This option is sometime referred to as "electronic readout". It is the most popular and useful of the meter options. It consists of a capacitance bridge sensing the position of the meter's beam and reporting this to a galvanometer or liquid crystal display (LCD) on the meter lid.
This option is required for various options such as analog and digital electrostatic nulling and variable damping.
The only noticeable difference between meters with CPI and those without CPI is the addition of the galvo or LCD on the top of the meter and the miniature phone jack outlet on the side of the meter. For more information see the Meter Details section of this manual.
Optional electronic levels are available for the gravity meter. They are hard mounted directly to the sensing element inside the white box. The existing bubble levels are not removed, but revert to a backup role. Each electronic level is a simple pendulum that is air damped. The position of the pendulum is sensed by a capacitance bridge with the two air dampers being the fixed capacitance plates and the pendulum the moving plate. These levels first became an option in 1979.
These levels are a compromise. They are much less expensive than the pendulum type electronic levels. However, they do not have the very fast response and perfect damping of the pendulum levels.
They consist of a bubble inside a curved glass tube. The fluid is a conductor and there are electrodes to sense the change in resistivity as the bubble moves along the glass tube.
They are located below the insulation and just atop the meters sensor. The standard spirit levels can be retained as a backup. This option was introduced in 1989.
The variable damper is an option for the gravity meter that allows the operator to adjust damping to suit different needs. The gravity meter is identical to the standard meter except for the variable damping feature. Damping can be set at standard damping or at 25 times standard damping. With care, damping can be set at intermediate values.
This option was developed for reading gravity atop frozen lakes. It is useful for observing gravity in areas of severe ground motion. For areas of severe ground motion, the CPI option with an RC filter should be adequate.
Meters with this option can be recognized by a large nylon plug on the bottom of the meter and two white button switches on the operator side of the black lid. The plug on the bottom side allows access to the engaging device.
When full damping is engaged, the method of reading the meter is slightly different. The two white buttons called "cheater switches", are used to electrostatically push the beam to the reading line. According to the direction and velocity the beam moves away from the reading line, the nulling dial is turned to the next estimate of the null reading. The beam is once more pushed back to the reading line and the procedure is repeated. With a little practice, the reading can be made almost as fast as normal readings with standard damping.
Under normal operation the gravity meter mechanically balances the force of gravity on the mass of the beam by adjusting the upward force of the main spring. Another method of nulling the forces on the mass is available. The majority of the gravitational force may be balanced by the main spring. That mechanical force can then be held constant by clamping the nulling dial and the fine adjustment of the nulling accomplished with electrostatic force on a capacitance plate affixed to the meter's mass.
With this option, the meter can be read to a microgal. The present design has a range limited to about two or three milligals. This can be an economical means of obtaining microgal precision with a Model G meter. The option is contained in a small housing that attaches to the left side of the meter. The meter continues to fit in the standard carrying case. The partition between the meter and battery compartments is repositioned slightly.
Electrostatic force is not linearly proportional to the voltage applied to capacitance plates. To attain a linear accurate system, a constant voltage is used and the percentage of time that the voltage is applied is varied to balance the beam. The electrostatic force is linearly (directly) proportioned to the percentage of time the force is applied. This is called pulse width modulation.
Below are three examples of pulse width modulation.
B causes two times as much force on the beam as does A and C causes three times as much force on the beam as does A. Since the voltage can be maintained at an exact amount, only the time the voltage is applied must be measured.
After the analog electrostatic nulling option is calibrated, the balancing force (gravity reading) can be read on a digital multimeter in microgals.
Besides field use, the option has another advantage. It can assist in recording earth tides and teleseismic surface waves. There are three analog outputs from the device. One is the same signal as sent to the LCD. The second is filtered with three stages of 10 sec. RC filtering. These can be recorded on a strip chart or digital data logger. The filtered signal is useful for measuring gravity under conditions of severe ground motion and is an alternative to variable damping. Connecting a voltmeter or data logger with more than 3-1/2 digit resolution allows precisions greater than 1 Gal to be obtained. The third output is the unfiltered beam motion and contains higher frequency information useful for recording teleseismic surface waves.
Any meter with the electrostatic nulling option installed can
always be used as a regular meter. There is a disabling switch on
the top of the analog nuller housing. Or, the device can be
removed from the meter. It is held to the side of the meter by
four screws. It is electrically connected to the meter by a
ribbon cable. If the unit is removed from the meter, disconnect
the unit from the ribbon cable and place the small jumper plug on
the end of the cable. The meter will now function as a normal CPI
meter. For convenience, the black lid can be raised temporarily
and the ribbon cable tucked out of sight under the meter lid.
The manual for this option has more detailed information.
This option is similar in principle to the Analog Electrostatic Nuller. However, it is not as portable and uses digital electronics. With digital electronics there is greater long term stability, flexibility in selecting filters and the capability of sending its data to a digital data logger or computer via its RS232C serial port.
The unit is in a box separate from the gravity meter and is connected to the meter by a small ribbon cable. When not in use, the unit can be disconnected from the meter, the ribbon cable tucked under the lid of the meter and the meter used in its standard mode.
If the meter is properly insulated from air temperature changes, the digital system makes a fine earth tide recording system. L and R is pleased to provide advice on the application and has computer programs available for processing and analysis of the data.
A power supply and clock are required for the digital nuller. They may be provided by the user. Many observatories have an accurate source of onesecond time pulses. L and R has two power supplies/clocks available. One is supplied by 115 or 230 VAC power, provides the regulated DC voltages need by the nuller and has a dividing circuit for obtaining onesecond pulses from the 50 Hertz or 60 Hertz AC power. The other L and R power supply/clock is supplied by unregulated 12 VDC power, has a DCtoDC power supply and a high precision quartz clock. If the AC power source has frequent interruptions or does not have good long term timing accuracy, the 12 VDC unit is the preferable unit.
The manual for this option has more detailed information.
The MVR (Maximum Voltage Retroaction) Feedback System was designed in 1989 by Dr. Michel van Ruymbeke of the Royal Observatory of Belgium. It has been installed on more than 30 L and R meters.
The MVR system requires disconnection of the CPI system. With the MVR system the gravity meter beam is grounded, eliminating some electrical noise. The surface mount electronics and small size allow the card to be placed inside the meter's white box. It has very low power consumption, linear calibration and DC voltage output.
If ordered in advance of construction, the range of the Model D meter may be 300 Milligals instead of the standard 200 milligals. This does not cause a reduction in meter sensitivity. All components are the same. The micrometer screw is allowed to travel over a 50% greater length. Laboratory calibration must be done over this extended range and a detailed calibration table is provided over this extended range
The coarse or ranging micrometer screw of the Model D meter may be equipped with a gearbox, counter and nulling dial. There are several advantages to doing this. The resulting meter has all the advantages of both the Model D and Model G and more.
The meter makes an excellent high precision geodetic meter. If there is a small amount of circular or periodic error in the coarse worldwide range screw, this can be minimized by turning the coarse screw exact integral revolutions. Final balancing can then be finished using the fine nulling screw, which by its design has minimal circular error. The reading technique is illustrated in the Primary Information section of this manual.
The coarse screw gearbox has a ratio of 100 to 1. Thus it is easy to make integral numbers of turns of the screw.
An existing Model D meter can be retrofitted with a gearbox, counter and nulling dial on the coarse screw. The existing gearbox on the fine screw is retained. It has a ratio of 32.5 to 1.
If the option is ordered before construction of the meter, a special gearbox can be utilized that has a ratio of 100 to 1 on the coarse screw and 50 to 1 on the fine screw. (The use of these special meters is described in more detail in a technical paper by Fett and Halliday.)
If older meters are to be used in very hot climates, the wiring of the instrument can be modified so special LED lights can be substituted for standard incandescent lamps. The LED's generate less heat. All new meter since 1996 are equipped with LED reading lights and LED level lights.
A low-power 3.5 digit red LED digital voltmeter is mounted on the black lid. The Meter Temperature function uses a precision centigrade temperature sensor. The temperature sensor is powered from a regulated 5 Volt supply derived from the 12 Volt system power. The temperature sensor is glued with epoxy cement inside an aluminum tube, which is then inserted in the standard thermometer well on the side of the heater box. The output reads in tenths of a degree. By turning a switch, the same voltmeter is used for measuring battery voltage.
All land meters constructed since January 1997 have this as standard equipment. Older meters may be retrofitted.
The meters may be mounted in a double oven housing for greater precision in field observations or stationary recording. The inner oven is the same as in the standard meter and is thermostated at the same temperature. The outer oven is thermostated at approximately 38°C.
There are several types of base plates available. All are 10.5 inches (28 cm) in diameter and have a concave upper surface. They are made of cast aluminum. All have three legs. Most have legs that are 2.5 inches (6 cm) tall. The standard base plate has a bullseye level bubble in the center.
After using the meter for some time, few operators continue to observe the base plate's level bubble. For that reason, L and R offers the base plate without the level or the level's machined hole, resulting in a cost savings.
Another variation of the base plate is available without the level, but with a central hole threaded 5/8inch national coarse. This allows the base plate to be mounted atop a standard surveyors tripod. This is of great advantage for underground observation work in the deep snow or vertical gradient observations.
Finally, there is a base plate with oak wood legs approximately 20 inches (50 cm) long. When using this baseplate, care should be exercised to make proper corrections for the height of the meter above the survey elevation marker.
A backpack is available for carrying the meter and its battery in the standard aluminum case.
The purpose of this device is to expedite gravity readings between stations where large gravity changes are encountered. This crank makes it possible to turn the dial about 10 times faster. It is a small cylindrical gear box with a small crank handle at one end and two pins at the other end. The pins are inserted into two holes on the top of the nulling dial.
The high speed crank can save time during geodetic surveys with the Model G meter when there are large differences in gravity between readings. It can also be useful with the Model D meter when there are moderate differences in gravity between observations. This is particularly true for periodic repeat surveys.
This charger has all the features of the standard charger/battery eliminator. Instead of the power being supplied by AC power, unregulated 12volt DC power is used. That power may come from a vehicle battery. It is particularly useful when working in remote locations where AC power does not exist or requires a motor generator. The L and R rechargeable batteries can be recharged promptly after discharge, thus extending their useful life. This charger will also work from 115 or 230 volts AC power.
The charger can also be modified to accept power from a solar panel. L and R can provide the solar panel and a field case for it.
On special order, L and R has built chargers that are powered by a solar/electric panel.
Several software packages are offered by L and R for reduction of land, sea, earth tides and borehole gravity data. When purchasing a software package, it is copywritten and remains proprietary. The owner is registered and L and R will provide improvements to the program at no additional charge except shipping.
GRAVPAC is a program for the entry, reduction, and tabulation of gravity survey data. The data are in the form of meter readings, times, and station coordinates and may be entered in segments as they become available during a field survey. Data reduction includes conversion of meter readings by the calibration factors, and correction for earth tides and drift.. Reportready tables are made of the raw data, observed gravity, freeair gravity, and Bouguer gravity. Provision has been made for multiple base stations, tares in the drift control, multiple Bouguerslab densities, and terrain corrections. GRAVPAC also determines absolute gravity at any temporary or secondary base stations inferred by their ties to bases of known absolute gravity. Both primary and secondary bases are then used to determine meter drift. The processed data can be stored on disk files and/or output to a line printer. GRAVPAC can also produce earthtide tables. The files are compatible with contouring programs such as SURFER. GRAVPAC is available in several versions for operating with MS_DOS and 32bit Windows.
GMODEL finds the gravity anomaly of models assembled from: infinitelylong horizontal polygons, finitelength horizontal polygons, vertical polygons, and spheres. The calculations may be at select locations along a profile or on a horizontal grid of points. For profile calculations, observed gravity may be entered and displayed with the modeled gravity. The user may then modify the model to improve the fit or print the results. Profiles are defined by the X, Y and Z coordinates of the end points and may have any orientation. The profile may pass over under or through the model. Thus, modeling for subsurface and borehole gravity is possible. Grid calculations may be made on a square 51 by 51 grid on a horizontal plane and are written to a disk file for subsequent contouring by an appropriate program, such as SURFER. GMODEL will also invert a profile of gravity observations into a single irregular interface such as might represent the contact between surficial alluvium and bedrock. For this option, the user controls the average depth and dip of the interface and the program finds its configuration. GMODEL runs on MSDOS compatible computers and 32bit Windows.
TERRAIN is sent with each GMODEL program. It is designed for computing nearstation terrain corrections. It can calculate the effects of irregular topography by use of Hammer charts or by a sloping wedge technique. The files generated are compatible with GRAVPAC.
Training is provided at the L and R facilities or in the field or at your facility. Training at our facilities may be on a onetoone basis or may be an organized course. The average course is two to three days in duration.