Selecting Measurement Instruments

Introduction

• Covers how to choose appropriate instruments for specific applications
• Includes a review of the main applications of measurement
• Requires knowledge of instrument characteristics and performance in various applications and operating environments
• Includes a review of the different instrument classes
• Focuses on the attributes of instruments that determine their suitability for different measurement requirements and applications
• Classifies instruments as active or passive based on whether they contain an energy source
• Distinguishes between null-type which requires adjustment to datum level, and deflection-type instruments that give output as pointer deflection or numerical display
• Classifies instruments as analog (output varies continuously) or digital (output in discrete steps)
• Indicators give visual/audio magnitude indication, used in process industries
• Instruments give a signal output, often part of automatic control systems
• Instruments are classified as smart or non-smart, with smart instruments being increasingly important in measurement applications
• Examines static characteristics (steady-state attributes): accuracy, measurement sensitivity, resistance to errors from environmental variations
• Focuses on dynamic characteristics of instruments by describing their behavior after a measured quantity changes, until the output reaches a steady value
• Considers instrument calibration

Review of Instrument Types

• Divides instruments into active or passive depending on how the output is produced
• Active instruments depend on an external power source
• Passive instruments outputs are entirely produced by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source
• A pressure-measuring device is an example of a passive instrument
• A float-type fuel tank level indicator is an example of an active instrument.
• External power in active instruments is typically electrical

Resolution and Cost

• Key difference between active/passive instruments is the achievable measurement resolution
• Adjustment of external energy input in active instruments allows greater control over measurement resolution
• Passive instruments have simpler construction and are cheaper
• Choosing between instrument types involves balancing required measurement resolution against cost

Null-Type and Deflection-Type Instruments

• The pressure gauge is a deflection type instrument
• A scale is a null-type instrument
• Weights are used to balance an unknown weight
• Weight measurement is based on the weights needed to achieve a null position
• Accuracy depends on different factors for each instrument type
• Accuracy the first type depends on linearity and calibration of the spring, while second type relies on the calibration of weights
• Null-type instruments are typically more accurate
• Requires calibration of weights versus springs
• Deflection-type instruments are more convenient
• Simpler to read the position of a pointer against a scale

Analog and Digital Instruments

• An analog instrument gives a continuously varying output as the measured quantity changes
• The output can have infinite values within the instrument's range
• A deflection-type pressure gauge is an example
• The pointer moves continuously as the input value changes, though visual discrimination is limited by scale fineness and size
• A digital instrument has an output that varies in discrete steps so it can have a finite number of values
• A rev-counter is an example; a cam opens/closes a switch with each revolution, counted electronically
• Only counts whole revolutions, cannot discriminate partial revolutions

Indicating Instruments and Instruments with a Signal Output

• Instruments can be divided into those that merely give an audio or visual indication of the physical quantity measured
• Instruments can give an output in the form of a measurement signal proportional to the measured quantity
• Indicating include null-type instruments and most passive ones

Indicators

• Can have an analog output or a digital display
• A liquid-in-glass thermometer represents an analog indicator
• A bathroom scale represents another common indicating device, which exists in both analog and digital forms.
• Major drawback is human intervention is required to read and record a measurement
• The displayed output are prone to error unless the human reader is careless

Signal-Type Outputs

• Are commonly used in automatic control systems or measurement systems where the output is recorded for later use
• The measurement signal can come from an electrical voltage, but it can take other forms
• Examples forms could be electrical current, an optical signal, or a pneumatic signal.

Smart and Non-smart Instruments

• Instruments are divided into those that incorporate a microprocessor (smart) and those that do not
• Benefits of smart sensors include higher signal-to-noise ratio, fast signal conditioning, auto-calibration, high reliability, small physical size, and detection and prevention of failure capabilities
• Smart sensors include different components like amplifiers, transducers, analog filters, excitation control, and compensation sensors
• Examples of smart sensors include electric current, level, humidity, pressure, proximity, temperature, heat, flow
• Output signal is ready to use
• Smart sensors are generally preferred over base sensors because they include native processing capabilities

Static Characteristics of Instruments

• The true temperature of the room, for example (20°C) does not really matter whether it’s within a small variation (19.5°C or 20.5°C)
• Our bodies cannot discriminate between such close levels of temperature
• Therefore a thermometer with an inaccuracy of 0.5°C is perfectly adequate
• Measuring a chemical process temperature, a of variation of 0.5°C might affect the rate of reaction or even the products of a process
• A measurement inaccuracy less than 0.5°C is therefore clearly required
• Accuracy of measurement is one consideration in the choice of instrument for an application
• Other parameters include sensitivity, linearity, and reaction to ambient temperature changes
• Attributes, and their specification, are collectively known as static characteristics
• Values apply when instruments are used under specified standard calibration conditions
• Allowance needs to be made for characteristics variations when instruments are used in alternative conditions

Static Characteristics: Accuracy

• Accuracy is how close the instrument output is to the real value
• Inaccuracy (measurement uncertainty)
• Inaccuracy is the extent to which a reading might be wrong, it's often a percentage of the total range of reading

Example Solution

• Example - Accuracy and inaccuracy
• A pressure gauge with 0–10 bar range has ±1.0% inaccuracy of the full scale reading
• Maximum error of this instrument: 1.0% of 10 bar = 0.1 bar
• Pressure with 1 bar will have maximum error of 0.1 bar
• Important - Measurement error in an instrument related to the full-scale reading means that measurements far lower than full scale reading will be amplified.
• Accuracy is greatest when the range of the instrument is appropriate to the values measured
• Measuring pressures with expected values between 0 and 1 bar should be measured with an instrument with a range of 0 - 1 bar.
• An instrument with a pressure range of 0 - 10 bar would not be appropriate

Precision

• Describes an instrument’s degree of freedom from random errors. if using a high precision instrument, the spread of readings will be less
• Precision should not be confused with accuracy as high precision doesn't imply high accuracy
• Low accuracy measurements from a high precision instrument are normally caused by bias in the measurements, and can be recalibrated

Repeatability / Reproducibility

• Repeatability - the closeness of output readings when the same input is applied repetitively over a short time, same measurement conditions, instrument, observer, location and use are maintained throughout
• Reproducibility - describes the closeness of output readings for the same input when there vary the method of measurement, observer, measuring instrument, location, conditions of use, and time of measurement
• Spread of output readings is called repeatability id measurement conditions are constant
• If measurements vary, they are called reproducibility
• Degree of repeatability/reproducibility in instrument measurements is an alternative way to express its precision

Precision example

• Room width measured 10 times, variations measured by Ultrasonic Sensor is greater than calibrated steel
• Precision of the ultrasonic sensor is ±0.003 m (±3 mm), but error is +9mm more than the ultrasonic rule

Tolerance

• It is closely related to accuracy and defines the maximum error expected
• It is not a static characteristic of measuring instruments but is mentioned because the accuracy of some instruments is given as a tolerance value

Tolerance Usage

• Describes the maximum deviation of a manufactured component from a specified value
• Crankshafts are machined with a diameter tolerance
• Electrical components like resistors may have ±5% tolerances

Tolerance Solution

• Solution - Tolerance
• Resistors bought in-store have +/- 5% manufactures tolerance, 1000 nominal resistance
• Minimum Likely Value - 1000 – 5% = 950
• Maximum Likely Value - 1000 + 5% = 1050

Range

• Defines the maximum and minimum values of quantity the instrument is designed to measures

Solution for Range

• Solution - Range
• If a micrometer is designed to measure dimensions between 50 and 75 mm, its measurement range is 75-50 = 25 mm

Threshold

• If the input is gradually increased from zero, the threshold is the time the input will need to reach a minimum before the change in the instrument output reading is detectable
• The minimum level of input is known as the threshold of the instrument
• Manufacturers specify this threshold for instruments: Some quote percentage, some quote "absolute values"

Example for Threshold

• Example - Threshold
• A car speedometer typically has a threshold of about 10 km/h
• Speeds from rest that are lower than this, will not register on the speedometer until the speed reaches 10 km/h

Resolution

• When there is a lower magnitude on the input, an instrument will show a particular output
• This will cause an observable change in the quantity produced for a change in the measured input

Resolution Specifications

• Is specified by sometimes as an absolute value specified as a percentage of the full scale deflection
• Finely its output scale is divided into subdivisions, which majorly influences this

Linearity

• It is normally desirable for instruments to have an output reading linearly proportional to the quantity being measured.
• Graph the output, then draw a "good fit straight line" , which is a mathematical leat squares line fitting technique
• Any outliers (deviations) is the nonlinearity, and a percentage is applied with the full scale reading

Example

• Assume the instrument shown in the instrument characteristic plot from the previous slide is a pressure sensor, input is bars from 1-9 and output is volts between 1 and 13
• Determine the following
  • Whats percentages in total the maximum non linearity in full scale deflection
  • What is the resolution of the sensor that the characterstic is given
• Solution
  • The sensor as maxium deviation data point = 0.5 volts
  • "Full Scale Deflection" = Length (Calculated for the fitted straight line) = 13.0
• Therefore the maximum non linearity - can therefore be expressee
• as 0.5/13 x 100 = 3.85 of the full scale deflections
• The resolution of the sensor graph as is the limited that is not detectable within the data
• The best eye determination is one tenth (0.1)

Sensitivity

• A change in the instrument output will change when the quantity being measured changes by the provided amount
• This can be shown as a ratio with the following equation:
• Scale Deflection
• Value of Measured Producing Deflection

Line Slope

• The line can be drawn in the "previous plot" for the sensitivity of the measurement
• If a value of the pressure transducer - 2 bar produces 10 degrees, its value is 5 degrees / per bar
• Example: Find resistance values of a platinum based "resistance thermometer", during ranges in temperature
• Determine the value of the measurement sensitive, during measurement

Static Sensitivity

• When plotting data on a graph, show the plotted values for each of the values
• Exisitng obvious "straight line relationships"
• For temperature @30 degrees
• The measurement sensitivity =7/30 = "0.233"

Sensitivity to disturbance:

• All calibrations, and specifications, are only valid with controlled conditions of temperatures, pressure, and more
• "Standard Ambient Conditions" are only defined in the instrument specification
• Certain "static instrument characteristics" will change in the "Ambient temperature"
• Sensitivity to disturbance is measured by the magnitude to this change
• The change with "environmental" affect instruments using two main ways
  • "Zero Drift"
  • "Sensitivity Drift"
• Zero drift is simply know by another phase bias

Zero Drift / Bias

• The effects of the zero reading of instruments is modified when theres a change with ambient conditions\
• It will cause any consistent errors
• The mechanical form of bathroom will cause zero drift
• If there is a 1kg already on said scale
• 70kg will measure 71, and 100kg will measure 101kg

Zero drift

• Zero drift can be "normally" removed using "calibration"
• In the case of "bathroom scale", a thumbwheel is given can turn the reading is zero, without anything being unloaded
• Most "typical unit is the zero drift" and can be measure in V/C
  • It is called "zero drift coefficient"
  • Related to "temperature changes", where characteristics of an instrument are sensitive to "environmental parameters"
• A typical in output the characteristic of a "Pressure Gauge"
  • Changes to zero drifty

Static Characteristics of instruments

• Shows the "output measurements" of voltages at "20 degrees and 50 degrees conditions"
• Use a 50 degree "environment" Determine the "zero environment", with values during = "zero coefficieint"

Data to understand the data

• Voltages (20 C)
  • 10.2
  • 20.3 -30.7 -40.8
• Voltages (50C) -10.5 -20.6 -31.0 -41.1
• The results - "20C" is correct - "20C" to the - "50C", where we assume the "20C environment"

Values Example to better solution

• The zero drift is + 0.3 volts @50C
• It is constant between pair of "output"
• The zero drift @30C - 0.01 v / degrees

* Sensitivity / DRif - ( Also - Also known - Scale Factor)*

• DEFINES - Amount Which instruments * is SENSITIVE to
  • Measure Variance as: " Ambient - Condition"
• Is quantified using using the "sentient - drift coefficents
• Defined That much the drift UNIT - CHANGE
• Parameters to which is sensitive

Components of Instruments

• Affect Environment "FLUCTATUONS"

• ex : modulus springs "Temp Dependent"

• if* - Instrumet drift/sensitive simultaneously

• Modification of characteristics is shown:

Hysteresis Effects

• The figure demonstrates the output of charactertestcs -
• When the quantly is increased " from + to - value" the output varies in the manner C(A)
• if - Variable - STREADIY , output - varies is curve "B"
• No co-indecndence between loading

Un_loading_

• Quantifies , maxim input - out put

• is normally expressed with a "%" , related to the full scale output readings.

• Instruments

Instruments containt:

• The Hysteeris - is common when Instruments contain "springs"

Silicone Based Soft Resistors::

Also

• "Soft Actuators"
• Smarts Materials

• Exisites the H (hystereres)

Dead space

• The range with different "Input Valyes" with "Change in Output Value"
  • AN INSTRUMENT that has exhibits can display "Dead space"
• The instruments that will not suffer hystersis

• can exhibits in the "Output Charactericts" __Backlash in GEAR is the cause for Dead Space AND results in the INSTRUMETS Output to Charaterstic

_"Backlass" :* Common CONVERTED sets using to covert"between" _"Translational" and __ROTATIONAL"

• Common way is "MeasuringTranslational "

Dynamic Characteristics of Measurement

• Measure with "Steady Stats Reading" to settlement and accuracy of reading

Characteristics of INSRUMENTS describe behaviour with measured quantity

• changes and the instrument with steady value is in response
• Same as before - the Dynamic has data sheets only UNDER SPECIFICAL*

Conditions/calibration

DYNAMIC "Values" can be expects

• Linearity - Inavriat Measures - THE LINEAR relation as between the Input and outputs + time is greater than -
• "measured quantity" + "outputs" +

Dynamic Characterises

• As "special and simplified cases, its applicable for the normal"
• Example** if with consideration for - STEP CHANGED with quantity , the reduce, The equation

Instrumentation is zero

• Assumed - Is assumed for : Equation (**) in assumed/known zero

Constant / sensitivity wise - Defined

• All instrument *beahaves - equation in a type of - "Zero Order" Type

• Change quantity of amount vs time *instrument of output, will amount immedietly with *new + (same instant of time )

Example Of PotentialMeter

• Measures - "POTENTIMET" which changes + instantaneous with - is dispalced along with "PotentialMeter" = TRACK"

Dynamic Characteristis ->First Order Instruments

• Cofficient 2 ..... AN (Except For, Ao, A1 = eq* *assume is ZERD

• "A1 * * " " "dQ_o" is over " "dT" * *+ + Ao_a = Bi_gi.

Instruments

• " Behaviours from _First = Order " instruments"*

A1073, is replaced to = DO , OPERATORS -

"a1DO+aoq =bio+ - AOgo = Bio Qi"

Rearagging

"Go = (b1/( 2,D)) "Qi - divide "

• "1 + aA/ (2/90,

DEFINING (k = b/0 as/ 2,0 Static - sensibility

• 7 (a1/0-2,0) as"Time Constant" With Systems - Equations
• Go = Ko divided - / to 1 + y / D

Dynamic with 1st Order

• (Eq , *** is solved, The output (Q) - response in steps - Time varying * is shown in this diagram

"TIME CONSTANT" OF STEP

• response is a time for THE OUTputs* " Quantity (q) - ro reach 63 - Final values" -

• THE "thermocouple: is good with "ONE ORDER"

Known for "room in temperature", + boiling water , the output in will RISE but + not. > "Intstantious" with "Indicators"