TSS vs Turbidity: Difference, Units and Sensor Selection

1. The Two Parameters Measure Different Things

Turbidity and TSS are often treated as two names for the same measurement. They are not. One describes how water looks to a light beam; the other describes how much solid material it carries.

1.1 Turbidity: How Much Light the Particles Scatter

Turbidity describes the optical clarity of water. A light beam enters the sample, suspended particles scatter it in every direction, and the instrument measures the light scattered at 90° to the beam. The result is reported in NTU (nephelometric turbidity units) or FNU (formazin nephelometric units).

ISO 7027 and EN 27027 specify an 860 nm infrared light source, which reduces interference from the colour of the sample. That is the design used in the RK500-07, together with a sapphire measuring window and four selectable ranges: 0–10, 0–100, 0–1,000 and 0–4,000 NTU.

Turbidity is very sensitive at low concentrations, which is why it is the standard choice for drinking water, filter outlets and clean process water.

1.2 TSS: How Much Suspended Solid Is Actually There

TSS (total suspended solids) is a mass concentration in mg/L. The reference method is gravimetric: filter a well-mixed sample through a glass-fiber filter, dry the retained solids at 103–105 °C to constant weight, and weigh them. Filter specifications and procedure follow the applicable method, such as Standard Methods 2540 D or EPA Method 160.2.

That method is direct, traceable and uses the same unit as discharge permits, but it is slow: sampling, filtration, drying and weighing cannot be done online. An online TSS sensor takes a different route — usually backscatter — and converts light intensity into mg/L through internal calibration and linearization. The RK500-20 is built this way.

1.3 Why NTU Cannot Be Converted to mg/L

Scattered light depends on more than the mass of solids in the water:

The familiar “NTU × 2 ≈ mg/L” is a local rule of thumb, valid only for one water body at one point in time. Change the location or the season and it stops working. The correct approach is to sample the water on site, measure TSS by gravimetric drying, record the sensor reading at the same moment, and build a site-specific TSS curve.

2. How an Online TSS Sensor Produces mg/L

Using the RK500-20 as the example, the measurement runs in three steps:

1. The sensor emits a light beam into the water.
2. Suspended particles scatter light backwards, and the receiver measures that backscattered intensity.
3. The intensity is compared with the internal calibration value and linearized into a mg/L output.

The sensor applies internal calibration and linearization, and the factory settings suit typical media. Where the particle characteristics are unusual — high organic content, strong colour, very fine solids — a field calibration is worth the effort:

• Take three to five samples near the sensor, covering low, medium and high concentration.
• Measure each sample by gravimetric drying.
• Regress the measured TSS against the sensor readings to obtain the local curve.
• Recalibrate when the process or the incoming water changes significantly.

3. The Two Sensors Side by Side

3.1 The Four RK500-07 Versions

The D version carries a ±2% FS accuracy because of its brush assembly; the other versions are ±1% FS.

4. Which One for Which Application

A practical distinction: turbidity responds faster and suits process control, trends and alarms, while TSS gives a mass concentration in mg/L. For regulatory reporting, the analytical method and monitoring requirements of the applicable permit or authority still apply; an online TSS sensor supports continuous monitoring but does not replace the reference method.

For a broader look at the turbidity side, see our turbidity sensor selection guide.

5. Installation: Bubbles and Reflections Come First

5.1 Turbidity Sensor

• Avoid bubbles. Do not mount above an air diffuser, below a weir or waterfall, or at a pump outlet. Bubbles scatter strongly and make the reading jump.
• Avoid strong vibration. It disturbs optical alignment and can eventually damage the front window.
• Watch reflections in pipes. In reflective pipework such as stainless steel, align the line between the two small holes with the flow direction and keep at least 58 mm between the probe and the pipe wall.
• Keep the optical window clean. Wipe gently with a soft cloth; never scrape the sapphire window with a hard object.

5.2 TSS Sensor

• Keep the probe more than 5 cm from the side wall and more than 10 cm from the bottom, as specified by the manufacturer, to avoid wall reflection and settled sediment.
• Place it where there is slow movement rather than a dead zone that collects sludge.
• Even with an automatic brush, check periodically that the bristles are not wrapped with fibres.

5.3 Common Requirements

• Process connections vary by model: RK500-07 Types A/B and D use NPT3/4, Type C is configured for UPVC pipe installation, and the RK500-20 uses NPT3/4. A 1 m mounting bracket is available for immersion; confirm dimensions and mounting conditions before ordering.
• IP68 allows continuous immersion; 5 m cable is standard, custom lengths available.
• Route signal cable separately from power cable and ground the shield at one end.

6. Calibration and Maintenance

6.1 Turbidity Calibration

• Use formazin standards; select the points according to the model, range and the manufacturer's calibration procedure. 0.02, 10, 100 and 1,000 NTU are common examples.
• Clean the optical window first, rinse with pure water and blot dry with a soft cloth.
• Let the standard stabilize in temperature and avoid bubbles.
• Follow ISO 7027 practice and record the date and slope of each calibration.

6.2 TSS Calibration

• Take samples on site and measure them by gravimetric drying, at no fewer than three concentration points.
• Sample as close to the probe as possible, and record the reading at the same moment.
• Configure the resulting curve in the sensor or the host system according to the manufacturer's calibration procedure, and repeat when the process or season changes.

6.3 Cleaning Intervals

The intervals below are recommended practice based on typical water quality; adjust them to the actual fouling rate.

7. Five Common Mistakes

1. “Multiply NTU by 2 to get mg/L.” It holds for one water body at best, and fails as soon as the source or season changes.
2. “A wider range is always safer.” A wider range costs resolution and accuracy at the low end. Clean water should use 0–10 or 0–100 NTU, not 0–4,000.
3. “Colour does not matter.” 860 nm infrared reduces colour interference, but high-colour, high-organic water still affects the reading.
4. “Bubbles are a minor issue.” Bubbles scatter far more light than particles do; they are the first cause of jumping readings in the field.
5. “Install it and forget it.” Fouling can cause drift, bias or unstable readings depending on the deposit, and by the time the alarm triggers the fouling is usually weeks old.

8. Frequently Asked Questions

Can turbidity and TSS be converted?

Only within one water body with a stable particle size distribution, and only through a site-specific curve built by gravimetric sampling. There is no universal factor.

What is the difference between NTU and FNU?

NTU is the nephelometric turbidity unit, historically associated with white-light methods such as EPA 180.1. FNU is the formazin nephelometric unit used with ISO 7027 and its 860 nm infrared source. Under their respective standard definitions the two units are numerically equivalent for formazin calibration, but readings from different instruments should still be compared by checking light source, method and optical design.

Why do two turbidity meters give different readings on the same sample?

Different wavelength, measuring angle, optical geometry and calibration standards all produce differences. Compare with the same standard, at the same temperature, and confirm both instruments follow the same method.

How often does a TSS sensor need calibration?

The factory calibration suits typical media. Where the solids change significantly — seasonal sediment, process switches — a field verification every three to six months is sensible; confirm the procedure with the manufacturer.

Should high-turbidity water use turbidity or TSS?

Above roughly 4,000 NTU a turbidity sensor is near its limit; TSS covers up to 40,000 mg/L. Running both gives a useful cross-check.

Which sensor for seawater aquaculture?

For turbidity, the RK500-07 B with a titanium body. For TSS, the RK500-20 uses 316L, so long-term seawater immersion should be confirmed with the manufacturer for the specific duty.

Does an automatic cleaning brush remove the need for maintenance?

It extends the interval substantially, but bristles can trap fibres and scale. A visual check once a month is still recommended.

Can 4–20 mA and RS485 be used at the same time?

The RK500-07 supports simultaneous 4–20 mA and RS485 output. The RK500-20 specification lists 4–20 mA and RS485 (Modbus-RTU); confirm simultaneous operation and wiring requirements with RIKA before integration.

Where should the probe be installed?

Turbidity: away from bubbles and vibration, with the reflection distance respected in pipework. TSS: more than 5 cm from the wall and 10 cm from the bottom. Both belong in slow-moving water, not in a dead zone.

Is it worth installing both?

For wastewater plants, river stations and recirculating aquaculture, yes. Turbidity gives a fast trend and alarm; TSS gives the compliance and mass-balance number.

9. Selection Checklist

Confirm these eight points before ordering:

1. Water type: drinking, wastewater, river, aquaculture or industrial
2. Whether you need NTU, mg/L, or both
3. Expected concentration range and peak value, which sets the range
4. Seawater or aggressive media, which sets the body material
5. Mounting: immersion, pipe or bracket
6. Whether automatic cleaning is required
7. Output and supply: 4–20 mA, RS485, 12–24 VDC
8. Cable length, 5 m standard

Turbidity and TSS Monitoring with RIKA SENSOR

RIKA SENSOR provides online optical sensors for turbidity and suspended solids, covering clean water through high-solids industrial effluent.

The RK500-07 turbidity sensor and the RK500-20 TSS sensor are both IP68, both output 4–20 mA and RS485, and both support immersion or in-line installation depending on the version.

For project selection, it helps to provide: water type, expected concentration range, operating temperature and pressure, installation method, cleaning requirements and output needs.