Selection and installation method of thermocouple
In conventional industrial applications, the thermocouple element is generally terminated on the connector; However, the reference connection point is rarely located on the connector, but a suitable thermocouple extension wire is used to switch to a controlled environment with a relatively stable temperature. Connection point type The connection point of the shell-type thermocouple is physically connected (welded) to the probe wall, which can achieve good heat transfer-that is, heat is transferred from the outside through the probe wall to the thermocouple connection point. It is recommended to use a shell-type thermocouple to measure the temperature of static or flowing corrosive gases and liquids, as well as some high-pressure applications. In insulated thermocouples, the thermocouple connection point is separated from the probe wall and surrounded by a soft powder. Although the response speed of an insulated thermocouple is slower than that of a shell-type thermocouple, it can provide electrical insulation. It is recommended to use insulated thermocouples to measure corrosive environments. Ideally, the thermocouple can be completely electrically insulated from the surrounding environment by shielding.
The open-ended thermocouple allows the top of the connection point to penetrate deep into the surrounding environment. This type provides the best response time, but is limited to use in non-corrosive, non-hazardous, and non-pressurized applications. The response time is expressed by a time constant, which is defined as the time required for the sensor to change 63.2% between the initial value and the final value in the controlled environment. The open-ended thermocouple has the fastest response speed, and the smaller the diameter of the probe sheath, the faster the response speed, but the lower the maximum allowable measurement temperature.
Extension wire thermocouple: An extension wire is a pair of wires that have the same temperature and electromagnetic frequency characteristics as the thermocouple connected to it. When the connection is appropriate, the extension wire transfers the reference connection point from the thermocouple to the other end of the wire, which is usually located in the controlled environment.
1. The measured temperature range;
2. The required response time;
3. Type of connection point;
4. Chemical resistance of thermocouple or sheath material;
5. Anti-wear or anti-vibration ability;
6. Installation and restriction requirements, etc.
Installation requirements for thermocouples
1. In order to have sufficient heat exchange between the measuring end of the thermocouple and thermal resistance and the measured medium, the measuring point position should be selected reasonably. Try to avoid installing thermocouples or thermal resistances near valves, elbows, pipes and dead ends of equipment.
2. Thermocouples and thermal resistances with protective sleeves have heat transfer and heat dissipation losses, in order to reduce measurement errors. Thermocouple and thermal resistance should have sufficient insertion depth:
(1) For thermocouples that measure the temperature of the fluid in the center of the pipeline, the measuring end should generally be inserted into the center of the pipeline (vertical installation or inclined installation). If the pipe diameter of the fluid to be measured is 200 mm, the insertion depth of the thermocouple or thermal resistance should be 100 mm;
(2) Temperature measurement for high temperature, high pressure and high speed fluids (such as main steam temperature). In order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, a shallow insertion method of the protective tube can be adopted. Or use hot-jacket thermocouple, shallow plug-in thermocouple protection tube. The depth of its insertion into the main steam pipe should not be less than 75mm;
The standard insertion depth of hot-sleeve thermocouple is 100mm;
(3) If you need to measure the temperature of the flue gas in the flue. Although the flue diameter is 4m, the insertion depth of thermocouple or thermal resistance is 1m;
(4) When the measured original insertion depth exceeds 1m, it should be installed as vertically as possible, or a support frame and protective sleeve should be installed.