# Static And Dynamic Characteristics Of Measurement Systems Pdf

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## Dynamic Characteristics of Instrument Systems: Dealing with Dynamic States

Facebook Twitter. Characteristics of electrical measuring instruments: The performance characteristics of electrical measuring instruments can be divided into two categories:. Some applications involve the measurement of quantities that are either constant or vary slowly with time.

Under these circumstances, it is possible to define a set of criteria that gives a meaningful description of the quality of measurement without interfering with dynamic descriptions that involve the use of differential equations.

These criteria are called static characteristics. The main static characteristics are i Accuracy:. It is the closeness with which an instrument reading approaches the true value of the quantity being measured.

Thus accuracy of a measurement means conformity to truth. It the important static characteristic of electrical measuring instruments. Accuracy can be specified in terms of inaccuracy or limits of errors and can be expressed in the following ways:.

The specification of this accuracy does not give any information about the accuracy at other points on the scale or in the words, does not give any information about the general accuracy of the instrument. The term precise means clearly or sharply defined. As an example of the difference in meaning of the two terms accuracy and precision, suppose that we have an ammeter which possesses high degree of precision by virtue of its clearly legible, finely divided, distinct scale and a knife edge pointer with mirror arrangements to remove parallax.

It is also the important static characteristic of electrical measuring instruments. However, the readings taken with this ammeter are not accurate, since they do not confirm to truth on account of its faulty zero adjustment. Zero stability defines the ability of an instrument restore to zero reading after the input quantity has been brought to zero, while other conditions remain the same.

This minimum increment in what is input is called resolution of the instrument. Thus, the resolution is defined as the smallest incremental of the input quantity to which the measuring system responds. Resolving power or discrimination power is defined as the ability of the system to respond to small changes of the input quantity. One of the major factors influencing the resolution of an instrument is how finely its output scale is subdivided. If the input to an instrument is increased very gradually from zero value, there will be some minimum value of input below which no output change can be observed or detected.

This minimum value of input defines the threshold of the instrument. This minimum value defines the threshold of the instrument. In specifying threshold, the first detectable output change is often described as being any noticeable measurable change.

It is primarily due to changes in operating conditions of the components inside the measuring system. The drift is noticeable as zero drift and sensitivity drift.

Zero drift is a deviation observed in the instrument output with time from the initial value, all the other measurement conditions are constant. This may be caused by a change in component values due to variation in ambient conditions or due to ageing. This is often called the zero drift coefficient related to temperature changes. It describes the closeness of output readings when the same input is applied repetitively over a short period of time, with the same measurement conditions, same instrument and observer, same location and same conditions of use maintained throughout.

It is affected by internal noise and drift. It is expressed in percentage of the true value. Measuring transducers are in continuous use in process control operations and the repeatability of the performance of the transducer is more important than the accuracy of the transducer, from considerations of consistency in product quality. The output signals and indications are checked for consistency over prolonged periods and at different locations.

Perfect reproducibility ensures interchangeability of instruments and transducers. For instance, the input applied to the instrument may not be sufficient to overcome the friction and will, in that case not move at all.

It is due to either static friction stiction , backlash or hysteresis. Dead zone is also known as dead band dead Space. All elastic mechanical elements used as primary transducers exhibit effects of hysteresis, creep and elastic after- effect to some extent. Hysteresis is the difference in the readings of an instrument, which fixed value of the input signal, which depends on whether that input value is approached from increasing or decreasing values of input. That is upscale and down scale deflections do not coincide when the measurement is made of the same value by the method of symmetry.

The non-coincidence between the loading and unloading curves is known as hysteresis. If the sensitivity is constant for all values from zero to full scale value of the measuring system, then the calibration characteristic is linear and is a straight line passing through the origin. If it is an indicating or recording instrument the scale may be made linear.

Linearity is the closeness of the calibration curve of a measuring system to a straight line. If an instruments calibration curve for desired input is not a straight line, the instrument may still be highly accurate. In many applications, however, linear response is most desirable. The range of indicating instruments is normally from zero to full scale value and the Span is simply the difference between the full scale and lower scale value.

But same instruments operate under a bias so that they start reading, for example, voltages from 5V to 25V only. The zero of these instruments is suppressed from indication by means of a bias.

In such case, the scale range is said to be from 5V to 25V and the scale span is i. Xiv Bias: Bias describes a constant error which exits over the full range of measurement of an instrument. The error is normally removable by calibration. While it is not, strictly speaking, a static characteristic of measuring instruments , it is mentioned here because the accuracy of some instruments, is sometimes quoted as a tolerance figure.

Tolerance, when used correctly, describe the maximum deviation of a manufactured component from some specified value. Total dynamic error is the phase difference between input and output of the measurement system. In the definition of fidelity any time lag or phase difference is not included. Fidelity needs are different for different applications. For other physical systems, electrical filters electronic amplifiers, the above criterion is relaxed with the result that their bandwidth specification extend to frequencies at which the dynamic sensitivity is It is usually specified as the time taken by the system to come close to steady state conditions, for a step input function.

Hence the speed of response is evaluated from the knowledge of the system performance under transient conditions and terms such as time constant, measurement lag, settling time and dead time dynamic range are used to convey the response of the variety of systems, encountered in practice.

System having small time constant attains its final output amplitude earlier than the one with larger time constant and therefore, has higher speed of response. This lag is usually quite small but it becomes quite significant where high-speed measurements are required.

Measurement lag is of two types. In retardation type, the response of the instrument begins immediately after a change in the measurand has occurred. In time delay type, the response of the system begins after a delay time after application of the input. For portable instruments, it is the time taken by the pointer to come to rest within - 0.

Smaller settling time indicates the highest speed of response. Settling time is also dependent on the system parameters and varies with the conditions under which the system operates. Tags Electrical measurements Measurements.

## Instrumentation and Process Control

The characteristics of measurement instruments which are helpful to know the performance of instrument and help in measuring any quantity or parameter, are known as Performance Characteristics. Performance characteristics of instruments can be classified into the following two types. The characteristics of quantities or parameters measuring instruments that do not vary with respect to time are called static characteristics. Sometimes, these quantities or parameters may vary slowly with respect to time. Following are the list of static characteristics.

Peter H. Before we can begin to develop an understanding of the static and time changing characteristics of measurements, it is necessary to build a framework for understanding the process involved, setting down the main words used to describe concepts as we progress. The basic entity needed to develop the knowledge is called data , and it is obtained with physical assemblies known as sensors that are used to observe or sense system variables. The terms information and knowledge tend to be used interchangeably to describe the entity resulting after data from one or more sensors have been processed to give more meaningful understanding. The individual variables being sensed are called measurands. The most obvious way to make observations is to use the human senses of seeing, feeling, and hearing.

## Dynamic Characteristics of Instrument Systems: Dealing with Dynamic States

The static characteristics of instruments are attributes that changes slowly with time. Static characteristics can be divided in to desirable and undesirable. The true length of a steel beam is 6 m. Three repeated readings with a laser meter indicates a length of 6. The accuracy based on maximum difference can be calculated as.

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Click here to visit Engineering Pro Guides. Facebook Twitter. Characteristics of electrical measuring instruments: The performance characteristics of electrical measuring instruments can be divided into two categories:. Some applications involve the measurement of quantities that are either constant or vary slowly with time. Under these circumstances, it is possible to define a set of criteria that gives a meaningful description of the quality of measurement without interfering with dynamic descriptions that involve the use of differential equations. These criteria are called static characteristics. The main static characteristics are i Accuracy:.

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