Measurement and Control Basics Fourth Edition

Chapter 7 - Temperature Measurement: Metal-Film RTDs

In the newest construction technique, a platinum or metal-glass slurry
film is deposited or screened onto a small flat ceramic substrate. It is then
etched with a laser-trimming system and sealed (see Figure 7-15). The
laser makes adjustment cuts to achieve the desired base resistance. The

Figure 7-15. Metal-film RTD

metal-film RTD substantially reduces assembly time and has the further
advantage of providing increased resistance for a given size. Because of
the technology with which the device is manufactured, the device size
itself is small, normally about one-half inch by one-half inch. This means it
can respond quickly to rapid changes in temperature. Film RTDs are currently
less stable than wound RTDs, but they are becoming more popular
because of their decided advantages in size and low production cost.

The common values of resistance for platinum RTDs range from 10 O to
several thousand ohms. The single most common value is 100 O at 0°C.
The resistance only changes about 0.4 percent per one degree change in
centigrade temperature. This change in resistance with temperature is
called the temperature coefficient (a). The standard temperature coefficient
of the platinum wire used in RTDs is 0.00385 O/°C in Europe and
0.00392 O/°C in America. These values are the average slope from 0 to
100°C. Electronic circuits are used to measure this small resistance change
with temperature.

 

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