Why the Spa Warms Faster Than the Pool

A Question at the Rec Center

1Dario Kowalczyk swims laps at the Riverside Rec Center three mornings a week. One Monday both the lap pool and the small spa next to it had been drained and refilled with cold water. By Tuesday the spa was warm again, but the lap pool felt chilly for most of the week. Dario asked the aquatics director, Ms. Fenna Oduya, why.

2Ms. Oduya said the spa holds about 6,000 L of water and the lap pool holds about 450,000 L. Both are warmed by heaters that run on the same schedule. Dario wanted to test how the amount of liquid affects warming, so Ms. Oduya helped him set up a small investigation in the staff room.

3Dario used a small electric immersion heater that always gives off energy at the same rate while it is switched on, so running it twice as long transfers about twice as much energy. Ms. Oduya handled the heater and plugged it in for every trial. Each trial used a foam cup with a lid, so little energy escaped. Dario measured the mass of liquid on a kitchen scale, started every liquid at 20 °C and stirred it gently with a plastic spoon while the heater ran. Right after the heater was switched off, he read the thermometer and found the temperature change.

4Dario also wondered whether the type of liquid matters. For two trials he used cooking oil from the rec center's snack bar instead of water. Each trial was repeated three times, and the table shows the average temperature change.