As well as an almost endless range of special versions suitable for extremely diverse applications and their associated measurement duties, there are some thermometers whose components are fully described by standard specifications. This type of thermometer - which always means the complete unit including the measuring element itself - has a modular construction: it is made up of the resistance sensor or thermocouple, the protection tube, the terminal head and the terminal block located inside it, as well as a process connection that could be, for example, a thread, flange, weld-in socket, compression gland or movable socket. The temperature detector or sensor denotes only the part of the thermometer that is directly affected by the measured variable, and contains the temperature-sensitive part of the thermometer [16].
Fig. 46: Section through an electrical thermometer
Measuring inserts are pre-assembled units comprising the sensor and the terminal block, where the resistance sensor is housed in a 6mm or 8mm diameter stainless steel insert tube. The insert tube is then inserted in the actual protection tube, such that the tip of the insertion tube fits flush with the bottom of the protection tube, to ensure good heat transfer. With this arrangement, the measuring insert can be easily replaced at a later stage. The fixing screws are spring-loaded to en- sure that flush-face contact is maintained even if the measuring insert and the protection tube ex- pand by different amounts longitudinally. The units are produced as single or dual probes, and their dimensions are laid down by DIN 43 762. Measuring inserts with an integral 2-wire transmitter are also produced.
In the case of measuring inserts for thermocouples, the sheath of a mineral-insulated thermocou- ple acts as the insertion tube at the same time. Again, the dimensions are based on DIN 43 762. If no measuring insert is used, the thermocouple or resistance sensor is mounted direct in the pro- tection tube, embedded in aluminum oxide, heat conducting medium or some other embedding compound suitable for the temperature range. After the sensor is inserted, the terminal block is fit- ted in the terminal head and the sensor connections soldered to it. The sensor cannot then be re- placed at a later stage; the complete thermometer must be replaced in this case.
Thermowells or pockets are often used to avoid this. These are a type of protection tube perma- nently attached to the machine or equipment; the thermometer is inserted into the pocket and se- cured with either a clamping screw or a wedge. Other versions have a G½ female pipe thread, so that a thermometer can be screwed in. The thermometer here may just consist of a measuring in- sert, but may also have its own protection tube. In the latter case, though, the response is then no- ticeably worse. The thermowell itself is either permanently welded to the machine (this is not possi- ble with protection tubes, because of their thin-wall construction) or has a male thread, normally a pipe thread. Thermowells like this are very useful and they are indeed widely used. For example, when a pocket is used, a thermometer can be removed without having to de-pressurize or drain a system. In contrast to thermometers with measuring inserts, where this is also possible in theory, there is no need to open the terminal head in this case.
Of course, the connecting cable must also be disconnected when replacing the thermometer. However, because of space limitations, it is often easier to remove the thermometer from the ma- chine before disconnecting the cable, and the use of a thermowell is clearly expedient here. As the thermowell comes into direct contact with the medium, it must meet the same requirements with regard to chemical and mechanical resistance as the protection tube would otherwise have to meet. Because of the materials available, pockets are mainly used in medium temperature applica- tions, such as boiler plant and the like.
8.1.1
Terminal heads to DIN 43 729
DIN 43 729 defines two versions of terminal head; they differ in size and also slightly in shape. The materials used can be cast iron, aluminum or plastic. The standard expressly indicates that the external shape and all features that do not affect the universal usability, as well as the method of fixing the cover, are left to the designer. So it is only necessary to comply with the specified dimen- sions to the extent that the universal usability is maintained. The dimensions specified for the over- all size of the terminal block should be regarded as minimum values, those for the total overall height as maximum values. Because of this, there are also a number of different versions suitable for special requirements. Neither does the standard specify the degree of protection, although IP54 (splashproof) is normal. But user requirements dictate that versions with IP65 protection, or plastic or stainless steel versions are also needed.
For the two types of terminal head, the nominal size of the diameter of the bore to accept the pro- tection tube is as follows:
Form A : 22, 24 or 32mm,
Form B: 15mm or M24 x 1.5 thread.
The smaller terminal head, Form B, is the most widely used; the 2-wire transmitter is designed for use with this version.
Fig. 47: Various terminal heads