PROCESS INDUSTRY JARGON RECAP 4-2

"Handle them carefully, for words have more power than atom bombs." -Pearl Strachan Hurd

PTOA Segment #99: Instrument Tech Must-Knows: Measurement Accuracy

Accuracy (of a detecting and measuring instrument): The capability of the device to consistently yield a true and accurate value of the detected and measured process variable.

Calibration (of a detecting, measuring or transmitting instrument): A unique procedure that must be followed to return an instrument to the status that makes it possible to detect, measure, or transmit the true value of a process variable.

(Total Instrument-Related Measurement) Error: The individual contributions of measurement error that originate from the following instrument-related sources:

The "inherent instrument measurement error" which will be determined by the manufacturer of the instrument under specified conditions.
The instrument's "measurement response lag."
The instrument's "reliability."

Manufacturer's Instrument Instruction Pamphlet/ap: The document or accessible software information that is written and provided by the manufacturer of any instrument and which answers all the FAQs about the instrument, including the calibration procedure.

Measurement Response Lag: The time interval between the real-world change in a process variable and the time that the process variable sensor can detect the change.

For example, PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order expertly understand that the time required for conductive heat transfer to occur over a physical boundary would contribute to instrument measurement response lag.

Repeatability (of an instrument): The ability of an instrument to yield the same measured value over and over again with successive measurements.

PTOA Segment #100: Instrument Tech Must-Knows: Measurement Error

Electrical Interference: A potential contributor to online measurement error that originates from electromagnetic forces being insufficiently isolated from each other.

Functionality of the Instrument: A contributing factor to the reliability of an instrument which is likewise a subset of the instrument-related instrument error.

Determining the functionality of the instrument requires pre-installation consideration of how the instrument will be used and its range of operation, accuracy, and power requirements.

Improper Manhandling (of an instrument): A potential source of online measurement error that originates from a human being mishandling the instrument.

For example, over-pressuring a Pressure Indicator (PI) results in inaccurate pressure readings and the PI must be replaced.

Instrument-Related Measurement Error: The sum total of measurement error caused by adding the contributing factors of Inherent instrument measurement error, instrument response measurement lag, and instrument reliability.

Inherent Instrument Measurement Error: A contributing source of instrument-related measurement error usually stated by the manufacturer as a "percent of span" or may be stated as how many units of variance can be expected over the range of the instrument.

Instrument Response Measurement Lag: A contributing source to the instrument-related measurement error which is defined as the amount of time between a changing process variable and the moment that the instrument sensor can actually sense the change.

Instrument Reliability: A contributing source to the instrument-related measurement error which is defined as the tendency of the instrument to continue operating and not malfunction and which will be minimized when pre-installation consideration is given to the functionality of the instrument, its physical limitations, and the physical environment that will surround the instrument.

Instrument Wear and Tear: The gradual degradation of an instrument over time that can contribute to online measurement error.

Miscalibration: A potentially contributing factor to online measurement error caused by a human being ... usually an Instrumentation Technician ... who unknowingly fails in his/her attempt to return an instrument to the status of being capable of returning the true and accurate measured process variable value.

Misreading (an Instrument): A potentially contributing factor to online measurement error caused by a human being ... usually the Process Operator ... that results in believing an inaccurate understanding of the measured process variable.

(Electrical) Noise: A potential contributor to online measurement error that is from undesirable electrical signals that distort or interfere with the originally desired electrical signal. Noise can be intermittent and therefore frustratingly hard to detect and eliminate.

Online Measurement Error: The sum total of an instrument's measurement error which are typically NOT quantified but which are caused by miscalibration, misreading an instrument, instrument wear and tear, and electrical interference.

Percent of Span: A phrase that is used to define an instrument's inherent-instrument error in terms of a percentage of the range of measurement; this range of inherent-instrument error is determined by the instrument's manufacturer under specified conditions.

Physical limitations (of the instrument): A potentially contributing factor to an instrument's reliability (which is a subset of instrument-related measurement error) and which can be minimized if pre-installation consideration is given to the instrument's dimensions, how the instrument will be mounted and connections to the instrument, etc. physical limitations of the instrument are particularly important when the instrument being installed is a retrofit into an existing housing, etc.

Physical environment that will surround the instrument: A potentially contributing factor to an instrument's reliability (which is a subset of instrument-related measurement error) and which can be minimized if pre-installation consideration is given to how well the instrument's materials of construction mesh with installation environment, whether or not the instrument needs to be encased or isolated, what type of NEMA ratings are required for the instrument to prevent sparking, etc.

Reliability (of an instrument): A contributing factor to the Instrument-Related Measurement error which is the instrument's ability to operate continuously and not malfunction and can be minimized by giving consideration to the instrument's functionality, physical limitations, and physical environment.

Span (of an Instrument): The accurate range of measurement for an instrument which is determined by the manufacturer.

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