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Getting Stronger Process Readings

 What do you think of when you see the term, “stronger” process readings? A reading is a reading, right? Well, yes, that’s true, but as Megan Wiens points out in the article, Getting Stronger Process Readings, in the December 2018 issue of Applied Automation, we should think of getting a reading as a series of steps, or as she puts it, a chain. Here are six steps she points out just to get a basic pressure reading, and the list simplifies the process in most cases:

  • The process fluid must have a path to reach the sensor.
  • The pressure sensor must be deflected in a measurable way in proportion to the fluid pressure against the atmosphere.
  • An electrical element must convert the deflection into an analog signal.
  • The transmitter must clean up the raw signal, scale it, and transmit it using a prescribed format, either analog or digital.
  • Wires, or perhaps radio, must carry the signal to the input/output (I/O) card of the automation system.
  • The automation system must convert the signal into appropriate engineering units for display and use the data in whatever equations it is performing for the larger control effort.

Those represent a lot of functions and they have to happen continuously in real-time. If anything, the electronic steps are much more complex than suggested here, but a Rosemount 3051S Pressure Transmitter handles them all in stride.

Naturally one of the potential weak links of the chain is the first bullet point because it is the one that depends most on the installation to be done properly.

For the sensor to be displaced, there must be a mechanism to allow the process fluid to press against it, which means there must be a process connection. The path to the sensor or isolator diaphragm must be clear and unimpeded. Any blockage can slow response or decrease accuracy. Some transmitters have the ability to determine when an impulse line is plugging and can alert operators of the problem forming. Slugs of gas in a liquid line, or vice-versa, can cause reading inaccuracy. Impulse lines should be clear and bled, although it can be beneficial to have some condensate in a steam pressure line.

 It’s important to remember how even the most sophisticated transmitter depends on the process connection. It can’t work without the connection, but a smart transmitter can at least indicate potential problems. This kind of intelligence has helped improve transmitters the most in recent years.

Today’s transmitters have additional capabilities such as self-diagnostics. While the main signal processing functions are going on, there also are internal functions evaluating the quality of power coming into the transmitter, the condition of the internal electronic components, sensor functionality, and other items, including plugged impulse lines as just mentioned. If a problem is present or developing, the diagnostics can send a warning via a digital wired or wireless protocol.

Whether you’re aware of it or not, there is a lot going on inside the transmitter, all designed to deliver the most accurate and reliable process variable possible. The article goes into more detail, so give it a full read. It gets into the ways everything has to interoperate, and even looks at improvements on the humble pressure gauge. It’s probably no surprise that Megan comes back to the importance of the instrument itself in the larger picture.

It’s important to know the weak links of a system, but building strong links is just as important. In many respects, having the best possible transmitter is the most important element. If the source of data is not reliable and accurate, the best infrastructure in the world won’t make it any better. The instrumentation available today from a variety of suppliers is sophisticated, stable, accurate, and reliable. If used in an environment capable of interacting with it fully, it can provide secondary variables and diagnostic information in addition to its primary variable.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Pressure Group and other specialty areas for suggestions and answers.

Posted by Deanna Johnson, Director Integrated Marketing Communications for Machine Automation Solutions