MJ-WQS-ORP is a simple yet reliable ORP sensor for water quality monitoring

Designed to be easily integrated into a system, it comes with an RS485 Modbus protocol.

Built with IP68 housing, it is suitable for long term deployment underwater.

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MJ-WQS-ORP is a digital output water ORP sensor developed and manufactured by MJ Instruments. With sensor body filled internally which acts as sensor weights as well as protecting the electronic components, the sensor is well suited for long-term deployment. The body is built with corrosive resistant material, allowing it to be deployed at industrial discharge for pollution monitoring. Different housing is also available upon request. With standard RS485 Modbus output, it is easy to integrate the sensor into the commercial controllers.

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ORP (Wiki)

Redox potential (also known as reduction/oxidation potential) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respectively. Redox potential is measured in volts (V), or millivolts (mV). Each species has its own intrinsic redox potential; for example, the more positive the reduction potential (reduction potential is more often used due to general formalism in electrochemistry), the greater the species’ affinity for electrons and tendency to be reduced. ORP can reflect the antimicrobial potential of the water.

In aqueous solutions, redox potential is a measure of the tendency of the solution to either gain or lose electrons when it is subjected to change by the introduction of a new species. A solution with a higher (more positive) reduction potential than the new species will have a tendency to gain electrons from the new species (i.e. to be reduced by oxidizing the new species) and a solution with a lower (more negative) reduction potential will have a tendency to lose electrons to the new species (i.e. to be oxidized by reducing the new species). Because the absolute potentials are next to impossible to accurately measure, reduction potentials are defined relative to a reference electrode. Reduction potentials of aqueous solutions are determined by measuring the potential difference between an inert sensing electrode in contact with the solution and a stable reference electrode connected to the solution by a salt bridge.

The sensing electrode acts as a platform for electron transfer to or from the reference half cell. It is typically platinum, although gold and graphite can be used as well. The reference half cell consists of a redox standard of known potential. The standard hydrogen electrode (SHE) is the reference from which all standard redox potentials are determined and has been assigned an arbitrary half cell potential of 0.0 mV. However, it is fragile and impractical for routine laboratory use. Therefore, other more stable reference electrodes such as silver chloride and saturated calomel (SCE) are commonly used because of their more reliable performance.

Although measurement of the redox potential in aqueous solutions is relatively straightforward, many factors limit its interpretation, such as effects of solution temperature and pH, irreversible reactions, slow electrode kinetics, non-equilibrium, presence of multiple redox couples, electrode poisoning, small exchange currents and inert redox couples. Consequently, practical measurements seldom correlate with calculated values. Nevertheless, the reduction of potential measurement has proven useful as an analytical tool in monitoring changes in a system rather than determining their absolute value (e.g. process control and titrations).


ORP Range

-1000 to 1000mV

ORP Accuracy


ORP Resolution


Temperature Range

0 – 50 Deg C

Temperature Accuracy

+- 0.2

Temperature Resolution


Max Pressure

50 Psig

Reference Solution

Ag/AgCl Double Salt Bridge


RS485 Modbus

Protection Rating


Power Supply

DC 12V