Sec. 2-1] 



TEM PER A T URE TRANSD UCERS 



173 



For precision work the reference junction should be kept at 0°C in 

 a distilled-water ice bath. In some industrial applications the refer- 

 ence junction is buried at a depth of 4 to G ft underground, where the 

 temperature is sufficiently constant throughout the year. For some 

 uses it is preferable to keep the reference junction at room tempera- 

 ture or in a thermostatically controlled constant-temperature bath, 

 usually at a temperature of about 50°C. 



Errors resulting from variations of the reference temperature may 

 be minimized or eliminated by manual or automatic mechanical 



X 



M ~ 



R\ 



: 



N 



R 



V 



Pz.W 



Fig. (2-1)14. Circuit for manual 

 compensation of the error resulting 

 from temperature variation of the 

 reference junction t R . 



Fig. (2-1)15. Circuit for automatic 

 compensation of the error resulting 

 from temperature variation of the 

 reference junction t R . 



(bimetallic-spring) adjustment of the meter or by electrical means. 

 Figure (2-1)14 shows a potentiometer setup for manual electric com- 

 pensation of the reference-temperature variations. With the contact 

 point N in a fixed position, the thermal emf of the junction P 2 is read 

 on the scale M after adjusting the sliding contact M until the gal- 

 vanometer G reads zero. The temperature of the reference junction 

 t R is read on a thermometer, and the influence of a temperature 

 variation is compensated for by manual adjustment of the sliding 

 contact N on a scale N, usually calibrated in temperature degrees. 

 Automatic compensation can be obtained by replacing the fixed 

 resistors R and R' and the slide wire N by a network containing a 

 temperature-sensitive element R t , as illustrated in Fig. (2-1)15. The 

 potential variation of the point N' compensates for the changes in 

 thermal emf resulting from a variation of the temperature t R . An- 

 other method of reference-point temperature compensation is shown 

 in Fig. (2-1)16. The three fixed resistors R x , R 2 , and R 3 and the 

 temperature-sensitive resistor R t form a normally balanced bridge, 



