Controls & Sensors: Vapor Sensing
January 30, 2009
Residential gas water heaters have long been considered safe when properly used. Yet the potential dangers resulting from the improper use and storage of gasoline and other flammable materials in the vicinity of the water heater have continued to be a concern among manufacturers of residential gas water heaters.
Due to the unique operation cycle of the power-vent water heater in which the blower is activated for up to 60 seconds at the beginning of every heating cycle as a system purge, flammable vapors may be drawn inside the water heater prior to ignition of the burner.
In 1995, the Water Heater Industry Joint Research and Development Consortium (WHIJRDC) was formed to determine whether a residential gas water heater could be made more resistant to accidents caused by mishandling or improper storage of gasoline or other flammable materials.
As a result, new Flammable Vapor Ignition Resistance (FVIR) requirements were developed, and the existing American National Standards Institute (ANSI) gas water heater standard was revised. By July 2006, all power-vent water heaters produced in the U.S. and Canada were required to comply with the new FVIR standard.
Given the potential for flammable vapor ignition inside the water heater, the Consortium recognized a need for a technology solution that senses the presence of flammable vapors and prevents the blower purge cycle from starting, thereby preventing the operation of the gas valve and ignition of the burner.
A new standardEmerson’s Therm-O-Disc business unit, a manufacturer of sensors, controls and electrical switches, was asked by the Consortium for a proposal to address this new flammable vapor sensing need and the company became a key contributor to the development of a flammable vapor sensor for the water heater OEMs’ power vent FVIR system.
Therm-O-Disc took on the challenge of developing a sensor that would detect the presence of a specific concentration of flammable vapors in less than a minute after a spill, and then would enable the control module to prevent the supply of gas and burner ignition - thereby preventing the ignition of vapors both inside and outside the water heater.
Over several years, Therm-O-Disc engineers worked to ensure that the sensor in the emerging FVIR power vent water heater design met the new standard. Specifically, this sensor was studied against four water heater test protocols in a specially designed and CSA-certified Flammable Vapor (FV) test chamber to evaluate product performance under harsh operating conditions dictated by the new standard. Also, along with the water heater OEMs, Therm-O-Disc was an active member of the Gas Appliance Manufacturers Association technical group that worked in conjunction with ANSI and CSA to develop the Flammable Vapor Sensors and Systems standard, ANSI Z21.94/CSA 6.31.
The technologyThis involvement led to the development of the 25VS flammable vapor sensor to detect vapors released during a typical gasoline spill. (See Fig. 1.) The sensor offers long life and good sensitivity to most gasoline components within a wide operating temperature range of 0 DegC to 65DegC. Additionally, the 25VS is certified by the CSA to the aforementioned ANSI standard and meets all requirements developed by the members of the Water Heater Industry Consortium. (See Fig. 2.)
The sensor technology is comprised of three primary components: a sensor film, an insert-molded base, and a molded cover. The sensor film is a proprietary mixture of silicone and carbon that is deposited on the insert-molded base and protected by a thermoplastic cover that is ultrasonically welded to the base. The custom-designed silicone system provides a robust foundation and support structure for the carbon. The carbon, which acts as a conductor, is dispersed throughout the silicone such that, in the absence of gasoline vapor, the carbon particles are in solid electrical contact. When the silicone encounters gasoline vapor, it expands, pulling some of the carbon particles apart and increasing the resistance of the sensor. The magnitude of the increase in resistance depends upon the gasoline vapor concentration - higher concentrations cause increased expansion of the silicone film, which results in a higher sensor resistance and vice versa.
Design considerationsThe 25VS flammable vapor sensor yields several unique benefits when compared with other vapor sensing technologies. Unlike active sensors that typically utilize metal-oxide or infrared technologies, the 25VS does not require an external power source to detect flammable vapors, only the amount of power used by the application’s control circuit to read the sensor’s resistance. This feature provides design flexibility and contributes to the efficient operation of the water heater control circuit to which it is connected.
Also, the 25VS delivers good sensitivity for a broad range of gasoline components, while being relatively insensitive to vapors from most household cleaners. Another benefit is that the 25VS has been designed to provide a long operational life in the application, in some cases up to 15 years depending on operating conditions and other factors.
The polymer-based 25VS was designed to accommodate a normal range of ambient conditions. However, the 25VS is moderately sensitive to ambient temperature, as such resistance does increase with temperature, but the water heater manufacturers have been able to accommodate this characteristic in their FVIR system designs.
New platformEncouraged by the successful launch of the 25VS, Therm-O-Disc has initiated a technology development program to expand the use of this technology to other applications and markets. Engineers are focusing on three areas that could help facilitate an expanded product offering: a more standardized design, significantly reduced temperature sensitivity, and the capability to sense a broader range of vapors in addition to gasoline. Fig. 3 shows one product design approach being explored for the next-generation flammable vapor sensor (NG FVS).
The next-generation design uses printed circuit board technology that allows the use of industry standard connectors. The new sensor also provides a temperature-compensated output that does not require further conditioning by the OEM’s system controller, making it easier to integrate into new applications.
The chart in Fig. 4 compares the sensor output for the 25VS against the temperature compensated NG FVS. Note that the 25VS output increases with temperature, while the NG FVS shows minimal change in signal output.
In addition, engineers are testing the capability of the vapor sensor technology to sense a broader range of vapors beyond gasoline, such as other hydrocarbons, flammable refrigerants, and toxic gases. Initial results are very promising and research and development work continues.
Built-in flexibilityThe new sensor, like the 25VS, is a passive device that continually senses for vapors. Its platform design features temperature compensation, which may enable applications outside the power-vent water heater market. The sensor’s common substrate allows for application of a variety of polymer systems for expanded vapor-sensing capabilities and application-specific packaging. Lastly, its compatible signal can be readily adapted as an input by any electronic control device using standard technology.
With the more flexible temperature-compensated platform design, the new sensor technology, along with continued research in this area, will lead to the development of a wide variety of unique vapor sensor products for a broad range of applications. In addition, Therm-O-Disc has vapor-sensor characterization capabilities over a wide range of temperature and environmental conditions. Testing capabilities also include chemical exposure, RGT, HALT, vibration and drop-testing that OEMs may use to support their application-specific design and testing.
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