HM Digital TDS-4TM Meter with Digital Thermometer The Digital TDS-4TM Water Meter is ideal for testing applications such as hydroponics and gardening, pools and spas, aquariums and reef tanks, water ionizers, drinking water and more The smallest and lightest TDS meter available on the market. Hold Function - Saves measurements for convenient reading and recording. Auto-off function - Turns off meter after 10 minutes of non-use to conserve batteries.