DM-1: In-Line Dual TDS Monitor from HM Digital

DM-1: In-Line Dual TDS Monitor

“Measure the tap water coming in vs. the purified water going out of your water filtration system at any time.

•Displays TDS levels of the feed water and product water with in & out electrode probes.

•Highly efficient and accurate due to its advanced microprocessor technology.

•Auto-Off function conserves battery power. The unit shuts off automatically after 10 minutes of non-use.

•Dual Range 0-9990 ppm (From 0 – 999 ppm the resolution is 1 ppm. From 1000 to 9990 ppm, the resolution is 10 ppm, indicated by a blinking ‘x10’ image.)

•Factory Calibrated: our meters are calibrated with a 342 ppm NaCl solution. Meters can be recalibrated with a mini-screwdriver.

•Comes equipped with Mur-lok® Quick-Connect T-fittings.

•Installs quickly and easily."


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133 tasting notes

Almost a year ago, we purchased this In-Line Dual TDS Monitor to monitor the performance of our new RO system: ( This meter monitors the TDS both before and after the RO system, which is important for brewing tea:

“Water with 50 – 150 ppm total dissolved solids (TDS) or 1 to 3 grains hardness provides the best results, according to the Tea Association of the USA."

Sensor Locations:
• For monitoring the TDS level of the untreated water, the “TDS In” sensor should be installed after all the pre-filters and just before the RO membrane.
• For monitoring the performance of the RO membrane, the “TDS Out” sensor should be immediately after the RO membrane.

These two sensor locations provide the two TDS levels necessary to calculate “RO Percentage Rejection” – the parameter that determines when to replace the RO membrane (about every 3 years). See the bottom ½ of this page:

1. If the water in your area requires an RO system and you wish to use this water for brewing tea, a re-mineralization filter such as this ( is highly recommended.
2. An additional fitting should be added to RO System that has a re-mineralization filter so the output sensor can be located either after the RO membrane for maintenance purposes, or after the re-mineralization filter to monitor the TDS of water used for brewing tea.

After using this In-Line Dual TDS Monitor for nearly a year, I’ve found it to be a simple and reliable device that provides consistent TDS data for monitoring the performance of both RO systems and re-mineralization filters. Highly recommended.

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