Because of the price, the relative humidity sensors, that base on polymers are most common. Depending on the type of the material, the polymer, that absorbs humidity changes its conductivity or dielectric constant.

BResist sensors, that base on the measurement of the conductivity changes of the sensor prove a large dependence on the temperature.
Much better parameters have capacitative sensors, that base on the dielectric contstant measurement.

A complication level of the systems of capacity and resistance measurement isn't large and doesn't influence on the choice of interface of th eappliance. Doubtless advantage is large accuracy of the measurement (about 2%) and very little size of the sensor.

The systems for conductivity measurement of the medium are separate systems, and consist of measuring electrode that is dipped in the medium and of the measuring converter. Because the signal from the electored is converted in the electronic module, the linkage of th eelements should be done by a special cable.

On account of the rule of the measurement we differentiate two types of the electrodes:

• conductive for little scopes of measurements
• inductive for large scopes of measurement (from 2000mS/cm)Zasada pomiaru

The rules of the measurement

The conductivity sensors that are applied in the low scope of  the scale consist of two electrodes that are made of rust-proof steel or of titanium. A converter assures lability of the voltage that is given to the electrodes. A curent that flowes in the circuit depends only on the resistivity of the liquid between the electrodes. The resistivity is measured and characterized by the conductivity of the water.

Choosing the elements of the measuring system, you should take measuring scope of the electrode into th eaccount. Each electodr has specified rate K, that is strongly connected with the scope.

Systems for PH/Redox rate measurements are (similar to the conductivity measuremet systems) separated systems, consist of an electrode that contacs with the medium and of the measuring converter.
Because the signal from the electrode is converted in the electronic module, the linkage of the elements should be done by a special cable.

Choosing the electrode you should take measuring pH scope and conductivity of measured medium into account.

A measurement of the pH rate bases on finding the difference of the potentials between two electrodes. The electrode of the reference assures direct contact between a sample and electrolyte. The measuring electrode is separated from the sample by the glass diaphragm, that is susceptible for H+ ions. A potencial difference between electrodes depends on the number of H+ ions, that are in the sample. The other way of finding pH rate is difference between the conductivity, that is measured in front of and at the back of stronly acidic cationic exchanger.

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Turbidity measurements base on the phenomenon of light dispersal

When the light beam meets any barrier (a particle) part of the energy scatters in all directions. It's connected with reflection and deflection of the light on the particle The intensity of the scattered light depends on the concentration of particulars in the medium.

A circonium method of oxygene measurement bases on using the properties of circonium oxide, which by high temerature and by the difference of oxygene concentration creates an electrochemical cell and generates (pursuant to Nernst equation) electric tension.

Circonium cells are characterized by a high stability and precision, reliabilty and mechanical resistance, little exploitation requirements, long motility, great vibration resistance, thermal shock resistance and by the resistance against reductive conditions and pollution of the electrodes (sulphur, vanadium and so on).

The cell method is applied for exhaust gases analyzers anf for measurements of oxygene vestigal amount.