DEEP-SEA REVERSING THERMOMETERS 



241 



Mean Pressure Coefficient ('Q') 

 The mean pressure coefficient (m.p.c.) used in calculating the depth from the reading obtained upon 

 reversal of an unprotected deep-sea thermometer, and referred to in the depth equation as ' Q ', is 

 defined as the increase in apparent temperature, in ° C, due to a pressure of i kg. per sq. cm. The 

 determination of this coefficient is made at the N.P.L. by supporting the thermometer in a stout steel 

 cylinder, containing a light oil. The cylinder is immersed in melting ice (0° C), and pressure is applied 

 and maintained constant for a suitable time to establish thermal equilibrium. The cylinder containing 

 the thermometer is then reversed, and the thermometer is removed and immersed in ice to the 

 reading. 



Prior to 1950 the m.p.c. given on all certificates issued by the N.P.L., in respect of this type of 

 thermometer, was determined at three points on the scale; since 1950, however, determination has 

 been made at only two points. The average pressures applied at the original calibration and the limits 

 of the scale reading (for existing N.I.O. thermometers) are shown in Table 4 below. 



-0.20 CORRECriON -0.10 



010 



;o' 



«■ 



Fig. 6 



Fig. 7 



Table 4. Limits of accuracy of mean pressure coefficient 

 Average test pressures in kg-fcm.^ 



j-point 

 determination 



2 -point 

 determination 



Range of 



increased 



thermometer 



reading in 



' C. {approx.)* 



13-17 

 24-30 



45-56 



Estimated accuracy 



of m.p.c. 

 (° C. per kg.jcm.^) 



+ 0-0005 



+ O-O002 

 + 0-O00I 



157-5 — 



283-5 284-74 



525-2 537-20 



* Different for each thermometer, as it is dependent on the diameter of the capillary and thickness of the walls of the bulb. 



In most unprotected thermometers the m.p.c. is remarkably constant over the range of pressure 

 covered by the calibration and is about o-i° C. per kg./cm.^. Reference to Wiist (1932) shows that out 

 of forty-seven thermometers of this type carried by the 'Meteor' only in one instance was there any 

 difference greater than o-ooio° C. kg./cm.^ in the m.p.c. at the original cahbration, over the whole 

 test range. On the return of the 'Meteor' the surviving thermometers were re-calibrated and the 

 change in the average m.p.c. noted. Differences varied between nil and 0001 3, with an average for 



