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TCLM.R1.1-1A

Instrument Performance Parameters

Annexure for Characteristics & Errors

Notes

1

• Static Characteristics-refer to characteristics of the system when an input is held

constant or varying slowly.

• Dnamic Characteristics-refer to characteristics of the system when input is

changing rapidly-In other words the transient response of the system to an input.

• Ran!om Characteristics-If repeated readings of the same quantity of the

measurand [ie the quantity being measured] are taken by the same instrument,

under same ambient conditions, they are bound to differ from each other. his is

often due to some inherent sources of errors of the instrument that vary randomly

and at any point of time it is very difficult to e!actly say, what would be its value. "

[e!ample #oise]

o characteri$e this kind of behavior, statistical terminologies are often used. %ost

common among them are Mean and Standard deviation.

DISCRIMINATI"N #or RES"L$TI"N% 

his is the smallest increment of the process variable which can be detected with

certainty by the instrument.

&or e!ample if a digital measurement has a ma! reading of ''' . Its resolution is ( in

'''.

he threshold of an instrument in a particular case of discrimination , is the minimum

that is necessary to cause a detectable change from $ero output.

It may be caused by backlash or noise

ER" STA'ILIT( 

 ) measure of the ability of the instrument to restore to $ero reading after the process

variable has returned to $ero and other variations[temperature ,pressure , humidity ,

vibrations etc] have been removed.

Instrument Turn!o)n 

 )n important performance parameter for transmitter instruments is something often

referred to as turndown or range down. *urndown+ is defined as the ratio of

ma!imum allowable span to the minimum allowable span for a particular instrument.

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uppose a pressure transmitter has a ma!imum calibration range of to pounds

per square inch /0I1, and a turndown of 23(. his means that a technician may

ad4ust the span anywhere between 0I and (5 0I..

*hat is $ncertaint+

#o measurement is ever guaranteed to be perfect. 6ncertainty of measurement isthe doubt that e!ists about the result of any measurement. 7y quantifying thepossible spread of measurements, we can say how confident we are about the result.

Ex,anation of the error s uncertaintEx,ressin/ $ncertaint

 ) measurement result is only complete when accompanied by a statement of itsuncertainty. ) statement of uncertainty is required in order to decide if the result is

adequate for its intended purpose and consistent with other similar results.

It does not matter how accurate a measuring instrument is considered to be, themeasurements made will always be sub4ect to a certain amount of uncertaint.

In order to e!press the uncertainty of a measurement, we need to evaluate asaccurately as possible the errors associated with that particular measurement.

&or e!ample - we might say that a particular stick is 2 centimetres long, plus orminus ( centimetre, at a '58 confidence level. his is written3

• 200 cm ±1 cm at a level of confidence of 95%

his means we are '58 sure that the length of the stick is between ('' centimetresand 2( centimetres.

0

•

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Iustrations of the use of Ran/e an! S,an Terminoo/ 

ypical 9anges #ame 9ange :ower 9ange 6pper 9ange pan

to ;( - to ( ;( (

2 to ;( uppressed $ero

range

2 to ;( 2 ;( <

-25 to to

;(

=levated $ero

range

-25 to ;( -25 ;( ;(25

-( to =levated $ero

range

-( to -( (

-( to -2 =levated $ero

range

-( to -2 -( -2 <

Instrument Performance Parameters

Dnamic Characteristics

he reason for dynamic characteristics is the presence of energy storage elements.

Inertia2 masses3 in!uctances

 Ca,acitances2 eectrica3 therma

>ynamic characteristics are determined by analy$ing the response of the sensor to a

family of variable input waveforms Impulse, step, ramp, sinusoidal, white noise 3

?hite #oise is a discrete signal whose samples are regarded as a sequence of serially

uncorrelated random variables

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Instrument Performance Parameters

Errors

Sstematic errors.

ystematic errors are 5iases in measurement which lead to the situation where the

mean of many separate measurements differs significantly from the actual value of the

measured attribute. )ll measurements are prone to systematic errors, often of several

different types.

ources of systematic error 67ero error8 may be im,erfect cai5ration of measurement

instruments, changes in the enironment )hich interfere )ith the measurement

,rocess and sometimes imperfect methods of observation can be either $ero error or

percentage error.

Ran!om error -he word random indicates that they are inherent un,re!icta5e , ,

they are scattered about the true value, when a measurement is repeated several times

with the same instrument. )ll measurements are prone to random error.

9andom error is caused by unpredictable fluctuations in the readings of a measurement

apparatus, or in the ex,erimenter9s inter,retation of the instrumental reading@ these

fluctuations may be in part due to interference of the enironment )ith the

measurement ,rocess.

he concept of ran!om error is closely related to the conce,t of ,recision.

Sstematic errors.-transfer 

ystematic errors are 5iases in measurement which lead to the situation where the

mean of many separate measurements differs significantly from the actual value of the

measured attribute. )ll measurements are prone to systematic errors, often of several

different types.

ources of systematic error 67ero error8 may be im,erfect cai5ration of measurement

instruments, changes in the enironment )hich interfere )ith the measurement

,rocess and sometimes imperfect methods of observation can be either $ero error orpercentage error.

Ran!om error -he word random indicates that they are inherent un,re!icta5e , ,

they are scattered about the true value, when a measurement is repeated several times

with the same instrument. )ll measurements are prone to random error.

9andom error is caused by unpredictable fluctuations in the readings of a measurement

apparatus, or in the ex,erimenter9s inter,retation of the instrumental reading@ these

fluctuations may be in part due to interference of the enironment )ith the

measurement ,rocess.

he concept of ran!om error is closely related to the conce,t of ,recision.

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:

T,ica Sam,e of a Process transmitter 

Aendor )7B and %odel ----Reference AccuracC.D58 of spanTota PerformanceC.(58 of span for C5 E& /2< EB1 temperature changes, up to( psi /F,' %0a1 line pressure for model--- 1, from (3( to 53(9ange down.Sta5iitC.(258 of 69: for 5 years for C5 E& /2< EB1 temperaturechanges, and up to ( psi /F,' %0a1 line pressure.Dnamic Performance Tota Res,onse Time 6T! ; Tc8G)9 output3 ( ms

&ieldbus and 0rofibus output3 (52 ms

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