Open the outer sphere by removing the six large bolts around the outer flange (12 bolts for the small sphere). Remove the heavy aluminum lid and you will see the inner sphere and gimbals mounted inside. Remove three machine screws around the outside top edge of the inner sphere, and carefully lift up on the top half of the sphere. You can now access the main electronics card and other components in the inner sphere as necessary.

The system can be controlled by the computer through the normal underwater cable while mounted in the outer sphere. If the meter is installed in the large outer sphere, you may remove the inner assembly from the outer sphere and operate the meter on a work bench. A special test cable is provided with the system to eliminate the main underwater cable. This cable is connected directly from the power control unit to the power and telemetry connectors on the gimbal wiring harness. Free the inner assembly by removing three bolts around the outer edge of the round plate. Disconnect all the electrical connections to the main cable terminator, the flood detector, and the pressure transducer. Lift the entire assembly out onto a level work area.

If you experience difficulties with the system electronics, you may wish to install the spare main electronics card as part of the debugging procedure. This can be difficult due to the numerous connectors, and care must be taken to note the correct position of all cables for re-installation. Also, each card has a unique cheat voltage calibration file which must be loaded in the system software if the card is changed.

Each main electronics card will have a four-digit serial number on the back of the card. The first two digits are the serial number of the U-series underwater meter, and the last two digits are the sequence number of the card itself. For example CN2401 is the first card for meter U24, and CN2402 is the spare card for the same meter. The cheat voltage calibration files for these cards will be on the software delivery disk labeled CN2401.CHT and CN2402.CHT respectively. The file corresponding to the card currently in use must be copied into the file UGS.CHT. Then use the command sequence FILES, LOAD, VOLTAGE_TABLE followed by the command sequence FILES, SAVE, METER_PARAM to place the correct calibration table into the system.

To remove the card, first locate the two mounting screws beneath the digital shaft encoder (blue ribbon cable). These screws attach the black plastic mounting bracket to the aluminum heater box. Remove the two screws, leaving the mounting bracket attached to the electronics card. There are nine in-line connectors which must be now be disconnected from the card. Six are on the top and sides, and three on the back. Lift the card up a few inches and disconnect three more connections to the Encoder, Main Power, and Status connectors. You should now be able to lift the card out of the inner sphere.

When replacing the card, be sure to place the bottom of the card in the card guide in the inner sphere, and that all connectors are placed on the correct connection. Card connection diagrams located in the circuit diagrams section of this manual may help to check connections. The black plastic mounting bracket must be moved to the spare card. Also, the four-pin jumper (with three connections wired together) on the side of the main card must be moved to the spare card.

Note that some of the connectors have the same wire color sequence, and it is possible to confuse them. The cross and long level connectors going to the back of the main card appear identical. The meter temperature sensor on the meter box has a connector (J6) identical to that for the level motors (J4) (BROWN-RED-ORANGE-YELLOW). The arrestment/measuring screw motors connector (J5) is identical to the position connector on the side of the main card (VIOLET-GRAY-WHITE-BLACK).

The electronic level settings on the gravity meter are carefully determined during manufacture, but can change with time during normal operations. The level settings should be checked occasionally and adjusted if necessary. These level checks must be performed on shore in a stable location. Should the inner sphere be open during the checking procedure, the electronic levels should be left well covered with dacron insulation since temperature affects their null point slightly.

The level setting procedure is quite different from that used on land meters. Since the electronic level positions are sent to the computer as the output of a 14-bit
A/D converter, they are represented by an integer number in the range 0 to 16,383. The reading for the true level position would be near the center of the range, or near 8,200.

In practice, the following Cross Level and Long Level checks will determine the exact A/D reading which represents the true level position. Once the true level position readings are determined, they must be entered into the Meter Parameters File. They are stored as the parameters Long_Level_Zero and Cross_Level_Zero.

Note that the computer control system must be set to the MANUAL mode by selecting the MANUAL command. Before selecting the MANUAL command, the STRIP CHART display mode must be enabled by using the EDIT, SWITCH_PARAM command sequence. This mode will allow you to observe changes in indicated gravity as the various level checks are performed. The current value of the Cross and Long Level Zero parameters as well as the current filtered level readings are displayed in the window at the top of the strip chart display.

The long level adjustment should be done first, and is quite different from the procedure used to check Model G or D land gravity meters. The long level adjustment determines the meter sensitivity, and the highly damped underwater meter is adjusted to infinite sensitivity. Infinite sensitivity means that if the meter counter is adjusted to give zero beam velocity ( vertical line on the stripchart ) at one beam position, then the beam velocity will be zero at any other beam position.

The Cross and Long Level Zero positions ( expressed in A/D bits ) will have been set at the factory, and these values should be used initially for this procedure. Note that you will need to be familiar with the commands to move the meter counter, cheat the beam position up and down, and how to tweak or slew the gimbals to a desired position. Also note the use of the Change command to switch between Counter-Cheat mode and Gimbals mode. When in Counter-Cheat mode, automatic leveling is enabled and the system will level to the current values of Cross and Long Level Zero in the meter parameters file. When in Gimbals mode (Tweak or Slew), automatic leveling is off and you can adjust the gimbal positions with the four arrow keys.

First, set the meter to MANUAL mode with Feedback OFF. Adjust the meter counter to give a vertical trace on the stripchart (zero beam velocity) and push the beam to the center line on the stripchart using the cheater commands. Then push the meter beam near the right side of the stripchart using the cheat up command. Observe the slope of the beam position trace for a minute or so. Then cheat the beam near the left edge of the stripchart using the cheat down command, and again observe the beam trace for a minute.

In both cases, the beam trace will remain vertical if the sensitivity is infinite. If the sensitivity is less than infinite, the beam trace will move back toward center. If the sensitivity is too high, the beam trace will move away from center. If the sensitivity is not infinite, a different long level setting must be tried and the above procedure repeated.

If the inner pot is open, the long bubble level can be used to determine the amount of long level change. The recommended move is one bubble division. If the inner pot is not open, one bubble division corresponds to about 400 A/D bits output from the electronic levels. Use the Gimbal move commands to tilt the meter to a new trial long level position. To increase sensitivity select a higher A/D output value or move the long level bubble one division towards the eyepiece stalk, and vice-versa to decrease the sensitivity.

Enter the new trial Long_Level_Zero value into the Meter Parameters file, and switch back to counter-cheat mode to re-enable automatic leveling. Check the sensitivity again as described above. After the correct long level position is determined, enter the reading (in bits) for that long level position into the Meter Parameters file as the Long_Level_Zero parameter, and save the file to disk.

The cross level adjustment procedure is similar to that for a land meter in that the correct setting is that which gives the maximum indication of gravity. Adjust the counter so that the beam trace in the stripchart display is moving vertically, indicating that gravity has been nulled by the spring force. We now change the cross level position an equal amount in both directions using the Gimbal move commands, and observe the effect upon the indicated gravity.

The recommended change is one spirit bubble division. Again, if the inner sphere is not open, the equivalent change is about 400 A/D bits on the electronic level readout. Change the cross level setting an equal amount in both directions, checking that the long level remains set to the Long_Level_Zero value previously determined and displayed at the top of the window. Observe the change in slope of the beam trace. A correct cross level setting results in equal and lower indicated gravity for both the left and right cross level tilt. Lower indicated gravity will cause the stripchart trace to move to the right of the stripchart display. If the change (slope of the line) is not the same in both directions, try a new cross level zero setting and repeat the check. The new trial value should be closer to the setting which produced the slope from vertical when the correct Cross_Level_Zero setting is determined, note the A/D reading for that position, enter this value into the Meter Parameters file as the Cross_Level_Zero parameter, and save the file to disk.

In order to reduce reading errors due to slightly incorrect leveling, it is important to use the correct reading line. To check the reading line, first make a reference reading at the current reading line. Then change the LONG level to both sides by one bubble division or 400 A/D bits, keeping the electronic cross level at the Cross_level_zero position previously determined and indicated in the window at the top of the screen. In both cases observe the response of the beam position trace.

If the current reading line is correct, the response to both long level tilts will result in less indicated gravity reading than the reference reading. If both test readings are not lower than the reference reading, the reading line may need to be adjusted. This will require setting the beam zero and reading line adjustments on the main electronics card. This is a complex procedure which also will require that the cheat voltage calibration be redone. It should only be done by trained personnel. In principle, a trial reading line is used either above or below the previously used reading line and the test repeated. Should the response become worse with the chosen trial reading line, assume a reading line in the opposite direction. The test is repeated until the indicated gravity becomes both lower and equal for both long level tilts.

The position of the cross and long gimbals is displayed in the main Telemetry Window. These positions are indicated in both degrees of tilt from horizontal and A/D bits (measuring the voltage drop across the small potentiometers located at one end of each gimbal bearing). Two values in the Meter Parameters file ( UGS.PRM ) need to be set in order for the tilt readings to read correctly. These are the CROSS GIMBAL ZERO and LONG GIMBAL ZERO parameters in A/D bits. To determine the correct value simply place the base of the meter in a flat level position and note the current indicated values on the Telemetry window. Enter these two values (in A/D bits) into the file using the EDIT, METER_PARAMS commands, then save the file to disk with the FILES, SAVE, METER_PARAMS command sequence. The correct gimbal tilt values in degrees should now be displayed in the Telemetry window.

Five electronic adjustments can be made with potentiometers on the main electronics card. These are the Meter Temperature, Ambient Temperature, Beam Zero, Beam Gain, and Reading Line adjustments. The two temperature adjustments are quite simple and are described below. The other three adjustments are quite complex due to interaction between themselves. They should only be performed by LaCoste and Romberg service personnel or someone with specific training in the procedure.

The underwater gravity meter temperature is fixed and controlled by a thermostating system inside the inner sphere. The optimum temperature is determined during instrument manufacture and should never need any adjustment. In any case, this temperature could only be changed at the factory through changing resistors connected to the thermostat module.

The meter contains a mercury thermometer and two electronic temperature sensors. The mercury thermometer is inserted into the top of the meter heater box. It can be read by removing a small amount of the Dacron insulation to expose the top of the thermometer. One of the electronic temperature sensors measures METER temperature, and the other measures AMBIENT temperature.

The gravity METER electronic temperature sensor is located on the side of the heater box. The temperature indicated by this sensor is displayed on the computer, and should match the temperature indicated by the mercury thermometer. The displayed meter temperature can be adjusted with the small potentiometer labeled METER TEMP located on the top edge of the RELAY (UW321) circuit board attached to the main electronics card. Simply turn the potentiometer adjustment screw until the temperature on the computer display corresponds to the mercury thermometer temperature.

The AMBIENT temperature sensor is located on the top edge of the RELAY electronics card, and looks like an ordinary integrated circuit. The displayed ambient temperature can be adjusted with the small potentiometer labeled AMBIENT TEMP located next to the sensor on the same RELAY circuit card. Simply turn the potentiometer adjustment screw until the computer displayed ambient temperature matches the true ambient temperature inside the inner sphere.

This is the equivalent of the CPI zero in the land meter, and is adjusted to give an A/D reading of 8,192 bits when the beam position is located on the mechanical reading line. The electrostatic feedback must be off when making this adjustment. Turning the potentiometer clockwise produces an A/D output with a smaller number.

This is the equivalent of the CPI sensitivity in the land meter, and may need to be adjusted to get a correct beam zero setting. Turning the potentiometer clockwise will increase the gain.

This adjustment is made to match the beam position to the mechanical reading line when the electrostatic feedback is in operation.

The Cheat Voltage Calibration file UGS.CHT is specific to the particular meter and main electronics card in use. These files are supplied at time of manufacture by LaCoste and Romberg. If major repairs are made to internal parts of the meter, or to the main electronics card, a new Cheat Voltage Calibration file will be required. This is done by the RUN, CHEAT_TEST command sequence from the main menu. The meter must first be unclamped and the counter moved to a point which requires a very small (<F128M>¢<F255D> 0.1 volt) cheat voltage value for balance. The computer will then automatically increase the counter by 0.05 counter units per reading, and record the cheat voltage value required to balance each counter setting.

This process requires about 3 hours and must be done in a very stable and quiet location to get accurate readings. The results can be viewed on the computer screen using the command sequence PLOT, VOLTAGE_TEST. If any points do not fall on the smooth curve, the calibration was probably contaminated by seismic noise and should be repeated. A sample cheat voltage calibration is shown:

The UGS software system contains a file where the last position of the measuring screw counter is recorded. It is possible that the recorded value will not match the true meter counter reading. This can happen if using multiple copies of the UGS software, and you do not use the most recent Meter Parameters file (UGS.PRM). While using one copy of the program, the meter counter is changed and the latest counter reading is recorded in the file. Later, if the Meter Parameters file is loaded from another copy of the program, an incorrect counter reading will be used. Also, the counter value is only recorded after exiting the GRAVITY screen. If the counter is moved while in the GRAVITY mode, and a power failure should occur, the correct Counter Reading would not be stored in the Meter Parameters file.

To check if the recorded value is synchronized with the meter counter, remove the inner sphere lid and read the meter counter located on top of the gravity sensor. Then check the COUNTER READING value stored in the file with the EDIT, METER_PARAM commands. If they do not match, enter the four digits of the meter counter and press ENTER. This new Counter Reading must then be saved to the meter parameters file by the command sequence FILES, SAVE, METER_PARAM and the software will be synchronized.

Meter depth is calculated by the computer using a depth gauge calibration function and a pressure to depth conversion factor. The calibration data for the standard depth gauge supplied by LaCoste and Romberg are stored in the computer file UGS.DTH when the system software is prepared. The calibration data consist of two points of known pressure and corresponding A/D reading in bits from the depth gauge output. The software does a linear interpolation between the points, and uses this linear function to compute pressure from any value measured by the gauge.

If a different depth gauge is installed, a new calibration file must be prepared and copied into the file UGS.DTH on the software disk. The calibration data are obtained by observing the depth gauge output ( in RAW A/D bits output mode ) at two known pressures in P.S.I.. This can be done in a pressure vessel, or by lowering the meter to a known depth in water and converting that depth to a pressure value. The depth gauge output can be changed to RAW mode by editing the parameter DEPTH OUTPUT located in the GRAVITY PARAMETERS file.