Selection of Temperature Meters【Forward】
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This post was last edited by RSchufaqi on 2017-9-26 at 10:14. 1. General principles (1) Units and scales The scale unit for temperature instruments is uniformly the Celsius temperature (℃). (2) Insertion length of the detection (measurement) element: 1) The selection of the insertion length should be based on the principle of inserting the detection (measurement) element to a position that is sensitive to temperature changes in the medium being measured and is representative of such changes. But under normal circumstances, for the sake of ease of interchange, the length of one or two gears is usually chosen uniformly for the entire device. 2) When installing on flues, furnaces, and equipment with insulated masonry, the appropriate type should be selected based on actual needs. (3) The material of the protective cover for the detection (measurement) element should not be inferior to that of the equipment or pipelines. If the protective sleeve of a finished product is too thin or not corrosion-resistant (such as in the case of armored thermocouples), an additional protective sleeve should be used. (4) Temperature instruments, temperature switches, temperature detection elements, and transmitters installed in flammable and explosive areas should be of the explosion-proof type. 2. Selection of on-site temperature instruments (1) Accuracy class 1) For general industrial thermometers: Class 1.5 or Class 1 should be selected. 2) Thermometers for precision measurement and laboratory use: Class 0.5 or Class 0.25 should be selected. (2) Measurement range: 1) The maximum measured value shall not exceed 90% of the upper limit of the instrument’s measurement range, while the normal measured values are around 1/2 of the upper limit of that range. 2) The reading of a pressure-type thermometer should be between 1/2 and 3/4 of the upper limit of the instrument’s measurement range. (3) Bimetallic thermometer 1) It should be given priority when the requirements regarding measurement range, operating pressure, and accuracy are met. 2) The diameter of the casing is generally set at φ100mm; in locations with poor lighting, at higher heights, or at greater observation distances, φ150mm should be used. 3) For the connection method between the instrument housing and the protective tube, a universal type is generally recommended; alternatively, an axial or radial type can be chosen based on the principle of ease of observation. (4) Pressure-type thermometers are suitable for use in low temperatures below –80°C, in situations where close observation is not possible, in environments with vibration, and for local or panel display where high precision is not required. (5) Glass thermometers are used only in special situations where high accuracy is required, there is minimal vibration, no mechanical damage, and easy observation is possible. However, due to mercury hazards, glass mercury thermometers should not be used. (6) Base-mounted instruments: For measurement and control (regulation) instruments installed on-site or at the control panel, base-mounted temperature instruments are preferred. (7) Temperature switch is suitable for applications where temperature measurement requires a contact signal output.
This post was last edited by RSchufaqi on 2017-9-26 at 10:11. 3. Selection of centralized temperature instruments (1) Detection (measurement) elements 1) Based on the temperature measurement range, select a thermocouple, thermal resistor, or thermistor with an appropriate rating. 2) Thermocouples are suitable for general applications. Thermal resistors are suitable for vibration-free environments. Thermistors are suitable for applications that require fast response times in measurements. 3) Depending on the requirements for response speed of the object being measured, the following detection elements with time constants can be selected: ① Thermocouples: 600s, 100s, and 20s ； ②Thermal resistance: four levels of 90–180s, 30–90s, 10–30s, and <10s ； ③Thermistor: ＜1s. 4) Depending on the operating environment conditions, select the junction box according to the following principles: ① Standard type: for locations with favorable conditions ； ②Splash-proof, waterproof: humid or outdoor areas ； ③Flameproof type: flammable and explosive environments ； ④Socket type: Suitable only for special occasions. 5) Threaded connections can generally be used; flanged connections should be employed in the following situations: ① When installing on equipment, lined pipes, and non-ferrous metal pipes ； ②Crystallization, scarring, clogging, and highly corrosive media: ③ Flammable, explosive, and highly toxic media. 6) Thermocouples and thermal resistors used in special applications: ① In environments with temperatures above 870°C, reducing gases with a hydrogen content of more than 5%, inert gases, and vacuum conditions, tungsten-rhenium thermocouples or gas-jet thermocouples should be used ； ②For the temperature of equipment, pipeline outer walls, and rotating surface surfaces, surface or armored thermocouples and thermal resistors are used ； ③For media containing hard solid particles, use wear-resistant thermocouples ； ④When multiple temperature measurements are required within the same protection sleeve of the detection element, multiple thermocouples should be used ； ⑤To save special protective tube materials (such as tantalum), improve response speed, or when it is necessary to detect bending of the sensing element, armored thermocouples can be used. (2) Transmitter 1) For measurement or control systems that are used in conjunction with instruments that display standard signals, a transmitter is selected. 2) Where design requirements are met, it is recommended to use transmitters that integrate measurement and transmission functions. (3) Display instruments: 1) For single-point display, conventional indicators are suitable; for multi-point display, digital indicators are preferable. When it is necessary to access historical data, conventional recorders are appropriate. 2) For the signal alarm system, it is advisable to use an indicator or recorder with contact-based signal output. 3) For multi-point recording, a medium-sized recorder is recommended (such as a 30-point recorder). 4. Selection of auxiliary equipment (1) When multiple devices share one display instrument, a switch with reliable quality should be selected. (2) Thermocouples are used to measure temperatures below 1600°C. When changes in the cold junction temperature prevent the measurement system from meeting accuracy requirements, and the accompanying display instrument does not have an automatic cold junction temperature compensation function, an automatic cold junction temperature compensator should be used. (3) Compensation wire 1) Depending on the number of thermocouples, their calibration class, and the operating conditions, a compensation wire or cable that meets the requirements should be selected. 2) Select different grades of compensation wires or cables according to the operating temperature: Grade ordinary for –20～＋100℃ ； ②For -40～+250℃, use the heat-resistant grade. 3) In areas with intermittent electric heating or strong electric and magnetic fields, shielded compensation wires or shielded compensation cables should be used. 4) The cross-sectional area of the compensation wire should be determined based on the round-trip resistance value over its installation length, as well as the maximum external resistance that the associated display instruments, transmitters, or computer interfaces can accept.
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