HM Digital

Dual Inline TDS Meter DM-1

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Why does BRS recommend this?

Using a RODI system without a TDS meter is like driving a car without a speedometer and gas gauge.  At some point you will end up in trouble.  With RODI systems it is TDS that you will want to monitor for performance and filter quality.  By using a dual TDS meter it will allow you to see your TDS readings in two places.  We will usually suggest putting one probe before the DI stage and one after the DI stage to show what your RO is producing for water quality and then how your DI is performing by removing all remaining TDS from the final product water.


The DM-1 Dual Inline TDS Meter from HM Digital is one of the most important reverse osmosis accessories. This tool monitors the TDS of your reverse osmosis system in two places and lets you know exactly how the system is performing.


BRS suggests: installing the dual TDS meter post membrane and post DI stage. These two readings will notify you when it's time to change the RO membrane and DI resin. This unit features a digital display and comes with two 1/4" GA Murdock Quick Connect Tee's with double o-rings to ensure a proper seal. As with all HM-Digital products, the DM-1 comes factory calibrated.


Replacement Battery Type - LR44G


TDS Meter Installation Instructions

More Information
UPC 891144000052
TDS Meter Type Dual
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Product Questions

Can this Dual line TDS meter be used on an RO/DI system that is not a Bulk Reef Supply RO/DI system? I have a 4 stage Aquaticlife 75 GPD system and want to make sure this will work.
Question by: David Householder on Mar 2, 2024, 1:37 AM
Hey there,
Thank you for reaching out to us. Yes this product can be used but make sure your line fitting sizes match up. Please let me know if you need anything further.
Answer by: Thomas Wofle (BRS Staff) on Mar 11, 2024, 11:42 AM
What does the flashing letters Lob mean on this unit when I turn it on it'll show the numbers and quickly start flashing these letter or I guess they are letters they are hard to read this unit has only been together producing water about 3 days. Thanks
Question by: d on Dec 17, 2022, 6:18 PM
LOB means low on battery and it can be replaced with an LR44 battery available at most home improvement stores.
Answer by: Scott F. (BRS Staff) on Dec 22, 2022, 3:14 PM
what if I only have the Bulk Reef Supply 4 Stage RO Only System - 75GPD what line do I hook up the dual TDS meters to ?
Question by: Mike Gosselin on Sep 21, 2019, 5:53 PM
For this, I would put on after the membrane and one after the DI resin area. If you decide to get the tripple I would also put one at the start to know my starting TDS.
Answer by: Tyler Schafer on Sep 22, 2019, 2:43 PM
How do I change the batteries on my TDS Meter?
Question by: Robert on Jun 6, 2018, 8:15 PM
Thanks for asking. Watch this video to see how it's done - https://youtu.be/uUpBA_Qzd-0?t=316
Answer by: Chad on Jun 7, 2018, 3:35 PM
What kind of battery does this meter require?
Question by: Adam on Aug 16, 2016, 12:00 PM
Hey Adam,
This TDS meter uses an LR44G Battery.
Answer by: Jason Smith on Aug 17, 2016, 9:41 AM
How low does the meter read? I need it to read <0.1 ppm TDS.
Question by: Jon on Apr 4, 2016, 8:06 AM
Great question!
These meters read from 0 ppm in increments of 1 ppm to 999 ppm. (9990 with calculation) This will not go be lower than 1 ppm measurements. Feel free to let us know if you have any further questions!
Answer by: Connor on Apr 6, 2016, 6:56 PM
Does the meter have to be replaced at some point? I noticed today that the filters are needing to be replaced but I'm getting an output reading of 0.0.
Question by: Jeff Ostrowsky on Jan 23, 2016, 9:31 AM
Great question!
The TDS meters on the units do not need replacement. In many cases they will last the life of the unit. It very well be that CO2 or other elements are reducing the DI resin faster than any TDS. Feel free to let us know if you have any further questions!
Answer by: Connor on Jan 28, 2016, 4:51 PM
I have this dual meter installed in my 4 Stage R.O. as recommended I am noticing a substantially higher levels of TDS in my R.O. system lately. Maybe 5 times what they were originally a year and a half ago now approaching 100... These levels are reflect straight out of the tap as well and post filter change... I have not changed my Membrane but it is a 18 month old Dow and my water production requirements are not large so it shouldn't be a problem.

My question is what is the best way to test ( Deionized Water) the meter and calibrate it if necessary?
Question by: Adam on Jun 21, 2015, 12:05 AM
Great Question,
We don;t recommend calibrating the units as we have not found them to be out of calibration since we have been using them. The calibration steps would also need to occur in a laboratory environment. If your TDS coming out of the membrane is in the 100's there is most likely an issue with the membrane, which possibly needs to be replaced or a bacterial bloom in the housing.
Answer by: Connor on Jun 22, 2015, 6:08 PM
Is the battery in this unit replaceable?
Question by: Dave on Jan 19, 2014, 1:58 PM
Hi Dave,
It sure is. It uses a pair of button batteries that can be replaced fairly easily.
Answer by: Bulk Reef Supply on Jan 19, 2014, 1:58 PM