ability to measure temperature change with change in depth. So far nothing has 

 been mentioned about the recorded vakies as related to the actual temperature values 

 of the ocean. 



The process of establishing reference values for the Bathythermograph trace is 

 simply one of transposing the slide containing the grid calibrations until the mark 

 which was made by the Bathythermograph pen arm at the zero depth level corre- 

 sponds to the zero depth level on the grid and the temperature value corresponds to 

 the actual temperature of the ocean, possibly recorded by a separate instrument. 



When the instrument is released by the manufacturer, or after it has been 

 checked, the positioning of the grid is made at that time so that the Bathythermo- 

 graph slides supposedly read not only the changes, but also the actual values rep- 

 resented there. 



Any departure of the trace thereafter with respect to its absolute position rela- 

 tive to the grid will produce a false reading. This false reading will be in error by a 

 determinate amount, i.e., the magnitude and direction of the error can be estab- 

 lished. These determinate errors which manifest themselves in terms of displacement 

 of the trace from the actual point at which it should be located are called depth and 

 temperature sets. 



It should be well noted that the Bathythermograph was not designed to correct or 

 maintain its position relative to the grid, but only to reproduce changes. The position 

 relative to the grid is a man-made one and is an arbitrary one. This attempt to posi- 

 tion the grid to read actual values and the correction techniques involved is a source 

 of indeterminate errors. These errors will be taken up in a later section. 



5.6. Instrument Failures 



5.6.1. General 



The greatest source of errors occur naturally when the instrument components 

 fail or become damaged. The most common sources of failure are listed below. 



5.6.2. Bending of Pen Arm 



The pen arm position relative to the grid is established at the point of calibra- 

 tion or manufacture. Should the pen arm be bent the result would be a displacement 

 of the trace and the resultant error in the set. This condition has been quite preva- 

 lent as a result of two things — (1) if the Bathythermograph is left in the sun the 

 heating element carries the pen arm beyond its normal limits and thus bends it, and 

 (2) if the Bathythermograph is retrieved from the water too fast, the pressure 

 element overrides and jams against the pen arm, thus bending it, in some cases. 



Corrective measures for both of these conditions have been suggested in the 

 literature. It merely suffices here to repeat that such corrective measures should be 

 employed so as to minimize these possible sources of error. 



5.6.3. Superimposition of Trace and Destruction of Trace 



At present the up and down trace are intended to superimpose on each other. 

 While this indicates the reproducibility of the instrument, it is often time disadvan- 

 tageous, whenever it is desirable to make accurate calculations with regard to tem- 

 perature structure. It is often impossible to determine which is the up and which is 

 the down trace and thus to follow it through continuously. It would be decidedly ad- 

 vantageous to design for scientific use an instrument which displaces the uptrace 



14 



