3. EQUIPMENT AND INSTRUMENTS 



103 



which cannot be taken by other types of 

 samplers. 



Dredges are of various desi^s. A section 

 of 10- or 12-inch steel pipe about 3 feet in 

 length can be used. One end of the sampler 

 is closed by welding steel bars across it so 

 as to retain large specimens. Wire mesh 

 screens of various size openings may be used 

 in the bottom of the dredge to retain sam- 

 ples of any desired minimum size and the 

 finer sediments will be washed out. 



Another type of dredge is constructed of 

 •4-inch steel plate, and is 1 foot deep, 2 feet 

 wide, and 3 feet long. The bottom is closed 

 and screened as above. 



A third type is constructed of a rectangu- 

 lar steel collar to which is attached a purse 

 of chain mail (Fig. 42). Interior of the 

 purse may be lined with screens as in the 

 other dredges or with netting, shrimp net 

 being commonly used. 



If specimens of the sediment or other fine 

 particles are desired, a small section of 2- 

 inch pipe with a canvas bag at one end may 

 be towed behind the dredge. 



Figure 42. — Chain mesh dredge. 



It is advisable to rig the dredge with a 

 short section of chain above the bridle which 

 is hooked to a weak link. Another cable 

 shackled above the weak link leads to the 

 after end of the dredge. If the dredge fouls 

 on fast rock, the weak link will break, the 

 second cable will dump the dredge and clear 

 it from the bottom. 



The samples should be transferred to wa- 

 ter-tight containers, labeled and stored or 

 shipped in accordance with instructions. 



3-132 Other oceanographic instruments. 



— In recent years, oceanographic and elec- 

 tronic laboratories have developed new types 

 of instruments for measuring temperature, 

 salinity, and velocity of sound in situ. New 

 designs in camera equipment permit the 

 taking of a series of photographs at one low- 

 ering to the ocean floor. Most of these in- 

 struments require a source of power at the 

 ship, or furnish information on dials aboard 

 the ship, which requires the use of multi- 

 conductor cables. The maximum depth at 

 which the instruments can be used is gov- 

 erned largely by the length of cable which 

 can be used, or by the pressures which the 

 instrument will withstand. A few of these 

 equipments are briefly described in the fol- 

 lowing paragraphs. 



3-133 Induction Conductivity Tempera- 

 ture Indicator (ICTI).— The ICTI was de- 

 veloped by the Chesapeake Bay Institute for 

 use in oceanographic studies in estuaries and 

 other shoal water areas along the Atlantic 

 Coast. The instrument measures conductiv- 

 ity and temperature in situ and salinities 

 can be computed from these data. The ICTI 

 consists of shipboard indicators, an under- 

 water unit for measuring conductivity, a re- 

 sistance thermometer, and a connecting cable 

 (Fig. 43). 



In the conductivity unit, there are two 

 cpaxially mounted iron-cored inductors which 

 are insulated from the water. The first cores 

 winding is connected to 115 V 60 cps, so 

 that a low voltage will be induced in any 

 one turn link which threads the hole in this 

 core. The total current flowing through the 

 hole is proportional to the conductivity of the 



