244 Table 220 



DIRECTIONS FOR USE OF STANDARD THERMOELEMENT CALIBRATIONS 



Deviation curves. — Standard tables such as are given on pages 245-247 have 

 no absolute significance; they are reference curves that, while representing 

 fairly well the e.m.f. functions for certain couples, are intended for use with 

 an appropriate deviation curve. The correction curve is determined for each 

 element by calibration at several fixed points — preferably three or more — given 

 in the tables. It is constructed by plotting A£ as ordinate (&E = E observed 

 minus E standard) against E ohs . In order to obtain the temperatures corre- 

 sponding to the measured e.m.f., the appropriate value of A£ (obtained from 

 its deviation curve by inspection) is subtracted algebraically from the observed 

 value of E before the latter is converted into degrees by means of the table. 

 The required accuracy may be secured by plotting the deviation curve on a 

 small scale; coordinate paper 20 by 20 cm is ample. There need be no fear 

 of error with this method even with deviations of several hundred /u,v, especially 

 if sufficient calibration points are taken. 



Fixed- junction correction. — Thermocouples have two junctions. The " busi- 

 ness end " is usually called the hot junction, and the other, the cold junction. 



The calibration tables are made on the assumption that the fixed junction 

 is maintained at o° C. The standard method is to use a vacuum- jacketed flask 

 filled with ice into which is inserted the junction protected by a glass tube 

 closed at one end and partly filled with kerosene. If it is not feasible to have 

 the fixed junction at o°, a fixed-junction correction must be applied. This 

 correction, in general, is not equal to the temperature of the fixed junction 

 and depends on both the temperature T of the fixed junction and the tem- 

 perature T of the variable junction. It may be applied by either of the follow- 

 ing two methods. 



(1) The e.m.f. corresponding to T may be added directly to the e.m.f. 

 Et-to and the resultant e.m.f. Et, converted into degrees by means of the 

 proper table (Tables 221-225). Thus if a platinum-platinrhodium couple gives 

 a reading of 6ooOju,v (microvolts), T being 50 , the value of £7-0, according 

 to Table 221, is 20,8/u.v which added to 6000 gives 6298 as the value of Et, 

 which by referring to the table corresponds to T=<yo^.6°. This method of 

 correction is mathematically exact. (2) Multiply the fixed junction tempera- 

 ture by a factor, /= (dE/dt) /(dE/dt), the ratio of the mean e.m.f. tempera- 

 ture gradient between o° and t t to the gradient at t, and add the product to /', 

 the uncorrected temperature, or t = t' + ft t . The e.m.f. temperature gradients 

 may be obtained by taking the reciprocals of the numbers corresponding in 

 successive vertical difference of the numbers in the vertical columns. 



Smithsonian Tables 



