Yoshihiko Murayama, Former Advisor, Nemoto & Co., Ltd.

* This article was serialized irregularly by Yoshihiko Murayama, a former advisor of our company.

　Early in the 21st century, the great cold waves that settled in the Japanese archipelago brought heavy snow from the Tohoku region to the Hokuriku region.
Then, an accident occurred in Kanazawa City where a man died of carbon monoxide (CO) poisoning in a light car buried in snow.According to the summary of the Ishikawa Prefecture Fire and Disaster Prevention Division, a total of 1 carbon monoxide poisoning accidents due to exhaust gas from private cars and the like had occurred in the prefecture by January 17, with 13 dead and 3 poisoning patients. Nine of them were children.In the case where two girls collapsed in the car, the morning edition of the 13th reported that the accident was a short time when the mother put the child in a car with an engine and went looking for a shovel for snow removal. There is.

Carbon monoxide poisoning is a deadly poisoning with a very high case fatality rate.Carbon monoxide is a gas generated when fuel etc. is incompletely burned due to lack of oxygen, and when breathing, it binds to hemoglobin in the blood in the lungs and loses the function of transporting oxygen of hemoglobin into the body, and as a result, first The brain nerve becomes oxygen deficient, causing deterioration of judgment ability and motor ability, resulting in death without taking evacuation behavior.

In the United States, there is a high risk of carbon monoxide poisoning because many households have fireplaces and cars inside the house, and in April 1992, the performance standard for carbon monoxide alarms was set to "UL standard".^*1The law requires the installation of alarms in ordinary households in each state or city. It is said that the spread of carbon monoxide alarms was promoted when a famous athlete caused a carbon monoxide poisoning accident about 7 years ago.Although unofficial information, it is said that 1996 million alarms were sold nationwide in 800.However, these alarms have a problem because a semiconductor type carbon monoxide sensor is used and its performance is not sufficient. About five years ago, one night in Chicago, hundreds of alarms issued false alarms all at once, and the UL standards were reviewed one after another in October 5 and October 1996. The alarm concentration standard was set very rigorously.As a result, it has become difficult for alarms using semiconductor-type carbon monoxide sensors to pass the certification.

On the other hand, in Europe, in response to the situation in the United States, in the examination of the establishment of the certification regulations for carbon monoxide alarms, stricter standards than the UL standards were stipulated, and in the spring of 2001, the "EN50291 standard" was also established.^*2It is expected to be promulgated as.

Nemoto's sensor development team has captured the global need for these reliable carbon monoxide alarms.
Worked on the development of a new carbon monoxide sensor that clears UL and EN standards.Then, we succeeded in developing a high-performance, highly reliable, and inexpensive "electrochemical carbon monoxide sensor" with a detection principle completely different from that of the semiconductor type.

The electrochemical gas sensor detects gas by extracting the energy generated by a chemical reaction (oxidation-reduction reaction) as electrical energy.The chemical reaction in the case of carbon monoxide is shown by the following chemical reaction catalyzed.

CO+H₂O-> CO₂ + 2H⁺ + 2e^-

　As shown in the operating principle diagram of FIG. 1, an oxidation reaction of carbon monoxide occurs at the detection electrode, and the hydrogen ions generated by the oxidation reaction react with oxygen in the air at the counter electrode to produce water. Generate.Since the current generated by this series of reactions corresponds to the gas concentration on the detection electrode side, the gas concentration can be detected by measuring this current.

 

 

Nemoto's electrochemical carbon monoxide sensor "NAP-701" has the following advantages over the conventional semiconductor and contact combustion type gas sensors.

1. Highly linear output characteristics can be obtained with respect to the gas concentration.
2. Excellent reproducibility.
3. Excellent gas selectivity.
4. Output fluctuation (drift) in the atmosphere is small, and stable output characteristics can be obtained.
5. Not affected by humidity.
6. Since it does not have a heating system, it consumes less electricity and enables a battery-powered gas alarm.
7. It is small and lightweight, and easy to install in portable devices.
8. Since it has no mechanically weak parts, it has high durability against vibration and shock.

 

As shown in Fig. 701, the gas sensitivity characteristics of the NAP-2 sensor have output characteristics that are linear with respect to the gas concentration, and can reliably detect gas in the low concentration region.

 

 

 

In addition, as shown in Fig. 3, the characteristics for humidity are almost unaffected by humidity, and even in a test placed in a high temperature and humidity of 50 ° C and 90% for one month, no abnormality occurs and the durability is high. Therefore, unlike a carbon monoxide alarm that uses a semiconductor sensor, there is no need to worry about the operation becoming unstable due to the influence of humidity.In general, electrochemical sensors that have stable performance in high humidity tend to have poor stability in low humidity, but NAP-701 has a low humidity by devising the structure. Above all, it maintains extremely stable characteristics.

 

　Nemoto plans to release the electrochemical carbon monoxide sensor NAP-701, and hopes that it will attract attention from all over the world.From now on, we hope that by improving production technology and further reducing costs, it will be widely used in the consumer sector, where demand is high, and will help prevent the dreaded carbon monoxide poisoning accident.

 

Note)
* 1 UL standard; “Underwriters Laboratories Inc. (UL)"(US Insurer Safety Testing Laboratory) is a product safety testing and certification organization in the United States, and refers to the standards established there. UL certification is globally recognized, and products that have been certified and bear the UL mark are trusted in each country.
* 2 EN standard = European Standard (EN); A unified European standard established jointly with "CEN (European Committee for Standardization)" and "CENELEC (European Committee for Electrotechnical Standardization)". EN50291 is under the jurisdiction of CENELEC.
(EN50291; Electrical apparatus for the detection of carbon monooxide in domestic premises – Test methods and performance requirements)

Copyright 1999 by Nemoto & Co., Ltd, Yoshihiko Murayama