But what you've done here
Is put yourself between a bullet and a target
And it won't be long before
You're pulling yourself away

("Bullet and a Target," Citizen Cope/Clarence Greenwood, 2002)

The next few PTOA Segments are targeted to PTOA Readers and Students who may be interested in pursuing a career as an Automatic Instrumentation Technician.

The information in this PTOA Segment applies to the measurement of any process variable, not just Temperature.

 

CRUCIAL DECISIONS ARE BASED UPON PROCESS VARIABLE MEASUREMENTS

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order expertly understand how accurate process variable readings make it possible for Outside Process Operators and Control Board Operators to manage process variables while efficiently transforming raw materials into value-added products

Even more important, the accuracy of instrument measurements that are rarely glanced at will suddenly take precedence when the daily routine is interrupted by a processing emergency.

Automatic Instrument Techs understand the importance of matching the optimal instrument to the processing environment and vigilantly maintain the installed instrumentation to support flawless process operations.

Determining the best instrument for the task at hand involves consideration of the following:

• How much measurement accuracy/repeatability is needed?
• How quickly does the instrument respond to changes in the process variable that is being measured, transmitted, recorded, controlled?
• How reliable is the instrument not to malfunction?

The ability and limits of instrument measurement accuracy are featured in this PTOA Segment #99.

Instrument reliability and response time are featured in PTOA Segment #100.

 

INSTRUMENT ACCURACY

There is no better graphic than ye olde bullseye target to illustrate the concepts of accuracy/repeatability and secondarily  demonstrate instrument error and the role of instrument calibration.

Each of the blue dots in the graphic to the right represent the measurement of a process variable like temperature, pressure, a flowrate or a level.

Each blue dot represents a measurement of the same process variable over a period of time that is steady, therefore the expectation would be that the dots would be on the target in the same place.

The accurately measured value of the process variable is represented as the exact center of the bullseye, dead center in the crosshairs.

Accuracy is achieved when the blue dot measurements are dead center, within the cross hairs.

 

The Best Instrument:

The lower right bullseye shows measurement results as a clump of blue dots in the center of the bullseye.

The instrument that yields such measurements can be  relied upon to give accurate measurements repeatedly, meaning over and over again.

Nobody should be too surprised that instruments that can be extremely accurate over and over again usually cost a lot more moola.

 

The Worthless Instrument:

From the sublime to the ridiculous, focus on the upper left hand corner  bullseye.

The instrument that yields the pattern of measurements shown in the upper left hand bullseye is neither accurate nor repeatable.

This measuring instrument is worthless; it has the same  probability of being accurate as Fred the Stickman has at throwing darts in the white, round bullseye shown in the picture on the left.

 

 

 

 

The Instrument That Is Begging to be Calibrated:

Focus on the bottom left bullseye.

The instrument measurements shown as blue dots in the bottom left target are in no way accurate but are definitely repeatable.

More than likely this instrument just needs to be calibrated so those blue dots will once again be dead center.

Calibration processes are specific to each type of measuring, indicating, recording, transmitting, and transduction component found in a control loop. Seek ye the manufacturer's instrument instruction pamphlet/ap. No kidding, every good Instrument Tech had to actually read the instruction manual, too.

If calibration does not improve the instrument's accuracy, then pat yourself on the back for solving the control loop problem and replace the instrument.

 

 

Some Measuring Instruments Are Just Disposed Of:

For a few instruments, calibration is not an option.

Over time, the bulb, capillary and enclosing glass stem of a liquid-in-glass themometer will fatigue and the temperature measurement read from the scale graduations will no longer be accurate.

Liquid-in-glass thermometers are specifically manufactured instruments and cannot be calibrated. When a liquid-in-glass thermometer is determined to be faulty, the next step is to properly dispose of it.

Rumor has it that the the common, garden variety Pressure Indicator (PI) gauge can be calibrated with an instrument called a dead weight tester.

Your Mentor has been around the processing world a long time and has never witnessed the dead weight tester that is in every IE shop actually be used for gauge calibration; apparently replacing the faulty PI gauge with a new one is more cost effective than assigning the man hours to perform the calibration work.

 

The Plausible Option Instrument:

Focus on the upper right hand corner bullseye.

Sometimes a process measurement does not need to be so accurate to still be useful.

The pattern of blue dot measurements shown in this bullseye illustrate a range of measurement error that may still be tolerable.

Determining whether or not the error of the instrument can be tolerated and learning the other sources of measurement error are the focus of the next PTOA Segment #100.

TAKE HOME MESSAGES:  The ability to  successfully use an indication or recording of a Process Variable starts with accurate measurement of the PV. All process operating decisions are derived from the integrity of the measurement.

Matching the best instrument to the process measurement application depends upon the needed accuracy, reliability, and response time of the instrument.

Accuracy is the ability of the instrument to measure the true value of the process variable over and over again.

(Instrument Measurement) Error is the tendency of the instrument to yield measurements that stray from the true value.

Measurements from an instrument that are repeatable but not accurate may indicate the need to calibrate the instrument.

Calibration procedures are specific to each instrument; calibration instructions are found in the manufacturer's instrument instruction pamphlet.

©2016 PTOA Segment 00099
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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