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.
environment Output/s Inputs System boundary
• 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
• A useful way of representing a system is as a block diagram. Ex. – Motor system – Amplifier system
Interconnected system Output from CD player
Output from amp
Input to amp
Input to speaker Output
Input CD player
Amplifier Sound
Electrical signals
Bigger electrical signals
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.
Importance of effective measurement in process industry Failure to measure effectively the level of liquid in bottom of the tower lead to --- Fire --- Explosion
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
Data Transmission Element
Variable Conversion Element
Variable Manipulation 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.
Functional Elements of an Instrument (Cont‘d)
Fluid Temperature Measured Quantity
Process/ Measured medium
Tubing
Temperature Tube
Primary Sensing Element
Variable Conversion Element
Pressure
Data Transmission Element
Pressure
Variable Conversion Element
Motion
Variable Manipulation Element
Motion
Data Presentation Element Observer
Spiral Bourdon Tube
Linkage Gear
Scale & Pointer
Performance Characteristics of Instruments
Static characteristics Dynamic characteristics
• 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
Performance Characteristics of Instruments (Cont‘d)
Static characteristics Static characteristics of an instrument includes; Accuracy Precision Repeatability Range Resolution
Others ( Sensitivity , Dead zone etc.)
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.
Sensor : Thermocouple Output: e.m.f. Input: temperature
• 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
Out Amplifier Voltage change
Larger voltage change
• 3. Data presentation • This presents the measured value in a form which enables an observer to recognize it. Input:
Output: Display
Signal from system
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. Display
Input Sensor
True value of variable
Signal processor
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
Temperature signal
Signal Processor
B Resistance change
Data presentation
C Current change
Measurement of pointer across a scale
Static Characteristics
Static characteristics of an instrument includes; 1. Accuracy Accuracy is the degree of conformity of the measured value with the accepted standard or ideal value, which we can take as the Accuracy is usually expressed in engineering units or as a percentage of the sensor range, for example: true physical variable. 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
Precise Not accurate
Not precise 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 b 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
Static Characteristics • 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.
Static Characteristics
Static characteristics of an instrument includes; 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.
Static Characteristics
Static characteristics of an instrument includes; 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.
Static Characteristics
Static characteristics of an instrument includes; 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.
Drift
• 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
Point 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
Calibration 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
Dead Zone
• 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.
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.
Static Characteristics
Static characteristics of an instrument includes; 5. Range/Span 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.
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.
Static Characteristics
Static characteristics of an instrument includes; 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.
Static Characteristics
Static characteristics of an instrument includes;
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
Actual relationship Assumed relationship Increasing Non
error error
Instrument Reading
Decreasing Value measured
Actual relationship Assumed relationship
Non
error error
Measured Value
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 Error
•
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)
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 Output reading
Dead space
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.
Performance Characteristics
Dynamic characteristics Dynamic Characteristics of an instrument includes;
1. 2. 3. 4.
Speed of response Fidelity Lag 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
Principle measurements desired in industry (a) Temperature (b) Pressure (c) Level
(d) Flow (e) Others ( Composition, pH etc.)
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
Traceability Chain
Calibration center standard
In-company standards Process instruments
• 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%
