A conductivity meter measures how readily a liquid conducts an electrical current, a reading that tracks directly with the concentration of dissolved ions in that liquid and makes it one of the fastest, most reliable ways to judge water purity. Pharmaceutical manufacturers and laboratories rely on this equipment because purified water and water for injection sit behind nearly every formulation, cleaning cycle, and analytical method in a facility, and a conductivity reading is often the very first signal that a water system has drifted out of specification. Browse new and used conductivity and TDS meters from verified sellers on Machinery Masters.

What Are Conductivity & TDS Meters?

A conductivity meter passes a small electrical signal between the electrodes of a conductivity probe or conductivity cell immersed in a sample and measures how easily that signal travels through the liquid. Pure water conducts electricity poorly, so a low conductivity reading generally signals a clean sample, while a rising reading points to dissolved salts, minerals, or other ionic contamination working their way into the system.

This is exactly why USP <645> conductivity testing anchors the purified water and water for injection monitoring that pharmaceutical manufacturing depends on, with the standard defining specific temperature-referenced conductivity limits a water system has to meet at each stage of testing.

A TDS meter, or total dissolved solids meter, takes a conductivity reading and converts it into an estimated concentration of dissolved solids using a standard conversion factor, giving a single number that's easier for many operators to interpret at a glance than a raw conductivity value. A resistivity meter measures the same underlying property from the opposite direction, reporting how strongly a sample resists the flow of electrical current rather than how well it conducts it, and resistivity is the preferred unit for describing extremely pure water where conductivity numbers would otherwise be inconveniently small.

All three readings, conductivity, TDS, and resistivity, describe the same core electrochemical property of a liquid, just expressed in whichever unit best suits the purity range and application being tested.

Conductivity & TDS Meter Brands

Mettler-Toledo, known for its InLab conductivity probe line and USP <645>-focused conductivity meters
Thermo Fisher Scientific, known for its Orion Star series conductivity meters and probes
Hach (a Veralto company), known for its conductivity and total dissolved solids meters for water quality testing
Hanna Instruments, known for its benchtop and portable conductivity and TDS meter ranges
Myron L Company, known for its Ultrameter series and dedicated conductivity, TDS, and resistivity instruments
Oakton Instruments (a Cole-Parmer brand), known for its benchtop and portable conductivity and TDS meters
Xylem Analytics, known for its WTW brand conductivity meters and probes for laboratory and process use
Emerson, known for its Rosemount conductivity transmitters for pharmaceutical water system monitoring

Please verify this brand list against current Machinery Masters marketplace inventory before publishing, as available stock changes over time.

Conductivity & TDS Meter Product Types

Conductivity and TDS meters divide into a handful of formats depending on where the measurement happens and how the reading gets expressed.

Benchtop Conductivity Meters

Benchtop conductivity meters offer the accuracy, multi-point calibration, and data logging capability that routine laboratory water quality testing and USP <645> compliance work demand, sitting at a fixed station rather than moving around a facility with the user.

Portable Conductivity Meters

Portable conductivity meters trade some benchtop precision for the ability to take a reading anywhere in a facility, suiting at-line process sampling and field water testing, where bringing a sample back to a fixed instrument would introduce delay.

TDS Meters

TDS meters convert a conductivity reading into an estimated total dissolved solids value, giving operators a single, easily interpreted number for tracking water purity trends across a facility's water systems.

Resistivity Meters

Resistivity meters report how strongly a sample resists electrical current rather than how well it conducts it, the preferred measurement unit for describing extremely pure water, where conductivity values would otherwise be inconveniently small to read.

Conductivity Probes and Cells

Conductivity probes and cells do the actual measurement work by contacting the sample directly, with cell constant and electrode material chosen to suit the purity range and application a specific water system or process requires.

Conductivity & TDS Meter Applications and Uses

Conductivity and TDS meters support water system qualification, in-process control, and quality assurance across pharmaceutical manufacturing and laboratory operations.

Pharmaceutical Water System Testing

Purified water and water for injection systems rely on conductivity testing under USP <645> to confirm water quality meets its defined limits at each stage, forming one of the most routine and consequential measurements a pharmaceutical facility performs.

In-Process Manufacturing Control

Manufacturing processes monitor conductivity in real time to confirm a rinse, dilution, or purification step is proceeding within its validated parameters, catching a deviation in water quality before it affects the final product.

Cleaning Validation

Cleaning validation programmes use conductivity measurement on final rinse water to confirm that cleaning residues have been adequately removed before equipment returns to production use.

Laboratory and Environmental Water Quality Testing

Laboratories use conductivity, TDS, and resistivity measurement to characterise water samples across research, environmental testing, and general quality control work where ionic content matters to the result.

Reverse Osmosis and Deionisation System Monitoring

Reverse osmosis and deionisation systems rely on continuous conductivity monitoring to confirm the treatment process is performing correctly and to signal when resin or membrane performance has started to decline.

How to Choose a Conductivity or TDS Meter

A handful of practical factors determine which conductivity or TDS meter suits a laboratory or facility application.

Match benchtop or portable format to where the measurement needs to happen, choosing benchtop for fixed-station laboratory work and portable for at-line or field sampling.
Confirm whether your application calls for a conductivity, TDS, or resistivity reading, since extremely pure water is generally better expressed in resistivity than in a very small conductivity number.
Check that the instrument supports the temperature compensation and referencing that USP <645> testing requires if the meter will be used for pharmaceutical water system compliance.
Consider probe or cell type and cell constant based on the purity range and sample type you're measuring most often.
For used equipment, request full calibration history, confirm probe condition and remaining service life, and verify accuracy against a certified conductivity standard before relying on the equipment for regulated use.

Frequently Asked Questions

What is the difference between conductivity, TDS, and resistivity?

Conductivity measures how readily a liquid conducts electrical current, TDS estimates the concentration of dissolved solids by converting a conductivity reading using a standard factor, and resistivity measures how strongly a sample resists current flow, the inverse of conductivity. All three describe the same underlying property of a liquid, and the choice between them generally comes down to which unit best suits the purity range being measured.

Why does USP <645> conductivity testing matter for pharmaceutical water systems?

USP <645> conductivity testing matters because it defines specific, temperature-referenced conductivity limits that purified water and water for injection have to meet, giving pharmaceutical manufacturers a fast, reliable way to confirm water quality without running a lengthier chemical test. It has become the standard method the industry relies on for both in-process monitoring and formal water system qualification.

Why is resistivity used instead of conductivity for very pure water?

Resistivity is used instead of conductivity for very pure water because as water approaches theoretical purity, its conductivity value becomes extremely small and harder to read meaningfully, while the corresponding resistivity value stays in a more practical, easily interpreted range. This makes resistivity the preferred unit for characterising high-purity water systems such as those feeding analytical instrumentation.

What is the difference between new and used conductivity and TDS meters?

New conductivity and TDS meters come with a manufacturer warranty and current calibration certification, while used equipment offers meaningful cost savings and suits organisations that can confirm calibration history and check probe condition. It is worth verifying accuracy against a certified conductivity standard before relying on used equipment for regulated water system monitoring.

How can I find conductivity and TDS meters on Machinery Masters?

Browse the Machinery Masters marketplace for new and used conductivity and TDS meters from verified sellers, and use the filters to narrow results by brand, format, and measurement range. Contact a seller directly to confirm specifications, service history, and pricing before you buy.