HM Digital DM-2 Dual In-Line TDS Monitor: Aquarium Controllers & Monitors
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HM Digital DM-2 Dual In-Line TDS Monitor

HM Digital DM-2 Dual In-Line TDS Monitor
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Inline TDS monitor for measuring aquarium RO unit performance
Real-time TDS measurement of tap water AND RO product water
Convenient way to ensure your RO unit is functioning properly

Determine how efficient your reverse osmosis unit is performing with a push of a button. HM Digital DM-2 Dual In-Line TDS Monitor is a "must have" for any aquarists who produces their own RO (reverse osmosis) water for aquarium use. Once installed to your RO production and collection system, you enjoy simultaneous precision monitoring of TDS (Total Dissolved Solids) levels of feed and product water lines!

HM Digital DM-2 Dual In-Line TDS Monitor offers a simple way to compare rejection rates and gauge reverse osmosis membrane performance. Highly efficient and accurate in-line TDS monitor boasts advanced microprocessor technology for precision measurement. Convenient features including extra large, easy-to-read LCD, digital calibration, low-battery indicator, auto shut-off, diagnostic error messaging, and more keep high-tech water quality testing simple. TDS Range of 0-9990ppm with a dual range resolution (1 ppm or 10 ppm) accurate within ±2%. HM Digital DM-2 Dual In-Line TDS Monitor comes equipped with 1/4" Mur-lok® Quick-Connect T-fittings and two AA batteries.


  • Displays TDS levels of the feed water and product water with IN & OUT electrode probes.
  • Auto-Off function conserves battery power. The unit shuts off automatically after 3 minutes.
  • Dual Range - Measures from 0-999 ppm, with a resolution of 1 ppm. From 1000 to 9990 ppm, the resolution is in 10 ppm increments, indicated by a blinking 'x10' icon.
  • Factory calibrated with a 342 ppm NaCl solution. The DM-2 features digital calibration.
  • Diagnostic error messaging.
  • Waterproof shielded sensor cables.
  • Strong water-resistant casing.
  • Variable mounting bracket.
TDS Range:
0-999: 1 ppm
10 ppm
(indicated by a
blinking "x10" icon)
Conversion Factor:
NaCl (Avg. of 0.5)
Factory Calibration:
342 ppm NaCl
Sensor Cable Length:
46" (including sensor)
Power Source:
2 x AA batteries
Auto Shut Off:
After 30 seconds
Battery Life:
Approximately 12-18 months
Base Unit Dimensions:
4.6" x 2.6" x 0.7" high

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Please click on "More Information" for Instructions.

More Info
The DM-2 can be configured in a variety of ways, depending upon your needs. Typically the IN Line (line 1) is connected to the source (tap) water and the OUT Line (line 2) is connected to the product (filtered) water. The DM-2 can also be configured with multiple systems, such as a RO/DI combination system.

To install the DM-2 to a water purification or filtration system:

  1. Insert the White Sensors fully into the bottom of the T-Fittings.
  2. Orient the Sensor Pins so that they are perpendicular to the direction of the "T." The water should flow over both pins equally. (You should be able to see both pins as you look through the fitting.)
  3. Disconnect water source.
  4. Snip the source (tap) water tube at the point between the source and the filter. Insert both ends of the tube into the top of the IN Line sensor's T-Fitting.
  5. Snip the product (filtered) water tube at the point between the filter and a dispenser. Insert both ends of the tube into the top of the OUT Line sensor's T-Fitting.
  6. The DM-2 monitor can be attached anywhere on or near the water system using the Mounting Bracket (which can be secured by screws or the adhesive backing).
  7. Reconnect the water source. Your monitor is now ready to use.

Note: Consult a professional plumber for specific bracket or connection questions.

Changing the Batteries
If the display is fading or the readings are incorrect, you may need to change the batteries.

  1. Unscrew the four (4) metal screws (not the orange plastic screws) on the rear of the unit and remove the Back Panel.
  2. Remove the batteries.
  3. Replace both batteries with two fresh AA batteries. Make sure the polarity is correct.
  4. Close the Back Panel and replace the screws. You will not need to recalibrate.


  1. Press the "POWER" button.
  2. To display the TDS level of the Feed (tap) water, press the IN Button. To display the TDS level of the Product (filtered) water, press the OUT Button.
  3. The displayed TDS will be most accurate after approximately 10 seconds.
  4. Determining filter effectiveness depends on your particular system. For an RO system, compare the IN water TDS level with the OUT water TDS.
  5. If the "x10" icon appears, this means the TDS level is above 999ppm. Therefore, multiply the reading by 10. For example, if the display shows 143 WITH the "x10" icon, the actual TDS level is 1430 ppm. (If the "x10" icon does not appear, the reading on the display is the actual TDS level.)
  6. Turn off the unit. It will automatically shut off after 30 seconds.

You monitor was factory calibrated to 342 ppm. This level is suitable for most tap water/filtered water applications, so it is ready to use out of the box. However, you may need to re-calibrate based on your needs as well as from time-to-time to ensure best results. To calibrate:

  1. Purchase a certified calibration solution that is correct for your needs. The calibration solution should be NaCl. 342 ppm NaCl is recommended HM Digital.
  2. Disconnect both T-Fittings from their tubes. Do NOT remove the sensors from the "T." Make sure the orientation of the sensor to the fitting is correct. Shake any water out.
  3. For better accuracy, calibrate to a flowing solution. If that is not possible, you can calibrate to a still solution. Turn on the monitor and place each T-Fitting (with the sensors in them) into the calibration solution. You will get a reading. Make sure the T-Fitting is completely filled with solution and there are no air bubbles. THIS STEP IS CRITICAL FOR PROPER CALIBRATION.
  4. If the reading on the monitor (for either line) does not match the Solution, adjust the reading up or down by gently turning the plastic Orange Screws on the rear of the unit clockwise or counterclockwise to raise or lower the reading.
  5. Adjust the Screws marked IN and OUT for their respective lines for large changes in calibration.
  6. Adjust the screw on the lower-right corner for fine-tuning (for both IN and OUT).
  7. If calibrating to a still (not flowing) solution, calibrate to 3% above the level of the calibration solution. This will accommodate for the lack of flowing water, which the monitor is programmed to do. For example, if the calibration solution is 342ppm adjust the screws until it reads 352 ppm. If you are calibrating to a flowing solution, calibrate to the level of the solution.
  8. Your monitor is now calibrated.

Care and Maintenance
Very little care is necessary for your DM-2

  • Never touch the Sensor Pins as skin oils may adversely affect the TDS measurement.
  • To clean the Sensor Pins, clean with rubbing alcohol and let air dry.
  • Avoid removing the Fitting as doing this often may strip the plastic off the sensor and potentially cause a leak
  • If you notice the readings are off from what they should be, replace the batteries or recalibrate.
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