Theory of industrial instrumentation

2년 전

Since I have already made an illustrated introduction on automation and its general components, I believe that some definitions need to be added at this point in order to lay the theoretical foundations for a detailed understanding of the future content. Everything described in previous articles concerning sensing devices and actuators can be framed within a branch of physics called industrial instrumentation. In order to understand the depth of the future content it is necessary to master certain concepts which I present below.

Industrial Instrumentation

DEFINITION It is the knowledge of the correct application of the equipment aimed at supporting the user in the measurement, regulation, observation and transformation of a given variable in a productive process.

To establish a technical control of the productive processes through different procedures.

Industrial Measuring Instruments

They are electrical, electronic, pneumatic or hydraulic equipment, which can achieve two main purposes:

First, they can give indications about something that is happening in the process by means of indicator needles, chart recorders or other means, but only as information. Secondly, they can give us the same indications as the previous case, but at the same time they can put into action some mechanism or variable to meet certain requirements of the process.


Because of its functions

They have no visible indication of the variables, such as pressure and thermostats, flow, pressure, level and temperature transmitters without indication.

They have an index and a graduated scale in which the value of the variable can be read.

They record the behavior of the variables over time with a continuous or dotted line.



They capture the process variable through the primary element and transmit it, remotely, in square inches or 4 - 20 mA DC electronics.

They receive a pneumatic input signal of 3 - 15 psi or 4 to 20 mA electronics from an instrument and after modification send it as a standard output signal.

They receive the signal from the transmitters and indicate or record it.

They compare the controlled variable with a desired value and take corrective action according to the deviation.

For its application - Tyres - Hydraulics - Electric - Electronics - Electromechanical - Mixed - Transducers - Amplifiers

Because of its location

  • Installed in the field
  • Locally installed
  • Installed on main board
  • Remotely installed
  • Indicators
  • Analyzers

For its technology

  • Discrete Systems
  • Direct digital control systems
  • Monitoring systems
  • Supervisory control and data acquisition systems




  • Sensing or capturing a given variable
  • Conditioning a given variable
  • Transmit a variable
  • Control a variable
  • Indicate the magnitude of a variable
  • Total up a variable
  • Enter a variable
  • Convert a variable
  • Alarm a variable by magnitude
  • Interrupt or allow a given sequence
  • Transmit a signal
  • Amplify a signal
  • Manipulate a process variable, among others


Digital systems
Digital systems operate on the binary number system, which implies that all circuit variables must be 1 or 0. The algebra used to solve problems and process information in digital systems is called Boolean algebra, which is based on logic rather than on the calculation of actual numerical values. Boolean algebra considers logical statements to be true or false, depending on the type of operation they describe and whether the variables are true or false. True corresponds to digital value 1, while false corresponds to 0.

Discreet systems
These are those made up of different units as opposed to the continuous magnitudes (length, time) or of a variation that takes place by integer quantities.

Specific heat
The amount of heat required to raise the temperature of a unit of mass of a substance by one degree. In the International System of Units, specific heat is expressed in joules per kilogram and kelvin; sometimes it is also expressed in calories per gram and degree Celsius. The specific heat of water is one calorie per gram and degree centigrade, i.e., one calorie per gram of water must be supplied to raise its temperature by one degree centigrade.

The amount of energy of a thermodynamic system that it can exchange with its environment. For example, in a chemical reaction at constant pressure, the enthalpy change of the system is the heat absorbed or released in the reaction. In a phase change, for example from liquid to gas, the enthalpy change of the system is the latent heat, in this case the vaporization heat.

A state function that measures the disorder of a physical or chemical system, and therefore its proximity to thermal equilibrium.

Measuring range
Spectrum or set of values of the measured variable that are within the upper and lower limits of the measurement or transmission capacity of the instrument; it is expressed by establishing the two extreme values. For example: the measuring range of a given temperature instrument is 100 to 300°C.

Range (span)
It is the algebraic difference between the upper and lower values of the instrument's measuring range.

It is the algebraic difference between the value read or transmitted by the instrument and the real value of the measured variable.

Uncertainty of the measure
It is the dispersion of values that can reasonably be attributed to the true value of the measured magnitude. The calculation of the uncertainty involves the statistical distribution of the results of measurement series, the characteristics of the equipment, among others.

It is the quality of a measuring instrument by which it tends to give readings close to the true value of the measured magnitude.

Accuracy is the measurement or transmission tolerance of the instrument (range where the measurement magnitude is allowed), and defines the limits of errors made when the instrument is used under normal operating conditions for a given period of time (normally 1 year).

Dead zone
This is the value field of the variable that does not vary the indication or the output signal of the instrument, i.e. does not produce its response. It is given as a percentage of the scope of the measure.

This is the ratio between the reading increment and the increment of the variable that causes it, after the state of rest has been reached.

This is the ability of the instrument to reproduce the positions of the boom or index or output signal by repeatedly measuring identical values of the variable under the same operating conditions and in the same direction of variation, across the entire field. Its maximum value (maximum repeatability) is generally considered and expressed as a percentage of the range; a representative value is +1%.

It is the maximum difference observed in the values indicated by the index or the pen of the instrument for the same value of any of the measuring range, when the variable goes through the whole scale in both directions, ascending and descending. It is expressed as a percentage of the scope of the measure.

Measuring range with zero elevation
This is the measuring range in which the zero value of the measured variable or signal is greater than the lower value of the range. For example, -10º to 30º.

Measuring range with zero suppression
This is the measuring range in which the zero value of the measured variable or signal is less than the lower value of the range.

Zero elevation
This is the amount by which the zero value of the variable exceeds the lower value of the field. It can be expressed in units of the measured variable or in % of the range.

Zero suppression
This is the amount by which the lower value of the field exceeds the zero value of the variable. It can be expressed in units of the measured variable or in % of the range.

A measure of the probability that an instrument will continue to perform within specified error limits over a specified period of time and under specified conditions.

Magnitude of the step changes in the output signal (expressed as a percentage of the full scale output) as the measurement is continuously changing throughout the field. It is also the degree to which the instrument can discriminate between equivalent values of an amount, or the smallest difference in value that the instrument can distinguish.

It is the ability of an instrument to maintain its performance during its specified storage and service life.

Serving temperature
This is the temperature range in which the instrument is expected to operate within specified error limits.

It is the property of the result of measurements made with an instrument or a standard, such that it can be related to national or international standards, through an interrupted chain of comparisons, with all the uncertainties determined.

In this way we establish the theoretical bases that will allow us to develop a little more technical subjects in the future, it is necessary to handle these concepts in order to understand the efficiency of the instruments associated with an automatic system. For today I finish this article not without first thanking @steempress for the support given in my previous articles, thanking you for reading and thanking God for the opportunity to share this article with you. Steemit is expanding to other blog ecosystems, will soon be official with Smart Media Tokens and is already possible for wordpress thanks to the @steempress plugin, a revolutionary initiative. If you wish to support the project I invite you to vote for @steempress as a witness by clicking here

All images used in this article are my own, photographs taken during my professional work. I allow the use of these images as long as they are mentioned to me in the article and the source is cited.




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