PT100 SENSORS FOR INDUSTRIAL USE
The use of a platinum resistance thermometer probe in industrial and scientific applications will normally
result in good, accurate temperature sensing without the need for special cables (unlike thermocouples).
To achieve the best possible accuracy and reliability, care is needed with installation of the probe,
connections, and choice of the host instrumentation.
CHOICE OF WIRE-WOUND OR FLAT FILM SENSING RESISTOR
The resistance element is produced in
one of two forms, either wire-wound or
flat film. Metal film resistors consist of a
platinum layer on a ceramic substrate; the
coil of a wire wound version is fused into
ceramic or glass. applications. Such sensors have fast
thermal response and their small thermal
mass minimises intrusion in the media
being tested. Such sensors are known
variously as flat film, thin film or chip
sensors.
WIRE-WOUND RESISTORS Fundamentally, every sensing resistor
is a two wire device. When terminating
the resistor with extension wires
during probe construction, a decision
must be made as to whether a 2,3 or
4 wire arrangement is required for
measurement purposes.
Various methods of detector construction
are employed to meet the requirements
of differing applications. The unsupported
"bird cage" construction is used for
temperature standards, and the partially
supported construction is used where
a compromise is acceptable between
primary standards and use in industrial
applications. Other constructional
methods include the totally supported
construction which can normally
withstand vibration levels to 100g, and the
coated wire construction where the wire is
covered with an insulating medium such
as varnish. The maximum operating range
of the latter method is limited by the wire
coating to usually around 250oC.
FLAT FILM RESISTORS
Flat film Pt resistors take the form of a thin
(1 micron) film of platinum on a ceramic
substrate. The film is laser trimmed to have
a precise Ro value and then encapsulated
in glass for protection.
A wide range of styles and dimensions
are produced to allow for different
In the sensing resistor, the electrical
resistance varies with temperature.
Temperature is measured indirectly by
reading the voltage drop across the
sensing resistor in the presence of a
constant current flowing through it using
Ohm's Law: V = R.I
The connection between the thermometer
assembly and the instrumentation is made
with standard electrical cable with copper
conductors in 2,3 or 4 core construction.
The cabling introduces electrical
resistance which is placed in series with
the resistance thermometer. The two
resistances are therefore cumulative and
could be interpreted as an increased
temperature if the lead resistance is
not allowed for. The longer and/or the
smaller the diameter of the cable, the
greater the lead resistance will be and the
measurement errors could be appreciable.
In the case of a 2 wire connection, little
can be done about this problem and some
measurement error will result according to
the cabling and input circuit arrangement.
For this reason, a 2 wire arrangement
is not recommended. If it is essential to
use only 2 wires, ensure that the largest
possible diameter of conductors is
specified and that the length of cable is
minimised to keep cable resistance to as
low a value as possible.
The use of 3 wires, when dictated either
by probe construction or by the input
termination of the measuring instrument,
will allow for a good level of lead
resistance compensation. However the
compensation technique is based on the
assumption that the resistance of all three
leads is identical and that they all reside at
the same ambient temperature; this is not
always the case.
Note: The wiring configuration (2,3,
or 4 wire) of the thermometer must
be compatible with the input to the
associated instrument.
HIGH ACCURACY OPTIONS (TOLERANCE CLASSES)
Assuming a 3 or 4 wire connection, and the
use of a class B sensing resistor, a standard
thermometer assembly will provide an
accuracy of around 0.5oC between 0oC
and 100oC. Considerable improvement
on this figure can be achieved by various
means including the use of closer
tolerance sensors.. The overall accuracy
of any measuring system (e.g. sensor,
instrument, interconnection, application
etc) is compromised by the sum of the
uncertainties in that system.
96
PECM Issue 27
Labfacility Ltd
Bognor Regis
Telephone: 01243 871280
[email protected]
www.labfacility.com