Chemical sensors and analytical electronics for your application

What does a pH sensor measure, and which probe should be selected?

pH expresses a solution's acidity or alkalinity in relation to hydrogen ion activity. A common glass electrode produces a potential together with a reference electrode; the electronics convert this into pH. The glass, reference structure and junction are selected for the sample. A probe for low-conductivity water may differ from one suitable for dirty process liquid.

The board must not load this signal, which requires very high input impedance. Moisture or contamination in cables and connectors, or leakage across the board surface, can distort results even when the sensor is sound. Project review also addresses probe removal, storage and replacement. The aim is consistent recommissioning after maintenance as well as a good first-day measurement.

EC and TDS: conductivity does not quantify each chemical separately

Electrical conductivity reflects the combined effect of ions in solution; it does not identify which ions are present. Different compositions can produce the same EC. Conductivity should therefore not be presented directly as nitrate, a particular heavy metal or overall purity. Many instruments also calculate TDS from conductivity using a selected factor, which depends on sample composition.

Two- or four-electrode cells and, in suitable processes, inductive conductivity methods may be considered. Cell constant, range, temperature correction, bubbles and surface deposits matter in selection. For example, EC and temperature records can help a fertilisation device, but they do not become separate laboratory results for each nutrient. Display labels should clearly reflect the measured quantity.

Which questions do ion-selective electrodes and ORP sensors answer?

An ion-selective electrode measures a potential sensitive to the activity of a particular ion. With suitable calibration and sample preparation, it can support concentration calculations; interference from other ions, pH and ionic strength can affect the result. Method-specific solutions may be needed to bring standards and samples to similar ionic conditions. Electrode and operating procedures are assessed separately for each ion.

ORP expresses a sample's oxidation-reduction potential relative to the electrode and reference used. Alone, it is not a selective concentration measurement for a particular substance. A project first turns 'Is a chemical present?' into an analytical target: which substance, in which sample and at what limit? If the method lacks sufficient selectivity, an alternative sensing or reference analysis approach is included in feasibility work.

Analogue front ends and isolation for chemical sensors

For high-impedance sources such as pH electrodes, low input bias current, suitable guard traces, clean PCB surfaces and shielding become important. Analog Devices' CN0326 example shows how temperature compensation and electrical isolation can be considered together in pH measurement. Evaluation-board results do not automatically transfer to a product using a different probe and enclosure.

A conductivity channel needs suitable excitation and impedance measurement; it does not use the same circuit as a pH input. Interactions between channels in the same sample are examined on the prototype. Design may include measurement sequencing, analogue filters, power domains and channel isolation where needed. Leakage introduced by protection components is included in the error budget. For a ready-made digital probe, verify the protocol and meaning of calibration data against manufacturer documentation.

Low-level signal measurement board development →

Why are sampling, temperature and flow arrangements decisive?

Good electronics cannot correct changes that occur before the sample reaches the sensor. Waiting time, air contact, bubbles, precipitation and adhesion to tubing are assessed for the application. A flow cell should provide representative sample around the probe, minimise dead volumes and allow cleaning access. Temperature at the measurement point may differ from that measured inside a distant cabinet.

Temperature compensation is not one operation that reverses every chemical change. Correction of electrode slope is distinguished from actual sample pH changes; the conductivity temperature coefficient must suit the sample. A VOMRO project can identify raw and corrected values in records and the interface. Temperature sensor position and simultaneous measurement planning are therefore defined at the outset.

Accurate sample temperature measurement →

How are calibration, cleaning and probe life managed?

Calibration follows the selected probe manufacturer's method using suitable standards. Consider buffers covering the expected pH range, appropriate conductivity standards and cell constants, or sample-appropriate standards and preparation for ion-selective measurements. Adjusting with a contaminated standard can make a sound device read incorrectly. Record standards' dates, temperatures and conditions of use.

Maintenance planning reflects the burden the sample places on the probe. User instructions cover cleaning-agent compatibility and storage. Firmware can retain calibration dates, slope or cell coefficients, stabilisation time and check results. Acceptance criteria are not left as 'the correct number on screen'; repeatability and comparison with a reference method on actual samples may also be defined.

How do you start developing a custom chemical sensor system?

A water monitor may combine pH, EC and temperature channels, while another project may need only one selective electrode. Method feasibility using actual samples and existing analysis results reduces uncertainty. If selectivity or maintenance burden is unsuitable, changing the method may be necessary instead of adding more sensors.

For discussion, share the target property or ion, sample composition, range, temperature, continuous or intermittent measurement, cleaning arrangements and data output. VOMRO uses this to define probe integration, electronics, firmware, interface and mechanical flow components. Projects requiring chemical method validation separately plan the relevant expertise and reference analysis. Source files, calibration arrangements and acceptance records are specified in the deliverables list.