An RTD (resistance temperature device) is a thermal sensor that measures temperature by relating the electrical resistivity of a material like platinum, copper, or nickel to its temperature - as a material's temperature increases, its resistivity increases in a predictable linear fashion. RTDs offer highly accurate and consistent temperature measurements and are commonly used to construct thermometers, with platinum being the most commonly used and stable material that follows a linear resistivity-to-temperature relationship over a wide range. RTDs require a stable current source and measure temperature by calculating resistance from the measured voltage.

1. Significance Of Resistance Temperature Device

2. A resistance temperature device (RTD) is a thermal
sensor that works on the measurement principle that
the electrical resistivity of any material varies directly
with its degree of sensible heat energy. The relationship
between the environmental temperatures and the
resistance of these devices is highly predictable. This
allows for a consistent and accurate measuring of the
amount of sensible heat in the detector. When the
devices are supplied with stable source of current and
resultant voltage drop across the resistor measured, its
force that opposes flow of current can be computed, and
its sensible heat also determined.

3. RTD is usually manufactured using a pure material,
mainly platinum, copper or nickel. The material used
always has a predictable variation of resistivity as its
internal energy changes. It is this predictable change
that is applied to determine its thermal energy changes.
Platinum is a noble metal having the most stable
conductivity versus resistivity relationship within a range
of different thermal conductivity range. Platinum is also
the best material for RTDs since it follows a linear
relationship in a highly repeatable manner.

4. Even though any value is achievable for nominal
resistivity, the most common is platinum 100 ohm.
Tolerance class also determines the accuracy of the
sensor. Different industry standards have also been put
in place to ensure accuracy is achieved. In addition,
using values of tolerance and nominal resistivity, the
functional features of their sensor can be defined.

5. Calibration may be performed beyond a hundred
degrees Celsius or below zero degrees Celsius. In this
case, the comparison method or the fixed point method
may be used. Fixed point calibration uses the best
accuracy calibration by using freezing point, triple point
or melting point of pure substances such as zinc, argon
and tin to generate a known repeatable fixed point
temperature.

6. Thin film elements have a detecting device which is
formed by depositing a relatively thin layer of resistive
substance, usually platinum, on a ceramic substrate. The
main disadvantage of this type is that they are less
stable compared to the wire-wound and coiled
counterparts. They also have different expansion rates
brought about by the substrate deposited that creates a
strain gauge effect.

7. Thermometers are constructed using RTDs in various
forms to offer greater accuracy, stability, and
repeatability in some cases compared to thermocouples.
Thermometers made using RTDs use the concept of
electrical resistance and require a constant power
source for effective operation. The opposition to flow of
current in these devices ideally varies linearly with
amount of heat sensible in their surroundings.

8. Platinum detecting wire should be kept free from any
contamination so as to ensure it retains its stability. A
platinum film or wire should be supported on a former
so that it gets minimal strains from its former or minimal
differential expansion, even though it is often resistant
to vibration. RTD assemblies manufactured using iron or
copper are also used in number of applications.

9. In industries where operations take place beyond 660
degrees Celsius, RTDs are not usually used as they get
uncontrollably contaminated. Their resistivity is
essentially zero at three Kelvins, therefore rendering
them useless. Compared to thermistors, RTDs have
slower response time and are less sensitive to small
temperature changes. A resistance temperature device
is, however, used to make thermometers which have
low drift, high accuracy and wide operation range.

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