Conductivity and TDS: Why a Meter Is Not the Lab Sheet

By SubcontractorHub Editorial Team·Published October 2026

This is a reference for the two lines dealers mix up on a well visit. The U.S. Geological Survey says total dissolved solids can be monitored in real time in groundwater by measuring specific conductance. Specific conductance measures the ability of water to transmit an electrical current, and that ability increases with the amount of dissolved ions. The same page defines dissolved solids as the sum of substances dissolved in the water. Conductivity and TDS stay in different sentences on the job ticket for that reason.

What the 500 mg/L public guideline means, and which processes EPA names for the dissolved load, is TDS in well water. Adding only the ions printed on the sheet is the TDS constituent sum. SubcontractorHub files the note. It does not turn a cup reading into a lab report.

Two Columns, Two Meanings

Why Conductivity and TDS Get Written as One Number

A handheld meter is fast. The lab bottle is slow. On a sales visit the meter wins the conversation unless someone labels it. USGS is comfortable calling conductance a surrogate for dissolved solids precisely because the current tracks the ions. A surrogate is allowed to be useful and still be the wrong name for the laboratory result.

The fetched USGS page does not give a factor such as a single decimal times microsiemens. Some meters apply one internally and print milligrams per liter. This reference will not guess that factor, and it will not tell you to type 0.65 or any other constant. If you use a meter, write the brand's mode in the note: conductance, or a TDS mode you did not convert by hand.

What Usually Sits Inside the Sum

USGS says calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate, and silica typically make up most of the dissolved solids. Combinations of those ions, sodium and chloride for example, are the salts people mean by salinity. “Most” leaves room for ions that are not on that list. A sum of the nine, when several are blank, is a partial sum.

Bicarbonate on a lab sheet is not the same as alkalinity reported as calcium carbonate. Chloride is not chlorine. Nitrate as the ion is not nitrate as nitrogen. The nitrate unit converter is the unit check for that one line. Do not drop an unconverted nitrate figure into a dissolved-solids story and treat it as milligrams of the ion.

How to Log a Visit Without Inventing a Factor

1. Name the method. Conductance, meter TDS mode, lab residue, or ions you added.
2. Name the unit. Microsiemens per centimeter and milligrams per liter are not synonyms.
3. Keep the temperature note if the meter showed one. Do not invent a compensation percentage.
4. Do not average the meter and the lab. They can disagree without either being a typo.
5. Point the 500 mg/L line at dissolved solids, not at the conductance cup. It is a public guideline, explained on the TDS page.

Where Else This Number Shows Up

Cooling-tower logs often use conductivity to watch cycles of concentration. That is a building-water habit, not a drinking-well pass line. The cooling tower water treatment page is the place for cycles. Do not paste a tower conductivity into a kitchen-tap quote.

The Langelier saturation index calculator asks for dissolved solids in milligrams per liter as one input. Use the lab figure when you have it. If you only have a meter, say so in the note beside the index. The index is a planning estimate. It does not repair a mislabeled sample.

What This Reference Will Not Do

It will not certify a well, set a private-well limit, or size a membrane. EPA's secondary 500 mg/L standard and the effects above it are discussed on the TDS page, including the statement that secondary standards are not federally enforceable. Repeat that limit as a public guideline. A county can have its own rule. Quote the county when you have it.

A water treatment dealer who writes “meter, TDS mode” or “lab residue” on the customer record can defend the number later. The privacy policy covers the site. It is not a lab method. No installed price and no removal percentage belong in this comparison.

Conductivity and TDS When the Numbers Disagree

A meter can read high on a hot afternoon and the lab bottle from that morning can print a lower dissolved-solids line. That is not automatically a bad meter or a bad lab. The samples are not the same moment, and the methods are not the same method. Write both, with the time and the place the water was taken. Do not delete the lab because the cup was easier to explain.

Temperature compensation, a dirty probe, and a meter set to a TDS mode are reasons a cup reading moves. This page still does not assign a percent to any of those. If the meter manual names a mode, copy that mode onto the ticket. If it does not, write “meter” and stop. Guessing a factor to force the cup to equal the lab hides the disagreement the next technician needs to see.

Use the lab residue or the ion list when you are comparing the water to the public 500 mg/L dissolved-solids guideline. Use conductance when you are watching a trend at the same tap with the same meter. Conductivity and TDS can both be filed. They should not be averaged into a third number that neither instrument printed.

Frequently Asked Questions