Temperature bimetallic thermometer

An industrial standard method has been developed to test the lightfastness of polymers in accelerated test equipment [103]. The apparatus consists of a quartz-xenon tube with a special optical filter between the light source and the specimen to produce light that resembles window glass-filtered daylight [104], Samples are mounted at a specific distance from the arc and are supported on a frame which revolves around the arc 1 to 5 times per minute for uniform exposure. A blower unit in the base provides a flow of air which makes it possible to maintain a black panel temperature of 45°C, measured by a black panel thermometer which is positioned at level with the samples. A black panel unit consists of a bimetallic thermometer mounted on a steel frame. Both faces of the frame plate and also the stem of the thermometer are coated with a heat-resistant glossy black enamel. The relative humidity level in the exposure cabinet is closely controlled. 

Liquid-in-glass thermometers used mercury or alcohol as the liquid that expands as it gets warmer. Most countries mandate the removal of any mercury-filled devices due to its extreme toxicity, but alcohol and other fillings are still used. The expansion rate is linear with temperature and can be accurately calibrated. Bimetallic thermometers bond two dissimilar metals with different coefficients of expansion to produce the bimetallic element in thermometers, temperature switches, and thermostats. Filled System Thermometers can be filled with either liquid or vapor. Liquid-filled units are the most popular although they require compensation for the weight of the liquid head and for capillary length. Vapor-filled elements cannot be used if the operating temperature crosses the vapor/liquid point. 

Bimetallic Thermometers. These use two metals of different coefficients of thermal expansion that are bonded together their heating bends the device, and an electrical contact can then be made or broken. Thermometer coils that expand/contract with temperature and make or break contact with a mercury electrode are used in household thermostats and industrial control apparatus. 

Bimetallic thermometers measure temperature by the change in physical dimension of the sensor. They have often been used [80]. The sensor consists of a composite strip of material, normally in a helical shape, formed by two different metals Differences in the thermal expansion of the two metals cause the curvature of the strip to be a function of temperature. The strip is used as a temperature indicator with a self-contained scale. Bimetallic thermometers have been used at temperatures from -185 to 425°C. The precision and accuracy of such thermometers are described in Ref. 81. 

The 2>4 sensitive element of the bimetallic thermometer is immersed only IW in the flowing stream within the line. Use when indicating temperature is of secondary importance. 

Bimetallic thermometer (bimetallic strip which shows a deflection due to differential expansivity that is proportional to temperature—frequently used for temperature control by coupling the bimetallic strip to a mechanical switch to control the chosen device). 

Bimetallic strip thermometers are preferred for monitoring oven temperatures. Mercury thermometers should not be mounted through holes in the tops of ovens so that the bulb hangs into the oven. Should a mercury thermometer be broken in an oven of any type, the oven should be closed and turned off immediately, and it should remain closed until cool. All mercury should be removed from the cold oven with the use of appropriate cleaning equipment and procedures (see Chapter 5, section 5.C. 11.8) in order to avoid mercury exposure. After removal of all visible mercury, the heated oven should be monitored in a fume hood until the mercury vapor concentration drops below the threshold limit value (TLV). 

Temperature measurement thermometers, thermistors, thermocouples, radiation pyrometers and bimetallic strips. 

The most widely used thermometer in polymer technology is the resistance temperature detector (RTD). For higher temperatures or with point measurements, the thermocouple is often preferred. Mechanical contact thermometers (expansion thermometers, rod thermometers, or bimetallic thermometers) are nearly always used only for local measurements. For optical thermometers, radiation pyrometers are used. 

The principle of the thermal expansion of solids is used in bimetallic thermometers, which consist of two metal strips of different materials with different coefficients of expansion firmly connected together. Temperature differences bend these bimetallic springs and they are usually wound in spirals or coUs. The useful measuring range is from —73 to - -537°C with an accuracy of +1% of full scale [4). Bimetallic strips are used for temperature compensation in mechanical instrument movements. They are also used in switching relays to indicate whether a limiting value is exceeded. 

Secondary standards are liquid-in-glass thermometers and base-metal thermocouples. They are calibrated by comparing them with primary-standard platinum-resistance thermometers or standard platinum-rhodium versus platinum thermocouples at temperatures generated in comparators. These secondary standards are used in turn for the calibration of other devices, such as liquid-in-glass thermometers, bimetallic thermometers, filled-system thermometers, and base-metal thermocouples, in which the highest degree of accuracy is not required. Optical pyrometers as secondary standards are compared with primary-standard optical pyrometers, and they are then used for calibration of r ular test pyrometers. 

There are three main families of temperature sensors thermocouples, resistance thermometers and thermistors. Some old reactors still have bimetalic thermometers (binary or with local readings), but it is suggested that they be replaced in order to allow the operator to follow closer any temperature transient from the control room. Other temperature sensors, like semi-conductor thermometers, consisting of doped germanium sensors, have a complex resistance temperature relationship and are useful only for very low temperature measurements. 

Changes in dimension. With such devices, a change in physical dimension occurs with a change in temperature. In this category are liquid-in-glass or other fluid-expansion thermometers, bimetallic strips, and others. 

No particular difficulties are experienced in controlling temperatures in the region from room temperature up to 300°C. Bimetallic strip devices, Wheatstone bridge circuits (thermistor activated) or mercury thermometers with capacitance devices connected to relays can be used for control of bath temperatures. For good control, the immersion heater should have the lowest possible heat capacity and the bath should have as large a volume as possible. Should the bath volume be small for whatever reason, the heat capacity of the heater should also be low to prevent flirther bath temperature rise after the current is shut off. A thin Pt wire, woimd aroimd a frame, may be used as a heater in these cases it may be placed directly in the bath without any Insulation. Vigorous agitation of the liquid in the bath is important. Bath temperatures somewhat below room conditions may be maintained by means of an immersed copper coil with a constant... 

Determine the radius of curvature of a metallic strip thermometer s bimetallic strip, composed of yellow brass and chrome. The two metals are sealed at 60 °C and each band has a thickness of 0.20 mm. The strip has a length of 15.0 cm when exposed to a temperature of 250 °C. J. Tran... 

Local measurements of wet chlorine temperature use tantalum-sheathed flanged thermowells with bimetallic dial thermometers. The insertion length should be selected to place the tip about one third of the diameter into the piping or into an elbow. 

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