RESISTANCE.pptVIP

 Chapter 6 RESISTANCE  THERMOMETRY Such procedures leave the wire relatively free from mechanical constraints as the wire experience differential thermal expansions . The sensor is also annealed to relieve winding strains . The diameter of the platinum wire is on the order of 0.1 mm , and it takes a length of about 2m to yield a satisfactory 25.5Ω resistance at the ice point. The resulting precision platinum sensors are usually about 1 in. long。  DETERMINATION of CALLENDAR TEMPERATURE of TIN . Although the Callendar temperatures of the triple and zinc points are identical with t68s , since △tS at these points are zero ( see equation 4 .12) , this cannot be said of the tin point . The temperature correction at the tin point is △tSN = 0 . 038937℃ as determined by a short iterative procedure . First, t is assumed equal to t68 and △t is calculated from (4 .12). Then , t is adjusted for this △t according to (6 .12) . The process is repeated until t does not change by more than some acceptable value , say 1 in the fifth decimal place . For the △t so determined ，△tSN = 231.92916℃ . 4. A contain resistance thermometer yields the following resistance at the fixed points of interest: Resistance temperature R,Ω Fixed point t68℃ 6.2268 oxygen -182.962 25.5500 ice 0 35.5808 steam 100 65.67700 Zinc 419.58 RESISTANCE THERMOMITER COEFFICIENTS . Platinum temperature at the zinc point : ptZN=100(RZN-R0)/(R100-R0)=4007.46/10.0308=399.5158℃ where Equation 6 .3 can be solved readily by hand , whereas（6.4) may be more amenable to machine solution . However , (6.4) may be manipulated still further to obtain Eggenberger’s tabular-graphical solution ; that is , to avoid solution of (6 .4) for each measurement