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6.7
Start-up and maintenance of
the TW probe
No start-up procedure is necessary for
TW probes supplied together with the
standard transmitter. When the electri-
cal connection of the transmitter to the
p ower supply unit has been made, the
TW probe is ready for use. However,
we recommend verifying whether the
transmitter's measuring range as set
by Pfaudler complies with your require-
ments. The measuring range setting is
indicated on a rating place each on the
transmitter and in the terminal box as
well as in the test report. If necessary,
the measuring range can be changed.
Upon final acceptance in the factory, a
functional test and a single-point calibra-
tion will be performed. A test report will
be prepared which is attached to each
probe.
If the TW probe is ordered and delivered
without a transmitter, it is the operator's
responsibility to ensure that a suitable
transmitter is used and configured (refer
to Sect. 5.3). The values comprised in
the characteristic of the resistance ther-
mometer are provided in the attached
test report.
Due to its special construction (resist-
ance thermometer fused into glass layer)
and non-critical operating conditions, the
probes are not subject to ageing. There-
fore, inspections (calibration) of the
resistance are not necessary for measur-
ing reasons. As a result, the TW probe is
maintenance-free.
If it is used for processes, however, which
are associated with the risk of product
deposits, it must be verified whether
or not product has accumulated on the
probe carrier. Deposits must be removed
using suitable means and methods.
© Pfaudler GmbH · OI 302-8 e
6.8
Calibrating the TW probe
6.8.1 General instructions
As mentioned before, the resistance ther-
mometer has been fused into the glass
layer, thus forming an integral part of the
probe carrier. Calibrating the resistance
thermometer separately is not possible
for this reason. It is most practical to
calibrate the TW probe with the probe
carrier installed because stable, repro-
ducible temperature conditions at the T
probe are most conveniently achieved.
Calibration outside the reactor or pipe-
line is very problematic because there
is considerable heat dissipation, which
results in instable temperature condi-
tions at the resistance thermometer, due
to the shape and size of the probe car-
riers. For calibration, the reactor or the
pipeline must be filled either with prod-
uct or with water. A steady temperature
at the transmitter must be ensured dur-
ing calibration. For this reason, the termi-
nal box should be closed.
For calibrating tubular probes, the reac-
tor must be filled until the immersion
depth of the measuring location is at
least five times the tube diameter. When
calibrating inside a reactor, the agitator
must turn slowly in order to ensure an
even temperature distribution inside the
liquid.
6.8.2 Calibrating the measuring
chain
In order to calibrate the entire measuring
chain including the sensor, transmitter
and display unit, immerse a calibrated
reference thermometer into the liquid
and compare its measured value (ref-
erence value) with the value measured
by the TW probe (actual value). If it is
necessary to calibrate the TW probe at
different temperatures, we recommend
calibrating it in icy water at 0 °C and in
boiling water at 100 °C.
The result will be reliable information
concerning the accuracy of the entire
measuring chain and the actual tempera-
ture conditions in the subsequent pro-
duction process.
Temperature probes type T/TW
Temperature probes type T/TW
If an excessive, inadmissible measuring
deviation is detected during calibration,
the individual elements of the measuring
chain must be verified or calibrated.
6.8.3 Calibrating the sensor
Actually, it is not necessary to calibrate
the sensor (resistance thermometer) for
technical reasons because the sensor is
not subject to ageing (refer to Sect. 5.7).
However, in some cases the sensor has
to be calibrated to comply with internal
or process-related requirements. For
calibrating the probe, a process calibra-
tor capable of measuring the resistance
of the resistance thermometer and the
product temperature in a four-wire cir-
cuit must be used. The measured values
(actual values) must then be compared
to the programmed values (values taken
from the characteristic).
6.8.4 Calibrating the transmitter
To calibrate the transmitter, an adjusta-
ble precision resistor must be connected
to the transmitter input. Then, the resist-
ance values from the characteristic are
input one by one, and the related temper-
ature (programmed value) is compared
to the output value of the transmitter
(actual value). The values of the charac-
teristic are indicated in the test report
that is attached to each probe supplied.
6.8.5 Calibrating the display unit
In order to calibrate the display unit,
an additional, calibrated display unit
is looped into the transmitter's output
circuit, and the two values displayed as
actual and programmed values must be
compared.
6.9
Explosion protection
The tubular with an integrated TW probe
have been approved for use in potentially
hazardous atmospheres of zone 0 in
accordance with the EC type examination
certificate No. PTB 03 ATEX 2132 X.
© Pfaudler GmbH 15
15
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