How to Use Suspended-Solids Sensors for Real-Time Process Control
A suspended-solids sensor provides a continuous indication of particulate concentration in water, slurry, or another liquid stream. When its signal is connected to a control system, operators can respond to changing solids loads within seconds rather than waiting for laboratory samples. This makes optical measurement valuable for dredging, sediment handling, clarification, filtration, dewatering, and industrial water treatment.
The measurement is often reported as total suspended solids (TSS), suspended sediment concentration, or an equivalent process-solids value. Although these terms are related, the sensor does not directly “see” mass concentration. Most instruments detect how particles scatter or absorb light, then use a site-specific calibration to convert the optical response into a useful engineering value.
Real-time control depends on much more than installing a probe in a pipe or channel. The sensing location, particle characteristics, calibration method, signal filtering, alarm limits, and control strategy all influence whether the measurement produces stable and actionable results. A carefully designed system can reduce chemical use, protect downstream equipment, improve product consistency, and make environmental compliance easier to document.
What the Sensor Actually Measures
Optical suspended-solids instruments generally use a light source and one or more detectors. Particles in the sample interact with the light, creating scattered or attenuated energy that changes with concentration. Backscatter sensors are often suited to higher solids levels, while transmission or attenuation methods can be effective in clearer water and lower concentration ranges. Some instruments combine several optical paths to extend the usable range.
The optical signal is influenced by particle size, shape, color, mineral composition, and refractive properties. Two samples with the same mass of solids can therefore produce different readings if their particle populations differ. Air bubbles, biological growth, oil films, and large debris can also alter the signal. For this reason, an instrument’s factory calibration is a starting point, not always the final relationship required for process control.
Turbidity and suspended solids should be treated as connected but distinct measurements. Turbidity is commonly expressed in NTU or another optical unit and describes the scattering behavior of a sample. TSS is generally expressed in mg/L or g/L and represents the mass of retained solids after a gravimetric laboratory procedure. A turbidity reading can serve as a proxy for TSS only after the relationship has been established for the specific water and process.
The water-quality glossary provides useful definitions for terms such as turbidity, suspended sediment, calibration, and optical measurement. Establishing a shared vocabulary helps operators, instrumentation engineers, and laboratory staff interpret the signal consistently.
Select a Measurement Point That Represents the Process
The best installation point is where the measured solids concentration reflects the process variable that needs to be controlled. In a treatment plant, this may be the clarified-water outlet, filter influent, sludge line, or final discharge. In a dredging operation, it may be a return-water channel, discharge line, overflow, or monitoring station downstream of the excavation zone.
Avoid locations with excessive turbulence, stagnant pockets, or sudden changes in flow direction unless the sensor is specifically designed for those conditions. A probe mounted immediately after a pump elbow may experience bubbles and unstable velocity profiles. A sensor placed too close to a discharge point may measure an unrepresentative plume or a stratified portion of the flow. Mixing distance, pipe diameter, access for cleaning, and safe maintenance should be considered together.
For open channels, mounting depth and orientation matter. The probe should remain submerged through expected changes in water level while avoiding contact with the bed, wall, or floating material. In a pipe, the sensor should be positioned where the solids remain suspended and the optical window is exposed to a representative stream. A bypass loop can provide easier access and controlled flow, but it must be designed to prevent settling and delays between the main process and the measured sample.
Dredging applications require particular attention to hydrodynamics and changing sediment conditions. A practical explanation of optical sensing in dredging shows why sensor placement and particle behavior affect plume-monitoring results. The same principles apply when the signal is used for an active control loop rather than passive observation.
Build a Calibration and Signal-Conditioning Plan
Begin by defining the process range. Identify the expected minimum, normal operating range, short-term peaks, and maximum concentration. Select a sensor whose optical range covers these conditions without operating continuously at the edge of saturation. If a process can change from clear water to dense slurry, a dual-range instrument or multiple measurement points may be more reliable than forcing one sensor to cover every condition.
Collect representative samples across the operating range. Laboratory TSS analysis can then be paired with simultaneous sensor readings to create a site-specific calibration curve. Samples should include different flow rates, particle sources, seasonal conditions, and process states where these factors are expected to change. A single grab sample may establish a rough relationship, but it cannot demonstrate how the instrument behaves during actual transients.
The conversion from optical signal to concentration may be linear over a limited range and nonlinear over a wider one. A polynomial, segmented curve, or lookup table may be appropriate, provided it is supported by sufficient data. The control system should retain the raw optical value as well as the calculated solids concentration. This makes it easier to identify a calibration shift, sensor fouling, or a change in particle characteristics.
Signal conditioning is equally important. A moving average can reduce noise caused by bubbles or turbulence, while a median or trimmed filter can suppress isolated spikes. Filtering should be short enough to preserve meaningful process changes. Excessive smoothing creates control lag and can allow a high-solids event to continue before the system reacts. A separate quality flag should identify cleaning, calibration, communication loss, and out-of-range conditions rather than disguising them as valid concentration data.
| Process requirement | Useful sensor arrangement | Control consideration |
|---|---|---|
| Low-to-moderate solids in clear water | Transmission or sensitive optical backscatter probe | Use a narrow calibration range and protect against fouling |
| High suspended-solids concentration | High-range backscatter or multi-range optical sensor | Check for saturation during peak loading |
| Variable dredging plume | In-situ probe with robust mounting and data logging | Link readings to flow, tide, location, and operating state |
| Slurry or sludge transport | Inline or bypass installation with accessible cleaning | Account for settling, abrasion, and pump-cycle changes |
| Compliance monitoring | Redundant measurement or periodic laboratory verification | Preserve timestamps, quality flags, and calibration records |
| OEM process equipment | Compact sensor with defined electrical and communication interfaces | Document scaling, failure states, and controller behavior |
Connect the Reading to the Control System
A suspended-solids sensor becomes a process-control instrument when its measurement is available to a programmable logic controller (PLC), supervisory control and data acquisition (SCADA) system, or other automation platform. Common interfaces include 4–20 mA, digital protocols, relay outputs, and serial or Ethernet communications. The selected interface should support the required distance, diagnostic information, update rate, and environmental conditions.
For a 4–20 mA output, assign the lower and upper current values to a clearly documented concentration range. Avoid using an undocumented scale that differs between the sensor, PLC, and SCADA display. The controller should also distinguish a valid low reading from a fault current, overrange condition, or loss of power. Digital communications can provide richer diagnostics, but they still require careful mapping and testing of registers, units, decimal places, and status codes.
Control logic should include permissives and interlocks. For example, a high-solids alarm might trigger an automatic diversion, reduce a pump speed, increase dilution water, adjust polymer dosing, or initiate a filter backwash. A low-flow condition may invalidate the reading and prevent the controller from acting on a stagnant sample. Maintenance mode should suspend automatic corrections while allowing the sensor signal and fault status to remain visible to operators.
The response time of the complete system includes sensing, signal filtering, communication, actuator movement, and process travel time. If water takes two minutes to reach the control point after a valve adjustment, a controller that reacts to the current reading may oscillate. Flow measurement, transport delay, and actuator limits should be incorporated into the control strategy. A simple deadband and timed response may be more stable than aggressive proportional-integral-derivative tuning in a slow or highly variable process.
Convert Measurements Into Useful Process Actions
The appropriate action depends on what the solids measurement represents. In a clarifier, a rising outlet concentration may indicate excessive hydraulic loading, poor flocculation, a disturbed sludge blanket, or a dosing problem. In a dewatering system, a change in feed solids can require adjustments to polymer, belt speed, pressure, or throughput. In dredging, the signal may control operating limits, pump settings, overflow routing, or alerts when a plume exceeds a permitted threshold.
Use staged limits rather than a single alarm whenever the process permits. A warning threshold can prompt inspection or a modest correction. A high-high threshold can initiate a protective action such as diverting flow or stopping a transfer pump. Hysteresis prevents the output from switching rapidly when the measurement hovers near a limit. Time delays can prevent brief bubbles or isolated particles from initiating an unnecessary shutdown.
Feedforward control can improve performance when another variable predicts solids loading. For instance, dredge pump speed, conveyor rate, flow rate, or upstream valve position may be used to anticipate a concentration change. The suspended-solids signal then provides feedback to correct the prediction. Combining measurement with operating-state data is often more reliable than expecting the optical sensor to explain every process change by itself.
Use trends as well as instantaneous values. A gradual increase may indicate fouling or a changing sediment source, while a sharp step may indicate a pump start, valve movement, or air entrainment. Store the concentration, raw optical signal, flow, control output, alarms, and sensor-status flags with synchronized timestamps. This record supports troubleshooting and demonstrates whether a control action actually improved the process.
Maintain Accuracy During Continuous Operation
Optical windows gradually accumulate deposits, especially in nutrient-rich water, wastewater, slurry, and marine environments. A cleaning schedule should be based on fouling rate rather than a fixed assumption. Wipers, air blasts, mechanical cleaning, or manual wiping may be appropriate depending on the application. Cleaning should be recorded as an event so that operators can distinguish a genuine process change from a restored optical response.
Inspect mounting hardware, cables, connectors, and junction boxes during routine maintenance. Abrasion can damage probes in dense slurry, while vibration can change the sensor angle or loosen a bracket. In open water, biofouling, sediment burial, floating debris, and changing water level require inspection procedures that account for field conditions. A robust enclosure does not eliminate the need to verify installation integrity.
Recheck the calibration when the process material changes, when laboratory comparisons drift, or when the optical signal reaches an unexpected range. A useful verification program compares online readings with properly collected and analyzed samples at low, normal, and high operating conditions. Sampling technique matters: the laboratory sample must represent the same water measured by the sensor at nearly the same time.
Diagnostic rules should identify implausible values, a frozen signal, rapid step changes, persistent maximum output, and disagreement between redundant sensors. The control system should move to a defined safe state when the measurement is unavailable. Depending on the process, that may mean holding the last valid output briefly, switching to a conservative fixed setting, reducing throughput, or stopping a transfer. The correct fallback should be chosen through process-risk analysis rather than left to default software behavior.
Practical Decisions Before Commissioning
A successful installation is easier when measurement, automation, and maintenance requirements are specified together. The manufacturer’s instrumentation information can help define suitable sensor technologies and application details, while product-management and support information is available through Campbell Scientific for the currently supported product line.
Before placing the system into automatic control, run it in observation mode. Compare sensor trends with laboratory results, operator observations, flow changes, and equipment states. This period exposes problems with placement, scaling, filtering, and delay before an incorrect signal can affect production or discharge performance.
Use these deployment priorities:
- Define the controlled variable, acceptable operating range, alarm levels, and safe response before selecting the output scale.
- Install the probe where flow and solids distribution are representative, with safe access for inspection and cleaning.
- Create a site-specific optical-to-TSS calibration using samples that cover normal operation and expected excursions.
- Preserve raw readings, converted concentration, diagnostics, calibration history, and control actions in the data system.
- Test sensor failure, communication loss, fouling detection, and manual override conditions before enabling automatic responses.
When the system is commissioned, tune the control loop gradually. Start with conservative limits and short observation periods, then adjust filtering, deadband, and actuator response using recorded process behavior. Operators should know which values are measured, which are calculated, and which indicate a sensor fault rather than a real solids concentration.
A well-integrated suspended-solids sensor turns changing particle loads into timely operational information. Review the application requirements, select the right optical range, establish a defensible calibration, and connect the result to control logic that accounts for process delay and sensor health. For dredging, water treatment, environmental monitoring, or OEM equipment, consult the available technical resources and Campbell Scientific support to move from continuous measurement to dependable real-time control.