German VALVOTERMICA sensors VALVOTERMICA models VALVOTERMICA prices

VALVOTERMICA sensors are devices that convert optical signals into electrical signals. Their operating principle is based on the photoelectric effect. The photoelectric effect refers to the phenomenon where electrons in a material absorb photon energy upon illumination, resulting in corresponding electrical effects. Based on the different manifestations of the photoelectric effect, it is classified into three categories: external photoelectric effect, internal photoelectric effect, and photovoltaic effect. Photoelectric devices include phototubes, photomultiplier tubes, photoresistors, photodiodes, phototransistors, and photocells. The performance and characteristic curves of photoelectric devices are analyzed.

VALVOTERMICA sensor classification

1) Contact type: The sensing element of a contact temperature sensor has good contact with the object being measured; it is also known as a thermometer. The thermometer reaches thermal equilibrium through conduction or convection, allowing its reading to directly indicate the temperature of the object being measured. Generally, the measurement accuracy is high. Within a certain temperature measurement range, thermometers can also measure the internal temperature distribution of an object. However, significant measurement errors occur for moving objects, small targets, or objects with very small heat capacity. Commonly used thermometers include bimetallic thermometers, glass liquid thermometers, pressure-type thermometers, resistance thermometers, thermistors, and thermocouples. They are widely used in industrial, agricultural, commercial, and other sectors. These thermometers are also frequently used in daily life. With the widespread application of cryogenic technology in national defense engineering, space technology, metallurgy, electronics, food, medicine, petrochemical industries, and research into superconducting technology, low-temperature thermometers for measuring temperatures below 120K have been developed, such as low-temperature gas thermometers, vapor pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, low-temperature resistance thermometers, and low-temperature thermocouples. Low-temperature thermometers require sensing elements that are small in size, highly accurate, and offer good reproducibility and stability. A carburized glass resistance thermometer, made by carburizing and sintering porous high-silica oxygen glass, is one type of sensing element for low-temperature thermometers and can be used to measure temperatures in the range of 1.6 to 300K. 2) Non-contact type: Its sensing element does not come into contact with the object being measured; it is also known as a non-contact temperature measuring instrument. This instrument can be used to measure the surface temperature of moving objects, small targets, objects with small heat capacity, or objects with rapid temperature changes (transient), and can also be used to measure temperature field distributions. The most commonly used non-contact temperature measuring instruments are based on the basic laws of blackbody radiation and are called radiation thermometers. Radiation thermometry methods include the brightness method (see optical pyrometer), the radiation method (see radiation pyrometer), and the colorimetric method (see colorimetric thermometer). Various radiation thermometry methods can only measure the corresponding brightness temperature, radiation temperature, or colorimetric temperature. Only the temperature measured for a blackbody (an object that absorbs all radiation and reflects no light) is the true temperature. To determine the true temperature of an object, correction for the material's surface emissivity is required. Material surface emissivity depends not only on temperature and wavelength but also on surface condition, coating, and microstructure, making precise measurement difficult. In automated production, radiation thermometry is often used to measure or control the surface temperature of certain objects, such as the rolling temperature of steel strips in metallurgy, roll temperature, forging temperature, and the temperature of various molten metals in smelting furnaces or crucibles. In these specific cases, measuring the surface emissivity of the object is quite difficult. For automatic measurement and control of solid surface temperatures, an additional mirror can be used to form a blackbody cavity with the measured surface. The influence of additional radiation can increase the effective radiation and effective emissivity of the measured surface. By using the effective emissivity to correct the measured temperature via the instrument, the true temperature of the measured surface can ultimately be obtained. The most typical additional mirror is a hemispherical mirror. Diffuse radiation from the measured surface near the center of the sphere is reflected back to the surface by the hemispherical mirror, forming additional radiation and thereby increasing the effective emissivity, where ε is the material surface emissivity and ρ is the reflectivity of the mirror. For radiation measurement of the true temperature of gaseous and liquid media, a method involving inserting a heat-resistant material tube to a certain depth to form a blackbody cavity can be used. The effective emissivity of the cylindrical cavity after reaching thermal equilibrium with the medium is calculated. In automatic measurement and control, this value can be used to correct the measured cavity bottom temperature (i.e., the medium temperature) to obtain the true temperature of the medium.

VALVOTERMICA sensor usage precautions

1) Errors introduced by improper installation: For example, the installation position and insertion depth of the thermocouple may not reflect the true temperature of the furnace chamber. In other words, the thermocouple should not be installed too close to the door or heating elements, and the insertion depth should be at least 8 to 10 times the diameter of the protection tube; if the gap between the thermocouple protection tube and the wall is not filled with insulating material, heat escape from the furnace or intrusion of cold air may occur. Therefore, the gap between the thermocouple protection tube and the furnace wall hole should be blocked with insulating materials such as refractory clay or asbestos rope to prevent convection of hot and cold air from affecting measurement accuracy; if the cold junction of the thermocouple is too close to the furnace body, the temperature may exceed 100℃; thermocouple installation should avoid strong magnetic and electric fields as much as possible, so thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors; thermocouples should not be installed in areas where the measured medium has little flow. When using a thermocouple to measure gas temperature in a pipe, the thermocouple must be installed against the flow direction and have sufficient contact with the gas. 2) Errors introduced by deteriorated insulation: If the thermocouple insulation is compromised, excessive dirt or salt residue on the protection tube and terminal board can cause poor insulation between the thermocouple poles and the furnace wall, which is more severe at high temperatures. This not only causes loss of thermoelectric potential but also introduces interference, with resulting errors sometimes reaching hundreds of degrees. 3) Errors introduced by thermal inertia: Due to the thermal inertia of the thermocouple, the instrument's indication lags behind changes in the measured temperature, an effect that is particularly prominent during rapid measurements. Therefore, thermocouples with thinner thermoelectrodes and smaller protection tube diameters should be used as much as possible. If the measurement environment permits, the protection tube can even be removed. Due to measurement lag, the amplitude of temperature fluctuations detected by the thermocouple is smaller than the amplitude of furnace temperature fluctuations. The greater the measurement lag, the smaller the amplitude of thermocouple fluctuations, and the larger the difference from the actual furnace temperature. When using a thermocouple with a large time constant for temperature measurement or control, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature fluctuation may be large. To measure temperature accurately, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the thermocouple's hot junction, the material density, and the specific heat. To reduce the time constant, besides increasing the heat transfer coefficient, the most effective method is to minimize the size of the hot junction. In use, materials with good thermal conductivity, thin-walled protection tubes with small inner diameters are typically employed. In more precise temperature measurements, bare-wire thermocouples without protection tubes are used, but these thermocouples are prone to damage and should be calibrated and replaced promptly. 4) Thermal resistance error: At high temperatures, if a layer of coal ash or dust adheres to the protection tube, thermal resistance increases, hindering heat conduction, causing the temperature reading to be lower than the true value of the measured temperature. Therefore, the exterior of the thermocouple protection tube should be kept clean to reduce errors.

VALVOTERMICA sensor industry applications

Temperature sensors are the earliest developed and most widely used type of sensor. The market share of temperature sensors greatly exceeds that of other sensors. People began using temperature for measurement in the early 17 century. Supported by semiconductor technology, semiconductor thermocouple sensors, PN junction temperature sensors, and integrated temperature sensors were successively developed in this century. When two conductors of different materials are connected at a point and that junction is heated, a potential difference appears at their unheated parts. The magnitude of this potential difference is related to the temperature at the measurement point of the unheated parts and to the material of the two conductors. This phenomenon can occur over a wide temperature range. If this potential difference is measured precisely and the ambient temperature of the unheated parts is measured, the temperature of the heated point can be accurately determined. Because it requires two conductors of different materials, it is called a "thermocouple." Thermocouples made of different materials are used in different temperature ranges, and their sensitivities vary. Thermocouple sensors have their own advantages and disadvantages; their sensitivity is relatively low, they are easily affected by environmental interference signals, and they are also susceptible to the temperature drift of preamplifiers, making them unsuitable for measuring minute temperature changes. Since the sensitivity of thermocouple temperature sensors is unrelated to the thickness of the material.

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