Description
The HM Digital DM-1 Dual Inline TDS Meter provides a simple, permanent way to monitor the performance of your Reverse Osmosis (RO), deionisation (DI), or other water treatment system.
Using two inline sensors, the DM-1 measures the TDS of the incoming feed water and the filtered product water, allowing you to quickly compare the two readings and see how effectively your RO membrane or filtration system is operating.
Simply press the power button and select IN or OUT to check either reading.
Ideal for domestic RO systems, commercial water treatment, aquariums, hydroponics and other applications where regular TDS monitoring is required.
Why Use a Dual TDS Meter?
With an RO system, the absolute TDS reading is useful, but the comparison between the incoming and outgoing water is even more valuable.
For example:
Feed water: 200 ppm
RO water: 10 ppm
This represents approximately 95% TDS rejection, giving you a quick indication of how well the RO membrane is performing.
Regular monitoring can help identify when membrane performance is beginning to decline, rather than relying purely on a replacement schedule.
Features
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Measures incoming and outgoing TDS
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Dual inline sensors for continuous installation
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Ideal for monitoring RO membrane performance
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Large, easy-to-read digital display
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Simple IN / OUT selector
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TDS measurement range up to 9990 ppm
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Accuracy of ±2%
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Automatic temperature compensation
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Factory calibrated at 342 ppm NaCl
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Can be recalibrated if required
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Automatic shut-off after approximately 10 minutes
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Battery powered – no external power supply required
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Supplied with 1/4" quick-connect T fittings
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Compact design suitable for mounting inside or beside an RO system
Specifications
Model: HM Digital DM-1
Type: Dual Inline TDS Monitor
TDS Range: 0–9990 ppm
Resolution:
0–999 ppm: 1 ppm
1000–9990 ppm: 10 ppm
Accuracy: ±2% of reading
Conversion Factor: NaCl, approximately 0.5
Calibration: Factory calibrated to 342 ppm NaCl; adjustable calibration
Sensors: 2 x inline TDS sensors
Connection Size: 1/4" quick-connect fittings
Sensor Cable Length: Approx. 62 cm
Power: 2 x LR44 batteries
Auto Shut-Off: Approx. 10 minutes
Dimensions: 76 x 20 x 47 mm
Weight: Approx. 79 g
Suitable For
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Reverse Osmosis systems
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RO/DI systems
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Drinking water filtration
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Deionised water systems
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Aquariums
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Hydroponics
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Water treatment equipment
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Commercial filtration systems
Easy Installation
The DM-1 includes two inline sensor fittings.
One sensor is normally fitted to the feed water entering the RO system, while the second sensor is installed on the RO product-water line.
Once installed, there is no need to collect water samples. Simply turn the meter on and select IN or OUT to view the relevant TDS reading.
For best accuracy, allow the reading to stabilise for approximately 10 seconds.
Monitoring RO Membrane Performance
The DM-1 is particularly useful for checking the rejection performance of an RO membrane.
You can calculate approximate membrane rejection using:
Rejection % = (Feed TDS – Product TDS) ÷ Feed TDS × 100
Example:
Feed TDS = 250 ppm
Product TDS = 10 ppm
Rejection = 96%
Tracking this percentage over time can provide a useful indication of changes in membrane performance.
Important
TDS measures the concentration of dissolved ionic material in the water. It does not identify individual contaminants or confirm that water is microbiologically safe.
The DM-1 should therefore be used as a monitoring tool for filtration performance rather than as a complete water-quality test.
What's Included
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HM Digital DM-1 Dual TDS Meter
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2 x inline TDS sensors
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2 x 1/4" quick-connect T fittings
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Batteries
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Installation / operating instructions