deduced that approximately 2.2A was the maximum permissible current. Arrangement and Experiments The initial arrangement consisted of a 25W resistor mounted on the bottom of an aluminium box; having been mounted on it with thermal paste and electrically insulating tape. A PT100 thermistor was used to provide a measurement of temperature between 20 and 30 degrees Celsius. The probe tip was mounted at the bottom of the box. Connected to an appropriate +15V power supply which then read out to a DAQ, the thermistor was then used to provide a temperature measurement. A voltage supply was connected to the resistor, and set to provide a voltage of approximately 4.7V. The current it provided was adjusted over a range of 2A; the currents fed in so as to power the resistor being in the region of 0.5A, 1A, 1.5A and 2A. Whilst the voltage supply was turned off, the current it provided was adjusted. A stop watch was then used to, from the second the current was turned on; record the voltage that was read out on the DAQ (which numerically corresponded to the temperature above 20 degrees Celsius). At first, when the temperature increase was most rapid, the readings were taken every 5-10 second and, afterwards, longer intervals were used. After waiting for some stabilization to occur, or at least having waited for the temperature profile of the thermistor probe to take on a smooth shape in which the growth had gradually began to slow down, the current was turned off. Temperature measurements were again taken using the DAQ Multi-meter, at appropriate time intervals. aluminium box PT100 temperature sensor to voltage supply 4.7 Ω + 5% resistor to Peltier cooler Fig. 1: The set-up for the first tests on the change of temperature; with the probe of the PT100 fastened to the inner base of the aluminium box with thermal paste and electrically insulating tape. The run was repeated for an approximate current of 2A fed into the thermistor probe; but this time with the lid placed on top of the aluminium box. Again, readings were taken as well as recorded time intervals, until saturation; or at least some indication of the beginning of temperature stabilization, was attained. Comparisons of this presumably weak form of environmental shielding, with the actual intended set-up, were needed to get a feel of whether or not stabilization was aided by use of the blocks. To loosely mimic the intended set-up, a 25W resistor was placed below one of the two horizontal aluminium blocks, with a thermistor probe sandwiched in between the two. For the purposes of making sure the probe of the PT100 thermistor was not damaged in this experiment, a small aluminium caging was used to surround the PT100 probe. For 1A and 2A supplied to the resistor, the temperature read-out on the DAQ was again taken in accordance with appropriate time intervals. aluminium blocks to Peltier cooler PT100 temperature sensor 4.7 Ω +5% resistor probe tip encased in aluminium grip Fig. 2: The arrangement for examining the behaviour of temperature in between the aluminium blocks, which will be shaped to correspond to that required to encase the voltage supply. Results and Analysis The plots for an approximate current of 2A fed into the 25W resistor are as shown. To inherit some understanding of the timescales needed for certain temperature changes and the variation of the profiles of the curves, the regions displaying exponentially decaying behaviour were extracted. Fig. 3: Temperature dependence on time, for no lid on the aluminium box. A delay of approximately 10 seconds before the current begins to decay; occurs from the moment the current (and hence power to the resistor) is switched off. Estimating from the plot, it takes approximately 1300 seconds for the temperature to decrease by 7 degrees Celsius, having begun to saturate at this point. Fig. 4: The region of the temperature profile shown in Figure 3 that’s displaying exponentially decaying behaviour. The time constant is ~260 seconds. Fig. 5: Time dependence of temperature for the experiment repeated for the lid on top of the aluminium box. A delay of approximately 40 seconds before the current begins to decay; occurs from the moment the current is switched off. Over a time span of approximately 2500 seconds, the temperature has saturated to approximately 25 degrees Celsius. Fig. 6: The region of exponential decay for the lid on top of the aluminium box. The time constant is ~640 seconds; a factor of two and a half times larger than the test in which there was no lid on the identical box. This illustrates the importance in completely surrounding the temperature sensor in order for it to retain the heat it senses. The amplitude of this exponential profile is 6.5 °C. Fig. 7: Time dependence of temperature when the PT100 probe tip was situated in between two aluminium blocks. A delay of approximately 100 seconds occurred after the current had been turned off; before the temperature read-out indicated a decrease. The temperature varies over a range of only 1 degree Celsius over an approximate time span of 3000 seconds. Fig. 8: The region of exponential decay is extensively elongated for the arrangement in which aluminium blocks surround the temperature sensor. The time constant is ~5700 seconds, which is a factor of 9 larger than that for the arrangement with the lid on top of the aluminum box. This illustrates the effectiveness of the aluminium blocks in stabilizing the temperature between them over relatively long time scales. The amplitude is 2 °C, conveying a temperature span of more than three times less than for the lid on top of the aluminium box. Indeed, the extent of the temperature over the experimental time scales was limited by the blocks. Conclusions The tests carried out show that the blocks are very effective in stabilizing the temperature in between them, by comparison with the first arrangement in which the thickness of environmental shielding was very limited indeed. Over a very long time (over 3500 seconds); the probe measured temperature changes that were just under 1 degree Celsius. With the aluminium plates not in their final shapes, only approximations can be made concerning the effect that the applied current, along with the shielding (from one test arrangement to the next), had on the temperature variations obtained, accompanied with the time scales on which these occurred. As long as the masses of he blocks are not massively reduced in the final arrangement for the voltage supply, we need to wait ~3 x10³ seconds to get a temperature increase of approximately 1 degree Celsius, for a heater having a power in the region of 25W.