40 OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



thermometers firmly without subjecting them to strain, the ends of the 

 tubes are fitted with coil springs packed with sponge rubber. 



Unprotected Reversing Thermometers. Reversing thermome- 

 ters, identical in design but mounted in open glass tubes, are employed 

 to determine the depths of sampling, because thermometers subjected 

 to pressure give a fictitious ''temperature" reading that is dependent 

 upon the temperature and the pressure. The unprotected reversing 

 thermometers are so designed that the apparent temperature increase 

 due to the hydrostatic pressure is about 0.01° per meter. An unprotected 

 thermometer is always paired with a protected thermometer, by means of 

 which the water temperature in situ, Tw, is determined. When Tw has 

 been obtained by correcting the readings of the protected thermometer, 

 the correction to be added to the reading of the unprotected thermometer 

 can be obtained from the equation 



J, ^ (Tw — tu){T'u + Vq,u) , J 



Here T'^ and tu are the readings of the unprotected reversing thermometer 

 and its auxiliary thermometer, and lu is the calibration correction. The 

 difference between the corrected reading of the unprotected thermometer, 

 Tu, and the corrected reading of the protected thermometer, T^, repre- 

 sents the effect of the hydrostatic pressure at the depth of reversal. The 

 depth of reversal is calculated from the expression 



D (meters) = —^ -i 



qpm 



where q is the pressure constant for the individual thermometer. This 

 constant is expressed in degrees increase in apparent temperature due to 

 a pressure of 0. 1 kg/cm^. p^ is the mean density in situ of the overlying 

 water. For work within any limited area it is usually adequate to 

 establish a set of standard mean densities for use at different levels. 

 Depths obtained by means of unprotected thermometers are of the 

 greatest value when the wire rope holding the thermometers is not vertical 

 in the water. When serial observations are made (p. 43), unprotected 

 thermometers are usually placed on the lowest sampling bottle and, if 

 possible, one on an intermediate bottle and one on a bottle near the top of 

 the cast. The accuracy of depths obtained by unprotected thermometers 

 depends upon the accuracy of the pressure constant and upon the 

 accuracy of the readings of the two thermometers. The probable error 

 is about ± 5 m for depths less than 1000 m, and at greater depths is 

 about 0.5 per cent of the wire depth. 



Thermographs. Many devices have been suggested for obtaining 

 continuous observations at selected levels or as a function of depth. 

 Thermographs are commonly used at shore stations and on vessels to 



