Measuring Principle; Temperature Probe Type Tw; Construction Of The Tw Probe Reactors - Pfaudler TW Series Operating Instructions Manual

6

Temperature probe type TW

6.1

Measuring principle

The temperature probe Type TW will
also be referred as the TW probe in the
present Operating Instructions.
As already mentioned, a resistance ther-
mometer serves as the sensor of the TW
probe. The measuring principle of resist-
ance thermometers is based on the fact
that the electrical resistance of all con-
ductors and semiconductors changes
with the temperature. The amount of
change in referred to as varies depending
on the material used. The relative change
of the electrical resistance as a function
of the temperature, ∆R/∆t, is referred to
as the temperature coefficient. This value
is not constant over the whole tempera-
ture range. Rather, it is a function of the
temperature. The relationship between
resistance and temperature is subject
to higher-order mathematical polynomi-
als for calculating the characteristic of a
resistance thermometer.
The special properties of platinum, such
as high measuring accuracy, high tem-
perature resistance, chemical resistance,
an approximately linear characteristic as
well as an excellent reproducibility of the
thermo-electrical properties, were reason
enough to produce most resistance ther-
mometers from this metal today. Since
platinum can also be easily glassed, it is
also used for the TW probe.
© Pfaudler GmbH · OI 302-8 e
The resistance values of resistance ther-
mometers made of platinum have been
standardized and are referred to as Pt 100,
Pt 500 and PT 1000. The number speci-
fies the nominal resistance in Ohm at a
temperature of 0 °C. Therefore, a Pt 100
has a nominal resistance of R
0
For these standardized measuring resist-
ances, basic value series (characteris-
tics) have been specified in EN 60 751.
The measured signal is evaluated by the
transmitter based on this characteristic
in order to determine the temperature.
For the TW probe, no Pt 100 to the EN
standard was used as resistance ther-
mometer. For technical reasons, the
TW probe cannot be produced with an
exact resistance value of 100 Ω. The re-
sistance of the TW probe is 100 ± 3 Ω.
Therefore, it is absolutely necessary to
use a programmable transmitter (refer
to Sect. 6.3) that compensates the de-
viation from the 100 Ω characteristic in
order to evaluate the measured signal.
Temperature probes type T/TW Temperature probes type T/TW
6.2 Construction of the TW probe
The TW probe can be installed in the
following probe carriers:
Paddle-type baffles
n
Thermometer well
n
Quatro-Pipe
n
= 100 Ω.
It is not possible to install the TW probe in
a ring probe, a C baffle or a valve shaft.
The resistance thermometer consists of
a very thin platinum band that is spirally
wound around the end of the probe car-
rier tube. The platinum band is fused into
the glass layer. The maximum possible
number of measuring locations on a
tubular probe depends on the size of the
probe carrier. For more details, please
contact the Pfaudler Instrumentation
department. The measuring locations are
always at the same level. Measuring loca-
tions at different levels can only be pro-
vided using the T probe (refer to Sect. 5)
The supply wires of the resistance ther-
mometers are also made of platinum.
The supply wires fused into the glass are
routed close to the terminal box where
they are connected to terminal wires. In
turn, the terminal wires are introduced
into the terminal box where they are con-
nected to the transmitter type TH02-Ex
(or terminals).
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