I can't get you out of my mind
I can't get you out of my mind
Can't get you out of my mind

("Can't Get You Out of My Mind, " by Aqualung, 2008)

 

PTOA SEGMENT #99: INSTRUMENT TECH MUST-KNOWS: MEASUREMENT ACCURACY

"Instrument Techie" PTOA Readers and Students must understand the Accuracy, Repeatability, and Reliability of each instrument that is used to measure any process variable.

 

Using the example of a bulls eye target, PTOA Segment #99 focused on defining and distinguishing a measuring instrument's "Accuracy" from its "Repeatability."

"Instrument Techie" PTOA Readers and Students learned that an instrument can exhibit the ability to repeat a measurement, yet still not be accurate.

 

 

The clarification between measurement "Accuracy" and measurement "Repeatability" conveniently provided the teaching moment that illustrated the use and need of "instrument re-calibration" ... the process and procedures used to restore an instrument's capacity to measure accurately as well as in a repeatable manner.

"Instrument Techie" PTOA Readers and Students were informed that the calibration procedures for each type of instrument are specific and will be found in the manufacturer's instrument instruction pamphlet/ap.

The Dead Weight Tester was introduced as the device that is used to calibrate mal-functioning Pressure Indicators (PIs). PIs will malfunction after being over pressured by Process Operators.

 

 

"Instrument Techie" PTOA Readers and Students were also alerted that some instruments ... like liquid-in-glass thermometers ... cannot be re-calibrated and thus must just be replaced after experiencing "normal wear and tear."

 

The bulls eye target example also conveniently introduced the concept of "Instrument (Measurement) Error."

"Instrument (Measurement) Error" can be visualized by observing the placement of separate process variable measurements around the true, accurate process variable value ... which is represented as the cross hairs in the center of the bulls eye.

In many instances, process variable measurements that are "close enough" to the center of the bulls eye will be good enough to convert raw materials into final products.

"Instrument Techie" PTOA Readers and Students were made aware that the information presented in PTOA Segment #99 applied to the measurement of all process variables, not just the Process Variable Temperature.

Accurate process variable measurement is likewise needed for the measurement of Pressure, Flowrate, Level and even less common process variables like Analytical Composition, Density or Specific Gravity, Speed of Rotation, and many others.

 

PTOA SEGMENT #100: INSTRUMENT TECH MUST-KNOWS: MEASUREMENT ERROR

PTOA Segment #100 was devoted to understanding the sources of "Instrument (Measurement) Error" ... which were divided into two broad categories: "Instrument-Related" measurement error and "On-Line" measurement error.

The information in PTOA Segment #100 applied to all instruments used to measure all process variables, not just those used to measure Temperature.

"Instrument Techie" PTOA Readers and Students learned that one source of "Instrument-Related" measurement had the lofty sounding label of "Inherent Instrument Measurement Error."

"Inherent Instrument Measurement Error" would be stated as a quantifiable amount of measurement span on the manufacturer's product data sheet.

Can we talk?

Your Mentor is the culprit responsible for tacking on the descriptor "Inherent" onto the phrase "Instrument Measurement Error."

That means you will not find the phrase "Inherent Instrument Measurement Error" in any text-book.

"Inherent" is defined as:

"existing in something as a permanent, essential, or characteristic attribute."

Your Mentor added the "Inherent" descriptor to delineate this source of error from the other two instrument-related sources of error discussed in PTOA Segment #100.

Throughout the PTOA, the descriptor is not always added; the truncated phrases of "Instrument Measurement Error" or simply "Instrument Error" will always be referring to what is defined as "Inherent Instrument Measurement Error" in PTOA Segment #100.

 

"Instrument Techie" PTOA Readers and Students learned the three steps that are required to convert the "Inherent Instrument Measurement Error" stated as a percentage of span into an actual number of process variable units ... the examples on this PTOA Segment were in degrees because the process variable under discussion was Temperature.

When stated as a percentage of span, "Instrument Techie"  PTOA Readers and Students were cautioned to ALWAYS convert the "Inherent Instrument Measurement Error" into process variable units ... because that's the only way to determine if the stated error will still be adequate for the application of the instrument.

Another source of "Instrument-Related" measurement error was due to "Instrument Response Measurement Lag"...

 

 

... which is the amount of time between a changing process variable and the moment that the instrument sensor can actually sense the change.

"Instrument Response Measurement Lag" was easy to visualize while considering how liquid-in-glass thermometers and fluid-filled bulb-capillary temperature-measuring instruments work:

The time required for convection and conduction heat transfer to occur through the glass stem or capillary bulb that separates the process stream from the working fluid causes a time delay between the changing process temperature and the moment when the working fluid expands or contracts.

Both "Instrument Techie" and Process Operator PTOA Readers and Students have been cautioned to understand "Instrument Response Measurement Lag."

"Instrument Techie" PTOA Readers and Students will visually understand "Instrument Response Measurement Lag" the first time they try to tune a control loop.

Process Operator PTOA Readers and Students must have a feel for how much "Instrument Response Measurement Lag" exists in each automatic control loop.

 

The Control Board Operator that makes a change in a control parameter prior to allowing the outcome of a previous change in control parameters to be sensed can bounce a process variable out of control.

The third source of "Instrument-Related" measurement error was "Instrument Reliability" ... the tendency of the instrument to continue operating and not malfunction.

"Instrument Techie" PTOA Readers and Students learned that there are three contributing factors to "Instrument Reliability:"

• The "functionality of the instrument" involves consideration of how the instrument will be used and the range of operation, accuracy, and power requirements for the instrument.
• The "physical limitations" refer to the instrument's internal and external dimensions and how the instrument will be mounted and connections to the instrument.
• The "physical environment that will surround the instrument" definitely impacts the materials of construction and whether or not the instrument need to be encased or otherwise isolated.

Can we talk again?

The language used in PTOA Segment #100 failed to get straight to the point with respect to how poor communications exacerbate the measurement error associated with "Instrument Reliability."

Error due to "Instrument Reliability" is directly related to how much communication takes place between design engineers and the instrumentation technicians and operators that must maintain and use the instrumentation that is eventually installed.

The "modifications" required for a hindsight installation are the root source or error due to "Instrument Reliability."

For example, an unprotected instrument installed within a corrosive environment will become unreliable; a non-intrinsically safe  instrument that is installed within a flammable atmosphere is downright dangerous.

The focus of PTOA Segment #100 then turned to sources of "On-Line" measurement error.

"Instrument Techie" PTOA Readers and Students learned that the root source of most "On-Line" measurement error comes from human error.

Human beings in the form of Instrument Techs can "miscalibrate" an instrument.

In the real world, Instrumentation Technicians verify with the Outside Process Operator and Control Board Operator that their recalibration effort in the field has returned the process value to what is expected in the DSC system and/or local instrument.

After all, it was the bogus measurement that caused the Process Operator or Control Board Operator to write a work order requesting the instrument to be checked for recalibration in the first place.

If that procedure is not followed, a "miscalibration" would typically go unnoticed until the next experienced Control Board Operator with fresh eyes reports to shift and notices what appears to be a bogus process variable measurement ... at which point s/he will write another work order to calibrate the instrument ... unaware that the instrument had recently been "recalibrated."

Process Operators can also misread the recorded process variable trends; however, errors caused by "Misreading the Indication" have been greatly reduced with digitized displays.

"Mishandling" an instrument was stated to cause measurement error as well as instrument deterioration due to "normal wear and tear."

An example of error caused by mishandling was described as the over pressuring of a pressure indicator (PI).

Allowing the bourdon tube in a PI to exceed its high range will permanently impact the elasticity of the bourdon tube; the PI will continue to indicate a pressure but the pressure indicated will be bogus.

One non-human source of "On-Line" measurement error was stated to be the "noise" caused by "electrical interference."

 

 

Measurement error cased by "noise" can be difficult to detect, isolate, and eliminate.

 

©2016 PTOA Segment 00123
PTOA Deja Vu Review 4-3

You need to login or register to bookmark/favorite this content.