Measurement and Control Basics Fourth Edition

Chapter 7 - Temperature Measurement: Reference Temperatures

We cannot build a temperature divider the way we can a voltage divider,
nor can we add temperatures as we would add lengths to measure distance.
Instead, we must rely on temperatures established by physical phenomena
that are easily observed and consistent in nature.

The International Temperature Scale (ITS) is based on such phenomena.
Revised in 1990, it establishes the seventeen reference temperatures shown
in Table 7-1. The ITS-90, as this new version is called, is designed so that
temperature values obtained on it do not deviate from the Kelvin thermodynamic
temperature values by more than the uncertainties of the Kelvin
values as they existed at the time the ITS-90 was adopted. Thermodynamic
temperature is indicated by the symbol T and has the unit known as the
Kelvin, symbol K. The size of the Kelvin is defined to be 1/273.16 of the
triple point of water. A triple point is the equilibrium temperature at
which the solid, liquid, and vapor phases coexist.

EXAMPLE 7-2

Problem: Express a temperature of 200°F in degrees Celsius and then
degrees Kelvin.

Solution: First, we convert 200°F to degrees Celsius as follows:

Now, we convert the temperature in degrees Celsius to Kelvin as follows:

Table 7-1. Defining Fixed Points of the ITS-90

Since these fixed temperatures are our only reference, we must use instruments
to interpolate between them. However, achieving accurate interpolation
can require the use of some fairly exotic transducers, many of which
are too complicated or expensive to use in process control applications.

 

 

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