Accessories · Feature
Your water has a number. Learn what it means.
The WaterVO TDS Meter — curiosity belongs in your pocket.
$19.99 WaterVO Black Friday special · normally $39.99 · save $20 until Black Friday, November 27, 2026.
WaterVO is a related-brand seller. Hydrology University provides educational backing; this feature is not a lab test or water-safety endorsement. WaterVO, a US-based company with local support in Newport Beach, California, fulfills orders and its 1-year warranty.
A TDS meter is not a safety test.
It cannot detect lead, bacteria, PFAS, microplastics or chlorine. A low number does not mean safe water, and a high number does not mean unsafe water. For safety questions, use a certified lab — start with our water testing guide.
The science, briefly
The science behind the number
Conductivity in, estimate out. The number is a proxy, not a list of what is in the glass.
Pure water conducts electricity poorly. Dissolved ions — such as calcium, magnesium and sodium — carry current, so water with more of them shows higher electrical conductivity (USGS).
A TDS meter measures that conductivity and converts it into an estimate of total dissolved solids. It cannot tell which ions are present, only roughly how much is dissolved (WHO background document).
Readings are shown in ppm, which in dilute water is approximately equal to mg/L.
Calibration and temperature matter. Conductivity rises with temperature, so meters compensate for it, and a meter that drifts out of calibration will drift in its numbers too. Compare readings taken the same way.
Context: EPA secondary drinking water standards · FDA guide to reverse osmosis
Reading the number
Same meter, four kinds of water
| Water | How to interpret it | Keep in mind |
|---|---|---|
| Tap | Varies widely by source and region; it reflects the minerals your utility's source water carries. | Record it once as your home baseline. |
| Carbon-filtered | Often close to the tap number. Carbon mainly targets taste, odor and some chemicals, not dissolved minerals. | A similar number does not mean the filter failed. |
| Reverse osmosis | Often under 50 ppm, because the membrane removes a large share of dissolved ions. | Compare to tap; a rising number over time can prompt a membrane check. |
| Bottled | Depends on the brand: purified waters read low, mineral and spring waters can read much higher. | A higher number can be by design, for taste. |
Where curiosity strikes
Where will you take your next reading?
Restaurant
Ask what “filtered” really means
Airport
Compare the refill station with your home baseline
Hotel
Track readings across your trip
On the go
Fits in a purse or pocket — an ideal stocking stuffer
Not all filtered water is equal
A $3 bottle and a $1,000 bottle are both “wine.”
“Filtered” works the same way. It might mean a $1 filter, or it might mean a WaterVO vapor machine. The word alone tells you very little.
A TDS meter gives you one honest number to compare. It is your first line of defense against guessing — only that. It is not a defense against contaminants.
Price comparison
About half the price
WaterVO TDS Meter
$19.99
Local US support · 1-year warranty
Amivra Purity Pen 2.0
$39.99
How to use it
Sample. Read. Record.
- 01
Sample
Use a clean glass. Let tap water run briefly, and test at room temperature when you can.
- 02
Read
Dip the probe to the marked line, stir gently, and wait for the number to settle.
- 03
Record
Write down the number, the source, the date and the temperature so later readings are comparable.
Questions
Does a low reading mean my water is safe?
No. A low number means fewer dissolved ions were detected. Lead, bacteria, PFAS, microplastics and chlorine can be present at a low TDS reading. Safety questions need a certified lab test.
Is 500 ppm a pass/fail line?
No. EPA lists 500 mg/L for total dissolved solids as a secondary standard, which is guidance for aesthetic effects such as taste and scaling, not a health-based safety limit.
My carbon filter barely changes the number. Is it broken?
Not necessarily. Carbon filters serve a different function from reverse osmosis: they mainly address taste, odor and certain chemicals, and leave most dissolved minerals in place, so the reading often stays near tap.
Find your number.
$19.99 WaterVO Black Friday special · normally $39.99 · save $20 until Black Friday, November 27, 2026.
WaterVO is a related-brand seller. Hydrology University provides educational backing; this feature is not a lab test or water-safety endorsement. WaterVO, a US-based company with local support in Newport Beach, California, fulfills orders and its 1-year warranty.