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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.     

 

 

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)   

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 :


 

 

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)  

Go to Section 8   (Connection Cables)

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