Bimetal thermostats

Laminates. Two or more layers of material bonded together form a laminated composite. Common examples of laminates are in automobile windshields (laminated glass) and bimetal thermostats (9). In both cases homogeneous, isotropic layers of materials are bonded together to form nonhomogeneous composite laminates (see Laminates). 

Bimetal thermostats, thermocouples, and resistance thermometers will be discussed in detail. 

The bimetal thermostat works on the concept that different metals expand by different amounts if they are subject to the same temperature rise. If two metals are fixed rigidly together, then a differential expansion takes place when the metals are heated, causing the composite bar to bend. The thermostat employs the bimetal bar to switch on or off a control device depending on the temperature. An illustration of a bimetal thermostat is given in Figure 2.10. 

The temperature range for bimetal thermostats is 0 to 400°C with an accuracy of 5 per cent, although the accuracy can be increased to 1 per cent [6], The deflection-temperature relationship is linear for many metal combinations over a particular temperature range only, and the materials must be chosen with care. These instruments are mgged, cheap, offer direct reading, and can work under conditions of vibration. 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

Universal waste thermostat Thermostat means a temperature control device that contains metallic mercury in an ampule attached to a bimetal sensing element. 

BIMETAL THERMOMETER. Thermostatic bimetal can be defined as a composite material, made up of strips of two or more metals fastened together, which, because of the different expansion rates of the components, lends lo change iLs curvature when subjected to a change in lemperaLure. 

The thermostatic bimetal approach is used widely in a variety of Ihermosiaiic-lype temperature-control situations, as found in healing and air-conditioning systems and in automotive cooling systems, among others. Bimetals are also used in thermal type time-delay relays and switches. 

PRIMARY ELEMENT - The portion of the controller which first uses energy derived from the controlled medium to produce a condition representing the value of the controlled variable for example, a thermostat bimetal. 

Bimetal - Two metals of different coefficients of expansion welded together so that the piece will bend in one direction when heated, and in the other when cooled, and can be used to open or close electrical circuits, as in thermostats. 

As solids are heated they expand at rates determined by their coefficients of expansion that vary with the different substances. The heat needed to raise the temperature of different solids varies and can be determined from its specific heat. Problems can arise where dissimilar metals are welded together that have different specific heats and different coefficients of expansion. Allowance has to be made for differential expansions where different metals are moving in contact and have different coefficients of expansion, such as in bearings. However use is made of these differences in bimetal strips for temperature measurements and in thermostats. 

Permselective memWane, chemical valve, affinity chromatography Thermostat, bimetal, pH control, functional electrode heat sensor, thermal actuator... 

It works by means of a set of heat sensitive bimetal points contained inside the thermostat housing. The bimetal points react to Fig. 44 250-600 degree the surface temperature adjustable thermostat... 

Detailed analyses of the effects of film thickness on substrate curvature in bimaterials date back to the early twentieth century, when interest in the use of thermostatic bimetals began to expand rapidly, as described in the historical note on thermostatic bimetals in Section 2.2.3. Timoshenko (1925) and Rich (1934) derived thermoelastic solutions for curvature and stress evolution in a bimetallic strip as a function of temperature change, for arbitrary variations in the relative thickness and elastic properties of the... 

The rationale for the choice of materials used in thermostatic bimetals was discussed in Section 2.2.3. 

Eskin, S. G. and Fritze, J. R. (1940), Thermostatic bimetals. Transactions of the American Society of Mechanical Engineers 62, 433-440. 

A thermostat is a device for maintaining a constant temperature at some predetermined value. The operation of a thermostat is often based on the principle of differential expansion between dissimilar metals that is, a bimetal strip, which causes a contact to make or break at a chosen temperature. Figure 3.64 shows the principle of a rod-type thermostat which is often used with water heaters. An invar rod, which has minimal expansion when heated, is housed within a copper tube and the two metals are brazed together at one end. The other end of the copper tube is secured to one end of the switch... 

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