Measurement Principles

Static Characteristics of Instruments

If you have a thermometer in a room and its reading shows a temperature of 20°C, then it does not really matter whether the true temperature of the room is 19.5°C or 20.5°C. Such small variations around 20°C are too small to affect whether we feel warm enough or not. Our bodies cannot discriminate between […]

What is Hysteresis Effects

What is Hysteresis Effects?

Figure illustrates the output characteristic of an instrument that exhibits hysteresis. If the input measured quantity to the instrument is steadily increased from a negative value, the output reading varies in the manner shown in curve (a). If the input variable is then steadily decreased, the output varies in the manner shown in curve (b). […]

Sensitivity to Disturbance - Zero Drift and Sensitivity Drift

Sensitivity to Disturbance – Zero Drift and Sensitivity Drift

All calibrations and specifications of an instrument are only valid under controlled conditions of temperature, pressure etc. These standard ambient conditions are usually defined in the instrument specification. As variations occur in the ambient temperature etc., certain static instrument characteristics change, and the sensitivity to disturbance is a measure of the magnitude of this change. […]

Resolution in Measurement Systems

When an instrument is showing a particular output reading, there is a lower limit on the magnitude of the change in the input measured quantity that produces an observable change in the instrument output. Like threshold, resolution is sometimes specified as an absolute value and sometimes as a percentage of f.s. deflection. One of the […]

Threshold

If the input to an instrument is gradually increased from zero, the input will have to reach a certain minimum level before the change in the instrument output reading is of a large enough magnitude to be detectable. This minimum level of input is known as the threshold of the instrument. Manufacturers vary in the […]

Linearity in Measurement Systems

Sensitivity of Measurement

The sensitivity of measurement is a measure of the change in instrument output that occurs when the quantity being measured changes by a given amount. Thus, sensitivity is the ratio: scale deflection / value of measurand producing deflection The sensitivity of measurement is therefore the slope of the straight line drawn on Figure. If, for example, […]

Linearity in Measurement Systems

Linearity and Non-linearity in Measurement Systems

It is normally desirable that the output reading of an instrument is linearly proportional to the quantity being measured. The Xs marked on Figure show a plot of the typical output readings of an instrument when a sequence of input quantities is applied to it. Normal procedure is to draw a good fit straight line […]

Range or Span in Measurement Systems

The range or span of an instrument defines the minimum and maximum values of a quantity that the instrument is designed to measure.

Tolerance in Measurement Systems

Tolerance is a term that is closely related to accuracy and defines the maximum error that is to be expected in some value. Even as 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. When used […]

Precision Repeatability Reproducibility

Precision, Repeatability and Reproducibility in Measurement Systems

Precision is a term that describes an instrument’s degree of freedom from random errors. If a large number of readings are taken of the same quantity by a high precision instrument then the spread of readings will be very small. Precision is often, though incorrectly, confused with accuracy. High precision does not involve anything about […]

Accuracy And Inaccuracy (Measurement Uncertainty)

The accuracy of an instrument is a measure of how close the output reading of the instrument is to the correct value. In practice, it is more usual to quote the inaccuracy figure rather than the accuracy figure for an instrument. Inaccuracy is the extent to which a reading might be wrong, and is often […]

Instrument Types and Performance Characteristics

Instruments can be subdivided into separate classes according to several criteria. These subclassifications are useful in broadly establishing several attributes of particular instruments such as accuracy, cost and general applicability to different applications. Active and Passive Instruments Null Type and Deflection Type instruments Analogue and Digital Instruments Smart and Non Smart Instruments

Smart and Non Smart Instruments

The advent of the microprocessor has created a new division in instruments between those that do incorporate a microprocessor (smart) and those that don’t. Smart sensors are usually at least twice as accurate as non smart devices, have reduced maintenance costs and require less wiring to the site where they are used. In addition, long-term […]

Indicating Instruments and Instruments With Signal Output

The final way in which instruments can be divided is between those that merely give an audio or visual indication of the magnitude of the physical quantity measured and those that give an output in the form of a measurement signal whose magnitude is proportional to the measured quantity. The class of indicating instruments normally […]

Analogue and Digital Instruments

Analogue and Digital Instruments

An analogue instrument gives an output that varies continuously as the quantity being measured changes. The output can have an infinite number of values within the range that the instrument is designed to measure. The deflection type of pressure gauge is a good example of an analogue instrument. As the input value changes, the pointer […]

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