How Can We Install Wireless Transmitters in Hazardous Areas?

Readers help a reader solve this control problem. Next month: Can We Use IDC Connectors?

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A Reader Writes:

Solutions:

It's the Ratings

www.ul.com/hazloc

One Method for Div. 1

www.microwavedata.com

February's Problem

Can We Use IDC Connectors?

Our I/O and terminal block vendor is pushing insulation displacement connection (IDC) technology. It looks slick but we're not sure how it will hold up in the plant. Does anybody have experience with IDC terminations in chemical plant environments? Is there any place they should be avoided? What IDC characteristics should we be looking for?

pstudebaker@putman.net.

Please send us your comments, suggestions, or solutions to this problem. Solutions will be published in our February issue, but we need your responses by January 6 to meet our editorial production deadlines. If you have a solution to this problem, or a problem of your own that you would like to pose to other colleagues, mail, fax, or e-mail it to CONTROL Problem Solving Ideas, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143; Fax 630/467-1124; e-mail

To install wireless transmitters in a hazardous area, you will need to purchase a wireless transmitter that is certified for a hazardous area. For a complete solution, the wiring and electrical connections for power, radio transmission and transmitters must also be approved for a hazardous location.

...To date the only wireless transmitter that is Class 1, Div. 1 (Group C and D) certified is manufactured by zed.i solutions. zed.i's product, the Smart-Alek System, is specific to gas volume measurement (for more information, see www.smart-alek.com). The device measures static pressure, differential pressure, and temperature as gas flows through an orifice plate. The field data is processed on-board, then transmitted over public telecommunication networks (satellite or cell phone) to a secure central data repository where it can be accessed via the Internet.

...In the process of approval for Class I, Div. 1 locations, zed.i had to submit both the components and the complete installation. In addition, the zed.i system architecture is easily customized to work with most existing automated measurement systems to create a seamless, integrated solution.

Toku Ito, P.Eng., Chief Technology Officer

zed.i solutions, www.zedisolutions.com

Power Is Key

There is a major obstacle to installing a wireless, level-measuring sensor in a hazardous environment. While there are numerous level instruments that are either explosion-proof or intrinsically safe, they require either 24 VDC or 120 VAC to operate. The selection of a sensor is important, but the transmitter (either radio or cellular) must also meet hazardous approvals and the sensor and transmitter need to be battery-powered.

...The first system that we designed used a solar panel and storage batteries mounted in explosion-proof housings with the appropriate barrier. This system powered a 7100 Series magnetostrictive level sensor with an intrinsically safe Class I, Div. 1 approval. While the system worked, it was expensive and had various maintenance problems that related to the solar panel and RF transmitter.

...We decided that this was not a practical solution, and designed a sensor that could be powered by conventional batteries. Once we had the level sensor available, we contacted a variety of companies that were developing 900 MHz broad-spectrum transmitters to establish an alliance. The need was for a transmitter that could provide power for both the radio and level sensor using a conventional battery.

...Our first products were developed for Xsilogy and Signet Scientific, but the system did not have hazardous approvals. Since then, we have developed level sensors for Innovative Sensor Solutions, World Telemetry (subsidiary of LPG Central), and SupplyNet (division of Alert Technology).

...The development of wireless instruments has been limited by the power requirements of the level sensors that are currently available. Obviously, battery life became an issue, and this meant that not only the power consumption of the radio and level sensor were involved, but update frequency had to be considered. For this reason, most wireless applications focus on inventory monitoring that does not need continual updates. With updates every 5 min., new lithium ion batteries will last between two and three years before they need to be replaced.

...While radar and ultrasonic instruments are very popular, their power requirements make it impractical to use them with standard-size batteries for distances greater than 6 ft. In addition, the filtering software used by these technologies to eliminate false signals is not suitable for the low power budget available in a wireless application, unless a solar panel is used. For that reason, the ultrasonic unit available has a recommended range of 6 ft. and the TDR radar is only about twice that.

...In large tanks, either pressure sensors or magnetostrictive probes must be used. When the application requires high accuracy for either distribution or inventory management, magnetostrictive probes offer accuracies of 0.1% of full scale, without calibration.

...There are a few companies that offer wireless level instrumentation with hazardous approvals. Barton has a TDR guided radar sensor and RobertShaw offers both a pressure transducer and ultrasonic unit that meet some type of hazardous approval. The Barton TDR and RobertShaw ultrasonic have limited ranges, while pressure sensors can be used in larger tanks. The accuracy and dependability of these sensors is also limited.

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