measurement systems

Measurement Systems

• A system can be defined as an arrangement

of parts within some boundary which work

together to provide some form of output

from a specified input or inputs.

• The boundary divides the system from the

environment and the system interacts with

the environment by means of signals

crossing the boundary from the

environment to the system, I.e. inputs , and

signals crossing the boundary from the

system to the environment, I.e. outputs

Inputs Output/s

System boundary

environment

• A useful way of representing a system is as

a block diagram. Ex.

– Motor system

– Amplifier system

Interconnected system

CD player Amplifier

Output

Sound

Output from amp

Input to speaker

Bigger

electrical

signals

Electrical

signals

Output from CD player

Input to amp Input

In drawing a system as a series of interconnected blocks, it is

necessary to recognize that the lines drawn to connect boxes

indicate a flow of information in the direction indicated by the

arrow and not necessarily physical connections.

• The purpose of an instrumentation system

used for making measurements is to give

the user a numerical value corresponding to

the variable being measured.

• An instrumentation system for making

measurements has an input of the true value

of the variable being measured and an

output of the measured value.

Failure to measure effectively the level of

liquid in bottom of the tower lead to

--- Fire

--- Explosion

Importance of effective measurement in process

industry

BP Explosion.avi

../Files-14-08-2009/USB_Files31-08-2009/BP Explosion.avi



Instrument

Typical components of instrument

A Sensor:

(measures a physical quantity and converts it into a signal)

A Modifier

(Change the type of signal)

A Display unit

(transmitting arrangement )

Perhaps the best advice for engineering students is that “instruments are always incorrect”!!!!.

Functional Elements of an Instrument

Process/

Measured medium

Primary

Sensing

Element

Variable

Conversion

Element

Variable

Manipulation

Element

Data

Transmission

Element

Data

Presentation

Element

Observer

Functional Elements of an Instrument (Cont‘d)

Typical Example:

• An instrument contain various parts that perform prescribed functions in converting a variable quantity or condition into corresponding indication.

a) Primary sensing element: of an instrument receives energy from the measured medium and produces an output depending in someway on the value of the measured quantity.

b) Variable Conversion element: merely converts the output signal of the primary sensing element (voltage or displacement) into a more suitable variable or condition useful to the function of the instrument

c) Variable manipulating element: manipulates the signal represented by some physical variables to perform the intended task of an instrument.

d) Data Transmission Element: transmits the data from one element to another, as simple as shaft and bearing assembly or as complicated as telemetry system

e) Data Presentation Element: performs the translation function such as simple indication of a pointer moving over a scale.

Primary

Sensing

Element

Variable

Conversion

Element

Data

Transmission

Element

Variable

Manipulation

Element

Data

Presentation

Element

Temperature

Measured

Quantity

Pressure

Variable

Conversion

Element

Pressure

Motion Motion

Fluid

Temperature Tube Tubing

Spiral Bourdon

Tube

Linkage Gear Scale & Pointer

Functional Elements of an Instrument (Cont‘d)

Observer

Process/

Measured medium

Static characteristics

Dynamic characteristics

Performance Characteristics of Instruments

• The performance characteristics of an instrument are very necessary for choosing the most suitable instrument for specific measuring task.

• Static Characteristics: considered for instruments used to measure an unvarying process conditions

• Dynamic Characteristics: for measuring quantities that fluctuates with time.

• The relative importance of each issue depends upon the specific application; for

example, one application might require excellent accuracy, while another might require only moderate accuracy, but high reliability.

• Generally, we find that the greater the requirements for good performance, the higher the cost for purchase and maintenance. Therefore, we must find the proper balance of performance and cost, rather than always specify the best performing sensor

Static characteristics

Static characteristics of an instrument includes;

Accuracy

Precision

Repeatability

Range

Resolution

Others ( Sensitivity , Dead zone etc.)

Performance Characteristics of Instruments (Cont‘d)

Elements of an instrumentation

system

• 1. Sensor – element which is effectively in

contact with the process for which a

variable is being measured and gives an

output which depends in some way on the

value of the variable and which can be used

by the rest of the measurement system to

give a value to it.

Input:

temperature

Sensor :

Thermocouple Output:

e.m.f.

• 2. Signal Processor

• This element takes the output from the sensor and

converts it into a form which is suitable for

display or onward transmission in some control

system.

• The term signal conditioner is used for an element

which converts the output of a sensor into a

suitable form for further processing

• Thus in the case of resistance thermometer

there might be a signal conditioner, a

Wheatstone bridge, which transforms the

resistance change into a voltage change,

then an amplifier to make the voltage bi

enough for display

Input:

Resistance

Wheatstone

bridge

Voltage change

Amplifier Out

Larger voltage

change

• 3. Data presentation

• This presents the measured value in a form

which enables an observer to recognize it.

Display

Input:

Signal from

system

Output:

Signal in

observable

form

• Alternatively, the signal may be recorded,

e.. On the paper of a chart recorder or

perhaps on magnetic disc, or transmitted to

some other systems such as control system.

Sensor Signal processor

Input

True value

of variable

Display

Record

Transmit

Output

measurement value

of variable

• Transducers are defined as an element that

converts a change in some physical variable into a

related change in some other physical variable.

• It is generally used for an element that converts a

change in some physical variable into an

electrical signal change.

• A measurement system may use transducers, in

addition to the sensor, in other parts of the system

to convert signals in one form to another form.

Sensor

A B C

Signal

Processor

Data

presentation

Temperature

signal Resistance

change Current

change

Measurement

of pointer

across a scale


1. Accuracy

Static Characteristics

Accuracy is the degree of conformity of the measured value with the accepted standard or ideal value, which we can take as the true physical variable.

Accuracy is usually expressed in engineering units or as a percentage of the sensor range, for example:

Thermocouple temperature sensor with accuracy of 1.5 K. Orifice flow meters with accuracy of 3% of maximum flow range

• Conditions:

– the same measurement procedure

– the same observer

– the same measuring instrument, used under the

same conditions

– the same location

– repetition over a short period of time.

Precision (repeatability) and accuracy (deviation)

Not precise

Not accurate

Not precise

Accurate

Precise

Not accurate

Precise

Accurate

Accuracy is a consequence of systematic errors (or bad calibration)

accuracy and precision may depends on time (drift)

Resolution and accuracy • Resolution expresses how many different levels can be

distinguished

• It is not related to accuracy

2.1.3.1 Analog mechanical position

potentiometer

capacitive

balanced transformer (LVDT)

(linear or sin/cos encoder) strain gauges piezo-electric

+cheap, -wear, bad resolution +cheap, -bad resolution +reliable, robust - small displacements

+reliable, very small displacements +extremely small displacements


• Accuracy is the extent to which the value

indicated by a measurement system or

element might be wrong.

• Accuracy is often expressed as a percentage

of the full range output or full-scale

deflection (f.s.d.)

• For example, a digital thermometer may

have an accuracy of ±0.1oC.

• Is quoted in its specification as :

– Full scale accuracy – better than 2%

• For example, a system might have an

accuracy of ±1% of f.s.d.

• If the full-scale deflection is, say, 10A, then

the accuracy is ±0.1 A.

• Accuracy is the indicator of how close the

value given by a measurement system can

be expected to be to the true value.

• The accuracy is a summation of all the

possible errors that are likely to occur, as

well as the accuracy to which the system or

element has been calibrated.

• Accuracy is needed for some variables, such

as product quality, but it is not required for

others such as level in a large storage tank.


2. Precision


Precision is the degree of exactness for which an instrument is designed or intended to perform.

It is composed of two

characteristics; 1. Conformity 2. Number of

significant figures

• The term precision is used to describe the

degree of freedom of a measurement system

from random errors.

• Thus, a high precision measurement

instrument will give only a small spread of

readings if repeated readings are taken of

the same quantity.

• A low precision measurement system will

give a large spread of readings.

• For example consider the following two sets

of readings obtained for repeated

measurements of the same quantity by two

different instruments: • 20.1mm,20.2mm,20.1mm, 20.0mm,20.1mm,20.1mm,20.0mm

• 19.9mm,20.3mm,20.0mm,20.5mm,20.2mm,19.8mm,20.3mm

• The results of the measurement give values

scattered about some value. The first set of

results shows a smaller spread of readings

than the second and indicates a higher

degree of precision for the instrument used

for the first set.


3. Repeatability

The closeness of agreement among a number of consecutive measurements of the same variable (value) under the same operating conditions, approaching in the same direction.


The term “approaching in the

same direction” means that the

variable is increasing (decreasing) to the

value for all replications of the

experiment.

• The terms repeatability and reproducibility

are ways of talking about precision ins

specific contexts.

• The term repeatability is used for the ability

of a measurement system to give the same

value for repeated measurements of the

same value of a variable.

• Common cause of lack of repeatability are

random fluctuations in the environment,e.g.

changes in temperature and humidity.

• The error arising from repeatability is

usually expressed as a percentage of the full

range output.

• For example, a pressure sensor might be quoted as

having a repeatability of ±0.1% of full range. Thus with a range of 20kPa this would be an error of ± 20Pa.

• The term reproducibility is used to describe the ability of a system to give the same output when used with a constant input with the system or elements of the system being disconnected from its input and then reinstalled. The resulting error is usually expressed as a percentage of the full range output.

• High precision does not mean high

accuracy. A high precision instruments

could have low accuracy.


4. Reproducibility

• The closeness of agreement among a number of consecutive measurements of the same variable (value) under the same operating conditions over a period of time, approaching from both directions.


The period of time is “long”, so that changes occurring over longer times of plant operation are included.

Reproducibility includes hysteresis, dead band, drift

and repeatability.

• Gradual change in instruments

measurements.

OR

• Measure of difference in repeatability.

• Under laboratory conditions drift of an

element can be determined by one of two

ways;

1. Point drift

2. Calibration drift

Drift

• By maintaining exact operating and load

conditions , monitoring of output variations

for a fixed input signals as a function of

time is called point drift.

• Used for stable process conditions

Point Drift

• By maintaining input signals, operating conditions, a load approximately constant comparison of calibration curves at the beginning and at specified intervals of time is called Calibration drift.

• Used for varying process conditions

Calibration Drift

• Dead zone is the largest range of values of a

measured variable to which the instrument

does not respond.

• This is sometimes called dead spot and

hysteresis.

Dead Zone

Backlash Backlash or mechanical hysteresis is defined as that lost

motion or free play which is inherent in mechanical elements

such as gears, linkages or other mechanical transmission

devices that are not rigidly connected.

Range represents the minimum and maximum values which

can be determined by an instrument or equipment.

Difference between upper and lower range is known as Span.

Span can be the same for two different range instruments.


5. Range/Span


If a chemical reactor typically operates at 300 C, the engineer might select a

range of 250-350 C.

Since the reactor will be started up from ambient temperature occasionally,

an additional sensor should be provided with a range of -50 to 400 C.


5. Linearity


This is the closeness to a straight line of the relationship between the true process variable and the measurement. Lack of linearity does not necessarily degrade sensor performance. If the nonlinearity can be modeled and an appropriate correction applied to the measurement before it is used for monitoring and control, the effect of the non-linearity can be eliminated.

Linearity is usually reported as non-linearity, which is the maximum of the deviation between the calibration curve and a

straight line positioned so that the maximum deviation is minimized

• Typical examples of compensating calculations

are the square root applied to the orifice flow

sensor and the polynomial compensation for a

thermocouple temperature sensor.

• The engineer should not assume that a

compensation for non-linearity has been applied,

especially when taking values from a history

database, which does not contain details of the

measurement technology.


6. Reliability


Reliability is the probability that a device will adequately perform (as specified) for a period of time under specified operating conditions. Some sensors are required for safety or product quality, and therefore, they should be very reliable.

If sensor reliability is very important, the engineer can provide duplicate sensors, so that a single failure does

not require a process shutdown

• Probability = number of occurrences of the event/ total number of

trials

• A high reliability system will have a low failure rate.

• Failure rate is the number of times during some period of time that the

system fails to meet the required level of performance, i.e.

• Failure rate= number of failures / number of systems

observed x time observed

• A failure rate of 0.4 per year means that in one year, if ten systems are observed, 4 will fail to meet the require level of performance.

• If 100 systems are observed, 40 will fail to meet the required level of performance.

• Failure rate is affected by environmental conditions.

• For example, the failure rate for a

temperature measurement system used in

hot, dusty, humid, corrosive conditions

might be 1.2 per year, while for the same

system used in dry, cool, non-corrosive

environment it might be 0.3 per year.

• Reliability is affected by maintenance and

consistency with process environment.

Also, some sensors are protected from

contact with corrosive process environment

by a cover or sheath (e.g., a thermowell for

a thermocouple), and some sensors require a

sample to be extracted from the process

(e.g., a chromatograph).

Calibration

Assigning standard values to an equipment is calibration.

The process for determination by measurement or comparison with a

standard of correct value scale reading on a meter or other measuring

instrument.

Error

• The term error is used for the difference

between the result of the measurement and

the true value of the quantity being

measured, i.e.

• Error = measured value – true value

• Thus if the measured value is 10.1 when the

true value is 10.0, the error is +0.1.

• If the measured value is 9.9 when the true

value is 10.0, the error is –0.1.

• Errors can arise in number of ways:

1. Hysteresis error

2. Non-linearity error

3. Insertion error

Instrument

Reading

Increasing

Decreasing

Non

Actual relationship Assumed relationship

error

error

Value measured

Measured

Value

Non

Actual relationship Assumed relationship

error

error

True Value

• Hysteresis error- is used for the difference

in outputs given from the same value of

quantity being measured according to

whether that value has been reached by a

continuously increasing change or a

continuously decreasing change.

• Thus, you might obtain a different value

from a thermometer used to measure the

same temperature of a liquid if it is reached

by the liquid warming up to the measured

temperature or it is reached by the liquid

cooling down to the measured temperature.

• Non-linearity error is used for the error that

occurs as a result of assuming a linear

relationship between the input and output

over the working range, i.e. a graph of

output plotted against input is assumed to

give a straight line.

• Few systems or elements, however, have a

truly linear relationship and thus errors

occur as a result of this assumption of

linearity.

• Linearity error is usually expressed as a

percentage error of full range or full scale

output.

Insertion error

• When a cold thermometer is put in to a hot

liquid to measure its temperature, the

presence of the cold thermometer in the hot

liquid changes the temperature of the liquid.

• The liquid cools and so the thermometer

ends up measuring a lower temperature than

that which existed before the thermometer

was introduced.

• The act of attempting to make the measurement

has modified the temperature being measured.

• This effect is called LOADING and the

consequence as an insertion error.

• If we want this modification to be small, then the

thermometer should have a small heat capacity

compared with that of the liquid. A small heat

capacity means that very little heat is needed to

change its temperature.

• Example of loading is when an ammeter is inserted into a circuit to make a measurement of the circuit current, it changes the resistance of the circuit and so changes the current being measured.

• Ex. Two voltmeters are available, one with a resistance of 1 K ohm and the other 1 Mohm. Which instrument should be selected if the indicated value is to be closed to the voltage value that existed across a 2 Kohm resistor before the voltmeter was connected across it.

• The 1Mohm voltmeter should be chosen. This is because when it is in parallel with the 2Kohm, less current will flow through it than if the 1 Kohm voltmeter had been used and so the current through the resistor will be closer to its original value. Hence the indicated voltage will be closer to the value that existed before the voltmeter was connected into the circuit.

• Static errors are generally of three types;

1. Mistake or gross error (human mistakes)

2. Systematic errors (instrumental or environmental errors)

3. Random or accidental errors (unknown)

Static Error

Range

• The range of variable of system is the limits between which the input can vary.

• For example, a resistance thermometer sensor might be quoted as having a range of –200 to +800oC.

• A meter might have dual ranges i.e. 0 to 4 and 0 to 20

• The range of variable of an instrument is also sometimes called its SPAN.

• The term dead band or dead space is used if

there is a range of input values for which

there is no output. For example, bearing

friction in a flow meter using a rotor might

mean that there is no output until the input

has reached a particular flow rate threshold

Dead space

Output

reading

Input of variable being

measured

Sensitivity

• Indicates how much the output of an

instrument system or system element

changes when the quantity being measured

changes by a given amount, i.e. the ratio

out/input.

• For example, a thermocouple might have a

sensitivity of 20uV/oC and so give an output

of 20uV for each change in temperature.

• Thus, if we take a series of readings of the

output of an instrument for a number of

different inputs and plot a graph of output

against input, the sensitivity is the slope of

the graph.

• The term is also frequently used to indicate

the sensitivity to inputs other than that

being measured, i.e. environmental changes.

• Thus a pressure measurement sensor might be

quoted as having a temperature sensitivity of ±0.1%

of the reading per oC change in temperature.

• Example: A spring balance has its deflection

measured for a number of loads and gave the

following results. Determine its sensitivity.

• Load in Kg 0 1 2 3 4

• Deflection in mm 0 10 20 30 40

• Fig. 1-20 shows the graph of output

against input. The graph has a slope of

10mm/kg and so this is the sensitivity.

Stability

• The stability of a system is its ability to give

the same output when used to measure a

constant input over a period of time.

• The term drift is often used to describe the

change in output that occurs over time.

• The drift may be expressed as a percentage

of the full range output.

• The term zero drift is used for the changes

that occur in output when there is zero

input.

Dynamic characteristics

Performance Characteristics

Dynamic Characteristics of an instrument includes;

1. Speed of response

2. Fidelity

3. Lag

4. Drift

• Speed of Response: is the rapidity with which an instrument responds to the change in the measured quantity.

• Fidelity: degree to which an instrument indicate the changes in the measured variables without dynamic error.

• Lag : is a retardation or delay in the response of an instrument to the changes in the measured quantity

• Drift: is a undesired change or gradual variation in output over a period of time that is unrelated to changes in output, operating conditions or load.

Other Issues

1. Maintenance

2. Consistency with process environment

3. Safety

4. Cost

Other Issues

Consistency with process environment

•Direct contact –

Sensors such as orifice plates and level floats have direct contact with process fluids.

Sheath protection –

Sensors such as thermocouples and pressure diaphragms have a sheath between the process

fluid and the sensor element

•Sample extraction –

When the process environment is very hostile or the sensor is delicate and performs a

complex physiochemical transformation on the process material, a sample can be extracted.

• Most sensors will function properly for specific process conditions. For example, many flow sensors function for a single phase, but not for multi-phase fluid flow, whether vapor-liquid or slurry. The engineer must observe the limitations for each sensor.

• Some sensors can have direct contact with the process materials, while others must be protected. Three general categories are given in the following.

• Sensors in direct contact must not be degraded by the process material.

• The sheath usually slows the sensor response.

• Samples must represent the fluid in the process.

Other Issues

Location of Measurement Displays

Local display

Local panel display

Centralized control room

Remote monitoring

• The measurement is displayed for observation by

plant personnel. Typically, the display uses analog principles, which means that the display presents the measurement as a position in a graphical format, which could, for example, be the height of a slide bar or the position of a pointer. Often, the value is displayed as a line on a trend plot that provides the values for some time in the past. In addition, the measurement can be displayed as a digital number to provide more accuracy for calibration. Finally, measurements that are transmitted to a digital control system can be stored in a historical database for later recall and for use in calculating important parameters useful in monitoring process behavior, for example, reactor yields or heat transfer coefficients.

• ------ A sensor can display the measurement at the

point where the sensor is located. This information can be used by the people when monitoring or working on the equipment. A measurement that has only local display involves the lowest cost, because the cost of transmission and interfacing to a digital system are not required. Note that no history of these measurements is available unless people record the values periodically.

• ---- Some equipment is operated from a local

panel, where sensors associated with a unit are collected. This enables a person to startup, shutdown and maintain the unit locally. This must be provided for units that require manual actions at the process during normal operation (loading feed materials, cleaning filters, etc.) or during startup and shutdown. Usually, the values displayed at a local panel are also displayed at a centralized control room.

• ------ Many processes are operated from a

centralized control room that can be located a

significant distance (e.g., hundreds of meters)

from the process. The measurement must be

converted to a signal (usually electronic) for

transmission and be converted to a digital number

when interfaced with the control system. A

centralized control system facilitates the analysis

and control of the integrated plant.

• ----- In a few cases, processes can be operated without a human operator at the location. In these situations, the measurements are transmitted by radio frequency signals to a centralized location where a person can monitor the behavior of many plants. Typical examples are remote oil production sites and small, safe chemical plants, such as air separation units.

The Smart Sensor Revolution

Digital conversion and transmission

Diagnostics

Signal conditioning

Configuration

• Currently, sensor technology is experiencing a dramatic

change. While the basic physics and chemistry of sensors are not changing, sensors are being enhanced by the addition of microprocessors at the location of the sensor. This change makes the following features possible that were not available with older technologies.

• ------ The “signal” from the sensor is no longer simply a single value representing the measured value. The sensor can transmit additional information, including diagnostics and corrected estimates of a variable based on multiple sensors, e.g., orifice pressures and density. All values can be transmitted digitally, which allows many sensor values to be sent by the same cabling, which reduces the cost of an individual cable for each measurement, as required with analog transmission.

• ---- - The sensor can provide sophisticated diagnostics of

its performance and warn when a measurement might be

unreliable.

• ----- The sensor can identify unusual signal characteristics

and eliminate noise or “spikes” according to methods

defined by the engineer.

• ------ The range of a sensor can be changed quickly to

accommodate changes in process operating conditions.

Principle measurements desired in industry

(a) Temperature

(b) Pressure

(c) Level

(d) Flow

(e) Others ( Composition, pH etc.)

Principle measurements desired in Industry

You have two

challenges

What variables

should be

measured?

What sensor should

be

specified for each

measurement?

Reactor with feed-effluent heat

exchange

Calibration

• Calibration should be carried out using

equipment which can be traceable back to

national standards with a separate

calibration record kept for each

measurement instrument.

National

Standards

Calibration

center

standard

In-company

standards

Process

instruments

Traceability

Chain

• Determine the sensitivity of the instruments that

gave the following readings:

• (a)

• Load kg 0 2 4 6 8

• Deflection mm 0 18 36 54 72

• (b)

• Temp oC 0 10 20 30 40

• Voltage mV 0 0.59 1.19 1.80 2.42

• [c]

• Load N 0 1 2 3 4

• Charge pC 0 3 6 9 12

• Calibration of a voltmeter gave the following data. Determine the maximum hysteresis error as a percentage of the full-scale range:

• Increasing input:

• Standard mV 0 1.0 2.0 3.0 4.0

• Voltmeter mV 0 1.0 1.9 2.9 4.0

• Decreasing input:

• Standard mV 4.0 3.0 2.0 1.0 0

• Voltmeter mV 4.0 3.0 2.1 1.1 0

• (a) 9mmkg

• (b)0.060 mV/oC

• [c] 3pC/N

• 5%

measurement systems

Documents

b variable conversion

instrumentation system

variable quantity

d data transmission

instrument c variable

suitable variable

output signal

measured value