German ALLEN BRADLEY sensors, ALLEN BRADLEY models, ALLEN BRADLEY prices

ALLEN BRADLEY sensors are sensors that can sense temperature and convert it into a usable output signal. Temperature sensors are the core component of temperature measuring instruments and come in a wide variety. They can be divided into two major categories based on the measurement method: contact and non-contact. Based on sensor materials and electronic component characteristics, they are classified into resistance thermometers and thermocouples.

ALLEN BRADLEY sensor classification

1) Contact type: The sensing part 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, so that the reading of the thermometer can directly indicate the temperature of the object being measured. Generally, the measurement accuracy is high. Within a certain temperature measurement range, the thermometer can also measure the internal temperature distribution of an object. However, significant measurement errors occur for moving bodies, 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. People also frequently use these thermometers 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 superconductivity, cryogenic 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. Cryogenic thermometers require sensing elements that are small in size, highly accurate, and have 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 cryogenic thermometers and can be used to measure temperatures in the range of 1.6 to 300K. 2) Non-contact type: Its sensitive element does not come into contact with the object being measured; it is also known as a non-contact temperature measuring instrument. This type of instrument can be used to measure the surface temperature of moving objects, small targets, objects with small heat capacity, or objects with rapid (transient) temperature changes. It can also be used to measure the temperature distribution of a temperature field. The most commonly used non-contact temperature measuring instruments are based on the basic laws of blackbody radiation and are called radiation thermometers. Radiation temperature measurement methods include the brightness method (see optical pyrometer), the radiation method (see radiation pyrometer), and the colorimetric method (see colorimetric thermometer). Various radiation temperature measurement methods can only measure the corresponding luminance 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. If the true temperature of an object is to be determined, correction for the material's surface emissivity must be applied. Since the surface emissivity of a material depends not only on temperature and wavelength but also on surface condition, coating, and microstructure, it is difficult to measure precisely. In automated production, radiation temperature measurement 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 together 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 energy 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. In the formula, ε is the surface emissivity of the material, 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 temperature at the bottom of the cavity (i.e., the medium temperature) to obtain the true temperature of the medium.

ALLEN BRADLEY sensor usage precautions

1) Errors introduced by improper installation: For example, the installation position and insertion depth of a 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 may escape from the furnace or cold air may enter. 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 the accuracy of temperature measurement. If the cold junction of the thermocouple is too close to the furnace body, the temperature may exceed 100 °C. Thermocouple installation should avoid strong magnetic and electric fields as much as possible; therefore, 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 the temperature of gas in a pipe, the thermocouple must be installed against the flow direction and have full 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 pull plate can cause poor insulation between the thermocouple poles and the furnace wall. This is more severe at high temperatures, causing not only loss of thermoelectric potential but also introducing interference. The resulting error can sometimes reach 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. This effect is particularly prominent during rapid measurements. Therefore, thermocouples with thinner thermoelectrodes and smaller protection tube diameters should be used as much as possible. When 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 the thermocouple fluctuation, 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 significant. 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 density of the material, 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, protection tubes made of materials with good thermal conductivity, thin walls, and 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. In this case, the temperature indication is lower than the true value of the measured temperature. Therefore, the exterior of the thermocouple protection tube should be kept clean to reduce errors.

ALLEN BRADLEY 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 the unheated parts. The magnitude of this potential difference is related to the temperature at the measurement point of the unheated part and to the materials 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 part is also 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 suitable for 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 susceptible to the temperature drift of preamplifiers. Therefore, they are not suitable for measuring minute temperature changes. Since the sensitivity of a thermocouple temperature sensor is unrelated to the thickness of the material.

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