SECTION 5.0 



ERRORS INHERENT IN THE INSTRUMENT 



The Bathythermograph is an instrument primarily designed to measure change of tem- 

 perature with change of depth. The word change is emphasized to point out the fact that the 

 values of temperature and depth registered by the Bathythermograph are arbitrarily estab- 

 lished by superimposed grids and correction factors. 



5.1. Reproducibility 



Given a Bathythermograph trace which represents a temperature versus depth 

 structure, the ability of the instrument to reproduce that trace (in shape but not 

 necessarily in the same coordinate location) is termed reproducibility. The rated re- 

 producibility is usually assumed to be ± 0.1 °F. and ± 2 feet in depth. The 

 instrument's best demonstration of reproducibility is obtained by the superim- 

 position of the up and down trace on a slide from a single cast where the vari- 

 ability of the ocean is low during the cast. 



Reproducibility can also be described as instrument precision. The reproduci- 

 bility or instrument precision is limited by the accidental errors which accumulate 

 in the mechanics of the instrument and by the limitations and the accuracy of 

 manufacture. 



The reproducibility of the instrument is also limited by other sources of error 

 which may creep into the picture, such as mechanical hysteresis, mechanical hesi- 

 tations, and others. These will be discussed in separate sections. The value for 

 reproducibility given above assumes that such other errors as may mar the repro- 

 ducibility are not present. This statement is true in a carefully handled and cali- 

 brated instrument. 



5.2. Accuracy of Registering Temperature Change with Change of Depth 



While the reproducibility of an instrument may be good, the accuracy can well 

 be poor. One will recall that accuracy is a measure of the instrument's ability to 

 reproduce the actual or true conditions. For instance, should the change in the 

 water be 10° and the instrument show a change of 15° repeatedly, the instrument 

 would be highly reproducible but poor in accuracy. 



Actually the accuracy of the Bathythermograph is governed by the assumptions 

 which are applied when the grid by which these temperature changes are meas- 

 ured is made up. Remember that we are concerning ourselves only with temperature 

 changes and not actual values. However, the grid must be employed to register tem- 

 perature changes as well as actual values. The limitations on the accuracy which are 



imposed by the assumptions involved in making up the grid can best be demon- 

 strated as follows : If it is assumed that an isothermal line is a straight line on the 

 temperature depth slide and the Bathythermograph registers this line with a slight 

 curve, the accuracy will be distorted by the amount of the curve. The accuracy 

 could be made as good as the reproducibility if the grid is made with a slight curve. 

 However, present techniques assume straight lines for isothermal conditions. 



Likewise, in depth characteristics it is assumed that a movement of so many 

 millimeters on the scale in shallow water will represent the same changes in deep 

 water. Any failure of the instrument to meet this requirement will produce in- 

 accurate change measurements. Discussions of the grid makeup will be deferred 

 until the section on Calibration. It is well to point out here, however, that the in- 



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