Temperature plays an important role in water balance because calcium becomes less soluble as water temperature increases and more soluble as the temperature decreases. In other words, warm water tends to be more basic and scale producing, while colder water is more corrosive. In a spa, where the water temperature is consistently over 100°F (38°C), calcium easily becomes less soluble and needs to be kept in check by closely monitoring the other water balance factors.
Most spas have a built-in thermostat that transmits the temperature to the spa's control panel. As a backup, it's a good idea to use a separate spa thermometer tethered to a handrail and dangled in the water.
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Click on Photos for Next Jacuzzi Bathroom Design Gallery ImagesThese thermometers cost about $5 and are available from spa supply outlets.