IEC 62397-2007 pdf Nuclear power plants – Instrumentation and control important to safety – Resistance temperature detectors

4.3.2 Resistance element material Platinum is used extensively for resistance thermometers in an NPP for both safety- and non- safety-related instrument applications. Platinum is a noble metal, relatively stable and unaffected by its surrounding environment. It resists corrosion, oxidation, and other forms of chemical attack. It is easily workable and can be drawn into fine wires. Platinum has a high melting- point, which shows little volatilization below 1 000 °C. Platinum can be obtained to a high degree of purity, which has a reproducible electrical and chemical characteristic over a wide range of temperatures. All this is evidenced by a simple linear and stable resistance temperature relationship that characterizes the platinum sensor. However, the electrical resistance of platinum wire is extremely sensitive to minute quantities of contaminating impurities and to strains; both of these characteristics may alter the simple resistance- temperature relationship. Other metals may also be used for resistance thermometers provided their accuracy, repeatability, response time, and reliability comply with the requirements of the applications. The sensing wire shall be mounted so as to be almost free of strains to avoid the strain gauge effect from causing extraneous changes in resistance. Furthermore, the thermometer shall be manufactured with the resistance element free of contaminants. 4.3.3 Seals and adhesives The RTD shall be hermetically sealed. The connector may or may not be an integral part of the RTD assembly. RTD used in a harsh environment, such as under high-temperature and/or radiation areas, shall be designed without organic material. The use of ceramic material is recommended. The tightness of the insulating termination shall be tested according to an adequate and proven procedure. Commonly suggested seals of the connector are glass-to-metal or ceramic-to-metal, which should have less than 10 –8 cm 3 /s leak rate when tested with helium at an atmospheric differential pressure.