Week 03 of 15 – Terms	Name: Answer Sheet

In your own words, write a definition for each of the following terms. Plagiarism will be checked and not tolerated.

Points: 5 pts/each = 19 * 5 = 95 pts

Dynamic

1. (of a process or system) characterized by constant change, activity, or progress. 2. A force that stimulates change or progress within a system or process.

Step change

The change from one value to another in a single increment in a negligible amount of time.

Time constant

This definition is straight from the ISA Definition of Terms Dictionary. If it is not fully understood at this time, as you proceed through the series of courses in the Certificate, it will become clearer. 1. The value t in an exponential response term. For the output of a first-order system that is forced by a step or an impulse, t is the time required to complete 63.2 percent of the total rise or decay. At any instant during the process, t is the quotient of the instantaneous ratio of change divided into the change still to be completed. In higher-order systems, there is a time constant for each of the first-order components of the process. 2. In process instrumentation, the value T in an exponential response term A exp (-t/T) or in one of the transform factors 1 + sT, 1 + jùT, 1/(1 + sT), 1/(1 + jùT), where: s = complex variable; t = time, seconds; T = time constant; j = square root of -1; ù = angular velocity, radians per second. Note: For the output of a first-order system forced by a step or an impulse, T is the time it takes to complete 63.2 percent of the total rise or decay. At any instant during the process, T is the quotient of the instantaneous rate of change divided into the change still to be completed. In higher-order systems, there is a time constant for each of the first-order components of the process. In a Bode diagram, break points occur at ù = 1/T. [ANSI/ISA-51.1-1979 (R1993); ISA-67.06-1984] 3. The length of time that is required for the output of a transducer to rise to 63 percent of its final value as a result of a step change of the measurand. [ISA-37.1-1975 R1982)]

Steady state

1. System response as time approaches infinity. 2. A characteristic of a condition, such as value, rate, periodicity, or amplitude, that exhibits only negligible change over an arbitrarily long period of time. Note: Steady state may describe a condition in which some characteristics are static, others dynamic. [ANSI/ISA-51.1-1979 (R1993); ANSI/ISA-77.42.01-1999] 3. A system in equilibrium, when all the potential opposing forces are at rest or balanced. In fiber optics, equilibrium mode distribution.

Dynamic error

1. Error which is caused by dynamic influences acting on the system such as vibration, roll, pitch or linear acceleration. This error may have an amplitude and usually a frequency related to the environmental influences and the parameters of the system itself. 2. A condition when the process variable and the instrument measuring it are both changing.

Transducer

This definition is straight from the ISA Definition of Terms Dictionary. 1. An element or device that receives information in the form of one quantity and converts it into information in the form of the same or another quantity. [ANSI/ISA-51.1-1979 (R1993)] 2. A device that is actuated by power from one system and transmits power in another form to a second system. An example is a pressure transducer that sends a signal that is proportional to the measured pressure. [ANSI/ISA-75.05.01-2000] 3. A device that provides a usable output in response to a specified measurand. Note: The term transducer is usually preferred to sensor and detector and to such terms as flowmeter, accelerometer, and tachometer. It is always preferred to pickup, gauge (when not equipped with a dial indicator), transmitter (which has an entirely different meaning in telemetry technology), cell, and end instrument. [ISA-37.1-1975 (R1982)] 4. A general term for a device that receives information in the form of one or more physical quantities; modifies the information and/or its form, if required; and produces a resultant output signal. Depending on the application, the transducer can be a primary element transmitter, relay, converter, or other device. Because the term transducer is not specific, its use for specific applications is not recommended. [ANSI/ISA-5.1-1984 (R1992)] See also primary element, signal transducer (signal converter), and transmitter.

Response curve

1. A curve graphically exhibiting the magnitude of the response of a sensitive device to a varying stimulus. 2. The graphical curve that is produced when a control that is at steady state has a step input applied that drives the control to another state. The response curve shows how well the control responded to the input change.

Accuracy

This definition is straight from the ISA Definition of Terms Dictionary. 1. The ratio of the error to the fullscale output or the ratio of the error to the output, as specified, expressed as a percentage. Note 1: Accuracy may be expressed in terms of units of measurand or as being within ±[number] percent of full-scale output. Note 2: The term accuracy should be used only for generalized descriptions of characteristics. It should not be used in specifications. The term error is preferred in specifications and other specific descriptions of transducer performance. [ISA-37.1-1975 (R1982)] 2. In process instrumentation, the degree of conformity of an indicated value to a recognized, accepted standard value or ideal value. [ANSI/ISA-51.1-1979 (R1983); ANSI/ISA- 75.05.01-2000] 3. The degree to which an indicated value matches the actual value of a measured variable. 4. Quantitatively, the difference between the measured value and the most probable value for the same quantity, when the latter is determined from all available data and critically adjusted for sources of error. 5. The deviation or error by which an actual output varies from an expected ideal or absolute output. Each element in a measurement system contributes to errors, which should be separately specified if they contribute significantly to the degradation of total system accuracy. 6. In an analog-to-digital converter, accuracy is tied to resolution. A 13-bit A/D, as used in the controller, for example, can resolve to one part in 213 or 8,192, so the best accuracy as a percentage of full-scale range is theoretically 1/8192, or about 0.0125 percent.

Precision

1. The degree of reproducibility among several independent measurements of the same true value. 2. The quality of being exactly defined or stated. 3. The value of the smallest incremental difference that can be measured by a given instrument or measurement system. 4. In an approximate number, the decimal position of the rightmost significant digit. See also repeatability and stability.

Repeatability

This definition is straight from the ISA Definition of Terms Dictionary. 1. The ability of a transducer to reproduce output readings when the same measurand value is applied to it consecutively under the same conditions and in the same direction. Note: Repeatability is expressed as the maximum difference between output readings. It is expressed as “within [number] percent of full-scale output.” Two calibration cycles are used to determine repeatability unless otherwise specified. [ISA-37.1-1975 (R1982)] 2. The closeness of agreement among a number of consecutive measurements of the output for the same value of the input under the same operating conditions and approaching from the same direction, for full-range traverses. [ANSI/ISA-67.04.01- 000; ANSI/ISA-75.05.01-2000] Repeatability is usually measured as a nonrepeatability and expressed as repeatability in percentage of span. It does not include hysteresis. [ANSI/ISA-51.1-1979 (R1993)] 3. In data processing, repeatability is normally considered to be a measurement of performance made over a very short period of time, short enough so that the gain and offset instabilities of system components are of no significance. In terms of the distribution of readings, repeatability is a measure of the maximum deviation of the readings from the mean value of the distribution. [ISARP55.1-1975 (R1983); ANSI/ISA-75.05.01-2000]

Steady-state accuracy

Steady-state or Static accuracy is the closeness of the approach to the true value of the variable when that true value is constant.

Dynamic accuracy

The closeness of approach of the measurement when the true value is changing.

Sensitivity

This definition is straight from the ISA Definition of Terms Dictionary. 1. The ratio of the change in the transducer’s output to a change in the value of the acceleration. Note: Where one sensitivity under defined conditions is the basis for determining deviations in performance, use “reference sensitivity.” 2. The ratio of the change in output magnitude to the change of the input that causes it after the steady state has been reached. [ANSI/ISA-75.05.01-2000] Note 1: sensitivity is expressed as a ratio in which the units of measurement of the two quantities are stated. (The ratio is constant over the range of a linear device. For a nonlinear device, the applicable input level must be stated.) Note 2: Sensitivity has frequently been used to denote the dead band. However, this usage is discouraged since it does not accord with accepted standard definitions of the term. [ANSI/ISA-51.1-1979 (R1993)] 3. The ratio of the change in transducer output to a change in the value of the measurand. Note: In the sense of the smallest detectable change in measurand use the term threshold. See threshold. [ISA-37.1-1975 (R1982)] 4. The ratio of change of the output to the change of the input. 5. Sensitivity is also defined as the least signal input that is capable of causing an output signal that has the desired characteristics. 6. The smallest change in the actual value of a measured quantity that will produce an observable change in an instrument's indicated or recorded output. 7. The minimum value of an observed quantity that can be detected by a specific instrument. 8. The degree to which a process characteristic can be influenced or changed by a small change in some physical or chemical stimulus.

Reproducibility

This definition is straight from the ISA Definition of Terms Dictionary. 1. In process instrumentation, the closeness of agreement among repeated measurements of the output for the same value of input made under the same operating conditions over a period of time, approaching from both directions. [ANSI/ISA-75.05.01-2000] Note 1: Reproducibility is usually measured as a nonreproducibility and expressed as reproducibility in percentage of span for a specified time period. Normally, this implies a long period of time, but under certain conditions the period may be a short time during which drift may not be included. Note 2: Reproducibility includes hysteresis, dead band, drift, and repeatability. See repeatability. Note 3: Between repeated measurements, the input may vary over the range, and operating conditions may vary within normal operating conditions. [ANSI/ISA-51.1-1979 (R1993)] 2. The ability of an instrument to duplicate, with exactness, measurements of a given value. Reproducibility is usually expressed as a percentage of span of the instrument. [ANSI/ISA-75.05.01-2000]

Hysteresis

This definition is straight from the ISA Definition of Terms Dictionary. 1. That property of an element such that the value of the output, for a given excursion of the input, depends upon the history of prior excursions and the direction of the current traverse. Note 1: Hysteresis is usually determined by subtracting the value of dead band from the maximum measured separation between upscale-going and down scale going indications of the measured variable (during a full-range traverse, unless otherwise specified) after transients have decayed. This measurement is sometimes called “hysteresis error” or “hysteretic error.” Note 2: Some reversal of output may be expected for any small reversal of input. This distinguishes hysteresis from dead band. [ANSI/ISA- 51.1-1979 (R1993)] 2. The maximum difference in output value for any single input value during a calibration cycle, excluding errors caused by dead band. This difference is sometimes called “hysteretic error.” [ANSI/ISA-75.05.01-2000] 3. The maximum difference in output, at any measurand value within the specified range, when the value is approached first with an increasing and then with a decreasing measurand. Note: Hysteresis is expressed in percentage of full-scale output during any one calibration cycle. Friction error is included with hysteresis unless dithering is specified. [ANSI/ISA-7.1-1975 (R1982)] 4. A phenomenon demonstrated by materials that make their behavior a function of the history of the environment to which they have been subjected. 5. The tendency of an instrument to give a different output for a given input, depending on whether the input resulted from an increase or decrease from the previous value.

Dead band

In process instrumentation, the range through which an input signal may be varied, upon reversal of direction, without initiating an observable change in output signal. Note 1: There are separate and distinct input–output relationships for increasing and decreasing signals. Note 2: Dead band produces phase lag between input and output. Note 3: Dead band is usually expressed in percentage of span. [ANSI/ISA-51.1-1979 (R1993)]

Drift

1. An undesired change in output over a period of time, where the change is unrelated to the input, environment, or load. See ANSI/ISA-S51.1-1979 (R1993). [ISARP67.04.01-2000.ISA-RP67.04.02-2000] 2. An undesired change in the output/input relationship over time. [ANSI/ISA-75.05.01-2000] 3. An undesired change in output over time that is not a function of the measurand. [ISA-37.1-1975 (1992)] 4. Drift is usually expressed as the change in output over a specified time with fixed input and operation conditions. It is usually used in the context of analog transducers, analyzers, and so on.

Rangeability

1. Describes the relationship between the range and the minimum quantity that can be measured. 2. The ratio of the maximum flow rate to the minimum flow rate of a meter. 3. The ratio of the largest flow coefficient (Cv or Kv) to the smallest controllable flow coefficient (Cv or Kv) within which the deviation from the specified flow characteristic does not exceed the stated limits. [ANSI/ISA-75.05.01-2000]

Turndown

Another way to measure rangeability, but in which the information is expressed differently.