PC-08Analytical · AT
Liquid Analysis Sensors & Analyzers
Water and process-liquid quality measured online — pH, ORP, conductivity, dissolved oxygen, turbidity, chlorine and more. The matrix decides the sensor; reliable readings depend on cleaning and calibration being planned from the start.
01Scope
What's included
pH, ORP, conductivity, dissolved oxygen, turbidity and related sensors.
- pH and ORPGlass and specialty electrodes with transmitters, holders and retractable assemblies.
- ConductivityContacting cells for pure water; inductive (toroidal) sensors for dirty or concentrated media.
- Dissolved oxygenOptical (luminescent) and amperometric sensors for aeration, fermentation and feedwater.
- Turbidity and suspended solidsNephelometric and absorption sensors for drinking water, filtration and sludge.
- DisinfectionFree and total chlorine, chlorine dioxide and related residual measurements.
- Wet-chemistry analyzers and panelsColorimetric analyzers, multi-parameter transmitters and sample panels.
02Compare
Measuring principles
Each parameter has its own sensor family; within it, the medium and the fouling risk decide the design.
| Principle | How it works | Suited for | Considerations |
|---|---|---|---|
| pH / ORP (potentiometric) | How it worksA glass membrane develops a potential that depends on hydrogen-ion activity, measured against a reference electrode; ORP uses a noble-metal electrode. | Suited forNeutralization, dosing control, water treatment and process quality. | ConsiderationsReference poisoning and fouling, temperature compensation and regular buffer calibration; consider glass-free or protected designs where breakage is a risk. |
| Conductivity — contacting | How it worksTwo or four electrodes measure the solution's ability to carry current. | Suited forPure and ultrapure water, condensate, boiler feedwater and low to medium conductivity. | ConsiderationsPolarization and fouling at high conductivity; the cell constant must suit the range. |
| Conductivity — inductive | How it worksTwo toroidal coils induce and sense a current in the liquid loop; no electrode touches the medium. | Suited forHigh conductivity, dirty or corrosive media — acids, caustics, CIP concentration, wastewater. | ConsiderationsNot for very low conductivity; nearby pipe walls affect the reading (installation factor). |
| Dissolved oxygen — optical | How it worksOxygen quenches the luminescence of a dye; the change in its decay gives the oxygen concentration. | Suited forAeration control in wastewater, fermentation and surface water, with low maintenance. | ConsiderationsThe sensor cap has a finite life; fouling still needs cleaning. |
| Dissolved oxygen — amperometric | How it worksOxygen diffuses through a membrane and is reduced at a cathode, producing a current. | Suited forTrace oxygen in boiler feedwater and established installations. | ConsiderationsMembrane and electrolyte maintenance; some designs depend on sample flow. |
| Turbidity and suspended solids | How it worksLight scattered by particles (nephelometric, typically at 90°) or absorbed along the path indicates turbidity or solids. | Suited forDrinking water, filter performance, wastewater and sludge. | ConsiderationsBubbles and fouling give false readings; the unit (NTU, FNU, mg/L) depends on the method. |
| Chlorine (amperometric) | How it worksChlorine is reduced at an electrode behind a membrane or in an open cell, producing a current proportional to concentration. | Suited forDisinfection control in drinking water, cooling water and food process water. | ConsiderationspH shifts the balance of free chlorine species, so a pH measurement often compensates it; sample flow must be controlled. |
| Colorimetric analyzers | How it worksReagents react with the target species; the colour change is measured photometrically. | Suited forSilica, phosphate, ammonia and hardness in power-plant chemistry and wastewater. | ConsiderationsReagent handling and consumption, cycle times, and sample conditioning. |
General engineering guidance. Limits and performance depend on the specific instrument and variant — confirm them against the manufacturer's datasheet for the selected model.
03Specify
What to specify
The parameters that decide the selection. State each with its unit; where a value is not known yet, say so — it becomes part of the engineering review.
Parameters and ranges
pH, µS/cm, mg/L or NTU — the range decides the sensor type, and sometimes the cell constant or optical path.
Medium
Clean water, wastewater, slurry, solvent or hygienic product — it decides materials and sensor design.
Process temperature and pressure
Electrodes and seals have limits; high temperature shortens sensor life.
Installation type
In-line, insertion, immersion, flow-through panel or retractable holder — decided by access and maintenance.
Flow and sample conditions
Velocity, bubbles and solids affect several measurements; some sensors need a controlled flow.
Fouling and cleaning
Oil, biofilm, scale and solids call for automatic cleaning or retractable assemblies.
Calibration regime
How often, by whom and where — laboratory-calibrated sensors reduce time spent in the field.
Wetted and hygienic requirements
Food and pharmaceutical processes need approved materials and cleanable designs.
Output and communication
4–20 mA per parameter, or a multi-parameter transmitter on Modbus, HART or Ethernet.
Area classification and utilities
Certified sensors in hazardous areas; drains, reagents and power for analyzers.
04Integrate
Outputs & communication
Interfaces typically offered in this category, and what each means for the control system. Availability depends on the specific model.
- 4–20 mAAnalog
- Current loop with a live zero: 4 mA is the bottom of the range, so a broken wire (0 mA) is distinguishable from a zero reading. Two-wire (loop-powered) on most transmitters; four-wire with an active output on line-powered instruments such as analyzers — the I/O channel must match. Accepted by practically every PLC and DCS analog input.
- HARTDigital over analog
- A digital signal (Bell 202 FSK, 1200/2200 Hz) superimposed on the 4–20 mA loop — configuration, diagnostics and secondary variables over the existing two wires.
- Modbus RTUSerial
- Serial client/server (formerly master/slave) protocol, typically over RS-485: several devices share one multi-drop cable and the client polls each for its values — common on analyzers, multivariable meters and packaged equipment.
- Industrial EthernetNetwork
- PROFINET, EtherNet/IP or Modbus TCP — the networks that connect controllers, remote I/O, drives and HMIs, and increasingly carry device data from the field as well.
- Relay / discreteDiscrete
- Switched contact or transistor output for alarms, interlocks and pump or valve control — it carries a state, not a value.
05Install
Installation practice
- Mount sensors where the liquid is representative and the sensor stays wetted; mount pH electrodes tilted from horizontal (commonly at least 15°) so the internal air bubble stays away from the glass membrane.
- Provide access for cleaning and calibration — retractable assemblies or flow-through panels allow maintenance without stopping the process.
- Control sample flow and pressure for flow-sensitive measurements such as chlorine and some dissolved-oxygen sensors.
- Keep sample lines short to limit lag and avoid degassing or temperature change before the measurement.
06Products
Liquid Analysis products
Status · no models listed
Specific models are not listed on the site.
Our engineers can recommend instruments against your process data. The selection checklist on this page is the information that decides it — send what you have.
How a review runs
AT01Share the process data
Medium, range, conditions, connection and the system it reports to.
02Engineering review
The measuring principle and the installation are checked against the application.
03Instrument options
Suitable devices are proposed with the data needed to confirm the selection.
Next steps
Where to go next
- 01SolutionAnalytical SystemsProcess and online analyzers with sampling, sample conditioning, analyzer panels and control-system integration.
- 02Resource4–20 mA and HART: How Analog Instrument Signals WorkLive zero, loop power, loop resistance and the digital layer HART adds.
- 03Related categoryGas AnalysisOnline gas analyzers with sample extraction and conditioning.
Engineering enquiry
Planning online liquid analysis?
Share the component to measure, the expected range, the sample conditions and the system it reports to. Our engineers can help match the analyzer principle and the sampling system to the application.