These XBT measurements are simultaneous with the scan of the thermistor chain, which 

 began when the XBT probe was dropped. This discussion compares these simultaneous 

 thermistor chain and XBT temperature measurements. 



Recall that the thermistor chain is a towed device used for measuring temperatures 

 every 10 seconds from the surface to a maximum depth of 242 m at 44 depths spaced 

 5.6 m apart. In addition, a sensor records the depth of the deepest thermistor. With the 

 towing speed and the maximum depth known, the depth of each sensor may be computed. 

 Unfortunately, during these measurements the depth sensor was inoperative. As a result, 

 measurements of the thermistor chain sensors were not available. To obtain information on 

 the sensor depths under tow at 3 knots, the average hydrocast and STD/SV temperatures 

 recorded in areas A and C were compared with the average temperatures measured by the 

 thermistor chain for all acoustic runs made in these two areas. Similar comparisons could 

 not be made for area B since no hydrocast or STD/SV measurements were made in this 

 area. The results of these comparisons are summarized in figures 29 and 30. The solid 

 curve connects the average hydrocast and STD/SV temperatures at standard depths and at 

 the maximum thermistor chain depth of 242 m. The horizontal bar connects the lowest 

 and highest temperatures observed at the indicated depth. The dots are the average therm- 

 istor chain temperatures for each of the 44 sensors. The comparison for area C measure- 

 ments suggests good agreement. The area A comparisons do not show good agreement. For 

 depths greater than about 170 m, the average thermistor chain measurements are 0.1 to 

 0.2°C lower than the hydrocast and STD/SV average measurements; and at shallower 

 depths, they are 0.2 to 0.7°C higher. However, all average thermistor chain measurements 

 fall within the hydrocast and the STD/SV measured range of temperature. In the vicinity 

 of the thermocline, good agreement would be observed if all thermistors were at a depth 

 about 10 m shallower than assumed. To accomplish this, the deepest thermistor would have 

 to be about 18 m shallower than the 242-m maximum depth. Experience in towing the 

 chain suggests that this amount of shoaling could not occur at a 3-knot towing speed. For 

 purposes of this study, it is concluded that accurate thermistor chain temperatures cannot 

 be established for area A; thus only the measurements made in area C will be used in the 

 following analysis. 



Figure 3 1 presents the measurements associated with the single XBT profile taken 

 on 12 February 1972. The left-hand figure is the average, for each of the 44 sensors, of 

 nine scans of the thermistor chain made while XBT 8 1 L was being recorded. The standard 

 deviations for these average thermistor chain temperatures are small, varying from 0.00°C 

 to 0.03°C. The right-hand figure is a plot, for each sensor, of the differences between the 

 XBT measurement and the average thermistor chain measurement. The largest differences 

 are associated with the thermocline, where small differences in depth cause large differences 

 in temperature. The largest difference, -0.5 1°C at 96 m, can be accounted for by a 2-m 

 depth difference. In the surface layer, to 79 m, and below the thermocline, 135 to 242 m, 

 the largest positive difference was 0.09°C, and the largest negative difference was 0.1 0°C. 

 The average differences and standard deviation for the in-layer differences were 0.007°C 

 and 0.04°C; for the below^hermocline difference they were 0.009°C and 0.04°C; and for 

 all 35 in-layer and below-thermocline differences they were -0.002°C and 0.04°C. Note 

 that the differences for the five deepest sensors are negative. The measurements made by 

 this single XBT probe are in excellent agreement with those made by the thermistor chain. 



78 



