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"Disaster Prevention Safety and Nemoto-CO Sensor for Water Heater" (2000/08/09)

  • 2000.08.09
  • Nemoto's technology

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

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

 Everyone knows that gas accidents are disastrous.Gas accidents include gas explosion accidents and gas poisoning accidents. For gas explosion accidents, gas leak alarms are widespread and disaster prevention measures are taken.However, for gas poisoning accidents, the "CO gas sensor" that detects carbon monoxide (CO) gas, which is caused by incomplete combustion of gas and causes poisoning, has been inaccurate or expensive. , The spread of alarms has not progressed, and the delay in response has become a problem.

With an emphasis on this situation, the Japan Gas Association formed a working group in November 1995 with three city gas companies and seven sensors including Nemoto, and proceeded with the development of CO sensors for water heaters.Then, in March 11, it was clarified that the sensors manufactured by Yazaki Keiki Co., Ltd. and Nemoto made it meet the target specifications and cost.

After that, Tokyo Gas Co., Ltd., Osaka Gas Co., Ltd., and Toho Gas Co., Ltd. proceeded with research and development aiming at further cost reduction of sensors in order to expand the models equipped with CO sensors.The results were announced at the city gas symposium held on May 2000, 5, and Nemoto's contact combustion type CO gas sensor was considered to satisfy the demands in terms of performance, durability, and price. It is applicable not only for commercial use but also for commercial use, and it is expected that it will be widely used in the future.*1

What must be considered in the development of a CO sensor for a water heater is that when both city gas and propane gas are incompletely burned, not only CO gas but also hydrogen gas is generated at the same time, and the exhaust gas temperature is 60 depending on the hot water supply temperature. It was to fluctuate in the range of ~ 200 ℃, and to reduce the cost to a price that could be popularized.

Photo.CO sensor for water heater NAP-78A
(Size: 46 mm in the longitudinal direction of the mounting bracket)

Nemoto's development team challenged these difficult problems and proceeded with research, thinking that it would be possible to solve it by improving the catalyst of the contact combustion type gas sensor, which has already had a market record of more than 10 years.Then, in November 1998, we finally succeeded in developing the world's first CO sensor for water heaters as an inexpensive general-purpose type.

The features of Nemoto's CO sensor for water heaters are as follows.

  1. The detection concentration range is 0 to 3,000 ppm, and the gas sensitivity has good linearity as shown in Fig. 1, and can be detected accurately.
  2. The operating temperature range is -25 ° C to 260 ° C.
  3. Durability has been obtained as shown in Fig. 13,000 in the actual machine test for 2 hours, and it has sufficient performance to withstand practical use for 10 years.
  4. It has excellent resistance to sulfur and does not require regular heat cleaning.
  5. Due to the improved catalytic function, there is no need for a circuit to compensate for the effect of exhaust gas temperature.


Straightness Durability characteristics
Figure 1. CO sensor gas sensitivity characteristics Figure 2. CO sensor durability characteristics
(Hot water supply stopped for 20 minutes and repeated for 5 minutes, CO: 1,000ppm, H: 500ppm)


リ ン ナ イ
Photo. Rinnai water heater with CO sensor Yukko 16






Nemoto's CO sensor for water heaters has already been installed in water heaters of Rinnai Corporation and Takagi Sangyo Co., Ltd., and has begun to be put into practical use, but we expect that it will become widespread in the future.






Shanghai City City Gas Poisoning Accident Statistics

Figure 3. Statistics of Shanghai City Gas Poisoning Accidents





Shanghai City Fuel Management Office Celebration Weihua's Report*2According to the report, the outbreak of gas poisoning accidents in Shanghai from 1990 to April 1998 is as shown in Figure 4.In Shanghai, the number of gas-using households has increased by more than 3 a year in recent years, and the fact that poisoning accidents are increasing rapidly along with it has been clarified, and it is pointed out that countermeasures are urgently needed.Such a situation would be a problem for many developing cities.It is estimated that the demand for CO gas sensors to prevent gas poisoning accidents will be enormous in Asia alone in the future, and the future of Nemoto's CO sensors may be promising.

* 1 Abstracts of the Urban Gas Symposium (The Japan Gas Association), P107 (2000)
* 2 Shiro City Public Utility: Volume 13, 1999 Phase 1, P19 (1999.2)

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