How to determine the quality of a temperature sensor?

Thread Content

Preface // We often come across temperature sensors in electrical circuits. So, when a temperature sensor fails, do you know how to detect it? What are the other testing methods? This article mainly introduces the detection of whether a temperature sensor is good or bad, as well as the methods for such detection. What is a temperature sensor? A temperature transducer is a sensor that can detect temperature and convert it into a usable output signal. Temperature sensors are the core component of temperature measuring instruments, and there are many types available. Based on the measurement method, they can be divided into two main categories: contact and non-contact. According to the sensor material and characteristics of electronic components, they are classified into two types: thermal resistors and thermocouples. Working principle of temperature sensors: Temperature sensors convert temperature into usable output signals by utilizing the patterns in which various physical properties of materials change with temperature. Modern temperature sensors are extremely small in size, which enables their widespread use in various fields of industrial practice and brings countless conveniences and functions to our lives. How to test whether a temperature sensor is working properly: 1. If there is a gauge available, connect the sensor to it and place the sensor in an ice-water mixture. Check whether the gauge shows a value other than 0 degrees Celsius, and see if the reading changes. 2. If there is no gauge, consider the temperature measurement range of the sensor; you might look into three-wire platinum resistance temperature measurement. 3. Place the sensor in an ice-water mixture and use a multimeter to measure its resistance. Platinum resistors come in several typical types: PT100, PT1000, and PT200. Their resistance values in an ice-water mixture are 100 ohms, 1000 ohms, and 200 ohms respectively. 4. Hold the sensor; the reading changes accordingly, with a consistent magnitude of change. Measurement methods of temperature sensors: Based on whether the temperature-sensing element is in contact with the medium being measured, the measurement methods of temperature sensors can be divided into two main categories: contact type and non-contact type. Temperature measurement during contact: The method of contact-based temperature sensing involves bringing a temperature-sensitive element into contact with the object whose temperature is to be measured, thereby allowing for adequate heat exchange. When this heat exchange reaches equilibrium, the temperature of the temperature-sensitive element becomes equal to that of the object being measured. The output value of the temperature sensing sensor then reflects the level of temperature of the object in question. Commonly used contact-type temperature sensors include thermal expansion temperature sensors, thermocouples, thermal resistors, thermistors, and temperature-sensitive transistors. The advantages of such sensors are simple structure, reliable operation, high measurement accuracy, good stability, and low cost ； The disadvantages include significant lag (as sufficient heat exchange is required during temperature measurement), difficulty in measuring the temperature of moving objects, the temperature field of the object being measured being affected by the sensor’s contact, and the temperature measurement range being limited by the properties of the material used in the temperature-sensing element. Common contact-type temperature sensors mainly fall into two categories: those that convert temperature into non-electrical quantities and those that convert temperature into electrical quantities. Temperature sensors that are converted into non-electrical quantities are mainly thermal expansion type temperature sensors ； Temperature sensors that convert to electrical quantities mainly include thermocouples, thermal resistors, thermistors, and integrated temperature sensors. Since thermocouples, thermal resistors, and thermistors all belong to the category of thermoelectric sensors – devices that convert temperature into potential and resistance values – and are currently widely used in industrial production, they will not be discussed here. It mainly introduces the principles and applications of thermally expanded temperature sensors and integrated temperature sensors. (1) Thermal expansion temperature sensors: Thermal expansion temperature sensors operate based on the principle that liquids, solids, and gases undergo thermal expansion when heated. Therefore, this type of temperature sensor can be categorized into three main types: liquid expansion type, solid expansion type, and gas expansion type. The alcohol thermometers and mercury thermometers commonly used in daily life are liquid-expansion temperature sensors. It consists of a thin, transparent glass tube with scales, into which a liquid (alcohol or mercury) is filled. As the liquid expands or contracts within the tube due to changes in temperature, the temperature can be determined by reading the scale value corresponding to the surface of the liquid. A solid expansion temperature sensor is composed of a bimetallic strip formed by fastening two pieces of thermally sensitive metal with different coefficients of thermal expansion together. To improve sensitivity, the bimetallic strip is often designed in a spiral shape; one end of this spiral-shaped bimetallic strip is fixed, while the other end is connected to a pointer shaft. When the temperature changes, the free end of the spiral bimetallic strip connected to the pointer rotates around the central axis, thereby causing the pointer to indicate the corresponding temperature value on the dial. Gas expansion temperature sensors measure temperature based on the principle that the pressure of a gas enclosed in a sealed container changes with temperature; temperature sensors made using this principle are often also referred to as pressure-type temperature sensors. As the temperature changes, the gas pressure inside the bulb also changes. This pressure is transmitted through capillaries, which in turn causes the Bourdon tube to move, thereby changing the position of the pointer on the dial. This allows for the determination of the temperature of the bulb, that is, the temperature being measured. (2) Integrated temperature sensor: Since the forward voltage drop across a transistor’s PN junction changes with temperature at a rate of approximately -2 mV/°C, and this rate remains relatively stable, and moreover the base-emitter voltage of a transistor has a basically linear relationship with temperature, these characteristics can be utilized to measure temperature. ①The basic working principle of an integrated temperature sensor: By integrating the temperature-sensing transistor with the excitation circuit, amplification circuit, and other components on a small silicon chip, an integrated temperature sensor is created. Compared to other temperature sensors, it offers advantages such as high linearity, high accuracy, small size, fast response, and low cost ； The disadvantage is its narrow temperature measurement range, generally ranging from -50°C to 150°C. ②+Current-output integrated temperature sensors: The outputs of integrated temperature sensors fall into two categories – voltage output and current output – with current-output types being more widely used. Non-contact temperature measurement: Whenever an object is heated, a portion of the heat is converted into radiant energy (also known as thermal radiation). The higher the temperature, the more energy is radiated into the surroundings, and there is a specific functional relationship between these two quantities. Since non-contact temperature measurement utilizes the thermal radiation of an object, it is often also referred to as radiative temperature measurement. A contactless temperature measurement system generally consists of two parts: a. an optical system, which focuses the radiation from the object being measured onto the detection element by aiming at it; b. a detection element, which converts the focused radiant energy into an electrical signal. +Non-contact temperature sensors can be classified into radiation-type temperature sensors, luminance-type temperature sensors, and colorimetric temperature sensors based on their input signals. The following are introduced separately. (1) Radiative temperature sensors: Radiative temperature sensors are divided into full-radiation temperature sensors and partial-radiation temperature sensors. ①A total radiation temperature sensor measures temperature by utilizing the relationship between the total radiation energy of an object across the entire spectrum and its temperature. Since it measures radiation across all wavelengths, it is desirable for the optical system to have broad spectral characteristics; furthermore, the thermosensitive detection element used should also be one that has no spectral selectivity. ②To improve the sensitivity of thermal radiation temperature sensors, detection elements with spectral selectivity are sometimes used, depending on the requirements of specific measurements. The common sensing elements of partial thermal radiation temperature sensors include photovoltaic cells, photosensitive resistors, infrared detection elements, etc. Below, we briefly introduce the temperature measurement principle of infrared temperature sensors. Any object in nature, as long as its temperature is above absolute zero, emits infrared light to radiate energy outward. The amount of energy emitted is directly related to the temperature of the object, and it can be expressed by the formula: E=3D(T4−T04), where E represents the total amount of infrared radiation per unit area and per unit time from the object at temperature T ； ――Stefan-Boltzmann constant: =5.67*10-8 W/m2K4 – the emissivity of an object, that is, the ratio of the radiative capacity of the object’s surface to that of a black body; T – the temperature of the object (K) ； T0――The ambient temperature surrounding the object (K). Infrared temperature sensors have a wide temperature measurement range, from -50 to over 3000. In different temperature ranges, the wavelength distribution of the electromagnetic wave energy emitted by the object varies; at normal temperatures (0–100), the energy is primarily concentrated in the mid-infrared and far-infrared wavelengths. The main optical system has two functions: it focuses the infrared light from the location being measured onto the detection element ； Limit the infrared emission surface that enters the instrument to a fixed range ； The detection element converts infrared energy into electrical signals ； The signal processing unit processes the signals output by the sensing elements using electronic and computer technologies, transforming them into various analog and digital information forms that are needed by humans ； The display unit converts the processed signals into numbers or charts that are readable by humans ； A targeting system is used to aim (or indicate) the area being measured; some infrared temperature sensors do not require targeting. (2) Luminance-type temperature sensor: A luminance-type temperature sensor utilizes the principle that the monochromatic radiant luminance LλT of an object changes with temperature; it measures temperature by comparing the luminance in a narrow spectral range of the object under measurement with the luminance of a standard radiator. Since the monochromatic radiation emission coefficient ελ of a real object is less than that of an absolute blackbody, i.e., ελ << 1, the monochromatic luminance LλT of a real object is less than that of an absolute blackbody. At temperature T, the monochromatic radiance L*λT of a perfect blackbody is given by the formula below, where c1 is the first radiation constant, c1 = 2C2; C2 is the second radiation constant, c2 = hc/k = 0.014388 m. (3) Colorimetric temperature sensors: Colorimetric temperature sensors measure temperature based on the principle that the ratio of the radiance at two wavelengths changes with temperature.