Temperature sensing element

According to the Code for Construction and Acceptance of Automated Instrumentation Projects GB50093-2002, when the pipeline does not meet the requirements for installing temperature sensing elements, an expansion tube should be added. Generally, pipes with a diameter of less than 50 should be equipped with 80 expansion sleeves.

What kind of temperature instruments are usually used on site? Thermometer?

Integrated thermometers or bimetallic thermometers are generally used on-site.

Types of thermometers (1) Liquid crystal thermometers: These use liquid crystals made with different formulations, each having a different phase transition temperature; when these phases change, their optical properties also change, causing the liquid crystals to appear to change color. If liquid crystals with different phase transition temperatures are applied to a piece of paper, the temperature can be determined from the change in the color of the liquid crystals. The advantage of this thermometer is its easy reading, while the disadvantage is its lack of precision; it is commonly used in aquariums for ornamental fish to indicate water temperature. (2) Resistance thermometers: The resistance of a metal increases as the temperature rises. When the temperature changes are small, its resistance is approximately linearly related to temperature; over larger temperature ranges, it can usually be represented by a simple quadratic polynomial. By measuring the resistance of a metal, one can determine the temperature. Such thermometers are usually made of platinum wire and can achieve precision of 10–3 degrees Celsius, making them suitable for accurate measurements. Due to its high melting point, platinum allows for a wider measurable temperature range, approximately from –250°C to 1200°C. (3) Gas thermometer: At a constant pressure, for gases with low density, there is a linear relationship between volume and temperature. A thermometer made using this relationship is called a constant-pressure gas thermometer. At a constant volume, for gases with low density, there is a linear relationship between pressure and temperature. A thermometer made using this relationship is called a constant-volume gas thermometer. (4) Rotary thermometer: A rotary thermometer is made of a coiled bimetallic strip. One end of the bimetallic strip is fixed, while the other end is connected to the pointer. Due to the different degrees of expansion of the two metal sheets, they curl to different extents at various temperatures; consequently, the pointer points to different positions on the dial. The temperature can be determined by reading the value on the dial. (5) Semiconductor thermometers: The resistance of semiconductors changes differently from that of metals; as the temperature rises, their resistance decreases, and the degree of this change is significant. Therefore, even small temperature changes can cause significant variations in resistance, resulting in thermometers with high precision; they are often referred to as temperature sensors. (6) Thermocouple thermometer: A thermocouple thermometer consists of two different metals connected to a sensitive voltmeter. At different temperatures, metal contacts generate different potential differences at the two ends of the metal. The potential difference is very small, so a sensitive voltmeter is required to measure it. The reason for the temperature can be known from the voltmeter reading. (7) Photometric pyrometer: When an object’s temperature is high enough to emit a large amount of visible light, its temperature can be determined by measuring the amount of thermal radiation it emits; such a thermometer is known as a photometric pyrometer. This thermometer is primarily composed of a telescope equipped with a red filter, along with a circuit that includes a small light bulb, a galvanometer, and a variable resistor. Before use, establish the relationship between the temperature corresponding to different filament brightness levels and the readings on the ammeter. When in use, align the telescope with the object to be measured, adjust the resistor so that the brightness of the bulb matches that of the object, and then the temperature of the object can be read from the ammeter.