- !A TDS meter measures electrical conductivity — not water safety. It cannot detect chlorine, chloramine, PFAS, lead, bacteria, nitrates or any specific contaminant. A safe glass of water and a contaminated one can produce identical TDS readings.
- ✓The ADWG has no health-based guideline for TDS at any level. The 600 mg/L figure is a palatability guideline describing taste, not safety. Brisbane tap water is ~370 mg/L. Adelaide is ~480 mg/L. Both are within guidelines.
- ✗When a salesperson shows a reading of 300–500 mg/L and calls it alarming, this is a sales tactic. The dissolved solids are primarily calcium and magnesium — minerals with no established health harm at these concentrations.
- ✓TDS meters are genuinely useful for: verifying RO membrane performance (should drop 85–95%), detecting sudden supply changes, initial bore water screening, and coffee machine water optimisation.
- →For real water quality concerns, a NATA-accredited lab analysis ($60–$150) identifies specific contaminants. A $15 TDS meter cannot.
What a TDS meter actually measures
A TDS (Total Dissolved Solids) meter measures electrical conductivity. When its two metal probes are dipped in water, dissolved ions carry an electrical current between them. The meter converts this conductivity reading into an estimate of total dissolved solids in milligrams per litre (mg/L) or parts per million (ppm).
The key word is total. TDS sums every dissolved mineral, salt, and ion into one number with no distinction between them. In typical Australian tap water, the vast majority of that number is calcium, magnesium, sodium, bicarbonates, and sulfates — all harmless or beneficial at concentrations found in mains water.
What a TDS meter cannot detect: bacteria, viruses, chlorine, chloramine, PFAS, lead (except at very high concentrations), nitrates at low levels, pesticides, or pharmaceutical compounds. A perfectly safe glass of water and a contaminated one can produce identical TDS readings. A dangerous glass of water can have a lower TDS reading than a safe one.
The ADWG has no health-based guideline for TDS at any level. The 600 mg/L figure referenced in Australian guidelines is a palatability guideline — it describes when water starts tasting different to most people, not when it becomes unsafe. There is no evidence of health harm from TDS at the levels present in any Australian capital city tap water.
The TDS sales demonstration — how it works
The TDS meter is the most widely-used tool in onsite water filter sales demonstrations across Australia. Understanding the script helps you evaluate what you’re actually being shown:
- The salesperson tests your tap water. The meter reads, say, 380 mg/L.
- They present this as alarming: “Three hundred and eighty parts per million of dissolved solids — you can see for yourself what’s in your water.”
- They test their filter’s output. The RO unit reads 18 mg/L.
- The contrast between 380 and 18 is presented as visual proof your water is problematic and their filter solves it.
What this demonstration withholds:
- A TDS of 380 mg/L is well within the ADWG palatability guideline and poses no established health risk. It primarily reflects calcium and magnesium — the hardness minerals.
- The meter cannot detect the contaminants that actually warrant concern: chloramine, PFAS, lead at typical concentrations, nitrates, bacteria.
- RO output at 18 mg/L is not inherently healthier — it has had beneficial minerals removed alongside anything genuinely harmful.
- The salesperson’s demonstration water sample may not be representative of the filter’s long-term performance on your specific water.
None of this means TDS meters are useless or that every RO system is an oversell. A high TDS can legitimately indicate hard water causing appliance damage, or — in bore water contexts — elevated specific minerals worth investigating. The problem is equating “has dissolved minerals” with “is dangerous” — a claim Australian drinking water science does not support for mains water in this TDS range.
Australian city TDS — context for the numbers
Source: City utility annual quality reports; ADWG 2022; independent water testing data
Source: ADWG 2011 (updated 2022); WHO Guidelines for Drinking Water Quality 4th Edition. Important: the ADWG sets no health-based guideline for TDS at any level. The 600 mg/L figure is a palatability guideline describing taste preference, not safety.
What TDS meters are actually useful for
Despite the misuse in sales contexts, TDS meters are genuinely useful for specific and well-defined purposes:
- Verifying RO membrane performance: An RO system should reduce TDS by 85–95%. If your RO system is producing water at nearly the same TDS as tap water, the membrane needs replacement. This is the most legitimate residential use of a TDS meter.
- Detecting sudden supply changes: If your tap water consistently reads 180 mg/L and suddenly reads 450 mg/L, something has changed — possibly a source blend shift (common in Perth as the desal/groundwater ratio varies seasonally) or a local issue worth reporting to your water utility.
- Initial bore water assessment: A TDS above 600 mg/L from a private bore is a signal to commission proper laboratory testing. The TDS meter is a starting indicator only — not a diagnosis.
- Coffee machine water optimisation: The Specialty Coffee Association recommends 75–250 mg/L TDS for coffee brewing. A TDS meter is exactly the right tool for this specific purpose.
When to get a proper water test
If you have genuine concerns about your water — from a salesperson demonstration, a utility notice, or a visible change — a NATA-accredited laboratory analysis is the appropriate tool. For approximately $60–$150, a lab test identifies specific concentrations of specific contaminants: lead, copper, nitrates, pH, hardness minerals individually, bacteria, and on request, PFAS or arsenic.
A TDS meter at $15–$30 tells you total electrical conductivity. These tools serve different purposes and are not interchangeable. For a genuine water quality concern, a lab test is the only tool that gives you useful information.
The ADWG sets no health guideline for TDS — only a 600 mg/L palatability guideline describing taste. Brisbane tap water is ~370 mg/L. Adelaide is ~480 mg/L. Both are safe. The dissolved solids are primarily calcium and magnesium from the natural mineral content of the source water.
Legitimate uses for a TDS meter: verifying RO membrane performance, detecting sudden supply changes, bore water screening, coffee water optimisation. If a salesperson used a TDS reading to recommend a filter purchase, the question to ask is: what specific contaminant does this filter address, and what NSF certification verifies that claim? See our certifications guide and sales tactics guide for the full picture.