strument is normally considered to have the same accuracy in measuring changes 

 of temperature with depth as quoted for reproducibility, that is dz 0.1°F or ± 2 

 feet in depth. 



5.3. Hysteresis 



The term hysteresis is borrowed from its more exact meaning (that is, a meas- 

 ure of the irreversibility of a closed work cycle) and is used here to describe the 

 failure of the up and down trace to superimpose upon each other in an ocean con- 

 dition of supposedly low variability. This failure to reproduce is manifested in these 

 particular cases by a space between the two curves, either in their entirety or in 

 any portion thereof, which is similar to the hysteresis curves obtained in irrevers- 

 ible work cycles. 



Hysteresis can evolve from several sources listed as follows: (1) mechanical 

 (2) response (3) ocean variability (4) change in lag conditions. 



5.3.1. Mechanical Hysteresis 



Should the pen arm be loose in its mount, it is quite possible that a play in the 

 pen arm of several degrees could be present. This will produce up and down traces 

 which are not superimposed but displaced with respect to each other by the amount 

 of play in the pen arm. Under special ocean conditions, notably where there may 

 be positive as well as negative gradients at different depths, the distortion as 

 well as displacement can exist. Likewise, a loose glass slide or loose slide holder 

 can produce mechanical hysteresis. 



Such a hysteresis is an indeterminate error unless one of the traces is repro- 

 ducible from slide to slide. One has then only to correct for the order of the magni- 

 tude of displacement between the up and down trace. 



Slides have been noted where mechanical hysteresis of this nature has ap- 

 proached 3°F. 



5.3.2. Response Hysteresis 



The failure of the instrument to respond rapidly to sudden changes in tempera- 

 ture conditions (more accurately termed large temperature gradients) will produce 

 hysteresis in that portion of the curve represented by displacement between the up 

 and down trace. 



This is most easily demonstrated by example. Consider a true state of condi- 

 tions in the ocean represented by a step-wise Bathythermograph ; that is, an isother- 

 mal section followed by a horizontal section representing say a drop of 10° followed 

 by another isothermal section. Hysteresis will result because of the failure of the 

 instrument to follow this rapid gradient change. The down trace will be below the 

 horizontal portion of this step while the uptrace will be above the horizontal por- 

 tion of the step. 



The response characteristics of the machine is rated such that the 63 % of the 

 temperature range can be traversed in the period of 0.27 seconds. This rate of re- 

 sponse should be adequate for almost all conditions which would be encountered in 

 the ocean. However, one must remember that the ability of the instrument to follow 

 the changes in gradient of the ocean is a function of the rate of lowering and raising 

 the instrument. This possible source of error is discussed in a later section under Op- 

 eration. 



11 



