Drinking Water Turbidity Measurement: Analyzer Selection, Sample Line and Calibration
Instrument and installation criteria for continuous 0.001 NTU monitoring at filter outlets and distribution networks.
Three Questions That Determine Instrument Selection
- Which method? EPA 180.1 white LED; ISO 7027 860 nm infrared LED. DW-739 complies with EPA, DW-739B complies with ISO.
- How many parameters? DW-739 for turbidity only; DW-2100P for turbidity, free chlorine and pH.
- Lens cleaning: Self-brushing IK-73X PLUS or wiper-equipped LT-73X chambers at frequently fouled measurement points.
What Filter Effluent Turbidity Indicates
Turbidity is the degree to which light is scattered by suspended particles in water and is expressed in NTU. It is not a standalone contamination indicator; its primary value is continuous verification of filtration step performance. A sudden increase is the earliest sign of filter media breakthrough or that backwashing is needed, preventing carryover of solids to downstream steps (softening, RO, UV). Because particles can shield microorganisms from chlorine or UV, low and stable turbidity is a prerequisite for reliable disinfection.
The target turbidity value and reporting frequency are defined in the regulations and permit conditions applicable to your facility; no limit value is stated on this page. What matters in analyzer selection is which method is required to measure that limit and the resolution of the monitored range.
Turbidity Analyzers for Drinking Water
| Instrument | Parameter and range | Method | Accuracy | Sample condition |
|---|---|---|---|---|
| Pyxis DW-739 | Turbidity, 0.001–10 and 10–40 NTU | EPA-180.1, white LED | Below 10 NTU plus/minus 0.005 NTU or 2%; repeatability plus/minus 0.001 NTU or 0.5% | FR-100: 7.25–30 psi; 200–1,800 mL/min; sample 4–40°C |
| Pyxis DW-739B | Same range | ISO-7027, 860 nm IR LED | Same | Same |
| Pyxis DW-2100P (US / EU) | Turbidity 0.001–10 and 10–40 NTU; free chlorine 0.01–5.00 ppm; pH 0.01–14.00; temperature | EPA-180.1 (US) or ISO-7027 (EU) | Turbidity same class; chlorine plus/minus 0.01 ppm or 1% of full scale; pH plus/minus 0.01 | FR-200: 7.25–30 psi; 600–1,000 mL/min; sample 4–40°C |
| Pyxis IK-739 PLUS | Turbidity 0–40 NTU | EPA-180.1 | 0.001 NTU or 1% of full scale; lowest resolution 0.002 NTU | 7.25–100 psi; 100–2,000 mL/min; sample 4–40°C; auto-brush |
| Pyxis LT-737 / LT-739 (sensor) | 0.001–5 / 0.001–40 NTU | EPA-180.1 or ISO-7027 | Annual drift: LT-737 below 0.01 NTU, LT-739 below 0.02 NTU (clean sensor) | FR-100 (200–400 mL/min) or FT-100 inline tee (40 L/min, up to 6.9 bar) |
| Hefer TU-311 | 0–4 / 40 / 400 NTU | IR, ISO-7027 / EN-27027 | — | Up to 6 bar; TU-410 flow cell |
Values are taken from manufacturer datasheets. For general selection and other range classes: online turbidimeter selection and online turbidity measurement.
Sample Line, Flow Chamber and Panel
Air bubbles, suspended solids and foam commonly encountered at drinking water inlets disturb low-level turbidity measurement. The FR-100 flow chamber on the DW-739 panel includes a rotameter and pressure-reducing valve, is self-regulating, and allows bubbles, coarse particles and foam to settle before measurement. The sample inlet is 1/4 inch OD, the overflow outlet goes to drain via a 20 mm line, and the drain connection is 1/2 inch NPT. The FR-200 dual-sensor chamber of the DW-2100P is designed for the same purpose; recommended flow is 600–1,000 mL/min, inlet 1/4 inch, overflow 3/4 inch, outlet 20 mm and drain 1/2 inch.
The panel display is a 7 inch color capacitive touchscreen and stores approximately 1 million data/event records in 4 GB of memory. Outputs are 2× 4-20 mA and RS-485 Modbus RTU/TCP; inputs are 2× 4-20 mA and RS-485 Modbus RTU. The DW-739 panel is powered by 96–260 VAC / 50–60 Hz (200 W), panel operating temperature is 4–45°C, sample water is 4–40°C, relative humidity is 20–90% (non-condensing) and altitude is below 2,000 m. The display is IP-65 and the sensor is IP-67 rated; the panel measures 790×450×231 mm and weighs approximately 15 kg. In NSF-certified applications, the analyzer's approximately 200–400 mL/min discharge flow can be sent to sanitary drain or returned to the pre-treatment inlet.
Calibration, Field Verification and Troubleshooting
| Topic | Detail |
|---|---|
| Calibration kit | LT-SOLID-CAL solid-state kit (formazin-free) or L-CAL portable liquid formazin kit (500 mL per calibration) |
| L-CAL application | The sensor is placed horizontally in the kit; the integrated air-bleed line vents bubbles |
| Zero point | FR-100 and FT-100 can be used for LT-73X zero-point calibration when drained and cleaned |
| Wireless | MA-CR Bluetooth adapter and uPyxis app; liquid-free calibration with the solid-state kit |
| Field verification | SP-505: 0–100 NTU, ISO 7027; SP-800: 0–200 NTU, 1 NTU detection limit |
| Symptom | Possible cause | Action |
|---|---|---|
| Sudden spikes at low turbidity | Air bubble or coarse particle | Check flow chamber and flow rate; drain and clean the chamber |
| Reading drifts upward over time | Lens fouling | Switch to a self-brushing or wiper chamber; programmable brush cycle (IK-73X PLUS) |
| Two instruments read differently | White LED (EPA 180.1) and 860 nm IR (ISO 7027) are different methods | Perform comparisons using instruments with the same method |
| Measurement needed at line end, pressure is low | Insufficient chamber inlet pressure | Use a low-pressure flow chamber (DW-739: 7.25 psi, DW-2100P: 7.5 psi minimum) |
If free chlorine and pH will be monitored alongside turbidity, see our chlorine and pH measurement guide. For filtration and treatment stages, see cartridge filter selection.
Frequently Asked Questions on Drinking Water Turbidity Measurement
Analyzer selection, measurement method, dead-end monitoring, calibration and data access.
How do you select a drinking water turbidity analyzer?
Three factors determine the selection: the reporting method (EPA 180.1 white LED or ISO 7027 860 nm infrared), the number of parameters to be monitored (turbidity only, or combined with chlorine and pH) and lens cleaning. Pyxis DW-739 / DW-739B is suitable for turbidity only; DW-2100P is suitable when turbidity, free chlorine and pH are required on a single panel; and the self-brushing IK-73X PLUS is suitable for lines subject to frequent fouling.
What is the turbidity limit in NTU for drinking water?
The target turbidity value and reporting frequency are defined in the regulations and permit conditions applicable to your facility; no limit value is stated on this page. What matters for instrument selection is which method (EPA 180.1 or ISO 7027) and what resolution are required to monitor that limit. The DW-739 series measures over 0.001–10 and 10–40 NTU ranges with 0.001 NTU resolution.
Why is turbidity monitored continuously at filter outlets?
A sudden rise in turbidity is the earliest sign of filter media breakthrough or that backwash is due. In addition, particles can shield microorganisms from chlorine or UV; low and stable turbidity is a prerequisite for reliable disinfection. Continuous monitoring makes excursions visible that would otherwise fall between spot measurements taken with a handheld meter.
Can measurements be taken at pipe dead ends and secondary water supply points?
Yes. The minimum inlet pressure of the DW-739 and DW-2100P flow chambers is low (DW-739 FR-100 chamber inlet pressure 7.25–30 psi; DW-2100P FR-200 chamber at least 7.5 psi), so manufacturer documentation specifies pipeline dead ends and secondary water supply sampling as applications.
How long can the sensor operate without recalibration?
A clean LT-73X sensor drifts less than 0.02 NTU per year (0.01 NTU for LT-737) and can operate for up to 1 year without recalibration. Calibration is performed using the formazin-free LT-SOLID-CAL solid-state kit or the portable L-CAL liquid kit using 500 mL of formazin per calibration.
Is remote data access possible?
Pyxis DW-739-G, DW-739B-G and IK-73X PLUS-G models include a CloudLink 4G gateway and can upload field data to the cloud. All panels store approximately 1 million data/event records in 4 GB of internal memory; 4-20 mA and RS-485 Modbus RTU/TCP outputs connect to existing PLC and DCS networks.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
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