November 2008

Wireless: ISA rules on symbols for instruments

There is not need for symbol changes or additions to accommodate wireless measurement or sensing, said ISA. The existing designations work.

ISA5 has its roots in the 1940s back in the formative year the organization. It is the standard that pertains to symbols that appear on drawings and plant floor plans.

With the advent of wireless instrumentation and with that technology becoming a viable and secure communication tool in the industrial sector, the ISA Standards and Practices Department is fielding numbers of inquiries regarding the need to have symbols specific to this technology and to have these new symbols as part of ISA 5.1.

Tom McAvinew, vice-president of the ISA Standards and Practices Department, reported the standard is currently undergoing revision.

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From the earliest ISA standard on the subject of symbols and identification, RP5.1, Instrumentation Flow Plan Symbols first published in 1949, through several revisions including the current ANSI/ISA5.1-1984 (Reaffirmed 1992), Instrumentation Symbols and Identification, it is clear the intent has always been to establish a system whereby devices are identified and symbolized by function rather than form. 

This means we can have device depictions and alpha character designations that are identical whether they are pneumatic or electronic, analog or digital-based with the distinguishing representation being the signal transmission symbol.

As for state-of-the-art wireless systems, the fundamental characteristic is obviously the signal-communication means-radio transmission.

Upon examination of the existing ISA 5.1 standard, there is a broken sine wave symbol in Section 6.2 defined as "Electromagnetic or Sonic Signal (Not Guided)" with a footnote including radio waves in the definition.

In addition, the current draft of the ISA 5.1 revision includes a "lightning bolt" symbol, which has been in use by some practitioners in one form or another in pre-"wireless" radio applications.

The upshot is there are two symbols available to represent the primary unique function of wireless instrumentation devices in automation systems.

No other symbols are required.

Nicholas Sheble (nsheble@isa.org) writes and edits Automation Update.