Category: General
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18.8 Instrument turndown
An important performance parameter for transmitter instruments is something often referred to as turndown or rangedown. “Turndown” is defined as the ratio of maximum allowable span to the minimum allowable span for a particular instrument. Suppose a pressure transmitter has a maximum calibration range of 0 to 300 pounds per square inch (PSI), and a turndown of 20:1.…
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18.7 Calibration Procedures in Linear, Non-Linear and Discrete Instruments
As described earlier in this chapter, calibration refers to the adjustment of an instrument so its output accurately corresponds to its input throughout a specified range. The only way we can know that an instrument’s output accurately corresponds to its input over a continuous range is to subject that instrument to known input values while measuring the…
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18.6 An analogy for calibration versus ranging
The concepts of calibration (trimming) and ranging are often difficult for new students of instrumentation to immediately grasp. A simple analogy useful for understanding these topics is that of setting a digital alarm clock. Suppose you purchase a digital alarm clock to wake you up at 7:00 AM in the morning so that you can get to school on…
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18.5 LRV and URV settings, digital trim (digital transmitters)
The advent of “smart” field instruments containing microprocessors has been a great advance for industrial instrumentation. These devices have built-in diagnostic ability, greater accuracy (due to digital compensation of sensor nonlinearities), and the ability to communicate digitally3 with host devices for reporting of various parameters. A simplified block diagram of a “smart” pressure transmitter looks something…
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18.4 Transmitter Damping Adjustments
The vast majority of modern process transmitters (both analog and digital) come equipped with a feature known as damping. This feature is essentially a low-pass filter function placed in-line with the signal, reducing the amount of process “noise” reported by the transmitter. Imagine a pressure transmitter sensing water pressure at the outlet of a large pump.…
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18.3 Calibration errors and testing
The efficient identification and correction of instrument calibration errors is an important function for instrument technicians. For some technicians – particularly those working in industries where calibration accuracy is mandated by law – the task of routine calibration consumes most of their working time. For other technicians calibration may be an occasional task, but nevertheless…
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Chapter 18 Basic Principles of Instrument Calibration and Ranging
Every instrument has at least one input and one output. For a pressure sensor, the input would be some fluid pressure and the output would (most likely) be an electronic signal. For a loop indicator, the input would be a 4-20 mA current signal and the output would be a human-readable display. For a variable-speed motor drive, the…
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17.2 WirelessHART Radio Communication Standard
An exciting development in industrial instrumentation is the WirelessHART radio communication standard, specifically designed for field instrument use (e.g. transmitters, valve positioners) as opposed to general data communication. The IEC (International Electrotechnical Commission) has codified the WirelessHART standard as IEC 62591. 17.2.1 Introduction to WirelessHART WirelessHART is a subset of the HART industrial instrument communication standard as of version…
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17.1 Basic Components Common to all Radio Systems
“Radio” systems use electromagnetic fields to communicate information over long distances through open space. This section explores some of the basic components common to all radio systems, as well as the mathematical analyses necessary to predict the performance of radio communication. 17.1.1 Antennas A radio wave is a form of electromagnetic radiation, comprised of oscillating electric and magnetic…
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16.6 H1 FOUNDATION Fieldbus segment troubleshooting
Feedback obtained from industrial users of FF reveal a common pattern: Fieldbus is a powerful and reliable technology, but only if it is properly installed. Poor installations, usually driven by a desire to minimize capital expenses, will cause numerous problems during commissioning and operation. One relatively easy way to avoid problems caused by short-circuits in…
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16.5 H1 FOUNDATION Fieldbus device configuration and commissioning
Fieldbus devices require far more attention in their initial setup and commissioning than their analog counterparts. Unlike an analog transmitter, for example, where the only “configuration” settings are its zero and span calibration adjustments, a FF transmitter has a substantial number of parameters describing its behavior. Some of these parameters must be set by the…
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16.4 FF function blocks
Data-processing modules within FF systems are known as function blocks. Sometimes these blocks serve merely to catalogue data, while in other instances the blocks execute specific algorithms useful for process measurement and control. These “blocks” are not physical entities, but rather abstract software objects – they exist only as bits of data and instructions in computer…
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16.3 H1 FOUNDATION Fieldbus Data Link layer
Like so many other industrial data networks, FOUNDATION Fieldbus is an “unswitched” or “broadcast” type of network. This means all data transmissions by all devices on a network are sensed by all the other devices. In other words, there are no private messages between two devices on a shared network: every device “hears” every transmission…
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16.2 H1 FOUNDATION Fieldbus Physical layer
Layer 1 of the OSI Reference Model is where we define the “physical” elements of a digital data network. The H1 FF network exhibits the following properties: Two-wire (ungrounded) network cable 100 ohm (typical) characteristic impedance DC power is conveyed over the same two wires as digital data 31.25 kbps data rate Differential voltage signaling…
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Chapter 16 FOUNDATION Fieldbus instrumentation
FOUNDATION Fieldbus is a standard for digital field instrumentation enabling field instruments to not only communicate with each other digitally, but also to execute all continuous control algorithms (such as PID, ratio control, cascade control, feedforward control, etc.) traditionally implemented in dedicated control devices. In essence, FOUNDATION Fieldbus extends the general concept of a distributed control…