Poolmaster 18305 9 5/8" Pocket Thermometer. Poolmaster’s Essential Collection consists of the essential must-have products for customary pool and spa maintenance constructed with durable materials designed to keep your pool sparkling. The Pocket Thermometer features a reservoir at the base that captures water which aids in measuring accurate water temperature. The Fahrenheit and Celsius readouts are large and clear making it easy to discern quickly your water’s temperature.