For most homes, the HM Digital TDS-4 is the TDS meter to buy: a pocket meter with automatic temperature compensation and a 0 to 9990 ppm range. Choose the Apera TDS20 for more resolution on low RO readings, or the HM Digital DM-2 in-line monitor to watch an RO system all the time. A TDS meter does not test for lead, bacteria, nitrate, arsenic or PFAS, so it cannot tell you whether water is safe.
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Use a TDS meter for one job: tracking how well a filter or RO membrane is working. For safety contaminants, use a water test kit or a certified lab, and read your utility’s report with our guide on how to read a water quality report.
TDS meters compared
| Check | HM Digital TDS-4 | Apera TDS20 | Apera EC60 | HM Digital DM-2 |
|---|---|---|---|---|
| Type | Pocket TDS meter | Pocket TDS tester | Pocket EC, TDS and salinity tester | In-line dual TDS monitor |
| TDS range | 0 to 9990 ppm | 0 to 100.0 ppm, 0 to 1000 ppm, 0 to 10.00 ppt | 0.1 ppm to 10.00 ppt | 0 to 9990 ppm, feed and product |
| Resolution | 1 ppm below 1000 | 0.1 ppm on the low scale | Set by EC scale; 0.1 µS/cm on the lowest | 1 ppm below 1000 |
| Accuracy | ±2% | ±1% of full scale | ±1% of full scale (EC) | ±2% |
| Temperature compensation | Automatic, 1 to 50°C | Automatic, 0 to 50°C | Automatic, 0 to 50°C | Not stated by HM Digital |
| Calibration | Factory, 342 ppm NaCl | User, 1 or 2 points | User, 1 to 3 points; TDS factor 0.40 to 1.00 | Factory, 342 ppm NaCl; each line can be recalibrated |
| Choose when | You want a simple meter for tap and filter checks | You read low RO numbers and want a waterproof meter | You want EC readings, an adjustable factor and a replaceable probe | You want to watch an RO system without dipping a meter |
| Exact purchase | Check HM Digital TDS-4 on Amazon → | Check Apera TDS20 on Amazon → | Check Apera EC60 on Amazon → | Check HM Digital DM-2 on Amazon → |
Specs are from the HM Digital TDS-4 page (hmdigital.com), the Apera TDS20 page (maker page: aperainst.com/tds20-value-pocket-tds-tester-meter), the Apera EC60 page (maker page: aperainst.com/ec60-premium-ec-tds-salinity-pocket-tester) and the HM Digital DM-2 page (hmdigital.com).
BEST BASIC TDS METER · 0 TO 9990 PPM, ATC
HM Digital TDS-4: best basic TDS meter
Choose it when you want a simple meter to check tap water and filtered water. HM Digital lists the TDS-4 with a 0 to 9990 ppm range, 1 ppm resolution below 1000 ppm, ±2% accuracy and automatic temperature compensation from 1 to 50°C, on the TDS-4 page (hmdigital.com). It is factory calibrated with a 342 ppm NaCl solution and uses an NaCl conversion factor of about 0.5. It has a hold function and shuts off after 10 minutes, and it runs on two LR44 button cells.
If you also want a water temperature readout, the HM Digital TDS-4TM (hmdigital.com) adds a digital thermometer with a 0 to 80°C range. HM Digital names RO and DI systems among its uses, to know when to change filters and membranes.
Skip it if you need to calibrate it yourself on a schedule, or if you want a waterproof meter. HM Digital does not list a waterproof rating on the TDS-4 page.
On Amazon, the TDS-4 is sold by a reseller under the IPW brand name, with HM Digital and TDS-4 in the listing title. Check that the listing shows model TDS-4 before you buy.
BEST FOR LOW READINGS · 0.1 PPM, IP67
Apera TDS20: best for low readings
Choose it when you check RO water, where readings are low and small changes matter. Apera lists the TDS20 with a 0 to 100.0 ppm scale at 0.1 ppm resolution, plus 0 to 1000 ppm and 0 to 10.00 ppt scales, ±1% of full scale accuracy and automatic temperature compensation from 0 to 50°C, on the TDS20 page (maker page: aperainst.com/tds20-value-pocket-tds-tester-meter). It supports 1 or 2 point calibration, is rated IP67 and floats.
Apera lists a two-year warranty on the meter and six months on the probe.
Skip it if you want a replaceable probe. Apera says the TDS20 probe cannot be replaced, so the meter is done when the probe wears out. The EC60 has a replaceable probe.
BEST ADJUSTABLE METER · EC, TDS AND SALINITY
Apera EC60: best adjustable meter
Choose it when you want conductivity readings as well as TDS, or you want to set your own TDS factor. Apera lists the EC60 with an EC range of 0 to 20 mS/cm, a TDS range of 0.1 ppm to 10.00 ppt, an adjustable TDS factor of 0.40 to 1.00 and 1 to 3 point calibration, on the EC60 page (maker page: aperainst.com/ec60-premium-ec-tds-salinity-pocket-tester). It has automatic temperature compensation from 0 to 50°C, an IP67 rating and a replaceable probe.
Conductivity in µS/cm is what the meter actually senses. A TDS number is that reading times a conversion factor. Reading EC lets you compare results with a lab or a water treatment dealer without guessing which factor they used.
Skip it if you only want a quick yes or no on a filter. The TDS-4 is simpler.
BEST FOR RO SYSTEMS · IN-LINE, FEED VS PRODUCT
HM Digital DM-2: best for RO systems
Choose it when you want to see RO performance any time without a cup and a pocket meter. HM Digital describes the DM-2 as a commercial-grade monitor that measures feed water against product water, on the DM-2 page (hmdigital.com). It reads 0 to 9990 ppm at ±2%, comes with two 1/4 inch T-fittings for quick-connect tubing (3/8 and 1/2 inch versions are available) and has 46 inch sensor cables. It runs on two AA batteries that HM Digital says last about two years, and each line can be recalibrated.
Installing it means cutting into the RO feed and product lines. If you are not comfortable with push-to-connect tubing, have a plumber or the RO installer add it. See our under-sink reverse osmosis system guide for the system side.
Skip it if you do not have an RO system, or if your tubing is a size HM Digital does not list. HM Digital does not list temperature compensation on the DM-2 page.
The Amazon listing title reads HM Digital Commercial In-Line Dual TDS Monitor, with no model number. Check that the product details show model DM-2 before you buy.
What TDS does and does not tell you
TDS stands for total dissolved solids: the minerals, salts and other matter dissolved in water, reported as one number in ppm or mg/L. Pocket meters estimate it from electrical conductivity. HM Digital lists an EC-to-TDS conversion factor of about 0.5 for its meters.
EPA lists TDS among its secondary drinking water standards at 500 mg/L, on its secondary standards page (epa.gov). EPA says secondary standards are guidelines for aesthetic qualities, are not federally enforced, and that these contaminants are not considered to present a risk to human health at the secondary level. The effects EPA lists above 500 mg/L are hardness, deposits, colored water, staining and salty taste.
A low TDS number is not a safety test. A TDS meter cannot detect lead, arsenic, nitrate, bacteria or PFAS, and dangerous levels of some of these are far too small to change a TDS reading. Test for those with a certified lab or a water test kit made for them.
High TDS often goes with hard water, but TDS and hardness are not the same number. Hardness counts calcium and magnesium only. Use our water hardness scale for that.
How to check an RO membrane with a TDS meter
- Read the feed water. Run the kitchen cold tap for 30 seconds, fill a clean cup and read TDS.
- Read the RO water. Open the RO faucet, let it run a little, fill a second clean cup and read TDS. Let both cups reach the same temperature.
- Work out rejection. Percent rejection = (feed TDS minus RO TDS) divided by feed TDS, times 100.
- Compare over time. Write the number on the RO housing when the membrane is new. A steady drop means it is time to change filters or the membrane.
| Feed TDS | RO TDS | Rejection | What to do |
|---|---|---|---|
| 300 ppm | 15 ppm | 95% | Write it down as your baseline |
| 300 ppm | 45 ppm | 85% | Compare with the baseline and the system maker’s rating |
| 300 ppm | 90 ppm | 70% | Check prefilters, pressure and the membrane |
Each RO system maker publishes its own rated TDS reduction on its performance data sheet. Compare against that rating, not a general number. Low water pressure, a worn prefilter or a bad check valve can all raise RO TDS before the membrane is worn out.
Calibration and temperature
- Temperature changes conductivity. All the pocket meters here list automatic temperature compensation, but readings are most reliable when both samples are at room temperature and inside the meter’s range.
- Calibrate with a standard. HM Digital factory calibrates with 342 ppm NaCl. Apera’s meters support user calibration: 1 or 2 points on the TDS20 and 1 to 3 points on the EC60. Check the meter against a fresh standard solution every few months, and after a drop.
- Use the same factor. A reading with a 0.5 factor and one with a 0.7 factor will not match. Note the factor when you compare results with a lab or a dealer.
- Keep the probe clean. Rinse it with distilled water after each use and shake it dry. Mineral film on the probe throws readings off.
TDS meter FAQ
Does a TDS meter tell me if my water is safe to drink?
No. A TDS meter reads total dissolved solids as one number. It cannot tell you whether lead, arsenic, nitrate, bacteria or PFAS are in the water. For safety, use a certified lab test and your utility’s water quality report.
What is a good TDS reading for drinking water?
EPA lists 500 mg/L as the secondary standard for TDS. Secondary standards cover taste, staining and deposits, not health, and EPA does not enforce them. A reading under 500 mg/L means fewer of those nuisance effects, not that the water is safe.
How do I check if my RO membrane needs replacing?
Read the tap water TDS and the RO faucet TDS at the same temperature, then work out percent rejection: feed minus product, divided by feed, times 100. If rejection falls well below what it was when the membrane was new, or below your system maker’s rating, it is time to service the system.
Do TDS meters need calibration?
Yes, they drift. HM Digital factory calibrates the TDS-4 with a 342 ppm NaCl solution. The Apera TDS20 supports 1 or 2 point calibration and the EC60 supports 1 to 3 points. Check the meter against a standard solution before you trust a reading.
What is the difference between ppm and mg/L on a TDS meter?
For dilute water like tap water, 1 ppm and 1 mg/L are treated as the same. HM Digital lists the TDS-4 range as 0 to 9990 ppm (mg/L).
How we chose
This is a document-based comparison from a licensed plumber’s point of view. We read maker product pages for each exact meter and compared range, resolution, accuracy, temperature compensation, calibration and probe design, and we used EPA’s secondary standards page for what TDS means. We favored meters that make RO checks easy. Sources were read October 14, 2026; sellers and availability change. We do not publish prices or claim hands-on testing. More about our selection method and Plumbing Picks.
Primary sources
- HM Digital TDS-4 page (hmdigital.com): range, resolution, accuracy, ATC range, NaCl factor, factory calibration, battery, auto-off.
- HM Digital TDS-4TM page (hmdigital.com): thermometer range, RO and DI use.
- HM Digital DM-2 page (hmdigital.com): feed vs product monitoring, range, accuracy, fittings, cables, battery, recalibration.
- Apera TDS20 page (maker page: aperainst.com/tds20-value-pocket-tds-tester-meter): ranges, resolution, accuracy, ATC, calibration points, IP67, probe not replaceable, warranty.
- Apera EC60 page (maker page: aperainst.com/ec60-premium-ec-tds-salinity-pocket-tester): EC and TDS ranges, adjustable TDS factor, calibration points, IP67, replaceable probe.
- EPA secondary drinking water standards page (epa.gov): TDS secondary standard of 500 mg/L, listed effects, non-enforceable, not a health risk at that level.