7. Configuration of Resistance Thermometers:
A standardized resistance thermometer consists of an enclosure and
a replaceable measuring insert, which is fitted to the enclosure.
The measuring insert in the enclosure is protected against moisture
and against chemical and mechanical stress.
7.1 Resistance Thermometer Enclosures
The enclosure mainly consists
of the connection head, the protection tube and a process connection, which is
used to attach the enclosure to the operating site. Protection tubes with both
an (outer) diameter that is constant across their entire length and with a
conically tapered measurement tip are available. A stud-end welded on to the
protection tube or a flange or a re-adjustable flange that can be moved on the
protection tube or a movable threaded bush is usually used as the process
connection. Exceptions are enclosures with
weld-in protection tubes, for which the protection tube simultaneously
serves as the process
connection.
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a. Connection head
b. Screw assembly: Used to attach
the connection head.
c. Measuring insert: See also Figure
6 and the text under 7.2 (below)
d. Stud end: Process connection, external
thread usually G1/2" or G1"
e. Protection tube: One-piece tube from
connection head to measurement tip (only in the case of enclosures with a weld-in
protection tube are extension tube and protection tube
separate items that are screwed to one
another)
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7.2 Resistance Thermometer Measuring
Inserts
The measuring insert mainly
consists of an insertion tube containing the resistance detector and the inner
cables as well as a ceramic connection base mounted to a flange plate (round
blank). Within the insertion tube, a ceramic insulation rod is used to insulate
the inner conductors from one another as well as from the insertion tube. A
mineral-insulated sheathed cable, which, in terms of structure and
manufacture, is the same as a sheathed cable for sheathed
thermocouple assemblies, can also be used instead of
the rigid insertion tube and insulation rod. In this case,
however, copper wires are used instead of the
thermocouple wires as inner conductors, with the resistance detector being located
in a rigid metal tube welded to the
sheathed cable. The flange plate is welded to the insertion
tube/ sheath tube. Depending on connection technique and configuration of the
resistance detector (single or dual), there may be up to
eight feeder clamps on the connection base (e.g. for
2 x Pt 100 in four-wire circuit). The connection base usually
has spring-loaded mounting screws, which are used to attach the measuring
insert to the enclosure. The spring system
ensures that the tip of the measuring insert is always in
contact with the base of the protection tube (even
where protection tube and measuring insert have different thermal
expansions), which ensures that there is always good heat transfer.
Figure 6 shows various configurations of measuring inserts
:
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a. Mounting screws with springs
b. Connection base
with feeder clamps
c. Flange plate (round blank)
d. Insertion tube
(rigid)
e. Ceramic insulation rod
f. Inner conductor (e.g. silver
or nickel wires)
g. Insulation powder
h. Resistance detector
i.
Sheathed cable (consisting of sheath tube, insulation and copper inner
conductors)
k. Metal tube (the diameter
of this rigid tube can also be greater than
the diameter of the sheathed
cable)
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