• NEMOTO HOME
• Company Profile
• Products & Services
• Web Showroom
• Technologies

Column No.4 Safety and Nemoto - (2) "Gas Leakage Detectors"
(January 27, 2000)

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

On August 16, 1980, 213 people were killed when a large gas explosion occurred in the underground shopping mall in front of the Shizuoka railway station (180 kilometers west of Tokyo). Descriptions such as, "A Pillar of Fire", "People Explode", "Station Entrance Resembles Battlefield" appeared in newspaper titles that evening. The exact cause of this disaster was never officially determined, yet in February of the following year, The Ministry of International Trade and Industry required the installation of gas leakage detectors in underground shopping malls and basements. As a result of this incident, public awareness regarding gas leakage and the need for gas leakage detectors was created.

A residential LP gas detector using a semiconductor type gas sensor had appeared on the market as early as August of 1969. In 1970, The High Pressure Gas safety Institute of Japan was formed and had begun inspection of residential LP gas detectors.

Semiconductor type LP gas detectors measure the change in electric resistance of an n-type semiconductor as LP gas molecules are absorbed on the surface of the semiconductor element. These detectors were not popular because other gases also affected the semiconductor, causing false alarms to occur with exposure to fumes from alcohol and other substances. Capitalizing on this situation, Nemoto began researching methods to make a sensor that would avoid these problems.

Nemoto became convinced that catalytic type gas sensors were the best as this type of sensor's output was directly proportionate to the concentration of gas. More importantly, catalytic type sensors had superior gas selectivity minimized the sensitivity to 'noise' gases. Nemoto began development work on a new catalytic type sensor in August 1978. After many tests on innovative catalysts and sensing materials, a highly efficient gas sensor was developed. A catalytic type gas sensor for municipal gas was made public in May 1979, and a sensor for LG gas in May 1980.

Domestic detector manufacturers were interested but hesitated to switch to Nemoto's catalytic type sensors because no data existed which could guarantee the life of the product. To obtain supporting evidence, Nemoto produced a gas detector using the new sensor and applied for UL approval. This detector received UL approval in February 1980. The number of gas detectors manufactured increased greatly after the Shizuoka explosion. Nemoto's gas sensor was developed just in time to capitalize on this growth.

Nemoto's catalytic type gas sensor has a sensing element as shown in the figure on the right. When a combustible gas contacts the surface of the element, the gas is oxidized by the action of the catalyst on the element surface, and the combustion heat heats up the core platinum filament. Since platinum's electric resistance is directly proportional to it's temperature, Nemoto's catalytic type sensors can not only detect the presence of a gas, but also measure its concentration.

The following are the features of Nemoto catalytic type sensors.

1. Linear outputs in the range of 0 - 100% LEL (Lower Explosive Limit)
2. Very fast response
3. Hardly affected by ambient temperature and humidity
4. Excellent long-term stability
5. Excellent reproducibility
6. The minimum detectable gas concentration is as low as 50 ppm (varies depending on gas type)
   

Twenty years have passed since Nemoto's catalytic type sensors were developed. Currently, Nemoto sensors are widely used in gas leakage detectors, densitometers, and other measuring devices. Nemoto sensors have been well accepted by customers and have the highest reputation. Nemoto continues to develop a variety of new sensors to meet the requirements of its customers. During the past twenty years, Nemoto has manufactured over 14 million gas sensors. Nemoto will continue to develop new sensors that answer to market needs and contribute to the protection of human lives and property.

Nemoto's catalytic type sensors

Objective gases	Model	Features	Applications
* Combustible gases Methane Propane Butane Hydrogen Alcohol	NAP-2A	High performance, Standard type	Municipal gas leakagedetectors Densitometers
	NAP-55A	Small-sized, Lower power consumption
	NAP-50A	Small-sized, Lower power consumption, Less sensitive to alcohol
	NAP-3A	High performance, Standard type	LP gas leakagedetectors Densitometers
	NAP-66A	Small-sized, Lower power consumption
	NAP-100AD	Quick response, Heavy duty	Industrial gas detectors
	NAP-100AH	Quick response, Heavy duty, High selectivity	Detectors for fuel cells
*Flu gas, Incompletecombustion CO gas Hydrogen gas	NAP-70A	Quick response, High selectivity, Less sensitive to alcohol, Lower power consumption	CO detectors
	NAP-78A	Quick response, Heavy duty, High accuracy	For gas boilers
	NAP-78R	Quick response, Heavy duty, High accuracy

Reference Information:
Relationship of the installation ratio of LP gas leakage detectors to the number of gas explosion accidents. (Courtesy of Japan Gas Detectors Manufacturers Association)

 

Copyright 2000 by Nemoto & Co.,Ltd , Yoshihiko Murayama, Hiro Yamamoto.