Device for avoiding local overheating on measuring transducers

In order to avoid local overheating on the surface of an intrinsically safe measuring transducer provided with current limiting resistors and embedded in a sealing composition, a body of a good heat conducting material is arranged within the sealing composition in the vicinity of the current limiting resistors.

BACKGROUND OF THE INVENTION 
The invention is directed to a device for avoiding local overheating of 
intrinsically safe measuring transducers embedded in a sealing composition 
and provided with current limiting resistors with the assistance of bodies 
made of a good heat conducting material. 
It is customary in chemical process engineering to raise the signals 
sensitive to disturbances of primary elements such as, e.g., 
thermoelements and resistance thermometers, to a signal level which only 
has a slight sensitivity to disturbance through the use of measuring 
transducers, also called transmitters, so that these signals can be 
carried over great distances. Thus, it is customary, for example, to use 
as the carrying signal a current impressed by the measuring transducer 
depending on the size of the measuring signal. 
It is suitable for reducing the disturbance to hold the cable between the 
measuring element and the measuring transducer as short as possible. 
Therefore, customarily such measuring transducers are mounted in the 
direct vicinity of the measuring assembly of, e.g., thermoelements. It is 
ideal to install the measuring transducer in the connection head of the 
assembly itself. 
There is frequently the danger of explosions in chemical plants, especially 
in organic chemistry plants. Electrical machining employed in these areas, 
therefore, are not allowed to contain any undesired ignition sources. In 
regard to this, there are a set of corresponding legal codes (e.g., 
Factory Mutual Approval Standards). 
Various types of protection are permitted to protect electrical machinery 
from explosion. For measuring apparatus, such as, e.g., measuring 
transducers, there has especially come into use the type of protection 
called "intrinsically safe". With this type of protection, care must be 
taken that the voltages and currents occurring in machinery in a space 
endangered by the possibility of explosion are held so small that their 
energy is not sufficient for ignition of explosive gas mixtures. In other 
words, care must be taken so that there is no local overheating in the 
measuring transducer in the area subject to the danger of explosion which 
would release an explosion if it came into contact with an ignitable 
mixture. Therefore, it is customary to tightly seal such measuring 
transducers with special sealing compositions. 
Local overheating also must be safely avoided if a disturbance occurs and 
if a partial or complete short circuit is present in the measuring 
transducer. Therefore, it is customary in this case to also embed in the 
sealing composition especially reliable prominent current limiting 
resistors. However, in the case of a disturbance, wattages are transformed 
in these resistors which lead to excess temperatures at the resistor which 
themselves are sufficient for ignition. It is true that through the 
sealing composition the ignitable mixture is kept away from the hot 
resistor, but care must be taken that the outer temperature of the sealing 
composition at no place reaches the ignition point. Futhermore, the 
sealing coposition cannot be allowed to be damaged at any place by the 
high excess temperature of the current limiting resistors so that no 
cracks are formed so that the ignitable mixture reaches the hot resistor. 
This creates difficulties in small structures in which it is easy to have 
localization of heat, such as in, e.g., transmitters which should be 
mounted in the connection head of assemblies. 
There is known from German OS No. 2937218 a device for avoiding local 
overheating on transmitters in sealing compositions, which transmitters 
are provided with current limiting resistors whereby the surfaces of the 
sealing compositions are provided with one or more good heat conducting 
boundary surfaces for the dissipation of heat. There are specially used 
for this purpose sheets and panels made of copper or aluminum. However, 
this apparatus has the disadvantage that there must be contact protection 
for the metal surfaces which can occur through ground. Therefore, in many 
cases, there must be provided additional measures for grounding which can 
lead to problems in high frequency sweep radiation since the potential of 
the inner circuitry of the measuring transducer customarily must be 
independent of the ground potential. 
Therefore, it was the purpose of the present invention to develop an 
apparatus for avoiding local overheating of "intrinsically safe" measuring 
transducers embedded in sealing compositions and provided with current 
limiting resistors with the assistance of bodies made of a good heat 
conducting material such that additional grounding is unnecessary. 
SUMMARY OF THE INVENTION 
This problem has been solved according to the invention by arranging the 
body within the sealing composition in the vicinity of the current 
limiting resistors. 
It is especially advantageous to arrange the bodies, preferably in the form 
of plates or hollow cylinders, in the direct vicinity of the current 
limiting resistors. 
It has been surprisingly found that local overheating on and in the sealing 
composition also can be safely prevented if a good heat conducting body is 
arranged within the sealing composition and thus can exert either no or 
only minimal cooling functions. The body made of good heat conducting 
material thereby is embedded in the sealing composition in such a manner 
that it does not enter the surface of the sealing composition at any 
point. Therewith, contact protection is automatically provided and 
grounding is not necessary. 
The body preferably consists of metal, such as copper or aluminum, however, 
it can also be constructed of a good heat conducting ceramic, for example, 
beryllium oxide, aluminum oxide, or aluminum nitride.

DETAILED DESCRIPTION 
As shown in FIG. 1, there is embedded within the sealing composition 2 
current limiting resistor 3 of the measuring transducer 1. In the vicinity 
of the resistors 3, there is arranged a body 4 made of a good heat 
conducting material, e.g., copper. The signals of the transducer 1 are 
transmitted via the connecting terminal 5. 
The heat conducting bodies may be in the form of plates or hollow cylinders 
in the direct vicinity of the current limiting resistors. 
Measurements have shown that even in the case of disturbance, there does 
not occur point to point overheating on the surface of the measuring 
transducer which could serve as the source of ignition in the presence of 
an explosive atmosphere. 
The entire disclosure of German priority application No. P.3443702.9 is 
hereby incorporated by reference.