NEMOTO ENVIRONMENTAL TECHNOLOGY
GAS SENSORS FOR INSTRUMENT MAKERS WORLDWIDE

THE TECHNOLOGY WE USE: CATALYTIC GAS SENSORS (PELLISTORS)

Nemoto is one of the world's largest manufacturers of Pellistor type flammable gas sensors, with over 1.5 Million sensors manufactured every year. Nemoto's Industrial standard range of gas sensing products brings together this highly automated high volume production capability with industrial standard product performance and specifications. The result is a new higher standard of technical performance with a repeatability of manufacture unrivalled by other OEM pellistor manufacturers.

Catalytic combustion has been the most widely used method of detecting flammable gases in Industry since the invention of the catalytic pelletized resistor (or "Pellistor") in the mid 1960's.

A Pellistor consists of a very fine coil of platinum wire, embedded within a ceramic pellet. On the surface of the pellet is a layer of a high surface area noble metal, which, when hot, acts as a catalyst to promote exothermic oxidation of flammable gases. In operation, the pellet and so the catalyst layer is heated by passing a current through the underlying coil. In the presence of a flammable gas or vapour, the hot catalyst allows oxidation to occur in a similar chemical reaction to combustion. Just as in combustion, the reaction releases heat, which causes the temperature of the catalyst together with it's underlying pellet and coil to rise. This rise in temperature results in a change in the electrical resistance of the coil, and it is this change in electrical resistance which constitutes the signal from the sensor.  

Pellistors are always manufactured in pairs, the active catalysed element being supplied with an electrically matched element which contains no catalyst and is treated to ensure no flammable gas will oxidise on it's surface. This "compensator" element is used as a reference resistance to which the sensor's signal is compared, to remove the effects of environmental factors other than the presence of a flammable gas.

Pellistor Drive/Measurement Circuit: A simple Wheatstone Bridge
to compare the resistance of two hot elements

The advantage of using this technique when detecting flammable gases for safety purposes is that it measures flammability directly.

Nemoto provides matched pair Pellistors conveniently mounted in TO4 size headers, or as complete flameproof Gas Detection Heads for use as field devices within fixed Gas Detection Systems.

 

THE TECHNOLOGY WE USE: ELECTROCHEMICAL GAS SENSORS

The introduction of Nemoto's electrochemical Gas Sensors represents the first of many diversifications into other gas sensing technologies.

The combination of the latest scientific design techniques and Nemoto's production engineering excellence has resulted in a sensor package which is guaranteed not to leak electrolyte throughout it's lifetime and either meets or exceeds all known performance standards for such devices.

Electrochemical Gas Sensors are used to detect and monitor low levels of toxic gases and oxygen levels in Industrial situations where it is essential to ensure that the air is safe to breathe. The most common type of Electrochemical sensor is the 3-Electrode fuel Cell.

Typical Electrochemical Sensor Layout

The air being measured diffuses into the cell through the diffusion barrier (capillary) and gas permeable membrane, when it comes into contact with the sensing electrode, the toxic gas present in the sample undergoes an electrochemical reaction. In the case of Carbon Monoxide, for example, the reaction is:

CO + H2O => CO2 + 2H+ + 2e-

The Carbon Dioxide generated diffuses away into the air, whilst the positively charged hydrogen ions (H+) migrate into the electrolyte. The electrons generated charge the electrode but are removed as a small electric current by the external measuring circuit.

This oxidation reaction is balanced by a corresponding reduction reaction at the Counter Electrode:

O2 + 4H+ + 4e- => 2H2O

So at one electrode, water is consumed whilst electrons are generated, and at the other, the water is recreated and electrons are consumed. Neither reaction can occur if no carbon monoxide is present. By connecting the two electrodes, the small electric current generated between them is measured as directly proportional to the concentration of CO in  the air.

The Reference Electrode controls the whole process. It remains totally immersed in electrolyte. It sees no gas and is not allowed to pass any current. The Reference Electrode always remains at the same electrochemical potential (it's Rest-Air Potential. The sensing electrode is electrically connected to the reference electrode ensuring it's potential will not change even when it is oxidising gas.