96 



HYDROGRAPHIC MANUAL 



for use in Nansen bottles, or Tanner-Sigsbee 

 reversing frames. 



Reversing thermometers are delicate in- 

 struments and must be handled with extreme 

 care at all times. A protected thermometer 

 is enclosed in a heavy glass case sealed at 

 both ends to protect it from pressure of the 

 water. The special features of a reversing 

 themometer are: a knife-edge in the capil- 

 lary tube which is made by an appendage 

 in the tube ; a gooseneck which may take the 

 form of a U-turn, S-tum, or a complete 

 circle, and a supplementary mercury reser- 

 voir at the opposite end from the main 

 reservoir. 



When the thermometer is reversed, or in- 

 verted, the extra weight of the mercury in 

 the enlarged section of the capillary tube in 

 the gooseneck breaks the mercury column at 

 the knife-edge, the mercury flows into the 

 supplementary reservoir, and extends into 

 the graduated stem where the temperature 

 is read when held in an inverted position. 

 Thus, when the thermometer is reversed in 

 watei at any depth, the temperature at the 

 point of reversal is obtained. Since the jacket 

 protects the thermometer from hydrostatic 

 pressure a true reading of temperature can 

 be made. If the thermometer does not have 

 an auxiliary, the temperature should be read 

 as soon as possible after it is brought to 

 the surface. 



3-118 Unprotected reversing thermome- 

 ters. — An unprotected reversing thermome- 

 ter is similar to the protected type in most 

 respects except that one end of the glass 

 jacket is open. The thermometer is in direct 

 contact with the water and is subject to 

 hydrostatic pressure. It has no mercury sur- 

 rounding the reservoir as does the protected 

 thermometer. It does not give a true tem- 

 perature reading but gives a reading which 

 increases in direct relation to the depth. By 

 pairing a protected and an unprotected ther- 

 mometer on one Nansen bottle the tempera- 

 tures can be used to determine the depth at 

 which the thermometers were reversed. 



3-119 Care of thermometers. — Deep-sea 

 reversing thermometers are expensive in- 



struments. Each one must be carefully cali- 

 brated at considerable additional expense. 

 Unless they are properly handled the mer- 

 cury column may be separated by gas bubbles 

 in the capillary and the calibration lost. The 

 following rules should always be observed 

 when using these instruments : 



(a) Avoid laying a reversing thermometer 

 in a horizontal position. 



(b) When not in use, each thermometer 

 should be placed in its individual cylindrical 

 case and stored in a padded carrying case 

 with the large mercury reservoir down. Al- 

 ways keep the carrying case in an upright 

 position. 



(c) All thermometers should be washed in 

 fresh water and dried before storing them 

 in the case. 



(d) Handle them gently, if the mercury 

 fails to return from the supplementary reser- 

 voir a light tap with the finger will bring it 

 down. 



(e) Never store the thermometers in the 

 Nansen bottle frames. If the thermometers 

 are left in the Nansen bottle frames during 

 a run between stations, the bottles should 

 be placed in the storage rack with the large 

 mercury reservoirs down. 



3-120 Corrections to observed tempera- 

 tures. — A calibration certificate is furnished 

 with each reversing thermometer from which 

 graphs can be drawn and corrections scaled 

 as needed. Precisely measured temperatures 

 are not required for computing corrections 

 to echo soundings, however, observed tem- 

 peratures should be corrected from the cali- 

 bration graph and the final results should 

 be accurate to one or two tenths of a degree. 

 Observed temperatures may contain small er- 

 rors from two different sources. One of these 

 is an intrinsic error of the thermometer due 

 to slight irregularities in the capillary tube 

 and slight errors of graduation. Corrections 

 for these errors are determined by calibra- 

 tion. The calibration curve retains its shape 

 but moves slightly with respect to the freeze 

 point and with age until the glass becomes 

 stable. 



The other error is due to the change in 

 the volume of mercury contained in the 



