Turbidity is measured by shining a light through a water sample and detecting how much of that light gets scattered by suspended particles. The standard instrument for this is a nephelometer (commonly called a turbidimeter), which reads the scattered light at a 90-degree angle and reports results in Nephelometric Turbidity Units (NTU). But depending on your purpose, whether testing drinking water compliance, monitoring a lake, or doing a quick field check, the method and equipment you need will vary significantly.
How a Turbidimeter Works
A turbidimeter passes a beam of light through a small vial of water. A sensor positioned at 90 degrees to the light source measures how much light bounces off particles suspended in the sample. The more particles present, the more light scatters, and the higher the turbidity reading. The instrument compares this scattered light intensity against a known standard to produce a number in NTU.
This 90-degree detection angle is the defining feature of nephelometric measurement, and it’s what the EPA specifies in its standard Method 180.1 for drinking water analysis. Benchtop turbidimeters used in labs typically use a tungsten lamp as the light source, while submersible field sensors often use a near-infrared LED instead. Both approaches measure scattered light, but the different light sources mean readings from different instruments aren’t always directly interchangeable.
Understanding Turbidity Units
NTU is the most widely used unit in the United States, especially for drinking water testing. You’ll also encounter a few others:
- FTU (Formazin Turbidity Units) are considered comparable in value to NTU but come from a different technique. Instead of measuring scattered light, an absorptometric method measures how much light gets absorbed as it passes straight through the sample. The numbers are similar, but the instruments work differently.
- JTU (Jackson Turbidity Units) come from an older visual method that’s no longer in common use. JTU values roughly approximate NTU but aren’t precise equivalents.
For most practical purposes, if you’re buying a turbidimeter for home, field, or lab use, you’ll be working in NTU.
Using a Benchtop Turbidimeter
For accurate, repeatable results, a benchtop nephelometer is the standard tool. The basic process is straightforward: fill a clean sample vial with your water, wipe the outside of the vial clean, place it in the instrument, and read the NTU value on the display. But several details make the difference between a reliable reading and a misleading one.
Air bubbles are one of the most common sources of false high readings. Even tiny bubbles scatter light just like particles do, inflating your result. Gently swirl or tap the vial to release trapped air before measuring, and degas the sample even if no bubbles are visible. Condensation on the outside of the vial is another problem, particularly when measuring a cold sample in a warm room. Wipe the vial with a lint-free cloth to remove moisture, fingerprints, and dust before inserting it into the instrument.
Dirty glassware also produces false results. Always use clean, scratch-free sample vials, and handle them by the cap or top edge to keep oils from your fingers off the optical surface.
Calibrating Your Instrument
A turbidimeter needs regular calibration against known standards to stay accurate. Formazin is the most commonly accepted primary standard. You prepare or purchase solutions at known NTU values, run them through the instrument, and adjust the readings to match.
Washington State’s Department of Health recommends calibrating with primary standards on a quarterly basis. A concentrated 4,000 NTU formazin stock solution can last up to a year, but once you dilute it to lower concentrations for calibration points, those diluted solutions should be discarded after one week or per the manufacturer’s instructions. Using expired standards defeats the purpose of calibrating in the first place.
Field Methods Without a Turbidimeter
Secchi Disk
A Secchi disk is a simple, weighted black-and-white disk lowered into a body of water on a measured line. You lower it on the shady side of a boat until it disappears from view and record that depth. Then you slowly raise it until it just becomes visible again and record that depth too. The average of these two depths is your Secchi depth, a measure of water transparency (the inverse of turbidity).
Secchi depth gives you a practical sense of how clear water is, even if it doesn’t produce an NTU number. For context, Crater Lake in Oregon has a Secchi depth of 44 meters, meaning you can see the disk nearly 150 feet down. The Sargasso Sea reaches 66 meters. Spirit Lake in Washington, after the Mount St. Helens eruption, dropped to just 1 to 2 centimeters. If you’re monitoring a lake or pond over time, tracking Secchi depth is an inexpensive way to spot changes in water quality.
Turbidity Tube
A turbidity tube (sometimes called a transparency tube) is a clear plastic tube with a black-and-white disk pattern printed on the bottom. You fill it with your water sample, look down through the tube, and compare what you see to a reference chart. The chart matches different levels of visual clarity to approximate JTU values. It’s a quick, low-cost option for stream monitoring, classroom projects, or situations where you need a rough estimate rather than a precise lab measurement.
Why the Oldest Method Was Replaced
Before electronic instruments existed, the standard tool was the Jackson candle turbidimeter. You looked down through a column of water at a candle flame and added water until the flame disappeared. The depth at that point gave you a reading in JTU. The problem: the lowest value it could measure was 25 JTU. Treated drinking water typically falls between 0 and 1 NTU, far below what the Jackson candle could detect. Electronic nephelometers replaced it because they can measure those very low levels accurately.
What Turbidity Numbers Mean
Turbidity isn’t just about how water looks. Higher turbidity often correlates with higher levels of disease-causing organisms, including viruses, parasites, and certain bacteria, because microbes can attach to or hide within suspended particles. That’s why the EPA treats turbidity as a critical indicator of filtration effectiveness rather than just an aesthetic measure.
For drinking water systems using conventional or direct filtration, EPA rules set two thresholds: turbidity can never exceed 1 NTU, and at least 95 percent of monthly samples must come in at or below 0.3 NTU. Systems using other filtration methods must stay below 5 NTU at all times, with specific limits set by each state. If you’re testing well water or a private supply, these benchmarks give you a useful reference point for what “safe” looks like.
Turbidity vs. Total Suspended Solids
Turbidity and total suspended solids (TSS) are related but not the same thing. Turbidity measures how much light particles scatter, while TSS measures the actual weight of solid material in a given volume of water. Research confirms a strong linear relationship between the two, meaning you can use turbidity readings to estimate TSS concentrations with reasonable accuracy.
The catch is that the conversion factor changes depending on the type of particles, the site, and whether it’s been raining. During dry weather conditions, the relationship stays fairly consistent. During storms, runoff introduces different types and sizes of particles, shifting the ratio. If you need precise TSS numbers, you’ll eventually need to filter and weigh a sample. But for ongoing monitoring, turbidity readings are a practical, faster alternative that tracks suspended solids trends reliably over time.