| Basic Information on Temperature Sensors |
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The seventh part of our series on the “Basic Principles of Sensor Technology”, produced incollaboration with Hans Turck GmbH & Co.KG, deals with temperature sensors. |
This article provides a better understanding of some important basic concepts that users will frequently encounter in practice, as well as an explanation of different temperature measuring principles. With the right sensors, companies can implement the automation in their production sites much easier and faster, whilst improving efficiency, quality and safety at the same time.
Together with pressure, temperature is one of the most important measuring variables in the process. The temperature in cooling circuits is not only monitored to ensure quality but also to monitor safety, and in the process industry, processes are controlled by the supply of thermal energy. The right sensor must be selected according to the application at hand. To ensure the optimum measuring result, it is therefore important to be familiar with the application and the disturbance variables involved, as well as to implement appropriate measures
What is hot - what is cold?
We are able to detect temperature differences and classify them thanks to the sensory apparatus of our own body. However, whilst we can define a body as cold or warm we are not able to quantify temperature itself. What, anyway, does temperature mean? Temperature describes the average kinetic energy per particle in a sample of matter. How can this type of energy be described in a scale in order to quantify temperature?
To define the temperature scale certain fixed points are used which are the temperature-dependent changes of state of certain materials. The two most known fixed points are absolute zero at 0K and the triple point of water (all three phases are present simultaneously) at 273.16 K.
These values make it possible to draw a straight line graph for temperature. Other fixed points were defined for values that are far from these points.
For example the triple point of neon at 24.5561 K or the freezing point of silver at 1234.93 K. Thermometers can then be calibrated with so-called fixed point cells. The values of the fixed points are defined in the latest version of the “International Temperature Scale of 1990” (ITS-90).
Measuring temperatures correctly
Temperatures can be measured using a wide range of methods. Starting from simple diodes that are found in virtually any electronic circuit for temperature monitoring up to high-precision noise thermometers, Thermometers can be divided into two groups: primary and secondary thermometers. What are the differences?
Primary thermometers are measuring instruments that enable the temperature to be determined without any calibration beforehand to other thermometers. In all cases, physical variables are measured that are linked with temperature by physical relationships. Examples of this are gas thermometers, noise thermometers or the measurement of black body radiation...
Author:
Klaus Ebinger
Product Manager, Process Sensors
Hans Turck GmbH & Co. KG, Mülheim/Ruhr
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