is difficult to design and adjust and is a problem for use in field 

 equipment where size and weight should be kept to a minimum. Compen- 

 sation for temperature is easier and is obtained by placing a tempera- 

 ture sensitive resistor close to the conductivity cell and connecting 

 it to the arm of the bridge on which the resistance of the cell is 

 measured. 



The literature contains many references to in situ conductivity 

 instruments, but the four described below have received the most 

 attention and mention in the literature. Also, several models of each 

 have been produced and used in the field. 



1. Salinity-temperature-depth recorder (STD) 



The salinity-temperature- depth recorder (STD), developed at the 

 Woods Hole Oceanographic Institution and first described in 1948, 

 provides a continuous trace of the three variables on a three- channel 

 strip chart recorder. An underwater unit, composed of a conductivity 

 cell, a nickel resistance thermometer, and a pressure-operated depth 

 element, is connected by a multi- conductor cable to the deckside 

 unit -which contains the amplifiers, salinity computing circuit, and 

 recorder. The recorder provides traces of salinity in two overlapping 

 ranges of 20 %o to 32 %o and 28 %o to 40 %o, temperature in the range 28° 

 to 90°F, and depth in two ranges to 1,200 feet. The lower limit of 20%o 

 in the salinity range restricts the usefulness of the instrument in 

 nearshore operations, and an accuracy of ±0.3%o limits its utility in 

 open ocean studies. Nevertheless, this instrument has been used 

 extensively by WHOI personnel. 



2. Conductivity-temperature indicator (CTI) 



This instrument was designed and constructed by the Chesapeake 

 Bay Institute primarily for estuarine studies. Thus,ithas a conductivity 

 range for salinities from 0%o to 35%o and a temperature range of -2° 

 to 32°C; it contains no depth measuring element. The underwater 

 element consists of a two-electrode, H-type conductivity cell and a 

 nickel resistance thermometer. Temperature in degrees centigrade 

 and conductivity in millimhos are indicated on a pair of four-digit 

 counters mounted on the housing for the amplifier and servomechan- 

 ism. 



A comparison was made at the Chesapeake Bay Institute between 

 readings from the CTI conductivity scale and conductivities computed 

 by using CTI temperature values and the titrated chlorinity values of 

 simultaneously collected water samples. This comparison indicated 



IV- 4 



