(fig. 6) are used as a standard. Both have been calibrated 

 by the U. S. National Bureau of Standards, Washington, D. C. 



Certain changes were made in this temperature- 

 measuring system to improve its reliability, accuracy, and 

 ease of use. An electronic dc null detector (fig. 6) was 

 substituted for the original reflecting galvanometer. The 

 advantages have been increased sensitivity, less zero drift, 

 faster response, and absence of alignment and vibration 

 problems associated with the light beam and prism, system. 



To check the Leeds and Northrup platinum re- 

 sistance thermometer, a triple -point-of- water cell* is used. 

 This cell is based on the temperature when the solid, liquid, 

 and vapor phases are in thermal equilibrium. The triple- 

 point temperature, fixed at 0.0100°C, is more accurate and 

 readily reproduced than an ice point. Immediately prior to 

 use of the cell, the Mueller bridge is cleaned and the various 

 controls are set according to the factory specifications. It 

 is then calibrated with a Leeds and Northrup calibrating 

 resistor of 10. 01 ohms. This resistor has been certified 

 by the U. S. National Bureau of Standards. The resistance 

 of the platinum thermometer at the triple-point-of-water 

 temperature is determined every three months. 



PRESSURE CHAMBER 



A pressure vessel (fig. 9) for determining the Q 

 factor of unprotected deep-sea reversing thermometers 

 was developed for use in the calibration tank. The Q factor 

 is defined as "the pressure coefficient of the unprotected 

 thermometer expressed in degrees Celsius increase in the 

 reading per 0. 1 kg/ cm^ increase in pressure. " ** The 

 engineering features and operation of the vessel with its 

 system of valves, tubes, pump, fluid medium separator, 

 and deadweight pressure gage (fig. 10), will be described 

 in detail in a future publication. 



*EQUIPHASE Triple Point of Water CELL, TRANS-SONICS, Inc., Burhngton, 



Mass. 



**Hydrographic Office PubUcation No. 607 



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