a diver-operated pneumatic core sampler

A Diver-Operated Pneumatic Core SamplerAuthor(s): Bryan WalkerSource: Limnology and Oceanography, Vol. 12, No. 1 (Jan., 1967), pp. 144-146Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2833168 .

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NOTES AND COMMENT

A DIVER-OPERATED PNEUMATIC CORE SAMPLER

A light, portable core sampler was needed to take undisturbed samples of the sediments of several high tarns that are in- accessible to vehicles or too shallow for the operation of the 6.1-m Mackereth core sampler (Mackereth 1958; Smith 1959). A light pneumatic sampler that could be handled by a diver was, therefore, devel- oped from Mackereth's design.

A preliminary survey of the tarns was made using aqualung equipment, a gauge to measure water depth, and an aluminum tube probe to measure the depth of sediment. Some of the tarns were less than 3 m deep and others had shallow deposits, so it was desirable to make two corers, one 2.4 m and one 4.3 m long. Both were constructed similarly, and procedures differ only after the core has been taken.

GENERAL DESCRIPTION

The core sampler (Fig. 1) is constructed in two parts, the body and the anchor chamber, for convenient transporation.

The outer tube, core tube, and air-bottle mounting were manufactured from high- impact rigid polyvinyl chloride; standard- sized tube and fittings were used where possible. Solvent cement was used for mak- ing the joints.

The anchor chamber was made of a steel cylinder closed at the upper end by a steel plate. Compressed air is carried in a cylinder of 750 liters capacity at 126 kg/cm2 and mounted on the outer tube close to the operating valves. The cylinder holds enough air to take approximately three cores when using the 2.4-m corer or two using the 4.3-m corer in a water depth 0 to 9 m. From 9 to 25 m, two cores can be taken with the 2.4-m or one with the 4.3-m corer.

The cylinder clamp on the outer tube will accommodate larger cylinders if necessary, and a charging connection is provided to allow a spare cylinder to be used if the

operating cylinder is emptied during the coring operation. The approximate weights of the corers, including the air cylinder, are 25 and 34 kg.

A buoyancy pack is fitted to the outer tube near the point of balance and adjusted until the corer just floats in a horizontal position. This enables the diver to maneu- ver the unit into any attitude when moving into position to take a core.

After taking the core, operating air trapped in the space vacated by the core tube floats the unit in a vertical position at the surface. A float is attached to the core tube to swing the unit to a horizontal position for towing ashore.

OPERATION

The coring position is first marked with a buoy. The corer is then assembled and placed in the water. The diver tows the unit to the buoy, dives with the corer to the bottom, and places the corer in a vertical position with the open end of the anchor chamber resting on the sediment surface (Fig. 2a). The plug is removed, and the chamber is pushed into the sediment until its top is even with the sediment surface. The plug is then replaced and the unit is firmly anchored (Fig. 2b).

The core is taken by opening the upper air valve, admitting air onto the top of the main piston, which pushes the core tube into the sediment until it reaches the bottom of its stroke. Maximum air pressure between the top cap and the main piston during this operation is approximately 5.6-7.7 kg/cm2, depending upon the con- sistency of the sediment. The bleed holes at the bottom of the inner tube prevent any buildup of pressure between the two pistons as the core tube descends (Fig. 2c). When the main piston passes the ports in the outer tube, the air emerges from them and the pressure falls, indicating to the

144



NOTES AND COMMENT 145

E 8

C

D

A

-j

-K

L W

N

H

F

FIG. 1. The core sampler. The body chamber consists of the outer tube (A), the main piston (B), to which is attached the core tube (C), the inner tube (D) attached to the top cap (E), and to which the inner piston (F) is attached at the bottom. The anchor chamber is pierced on the top to allow the core tube to pass through; a rub- ber plug (G) is also fitted in the top. There is a flange (H) at the bottom of the outer tube to which the anchor chamber is bolted. On the outer tube is mounted the compressed air tank (L), which is near the operating valves (I, J, K). The ports (M) are in the outer tube and the bleed holes (N) are at the bottom of the inner tube.

diver that the core has been taken; he then closes the upper, air valve.

The lower air valve is then opened and air enters the anchor chamber. This forces the chamber out of the sediment and the now buoyant corer rises to the surface and floats (Fig. 2d).

Once ashore, the core is removed. On the 2.4-m corer, the core tube is detached from the main piston and replaced with a clean, empty tube.

To extract the core on the 4.3-m unit, a hand pressure pump is used to pump water, via the inner tube and the bleed holes, to the bottom of the main piston. This forces the main piston, together with the core tube, back up the tube. The inner piston is static, so the sediment core is extruded onto V-shaped wooden trays as the core tube moves back into the outer tube. Be- fore pumping out begins, the air valves must be opened to prevent a buildup of pressure between the top of the piston and the top cap.

The corers have been highly reliable, anid with a minimum of instruction an ex- perienced diver has had little trouble in handling them. Approximately 100 cores have been taken at depths ranging from 3 to 25 m. Some practice is necessary to push the anchor chamber into the surface sediment, and the author has found that after placing the corer in a vertical position (by standing on the top of the anchor chamber and grasping the outer tube), it is possible to set up a vibrating motion by alternately pressing down and releasing which pushes the chamber into the deposit. For stiffer sediments, it would be a simple matter to attach an air lift pump of the type used on the Mackereth corer. Very little air would be required for pumping out the chamber.

The author wishes to thank John Macke- reth for his assistance during the construc- tion of the corers, Mr. H. C. Gilson for helpful criticism of the manuscript, and the valued cooperation of his associates on the staff of the Windermere Laboratory.

BRYAN WALKER The Freshwater Biological Association, Far Sawrey, Ambleside, England.



146 NOTES AND COMMENT

a b cd

-K

I' -

L~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~A

I

~~______ I . 11;i2 FIG. 2. Operation of the core sampler. a. G open, J and K closed. The corer is pushed into the

deposit by the diver. b. G and K closed, J open. Piston and core tube moves down to take the core. c. J closed. Core taken and air released from ports. d. J closed, K open. Air fills anchor chamber and floats the corer to the surface.

REFERENCES

MACKETETH, F. J. H. 1958. A portable core sampler for lake deposits. Limnol. Oceanog., 3: 181-191.

SMITH, A. J. 1959. Description of the Mack- ereth portable core sampler. J. Sediment. Petrol., 29: 246-250.



a diver-operated pneumatic core sampler

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