HM Digital Meters, handheld and portable TDS, EC and pH testers, in-line monitors, controllers and demonstration devices for reverse osmosis (RO) water filters and water purification systems are superior in construction and performance, yet made available at a very economic cost. HM Digital, Inc. is the only manufacturer of economical in-line dual TDS meters that measure total dissolved solids (TDS) levels of feed and product water lines simultaneously, an effective way to compare rejection rates and gauge reverse osmosis membrane, water filter and water purification performance.