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Nemoto Technologies

Column No.9 Safety and Nemoto - (7) "Electrochemical CO gas sensor"
(April 1,2001)

Written by; Y. Murayama
Technical Advisor
Nemoto & Co., Ltd., Tokyo

During the winter of 2001, a cold wave hit the Northern Japanese Islands and brought a very heavy snow fall. In Kanazawa City a man was trapped in his car by the heavy snow and killed due to CO poisoning. According to statistics, 13 further CO poisoning accidents happened in Ishikawa Prefecture during the period from January 01 to 17, 2001, where 3 people were killed and 13 others were hospitalized. It was also reported that 9 out of the 13 were children. Newspapers on the 18th reported that two girls had fainted in a car during the short period that their mother had left the car looking for a snow shovel without shutting the engine off.

Carbon Monoxide poisoning has a very high mortality ratio. Carbon Monoxide gas is generated when a fossil fuel combustion is incomplete. When CO is inhaled, it combines with the hemoglobin of the blood and obstructs the hemoglobin's function of carrying oxygen to the cells in the body. As a result, first the brain suffers anoxia (loss of Oxygen), then loss of judgment and paralysis of the limbs occurs, soon after that death occurs.

As most American homes have fireplaces and attached garages, there is a high risk of CO poisoning accidents. In order to protect the masses, a UL standard*1 was issued in April 1992 covering residential CO gas detectors, and many city and state governments set up regulations based on this standard for compulsory installation of CO detectors in domestic premises.

The CO poisoning accident of a famous athlete about 7 years ago may have accelerated the spread of residential CO detectors in the U.S. It was said that 8 million units of CO detectors were sold in 1996. However these detectors were made with an older technology based on semiconductor CO sensors. These created some problems because of the insufficient sensitivity of the sensors used. The UL standard was revised in October 1996 and again in October 1997 to have higher quality standards. These revisions have made it difficult for semiconductor type sensors to pass the revised UL standard.

In Europe the quality standards of CO gas detectors have been discussed at length. Basing their results on the American experience, the new EN50291standard*2 which will be issued during Summer of 2001, is much stricter than the current UL standard.

The development team at Nemoto became aware of a global need for more accurate and reliable CO gas sensors, and started to develop a new sensor that could pass the revised UL and the new EN standards. The team succeeded in developing a new "Electrochemical" CO gas sensor that has a completely different detection principle from the semiconductor types.

This new sensor has a high performance, is highly reliable, and less expensive.

Electrochemical sensors detect gases by measuring the electric that is generated by chemical reactions (oxidation/reduction). The chemical reaction in the case of CO gas generation is :

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

fig.1:Detection principle of electrochemical sensor
Figure1. Detection principle of electrochemical sensor

As shown in Fig.1 (Detection principle), carbon monoxide is oxidized on the detection electrode, and hydrogen ions are generated. On the counter electrode, hydrogen ions (equal to the hydrogen ions on the detection electrode) react with oxygen from the air and generate water. The electricity generated by this reaction chain is proportional to gas concentration, thus the gas concentration can be determined by monitoring the generated electricity.


Photo:NAP-701
Photo. Electrochemical CO gas sensor"NAP-701"

Nemoto's new CO gas sensor "NAP-701" has the following features that the conventional semiconductor/catalytic type sensors do not have.

  1. Linear output proportional to gas concentrations.
  2. Excellent reproducibility.
  3. Excellent gas selectivity.
  4. Stabilized output characteristic with the least variation of output in air (drift).
  5. Hardly affected by humidity.
  6. No heater used, thus very low energy consumption, enabling battery operation.
  7. Small size and lightweight. Can be used in portable detectors.
  8. Mechanically tough. High durability against vibration and impact.
fig.2:Gas sensitivity of NAP-701
Figure 2. Gas sensitivity of NAP-701

The gas sensitivity characteristics of NAP-701 are shown in Fig.2. The liner output feature enables accurately detection of low levels of gas concentrations.



fig.3:Humidity dependency
Figure3. Humidity dependency of NAP-701

As shown in Fig.3 (Humidity dependency), this sensor is hardly affected by humidity. Even after storage for a month under 50°C/90% R.H., no defects have been found. Contrary to semiconductor type sensors, users do not have to be concerned about instability under highly humid conditions. Generally speaking, electrochemical sensors that are stable under highly humid conditions would be unstable under low humid conditions, however, NAP-701 has a specially designed structure to maintain the stable functions also under low humidity conditions.


This new electrochemical CO gas sensor is now available for sale, and has generated interest from users all over the world. It is hoped that further price reductions will be possible once production capacity is increased.



Remarks:

*1 UL Standard; "Underwriters Laboratories Inc.(UL)"

*2 EN Standard; European Standard (EN) = Unified standard for EC countries ruled in conjunction with CEN (European standardization committee) and CENELEC (European electrical standardization committee). EN50291 is controlled by CENELEC.
(EN50291: Electrical apparatus for the detection of carbon monoxide in domestic premises - Test methods and performance requirements)

Copyright 2001 by Nemoto & Co.,Ltd , Yoshihiko Murayama, Hiro Yamamoto.

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