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Datalogging: The cooling effect of evaporation

This investigation looks at how evaporation can cause cooling. We have used two cups, two dataloggers, some paper towels and some water to measure the effect. We set up one cup with a wet towel on the top, and as a control, we had one with a dry towel.

We don’t need much for this investigation. We used two polystyrene cups, two pieces of paper towel, two  temperature dataloggers and some tepid water.

Within the ‘Mini-datalogger’ software it is possible to configure the datalogger to record temperature samples at varying intervals. We decided to set the dataloggers to record every two seconds, using the ‘single mode’, so we didn’t accidentally lose our data. We also chose to start logging a few minutes into the future, to make sure that we didn’t affect our data too much.

We placed one datalogger into each cup, with the circuitry near the middle. This should allow the air to circulate around the datalogger, giving us the most accurate reading.

Then we wet one of the paper towel squares with tepid water. We had to allow as much water to drip off as we could, so that we didn’t risk getting our dataloggers wet.

We then left them in the cupboard for a few hours, to allow the water to evaporate.

After around three hours, the paper towel was still very wet, so we decided to increase the airflow over the paper towel, by using a fan. This should cause more water to evaporate.

I repeatedly checked on the cups over the final 1hr 30mins as the paper towel on the dry cup kept flying off.

When I returned for the final time, the paper towel on the wet cup had blown off. This definitely proved that the water had evaporated, and the towel was almost dry.

Our initial results looked promising. The cupboard where they were being kept wasn’t heated so the temperature dropped considerably, and after 45mins – 1hr they levelled out. Interestingly, the cup with the wet towel was already lower in temperature than the dry one, and this continued to be the case throughout the investigation.

At around 13:40, the temperature of both cups increased – this was after the fan had been switched on. Due to the nature of the room, and the lights, we believe that the fan had caused the air to circulate, and even out the temperature. We are going to investigate this hypothesis. EDIT: See results.

As we mentioned previously, the wet paper towel had blown off, and it looks like this happened around 5 minutes before the end of the investigation, as there was a sudden rise in temperature recorded on the wet towel datalogger.

So far, it appears that the wet towel does help to cool the contents of the cup down, by evaporation. To see whether we can recreate and improve on our results, we will repeat the investigation.

This time we left the resources for the investigation in the room, for 45 minutes before the dataloggers started recording. This allowed everything to get closer to room temperature. After the first 15 minutes of datalogging, we then placed the wet and dry towels over the dataloggers.

The results this time did match our expectations. There was an initial rise of both temperatures, likely caused by the opening of the door. The wet towel cup continued to rise in temperature, after the dry cup had stopped. This is likely to be caused by the water used being slightly above room temperature – even after sitting out for 45minutes.

The wet towel cup then dropped in temperature, where it stayed about 0.6 degrees celsius below the dry towel cup temperature. The temperatures gradually rose in both cups until around 13:00 – this was when we checked the cups and obviously forgot to close the door! Interestingly, the dry towel cup temperature rose by more than the wet towel cup. The wet towel cup was then consistently around 0.8 degrees celsius below the dry one.

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