![]() When detecting water level, response time is typically 0.5 second and accuracy is within 2 mm. Thermal level and interface switches have no mechanical moving parts and are rated for pressures up to 3,000 psig and process temperatures from -75 to 175☌ (-100 to 350☏). They can also detect thermally conductive foams if spray-cleaned after each operation. Therefore, these switches can be used in difficult services, such as interfaces, slurry, and sludge applications. Since all process materials have a characteristic heat transfer coefficient, thermal level switches can be calibrated to detect the presence or absence of any fluid. This level switch is actuated when a change in bridge balance occurs. ![]() Their outputs will be nearly equal and the bridge will be in balance. When both probes are submerged in the process liquid, their temperatures will approach that of the liquid. While the sensor is in the vapor phase, the heated probe will be warmer than the reference probe, and the bridge circuit will be unbalanced. The outputs of both sensors are fed into a Wheatstone bridge (Figure 10-1). One probe is heated and the other provides an unheated reference. The switch is actuated when thisĪnother type of thermal sensor uses two resistance temperature detectors (RTDs), both mounted at the same elevation. When the probe is submerged, the liquid absorbs more heat and the probe temperature drops. As long as the probe is in the vapor space, the probe remains at a high temperature, because low-conductivity vapors do not carry much heat away from the probe. One of the simplest thermal level switch designs consists of a temperature sensor heated with a constant amount of heat input. Thermal level switches sense either the difference between the temperatures of the vapor space and the liquid or, more commonly, the increase in thermal conductivity as a probe becomes submerged in the process liquid. Of the other level sensor technologies, refractory floats, refractory bubbler tubes, and proximity-type capacitance detectors also are used in molten metal service. Of the three level-switch designs discussed in this chapter, only the laser-based optical level switch is appropriate for use in molten metal level detection. Thermal switches can continue to work when lightly coated, but build-up does usually add a thermally insulating layer, ultimately slowing response time. Optical level switches are available with automatic washers to remove the build-up of coating after each high level episode. Vibrating and tuning fork probes can tolerate a fair amount of material build-up, or, if coated with PFA, can be self-cleaning in some less difficult services. The performance of vibrating probe and tuning-fork sensors is also questionable in such services, but their vibrating nature can help to collapse the bridges or to break up the rat-holes. When solid materials rat-hole or bridge, few level sensors (except loadĬells or radiation devices) work well. The low thermal conductivity of solids and the dusty atmospheres that are likely to exist in the vapor space of solids bins tend to exclude the use of optical and thermal switches from most solids level measurement applications. They provide excellent performance as high or low level switches and can be mounted from the tops or sides of tanks. Vibrating probe-type sensors are often used to detect solid materials such as powders, bulk solids, grain, flour, plastic granules, cement, and fly ash. ![]() ![]() ![]() They do not require recalibration between batches and can be cleaned in place. These switches are good candidates for use in multiple purpose processing equipment where they must be compatible with a variety of process materials and process conditions. Although some can detect other process properties besides level, their main purpose is to measure the presence or absence of material at a particular level in a tank. In some specialized applications, all three of these switches have been tuned to identify specific materials or to determine when a material reaches a particular viscosity, density, opacity, or thermal conductivity condition.Īll three level switch designs are simple, straightforward, and reliable. The optical level switch is also suited for detecting high foam levels, if it is spray washed after each event. Typically, they are used in applications that either cannot be handled by the more common float and probe-type devices, or when ultrasonic, nuclear, radar or microwave designs would be too sophisticated, expensive, or otherwise unsuited for the task.Īll three types can be used to detect liquid levels or interfaces between liquids. Thermal, vibrating, and optical level switches are specialty devices developed to solve specific level detection problems. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |