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The inner sphere contains the gravity sensor, the thermostat module, the levels, the shaft encoder, and three piggybacked electronic cards.

Inner Sphere


The gravity sensor is a Model G sensor modified with an extra large air damper on its beam. The sensor is supported by gimbals so that it can be leveled. Two small motors drive these gimbals. Other motors operates the sensor clamping/unclamping mechanism and the mechanical nulling system of the sensor.


This is a small 2 centimeter molded cube that controls the heater windings of the sensor. It senses temperature with two thermistors that are inserted into wells on opposite corners of the sensor heater box. These two thermistors, together with two fixed resistors, form a thermistor bridge. The fixed resistors are selected so the thermostat will maintain the meter at the optimum temperature for that meter.


There are two sets of levels. One set consists of spirit bubble levels for the convenience of trained service personnel while doing repair and adjustment. The other set are electronic levels which contain a conductive fluid and three electrodes. Each electronic level is enclosed in a small aluminum housing located below the spirit levels. The position of the bubble is sensed by a resistance bridge. The levels have an operating range of about ten arc minutes to either side of level. The levels are adjacent to the thermostated sensor and vary in temperature less than one degree.

The principle of adjusting the levels is the same as for the land meter. The cross level is oriented 90 degrees from the beam direction. It should read zero (or level) when the sensor indicates a maximum for gravity at that geographical location. The sensor is then measuring the full force of gravity. If the sensor is tilted more and more to one side or the other of that position, it will be sensing less and less of the full value of gravity.

The long level (or longitudinal level) is parallel to the sensor's beam. It is set to give a predetermined mechanical sensitivity to the sensor while the reading line is selected to give the maximum value of gravity.


This device measures the position of the gravity meter measuring screw and sends that information to the main electronics card. It is the small black cylinder mounted on top of the gravity sensor and connected through a small blue 10­conductor ribbon cable. This part is manufactured by Hewlett Packard in the HEDS­5000 series of incremental optical encoders.


There are three standard options for the outer sphere.


This is a heavy aluminum sphere capable of use to great depth. These spheres have been fabricated in standard thickness and extra heavy thickness. The extra heavy sphere has been tested to a depth of 1,400 meters. These spheres have an inside diameter of 51 centimeters and a flange diameter of 65 centimeters. They are mounted on a triangular bottom casting that accepts three large metal feet. Each side of the triangle is 1.0 meter in length. With the feet mounted, the sphere stands about 75 centimeters high. All parts are cast of a corrosion resistant aluminum alloy. The weight of the standard large diameter sphere with all inside components is about 160 kilos (350 pounds) out of the water.

Underwater Sphere


This sphere is designed for use from small boats in shallow water, usually less than 60 meters deep. The configuration is similar to the large sphere. Its inside diameter is 38 centimeters and flange diameter is 46 centimeters. With feet mounted, its triangular base is about 87 centimeters on a side and it stands about 60 centimeters high. Its weight with all inside components is about 80 kilos (175 pounds) out of water.


This light weight housing is for use in situations such as ice, swamps, or suspended below a helicopter. It is usually hung from a cable or rope less than 30 meters in length. The gimbals are mounted on a flat triangular base made of plywood and covered with a fiberglass housing. The base is about 75 centimeters on each side and the unit is about 60 centimeters high. Its weight is about 30 kilos (65 pounds).


Lead weights are attached above the three feet of the outer sphere. The metal feet and weights are bolted to the sphere using 1/4 inch-20 thread bolts of various lengths. The weights are usually not shipped with the meter due to the shipping costs. They can be made by purchasing bulk lead and melting it into a large cast iron frying pan as a mold. Between 20 and 25kg of lead per foot is typical for the large outer sphere used in offshore surveying. If problems are encountered with soft bottom conditions or strong currents, additional weight may help the meter to plant itself more firmly on the bottom.



The gravity sensor and inner sphere are supported by gimbals which allow the meter to be leveled if the base of the housing is within about 16 degrees of level. The meter is leveled automatically with DC motors controlled from the computer system on the surface.

The outer gimbal ring rotates in ball bearings which are mounted to the gimbal base plate. Rubber stops on the inner sphere limit the motion of the outer gimbal ring to about 30 degrees. This gimbal ring levels the gravity meter about an axis perpendicular to the rotation axis of the gravity meter beam. This axis is called the cross direction.

The inner gimbal ring rotates in ball bearings which are fixed to the outer gimbal ring. Its axis rotation axis is perpendicular to the rotation axis of the outer gimbal ring. Rubber stops on the outer ring limit the motion of the inner ring to about 30 degrees. The gravity meter inner sphere is bolted to the inner gimbal ring. The inner ring levels the gravity meter about an axis parallel to the rotation axis of the gravity meter beam. This direction is called the long direction.



Different types of depth gauges are available depending upon the depth range and accuracy required. Accuracy of these pressure transducers is usually a percentage of the full depth range, therefore it is best to use the smallest range device which can cover the survey requirements.

The transducer is located on the inside of the outer sphere and connected to a small threaded post in the lower half of the sphere.

A standard transducer is the Bourns Absolute Pressure Transducer. They are available in many ranges and have an accuracy of approximately 1% of full range.

A higher precision can be obtained with the CEC5500 Pressure Transducer manufactured by the CEC Instruments Division of IMO Industries, Inc. This device is accurate to ć .1% of full range and is available in 10 pressure ranges: 0 to 15, 25, 50, 100, 250, 500, 1000, 1500, 2500 and 5000 psi. The CEC transducer has a lower voltage output than the BOURNS device, therefore a small IC (OP­AMP) must be attached to the CEC 5500.


The flood detector is the two­connection terminal strip found on the inside bottom of the outer sphere. If any water enters the sphere, it should collect at the bottom and cover the terminal strip. Two wires connect the flood detector with the main electronics card, which in turn reports the status of the flood detector to the controlling computer.


To quickly determine whether the slope of the ocean bottom is greater than the possible range of gimbal adjustment, three mercury switches are fixed to the bottom of the gimbal system. These switches are connected in parallel and adjusted so that at least one switch will close when the slope of the bottom is too great. This condition is indicated on the computer display, and the gravity meter must be moved to another position to find an acceptably flat location.


Cables and cable terminators are not supplied as standard equipment with the underwater meter, and must be ordered as an option. According to the survey requirements, either one or two cables are required. The underwater meter must be supported by a cable or rope for raising and lowering, and electrical power and signals must be transmitted to the meter. A seven conductor cable is required to control and power the meter. Two conductors are used for power ( 22 Gauge for 230 VAC and 14 Gauge for 12 VDC ). Four conductors are required for the upward and downward bound RS­422 control signals from the computer. The last conductor is used for electrical ground.

In shallow water operation it may be convenient to use a rope or steel cable for hoisting and a separate flexible cable for the electrical functions. This method has the advantage of not requiring a winch with electrical slip rings. It is practical in calm water to a depth of 30 meters (100 feet). LaCoste and Romberg can supply the flexible urethane jacketed cable with a molded cable terminator. The hoist line can be attached to the top of the outer sphere, and the electrical cable attached to a connector on the side of the sphere. A better alternative is to attach the hoist line to a three­cable sling. The three cables are attached to the three feet of the sphere. This frees the top of the sphere for the electrical cable to go straight to the sphere without the bend required to enter through the side of the sphere.

For deep water situations both the hoisting and electrical requirements are handled by a single cable. Seven­conductor armored logging cable is suitable for this purpose. The standard size is 7/16 inch diameter. This underwater cable requires a terminator which will connect to the top of the gravity meter outer sphere. Standard logging cable terminators have not proved reliable in underwater gravity surveying. When the meter is placed on the bottom and the cable goes slack, a strong twisting motion is applied to the cable terminator. Cable conductors break from the twisting motion, and need to be replaced. LaCoste and Romberg can supply a custom stainless steel terminator which is more resistant to twisting problems than commercially available products.



There are a total of six electronic circuit boards in the underwater gravity meter. Three cards are located inside the inner sphere and are piggy­backed together to save space. These three cards are the main electronics card, the relay card, and the optical encoder card. The main card contains the analog feedback circuitry, the RS­422 driver, and the two PLA (programmable logic array) chips which receive and transmit the telemetry data. The Relay Card contains motor control miniature relays for leveling the gimbals, clamping the meter, and turning the measuring screw. The Optical Encoder Card processes data received from the encoder mounted on the meter measuring screw gear box.

Three other cards are located outside of the inner sphere. Two of these are standard DC power supplies manufactured by Lambda Electronics. These cards take the 230 VAC coming down the power cable and convert it to 5, 12,and 15 VDC as required by the various motors and electronics. The last card provides transient protection for the system electronic components. The circuit cards, motors, relays, and other electrical components are interconnected by various wiring harnesses. Circuit diagrams and wiring diagrams are included in this manual for reference.


The underwater gravity meter is controlled by an IBM­compatible personal computer software program called "UGS" ( Underwater Gravity System ). The program is written in a mixture of "C" language and assembly language code and provides multitasking, a command­driven user interface, and a windowing display capability.

Operation of the UGS system is described in a the System Controller chapter of this manual. The hardware requirements for the computer are as follows:


There are two power control units, one for use with 115 or 230 volts AC power source and the other for use with 12­volt DC power source.

The AC power control unit contains an isolation transformer to reduce the risk of electrical shock from the system and provides AC power for the computer power supply and battery charger. It also receives RS­232 commands from the computer and translates them into stronger RS­422 signals to go down to the underwater system, and translates the returning RS­422 signals from the underwater system to RS­232 for the computer.

A switch inside the unit must be set for the proper AC power source voltage. Remove all connecting cables from the unit, then remove the seven pan head screws which secure the face plate to the case. Be sure all power is removed before lifting the face plate from the case. Set the switch to the proper 115 or 230 volt position. Replace the face plate and screws before connecting the cables or applying power to the AC power control unit.

Control Box

The DC power control unit is a small handheld box that accepts 12­volt DC power and transfers it to the underwater system. It also accepts RS­232 signals from the computer and translates them to RS­422 for the underwater system as well as translates RS­422 signals returning from the underwater system to RS­232 for the computer.

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