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General
Description for RTD Elements |
Temperature is one of the
important basic or fundamental
quantity of a matter and it
defines the thermal energy of
the matter. It is measured in
various units like °C, °F,
and °K.
All these units are quite well
defined by International
Temperature scale ITS 90.
Among various methods of
temperature measurements,
thermocouples, resistance
thermometers and thermistors
are most popular. The reason |
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being they
provide electrical signals, which can be
connected to, sophisticated indicating,
controlling and recording instruments.
Over the range of –200 deg c to +962 deg
C, accepted standards is the Standard
PRT i.e. Platinum Resistance
Thermometers. The basic principle of
resistance thermometry is that the
resistively of metal depends upon it’s
temperature. It is an absolute method of
measurement (As compared to thermocouple
which measures temperature difference)
For a particular metal, a curve can be
obtained for resistance versus
temperature. The base metal used in
resistance thermometry are Platinum,
Copper and Nickel. Since early days,
resistance thermometry has gone to
considerable changes and they are now
extensively used in industries. At
temperature below 650 deg C, the
industrial resistance thermometer are
now more accurate and reliable than any
other sensors. The increase in use of
microcontroller based linearised
instruments increases the choice of RTDs
.
Among all the base metals, Platinum is
now a days the best choice in the
industries. Platinum being a noble metal
retains its characteristics for longer
time. For wide temperature range
Platinum Resistance thermometer follows
the equation as defined below,
2
Rt = Ro(1+At +
Bt )
Where Rt = Resistance at temperature t
Ro = Resistance at temperature = 0 Deg
C'
A and B are alpha and beta values of
Platinum resistors.
Various
construction of RTD elements
RTD elements are available in three type
of construction
1. Thin film type
2. Thick film type
3. Wire wound element
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Film
Technology |
Thick film is produced by spreading
Platinum paste through a silk screen
onto a ceramic substrate.
Thin film is produced by evaporation of
metal onto a substrate and then by
suitably itching with LASER technology.
The basic advantages of both types are
they are mass produced elements, they
are cheaper as well as they can work in
environment. Where extreme vibration is
presence
The
disadvantage of these elements are
1. The film is covered rigidly by glass layer and so the Platinum
is not free to expand. The result is
they can not maintain characteristics
specially at higher Temperature
2. The film contains very little metal and hence there are more
chance of contamination from the
impurities of the glass covering.
3. The unit have very little size and mass is also small. Hence
they are subject to self heating error
when sensing current is pass through it.
4. Control of homogenous of metal is difficult as compared to wire
used in wire wound elements
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Wire
Wound Elements |
Wire wound elements are basically a
“hand made “ product which involves lot
of skilled Labour.
It is manufactured by placing a coil of
thin wire (7 microns to 35 microns) into
a ceramic insulator. The resistance of
the coil is suitably adjusted to the
ohms corresponding to that particular
temperature. Ideally, this coil should
be freely suspended for natural
expansion and contraction. However, such
construction is not immune to vibration
and hence it is not adopted for
industrial application. For industrial
application, normally partially
supported or fully supported
construction is used which are quite
immune to vibration. The extended leads
are quite rugged as compared to the
alpha wire and normally made from Pt
base metal (for Platinum resistors). The
choice of ceramic insulator is also very
important as any impurities in the
insulator may detoriate the
characteristic of the alpha wire.
Techno offers wide range of wire wound
RTD Elements.
These elements are manufactured in India
under strict quality control.
Techno supplies these elements with
tolerance value of class A and Class B.
Tolerance of class A & B or defined by
following equation
for class
A.
Tolerance
= ± (0.15 ± 0.0025(T))
for class
B.
Tolerance
= ± (0.3 ± 0.005(T))
Where T = Temp in °C'
These elements are manufactured for use
between –200 deg C' to +500 deg C'
All these elements are supplied with
Platinum leads or Platinum based metal
leads of
10 mm length. For measuring tolerance,
the measuring wires are connected to the
lead at 5 mm from the closed end.
Normally, 1mA sensing current is
recommended for sensing purposes.
A 2.8 mm x30 mm element will have self
heating characteristics of 0.01 per deg
c/mW. In air, it increases about 30
times.
With adequate support, the elements will
withstand vibration level of 30g over
the frequency range of 10 Hz to 30 Hz.
( Sensing
Length at element)
S.L. = T.L. -3mm
Click Here for Specification of RTD Elements
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