September 5, 2008
Choosing a personal gas monitor for your employees? Consider a few good points. To ensure the best gas detection per dollar invested, consider three basic criteria: reliability, ease of use, and durability, or “RED”, before buying. All features and benefits should fulfill at least one of these criteria.
Gas detectors are potentially life-saving devices and should be reliable and trustworthy. Your gas detector should perform two main functions:
- detection of the gas or vapor being monitored, and
- providing an alarm to alert the user.
Another newer feature is a manual call point alarm (MCP), which allows users to manually activate alarms, alerting those around them to potentially hazardous situations. These new options provide additional value, helping to keep workers safe.
Critical seconds are lost when gas detectors equipped with small and sometimes cluttered displays offer confusing information. A display should be clear, readable and unambiguous at a glance within many environmental conditions. Check for detector stability when units are powered on within fresh air, or when conditions change from air-conditioned to hot and humid environments. False alarms are not only inefficient (resulting in downtime), but also annoying, and may result in workers eventually ignoring perceived false alarms when potentially hazardous situations are at hand.
Make sure that visual alarm LEDs are bright, clear, and visible from most angles. Audible alarms should be distinct within loud and noisy areas. Depending on where worn, a minimum requirement of 90-95 dB at 1ft is often adequate for industrial environments; however, if your environment requires hearing protection, consider gas detectors offering vibrating alarms.
Cold temperatures may affect battery run time as well as pump and sensor performance. Sampling distances for built-in pumps, run time and sensor response often do not match published data when units are subjected to extreme conditions. Make sure to choose a reputable manufacturer that will stand by instrument performance claims.
Ease of use
Using portable gas detectors should never be a chore for your personnel. If users don’t understand use of gas detectors due to complexity or operational difficulty, the detectors may not be used at all. Look for instruments with ease-of-use design, interaction, and maintenance, with minimal required user training.
Ergonomic design, intuitive menu-driven displays, and large, tactile buttons are critical for easy operation with gloved hands.
Accessory choices should include carrying attachments, multi-unit chargers, and automated calibration systems for easier field operation, record keeping and maintenance.
Unnecessary or impractical features such as small operating buttons will likely increase chances for human and instrument error as well as increase service and repair costs. Gas detector design should be simple and easily adaptable to many work environments, and backed by knowledgeable technical support and service.
Even under the most extreme conditions, gas detectors should do what they are designed to do: detect and alarm. Cold, hot, dusty, humid, and corrosive environments are just a few examples of conditions under which gas monitors should operate properly. If harsh conditions are a concern, make sure that your choice of gas detector can withstand severe circumstances reliably.
Radio frequency interference (RFI) and electromagnetic interference (EMI) can cause gas detectors to display false readings and sound false alarms when operating near two-way radios. Instruments should minimize RFI and EMI effects to produce the most accurate results.
A high ingress protection (IP) rating against dust and water is critical for outdoor use, as detectors may be forgotten on-site and not retrieved for several days. IP ratings are not agency-regulated, allowing manufacturers to self-certify for dust and water ingress; ask for third-party testing certification to ensure robust design.
As units are often handled roughly in the field, it’s critical that units can withstand such use as well as accidental drops. Some manufacturers drop-test their instruments on concrete from six feet high or more to indicate fundamental durability. You get what you pay for; rugged designs are typically slightly costlier than flimsy ones. Look for manufacturers offering back-to-back warranties and optional extended warranties to keep your calibration, service, repair, and overall cost of ownership under control.
Think RED for effective personal gas detector design; reliability, ease of use, and durability. Make sure to weigh and rank options when choosing your next gas detector, as applications vary widely. Select the instrument that best fulfills your application’s requirements and conditions by understanding which features are really necessary. A sub-standard gas detector can be an expensive investment and ultimately, a liability as well.
SIDEBAR: HART Protocol Practical digital communication tool for analog installationsBy Rebecca Schulz and Leslie Mitchell
Communications protocols are defined as computing standards that control connections and data transmissions. Technologies range from copper wires to infrared light to digital streams.
As with any good tool, the most well thought-out protocol is impractical without information delivery, correct implementation, and the means to retrieve discarded data. Three network protocol design principles have been developed to maximize communication protocol usefulness and consistency: effectiveness, reliability, and resiliency.
Communications protocol effectiveness is achieved through layering; sectioning directives into smaller related functions or sub-tasks that send and receive information via pathways to layers above and below. “Software architecture” arose from the need for protocol layering consistency, also known as reference models.
Some common reference models include the Simple Mail Transfer Protocol (SMTP) used as an email protocol, the TCP/IP protocol suite (Transmission Control Protocol and Internet Protocol) used for Internet communications and for many commercial networks and the Open Systems Interconnection (OSI) model, developed by the American National Standards Institute (ANSI) in 1977.
Since it’s assumed that data transmission errors may occur, transmission reliability involves both the quantity of lost or discarded data as well as the means to detect and correct errors. This process uses data summary packets, which are included along with larger transmissions to detect omissions, corruptions, and degradation of data, and then resend this information. High overall transmission performance is achieved primarily through fiber network connections.
Resiliency concerns network failure, where communication links degrade significantly or crash altogether. Links are tested often and sometimes rerouted when failure is detected.
Digital protocols in industry
Industrial automation protocols present their own challenges. 4-20 mA analog current loop wiring systems have been used for industrial process control instruments since the 1950s and are still commonplace today. Industry found its implementation to be practical and cost-efficient, as its noise rejection ability allows cables to run over relatively long lengths. Analog current loops use conductors in pairs to monitor or operate automation devices remotely.
Fieldbus all-digital industrial network systems were widely implemented in the late 1990s for process control. Fieldbus technology is an open system and designs are not necessarily interchangeable. Fieldbus technology’s advantages over 4-20mA analog systems, such as transmission speed, and use of both multiple analog and digital points at times was offset by its complexity, as it required additional user training, more costly system components, and lacked compatibility among fieldbus vendors.
Addressing the analog/digital gap
HART (Highway Addressable Remote Transducer) Communications Protocol grew out of fieldbus as a practical way to bridge the analog/digital gap. HART communicates by sharing wires used by legacy 4-20 mA analog installed systems. As a result, HART Protocol is used widely, as many 4-20 mA systems continue to function on a global scale.
HART was developed in the mid-1980s as a proprietary protocol, but in 1986 was released as an open design. There are two main operational modes of HART instruments:
- analog/digital, the 4-20 mA loop current holds overlayed signals and allows for only one instrument for each cable signal pair.
- multidrop, which uses only digital signals, as the analog loop current is fixed at 4 mA. Multidrop mode allows for up to 15 instruments per signal cable.
The core of HART’s worldwide acceptance is based upon its practicality; simultaneous analog and digital compatibility that makes use of existing wired systems. Additional advantages factor in as well, such as HART-smart devices that assist with operational efficiency, and system problem-detection capability. HART’s advanced diagnostics help to increase safety integrity levels (SIL). HART provides cost-effective communication technology for many process automation control systems, as many device types are available and its usage training requirements are minimal.
HART 7 Specification
HART 7.0 Specification was released in September 2007; users must upgrade to HART 7.0 to use HART-enabled devices. Retrofit adapters are available for existing 4-20mA HART devices. HART 7.0 Specification enhances communication through new key features including:
- expansion of Manufacturer ID Codes from 8-bits to 16-bits.
- Time Stamping of process variable values at the Field Device.
- New Common Practice Commands support set-up and operation of I/O Systems, expedite Device Configuration upload, and setting the time of day, among others.
- burst mode communication allowing for bursting of multiple HART messages
- improved time interval/frequency specificity.
For much more information visit the HART Communication Foundation at www.hartcomm2.org/.
Rebecca Schulz is a product line manager for industrial, oil, gas, and petrochemical gas detection technologies and has been with MSA (www.msanet.com) for two years.
Leslie Mitchell is a marketing/technical writer for MSA.