Online Turbidimeter Selection: Sensor, Panel Analyzer, Range, and Method
Key questions that determine instrument type in inline turbidity measurement and the measuring range of each model.
Three Questions That Determine Online Turbidimeter Selection
- In what range? Ultra-low range starting from 0.001 NTU in filter effluent and drinking water; 0–100 or 0–1,000 NTU in process water; hundreds to thousands of NTU in dirty water.
- Sensor or panel? A sensor with 4-20 mA / Modbus output if a PLC is available; a panel analyzer when a display, data logging, and a ready-to-use flow chamber are required.
- Which method? White LED for EPA 180.1 compliance, 860 nm infrared LED for ISO 7027 compliance.
How Nephelometric Measurement Works
An online turbidimeter measures the portion of light passing through the sample that is scattered by suspended particles. In nephelometric measurement, the detector is positioned at a 90° angle to the light beam and the scattered light intensity is read in NTU. If the light source is a white LED, the method complies with EPA 180.1; if it is an 860 nm infrared LED, it complies with ISO 7027. Infrared light is unaffected by color variations in the sample; the same sample may read differently under the two methods, so an instrument matching the reporting standard is selected.
Pyxis LT-73X uses a 90° surface scatter configuration with a flat quartz glass tip; the flat surface reduces air bubble interference. The Hefer TU series is nephelometric with an IR light source (ISO-7027 / EN-27027). The Pyxis LT-635 TSS sensor, on the other hand, operates by absorption of 850 nm infrared light rather than scattering; it outputs suspended solids (mg/L) rather than turbidity and is calibrated via sample-specific gravimetric analysis.
Sensors and Panel Analyzers
| Instrument | Type | Range | Method | When to use |
|---|---|---|---|---|
| Pyxis LT-73X (LT-736/737/739) | Sensor, FR-100 chamber or FT-100 inline tee | 0.001–5 / 0.001–40 / 0.002–1,000 NTU | EPA-180.1 (white) or ISO-7027 (IR) | Filter effluent, drinking water, ultra-low range |
| Pyxis DW-739 / DW-739B | Panel, FR-100 chamber, 7" display | 0.001–10 and 10–40 NTU | EPA-180.1 / ISO-7027 | Drinking water network, secondary water, plug-and-play |
| Pyxis IK-739 PLUS / IK-736 PLUS | Panel, self-brushing FT-100 PLUS, built-in flowmeter | 0–40 / 0–1,000 NTU | EPA-180.1 | Process and raw water where lens fouling is frequent |
| Pyxis DW-2100P | Panel, turbidity + free chlorine + pH | 0.001–10 and 10–40 NTU | EPA-180.1 (US) / ISO-7027 (EU) | Three parameters on a single panel |
| Pyxis ST-730 series | Inline sensor, CPVC or stainless steel | 0–10 / 0–100 / 0–1,000 / 0–10,000 NTU | White LED; ST-730B/735: IR above 100 NTU | Process and cooling water, RO, pressurized line (stainless steel up to 290 psi) |
| Pyxis LT-631/632/633 | Immersion, motorized wiper | 0–500 / 0–1,000 / 0–4,000 NTU | Dual IR, ISO-7027 | Raw water, discharge, settling basin |
| Hefer TU-311 / TU-341 / TU-321 | Loop-powered sensor, TU-410 flow cell | 0–4/40/400 NTU; 0–100/1,000/10,000 NTU | IR, ISO-7027 / EN-27027 | Sensor connecting to existing PLC; TU-321 with compressed air cleaning |
| Hefer TU/TSS 3000C | Panel analyzer | 0–4,000 NTU / 0–10,000 mg/L | ISO-7027 / EN-27027 | Measurement with display and fouling compensation; NTU or mg/L |
| Hamilton Dencytee Arc RS485 | Bioprocess sensor | 0–4 AU; 0–30,000 NTU | 860 nm transmission/reflection | Crystallization, filtration, dissolution |
See our online analyzers page for all model categories and details.
Mounting, Cleaning, and Calibration
| Topic | Source specifications |
|---|---|
| Flow chamber (LT-73X) | FR-100: 200–400 mL/min by-pass flow, for drinking water; FT-100 inline tee: up to 40 L/min and 6.9 bar |
| Panel sample condition | DW-739: 7.25–30 psi, 200–1,800 mL/min. IK-73X PLUS: 7.25–100 psi, 100–2,000 mL/min, 4–40°C. DW-2100P (FR-200): 7.25–30 psi, 600–1,000 mL/min |
| Cleaning | Chamber with wiper (FR-100-PLUS / FT-100-PLUS), motorized brush on IK-73X PLUS, wiper on LT-63X, compressed air on TU-321 (up to 1 bar) |
| Calibration | L-CAL liquid formazin kit (500 mL), LT-SOLID-CAL solid-state kit; for the ST-730 series, the manufacturer recommends cleaning and calibration at least once a month |
| Wireless | uPyxis app, MA-CR / MA-WB Bluetooth adapter |
For field verification with handheld instruments (Pyxis SP-505 / SP-600 and SP-800), see our handheld water analyzer guide. For drinking water applications, see drinking water turbidity measurement.
Frequently Asked Questions About Online Turbidimeters
Online vs laboratory difference, sensor vs panel, EPA and ISO, bubbles, and calibration.
What is the difference between an online turbidimeter and a laboratory turbidimeter?
In a laboratory turbidimeter, a sample is taken into a cuvette and read at a single point. An online turbidimeter is installed in a flow chamber or inline tee where the sample flows continuously; it sends the result to the control system via 4-20 mA and Modbus and generates an alarm when a threshold is exceeded. A handheld instrument is used for field verification: Pyxis SP-505 measures 0–100 NTU using the ISO 7027 method, while SP-800 measures in the 0–200 NTU range with a 1 NTU detection limit.
Sensor or panel analyzer?
If an existing PLC or controller is available, a sensor is sufficient: Pyxis LT-73X and ST-730, and Hefer TU-311/341 output 4-20 mA and RS-485 directly. When a display, data logging, flow monitoring, and a ready-to-use flow chamber are desired, a panel analyzer is selected: Pyxis DW-739 (0.001–40 NTU), IK-73X PLUS (self-brushing), DW-2100P (turbidity + chlorine + pH), and Hefer TU/TSS 3000C.
Should EPA 180.1 or ISO 7027 be selected?
The required compliance method determines the choice. EPA 180.1 operates with a white LED, and ISO 7027 operates with an 860 nm infrared LED; results may differ on the same sample. Pyxis LT-73X, DW-739, and DW-2100P are available in versions for both methods; ST-730B and ST-735 use a white LED below 100 NTU and an infrared LED above it. The Hefer TU series complies with ISO 7027 / EN 27027.
How do air bubbles distort the measurement, and how are they prevented?
In low ranges, bubbles scatter light and are read as turbidity. The Pyxis FR-100 flow chamber separates air bubbles and coarse particles (including iron/manganese precipitates) prior to measurement and operates with a 200–400 mL/min by-pass flow. The flat-surface quartz tip of the LT-73X sensor also reduces bubble interference and simplifies cleaning.
How often is an online turbidity sensor calibrated?
It depends on the sensor and fouling: a well-cleaned LT-73X provides stability without calibration for up to 1 year, and LT-63X for up to 1 year; for the ST-730 series, the manufacturer recommends cleaning and calibration at least once a month. Calibration is performed using formazin standards (L-CAL liquid kit for LT-73X, 500 mL) or the formazin-free LT-SOLID-CAL solid-state kit.
Can a cell density sensor be used instead of a turbidity sensor?
Hamilton Dencytee Arc RS485 measures transmission and reflection at 860 nm; it measures total cell density and turbidity (0–30,000 NTU, 0–4 AU) and can be used as a turbidity sensor in crystallization, filtration, and dissolution processes. For measurements at the 0.001 NTU level such as water treatment effluent, the ultra-low-range LT-73X is selected.
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