Measurement is finding a number that shows the size or amount of something. There are two main 'Systems of Measurement': Metric and US Standard.
In science, a measurement is a collection of quantitative or numerical data that describes a property of an object or event. A measurement is made by comparing a quantity with a standard unit. Since this comparison cannot be perfect, measurements inherently include error, which is how much a measured value deviates from the true value.

Level Measurement Instrumentation
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Measurement And Instrumentation Principles By Alan S Morris

It is useful to anyone studying measurement systems and instrumentation but it is provided mainly in support of the EC module D227 – Control System Engineering. This tutorial is mainly descriptive. Explain the basic working principles of a variety of temperature sensors.

Instrumentation is the science of automated measurement and control. Applications of this science abound in modern research, industry, and everyday living. From automobile engine control systems to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation is everywhere around us. This piece will focus on the fundamental principle of measurement terminology.

The objective of any measurement endeavour is to be able to measure a given process variable in order to possibly control it. Hence what you cannot measure, you cannot control.

Instruments are used to measure and control the condition of process streams as they pass through a Plant. Instruments are used to measure and control process variables such as: Temperature; Flow; Level; Pressure; Quality. Automatic instrument control systems are most commonly used to continually monitor these process conditions and correct them, without operator intervention, if there is a deviation from the process value required. The main reason for using automatic controls is that production is achieved more economically and safely. In fact, some of our processes could not be controlled in a stable condition without automatic control systems.

Although I have laid out the basic foundation for measurement and control here, you might check out:Measurement and Control Basics This book is very good for learning the basics of measurement and control.

Common Measurement Terminology:

Measured Variable:
Is the physical quantity or condition, which is to be measured. Common measured

variables are: Temperature, pressure, rate of flow, level, speed, etc

Measured Signal:
Is the electrical, mechanical, pneumatic, or other variable applied to the input of a device. In a thermocouple, the measured signal is an E.M.F, which is the electrical analogue of the temperature applied to the thermocouple. A measured signal is normally produced by the primary element (sensing element) of an instrument.

Input Signal:
Is a signal applied to a device element, or system. The pressure applied to the input connection of a pressure transmitter is an input signal.

Output Signal:
Is a signal delivered by a device, element, or system. The signal (3 to 15 psig, 4 to 20 mA dc, etc) produced at the output connections of a transmitter is an output signal.

Range

Is the region between the limits within which a quantity is measured, received or

transmitted, expressed by stating the lower and upper range-value.

Is the algebraic difference between the upper and lower range-values.

Range: -20 to 2000C, Span is 2200C; Range: 20 to 1500C, Span is 1300C.

Suppressed zero, is used when lower range-values is greater than zero

Elevated zero, is used when lower range-values is less than zero

Rangeability or turndown
Ratio of the maximum adjustable span / the minimum adjustable span for a given

Repeatability
Ability of an instrument to give identical indications or responses for repeated

applications of the same value of the quantity measured under the same conditions of used. Good repeatability does not guarantee accuracy.

Uncertainty

Range of values within which the true value lies with a specified probability

Uncertainty of +/-1 % at 95 % confidence means the instrument will give the user a

Is the ratio of the change in transducer output to the corresponding change in the

measured value, i.e. sensitivity = (change of output signal) / (change of input signal). For example: A pressure-to-current converter could have a sensitivity of 0.1 mA / mbar.

Accuracy

Accuracy is the conformity of an indicated value to an accepted standard value, or

true value. It is usually measured in terms of inaccuracy and expressed as accuracy.

It is a number or quantity, which defines the limit that errors will not exceed, when the device is used under reference operating conditions. The units to be used must be stated explicitly. It is preferred that a + and - sign precede the number or quantity.

Accuracy can be expressed in a number of forms:

Accuracy expressed in terms of the measured variable

Accuracy expressed in percent of span.

Accuracy expressed in percent of the upper range-value

Accuracy expressed in percent of actual reading

Accuracy for an instrument loop:

elements:

A transmitter with an accuracy of 0.5 %

Absolute accuracy: +or-1+ or -0.5+ or -1 = + or -2.5 %

Most likely accuracy (Root Mean Square) = SQRT[(±1)2 ±(0.5)2±(1)2] = 1.5 %

Absolute Error

Algebraic difference between the indication and the true value of a quantity to be

measured. Absolute Error = indication - true value. ΔX = X’ – X

Relative Error

Ratio between the absolute error and the true value of the quantity to be measured.

Hysteresis
Hysteresis is the difference in the output for given input when the input is increasing and output for same input when input is decreasing. When input of any instrument is slowly varied from zero to full scale and then back to zero, its output varies as shown in the diagram below

This is where the accuracy of the device is dependent on the previous value and the direction of variation. Hysteresis causes a device to show an inaccuracy from the correct value, as it is affected by the previous measurement.

Linearity expresses the deviation of the actual reading from a straight line. If all outputs are in the same proportion to corresponding inputs over a span of values, then input output plot is straight line else it will be non linear (see diagram below) For continuous control applications, the problems arise due to the changes in the rate the output differs from the instrument. The gain of a non-linear device changes as the change in output over input

varies. In a closed loop system changes in gain affect the loop dynamics. In such an

application, the linearity needs to be assessed. If a problem does exist, then the signal needs to be linearised.

When the output of a device is expressed as a function of time (due to an applied input) the time taken to respond can provide critical information about the suitability of the device. A slow responding device may not be suitable for an application. This typically applies to continuous control applications where the response of the device becomes a dynamic response characteristic of the overall control loop. However in critical alarming applications where devices are used for point measurement, the response may be just as important. The diagram below shows response of the system to a step input.

Ambient

Measurement & Instrumentation Principles