Good TDS meter??


New member
So I have the HM-3 digital TDS meter. Filter guys sell it for $24. I see there are other options, much higher price and probably better.

What is the difference? Is the cheap one good enough? I am guessing the difference is ATC, but how much of a difference does this make?

Thanks for the help/


Bomb Technician (EOD)
Premium Member
The HM-3 digital TDS meter does have ATC although it is not listed there. The meter you have is fine. The HM-COM 100 is the best there is in hand-held pocket meters.

HM-3 digital TDS

TDS Range: 0 - 9990 ppm (mg/L)
Temp. Range: 0 - 80 degrees Celsius
Resolution: 1 ppm, 1 degree Celsius (also available in 10 ppm)
Accuracy: +/- 2%
ATC: Built-in sensor for Automatic Temperature Compensation of 1 to 50 degrees Celsius (33 to 122 degrees Fahrenheit)
Power source: 2 x 1.5V button cell batteries (included)
Battery life: 1000 hours of usage
Dimensions: 15.5 x 3.1 x 2.3cm (6.1 x 1.25 x 1 inches)
Weight with case: 76.5g (2.7 oz)
Weight without case: 56.7g (2 oz)


New member
I have been using an ESD (Engineered Systems and Design) TDS/Conductivity meter to test my RO/DI water. It is an older model I got from a local environmental lab that was closing shop. It had a probe that tests the sample. The problem is that it tests my purified water at 0.65 mg/l after going through a Dow Filmtec 75 GPD filter and two post mixed bed DI filters. My feed water test around 200-225 on average. Unfortunately the unit can not be calibrated. Would the TFG TDS unit mentioned above work to monitor the RO/DI filter? Also how accurate are the inline models of meters? I read once that they can be notoriously inaccurate due to many outside influences.

Randy Holmes-Farley

Reef Chemist
Premium Member
Does it read 0 ppm TDS in air?

IMO, the inline meters are a better way to go, in general, since the issues of contamination disappear, and temperature compensation and perfect calibration are not needed for RO/DI monitoring, IMO.


New member
I have the HM Digital TDS-4 model and it has the calibration pot for doing cals with the calibrated solutions. It too has ATM with 3 year warranty.