Report

On

 

BUILDING AN INEXPENSIVE SEISMOMETER

 

 

 

 

 

 

 

Submitted

to

Dr. John Lahr

&

Dr. Thomas Boyd

 

 

 

 

 

 

  

 

Prepared by

Beaver Design

 

 

 

 

 PC050044.JPG (410842 bytes)

 

 

 

 

 

 

December 3, 2002


 

TABLE OF CONTENTS

 

EXECUTIVE SUMMARY………………………………………………………………… iii

 

INTRODUCTION …………………………………………………………………………   1

 

MODELS DISCUSSED AND THE MODEL CHOSEN …………………………….   2

 

PARTS OF THE LEHMAN MODEL …………………………………………………..   2

 

DESCRIPTION OF THE SUBSYSTEMS ……………………………………………..   3

          The Base …………………………………………………………………………..   3

          The Boom ………………………………………………………………………….   5

          The Light Sensor ………………………………………………………………..    6

          The Damping Mechanism ……………………………………………………..   8

 

PUTTING IT ALL TOGETHER ………………………………………………………..    9

 

ASSEMBLY OF THE SEISMOMETER ………………………………………………   9

 

CONCLUSION …………………………………………………………………………..   11

 

REFERENCES …………………………………………………………………………..   12

 

FIGURES AND TABLES ………………………………………………………………..  13

          Matrix 1.  Models of Seismometers ………………………………………….  13

          Matrix 2.  Different Damping Mechanisms ………………………………..  13

          Figure 1.  The First Seismometer …………………………………………….  14

Figure 2.  The Lehman Model …………………………………………………  14

          Figure 3.  Modified Lehman Model …………………………………………..  15

          Table 1.  Items and Cost of the Base ………………………………………..  16

Table 2.  Weights of Candidate Metals ………………………………………  16

 

APPENDIX ………………………………………………………………………………….  17

 

 

 

 

 

EXECUTIVE SUMMARY

 

The first seismometer was invented in 132 A.D. by a Chinese philosopher.  The seismometers seen today do not resemble the first seismometer.  Technological advancements have improved the design and how the seismometer works.  The seismometer built by Beaver Design uses a computer to record data.  This would not have been possible back when the first seismometer was invented. 

 

The seismometer we built is a modification of the Lehman model.  We took the Lehman model and removed the pickup coil and replaced it with a light sensor.  Since we were building an inexpensive seismometer we could get accurate results and an efficient seismometer using the light sensor. 

 

The seismometer is made up of four subsystems: the base, boom, light sensor, and damping mechanism.  The base is a piece of laminate wood with a metal rod attached to the one end.  The boom is a piece of square aluminum that has only three sides, the bottom of the boom being open.  The end of the boom that rests against the metal rod has a razor blade that acts as a pivot so the boom has a pendulum motion.  Supporting the boom is a steel chain that is screwed onto the boom and is attached to the top of the metal rod.  Two sand filled balloons were added, as the weight on the far end of the boom.  An emitter is located on the boom at the one-meter mark and there are two detectors on the same side of the boom but equidistant on both sides of the emitter.  To the end of the boom is the damping mechanism.  It is composed of a piece of wood attached to the boom with a piece of aluminum attached to it.  On both sides of the aluminum, attached to the base, are magnets creating an electric field when the boom swings.

 

The cost to build the seismometer is less than the constraint we were given of $150.  The materials used are durable and will work efficiently for the purpose of the seismometer.

 

The project was taken on with the idea that we would be aiding the teaching of seismology and earthquakes and helping scientists further their research of earthquakes.    

 

 

 

Report on

Building an Inexpensive Seismometer

 

INTRODUCTION

 

The first seismometer wasn’t even a seismometer at all.  The first seismometer was a seismoscope invented by the Chinese philosopher Chang Hκng in 132 A. D.  The seismoscope looked like a wine jar with eight dragonheads on it.  In each dragon mouth there was a ball.  Below each dragon head there was a toad.  

 

Chang Heng's seismoscope with dragon heads

Figure 1.  Chang Hκng's seismoscope.

 

When there was an earthquake the ball would drop out of the mouth of the dragon into the mouth of the toad.  The dragon that the ball dropped from determined the direction of the earthquake.  Reportedly the instrument detected an earthquake four hundred miles away that was not felt at the location of the seismoscope [1].

 

During the eighteenth and nineteenth centuries the invention of more effective seismometers took place.  Some of them even looked like the ones around today only simpler in design because of the available technology of the time.  These seismometers, some were still seismoscopes, were built to better sense and determine the location of the earthquake although they did not have the ability to detect earthquakes at great distances [1]. 

 

Seismometers have come along way since the first one was invented.  The Lehman model, which was invented in the 1970s, is one of the current instruments for detecting and recording earthquakes.  The seismometer went from a jar with dragonheads, balls, and toads to an apparatus that is of a cantilever type with a strip chart recorder [2].  Seismometers today are hooked to computers that record the data rather than using a pen and paper.  The cost of a current seismometer without the computer is $500.  The cost of the seismometer makes them difficult for schools to obtain.  This brings up the goal of this project.

 

Beaver Design will design and build a seismometer that will be capable of detecting earthquakes of magnitude 6 or greater from anywhere on Earth.  The mass produced seismometers will be distributed to schools and remote locations all over the world.  This will allow students and teachers to participate in the study of seismology, aid seismologists and other scientists in furthering their research, and help rescue crews know the site of destruction site more quickly.  The assembly of the product will be simple and fairly easy to use.  Our goal is to design and build the sensor and transducer of the seismometer for under $50.  The entire seismometer will be built for less than $150.

 

In this report there is the discussion of the different models that were considered.  Also included is the division of the model chosen into subsystems with descriptions of each subsystem and how each subsystem is built and how each one connects with the other subsystems.  All tables and figures used are at the end of the report and are referenced in the report.

 

 

Models Discussed and the Model Chosen

 

The first step we took was to decide which model we were going to use or whether to come up with our own design.  After comparing several designs we chose the Lehman model.  This model we determined was the easiest to assemble, the cost we could build it for would be lower than the other models we considered, it is durable, and it is efficient.  We will be modifying this model by removing the pickup coil and replacing it with a light sensor.  The other models were the Hall-effect, optical, and Piezo-electric.  The Hall-effect, was determined to be the least expensive and least efficient.  The fact that it would be the least efficient was the determining factor in not choosing this model.  The optical model was determined to be not as efficient but if it were used as the sensor in another model it would be more efficient.  The Piezo-electric model was determined to be the most expensive and harder to assemble than the other models.  The harder the model is to assemble makes it not as effective for this project since it would be built by teachers with little training.  After determining to use the Lehman model we broke it into subsystems.  We came up with the following subsystems: the base, the boom, the light sensor, and the damping mechanism.  We were going to make the cover that would protect the seismometer a subsystem, but then decided that we needed to divide the seismometer into more parts.  (Matrix 1)

 

The design we chose is a modification of the Lehman model [3]. 

 

 

Figure 2.  The Lehman Model.

 

 

 

 The modification we are making to the Lehman model is we are replacing the magnet and coil wire sensor with a light sensor (Figure 3).  

 

Figure 3.  Modified Lehman Model

 

 

In order to build our design the system was divided into subsystems.  The subsystems are the base, boom, light sensor, and damping mechanism.

Parts of the Lehman Model

We have decided to build the base out of laminated wood.  The laminate on the wood will prevent a warping effect, which could greatly distort the data recorded. This base will extend from one end of the seismometer to the other. The housing for the sensors is very important because we want to prevent them from shifting and giving false data. Thus the sensors and the housing must be placed precisely to mitigate any problems. The base will also include one upright attached to the rear side, about 5in from the edge to support the steel chain that reinforces the boom. The upright will be mounted with one 1/2in pipe flange bolted to the base. This will stabilize the upright and give the system the stability it needs to get accurate readings.

The boom is mounted to the uprights by the lower crosspiece approximately two inches above the base. Attached to the boom will be a wood stick perpendicular to the boom. This wood stick will be used to interfere with the light beam when the boom moves. This wood stick will be anchored to the boom while pointing away from it.

We have determined that we will use a light source and a light sensitive transistor, which will detect motion of the seismometer. A phototransistor is a light sensitive transistor; when a current passes through the transistor, and a light strikes the transistor, then current will be amplified according to how much light struck the transistor [4]. This solution will allow the seismometer to be calibrated more efficiently, and also not require the use of an amplifier. If we use enough current going into the transistor, the output current would not need to be amplified. By using this design, we could reduce the cost of the current Lehman model in two ways. First of all, the parts for the light sensor setup are much less expensive than the current magnet and coil assembly. Secondly, it may eliminate the use of an amplifier. We are looking into massaging the signal so that it is pre-filtered as well before being converted to a digital signal.

 

The damping mechanism prevents the loss of the periods and the arrival times of the individual waves of the earthquake.  The damping mechanism could be either a magnet or a fluid of some type.  The best option from research appeared to be the magnet.  If we were to use a fluid, like motor oil, it would be cheaper initially than the magnet but because it would have to be maintained at a certain level to keep the desired period it would end up costing more in the long run.  The other negative to the fluid method is the calibration.  To have a certain period the level of the fluid would have to be the same.  To set the desired period would require filling the container with the fluid and then testing it.  If the period were incorrect the fluid would have to be added or removed to reach the desired period.  Every time the fluid level drops or needs to be corrected the seismometer will need to be calibrated.  The known factor in the damping mechanism is the use of magnets, what needs to be determined is the strength of the magnets to be used, the set up with the boom, and what material will be used to create the magnetic field with the magnets to oppose the booms motion to stop its swinging. (Matrix 2)

 

 

Description of the Subsystems

         

          The Base.  The base is a very important part of this system. If the base fails to complete its purpose of holding the seismometer stable, then the whole system will be useless. The base for our model is closely related to that of the Lehman Model. However, there are several modifications we have made.  

The frame for our model consists of a base and one vertical pipe. This differs from the Lehman Model in that there is only one vertical support. To compensate for the lost support, this pipe is made of steel to provide strength and stability for the boom. The vertical support is 2 ft. tall and is anchored to the base with a metal angle and several screws. A pipe flange will also work well.

The base consists of a 5 ft. by 11 in. piece of 1/2 in. laminated board. Laminated board is both cheap and sturdy enough to perform the duties required. Each of the other parts of the seismometer may be mounted on the base. Attached to the bottom of the base are three 1/2 in. hex nuts. These nuts are placed with the center of the nut 1 in. from the edge of the board. Two nuts are placed near the vertical support and the other is placed near the tip of the boom. We chose the triangular contacts over rectangular ones because, not only does it cut the cost down, but it also provides excellent stability with three points of contact.

The vertical support is going to be 24 in. tall and 1/2 in. in diameter. It is made of a black galvanized steel gas pipe. This part is attached to the base on the centerline of the board. At the top of the upright a screw will be placed in a vertical position to provide an attachment for the support chain. The pipe is supported by a metal angle, which is mounted to the base with four screws. The angle is attached to the pipe by a u-joint bolt. The pipe is also set into the wood about Ό in. deep. This helps stabilize the base of the support.

This base is safe and strong. The reason we picked steel for the support was because of its strength and rigidity. Also the wooden base will not become magnetized. The total cost of the base comes to approximately $10 (Table 1).

 

The design for the base can come preassembled but there is really no need for this. All the parts required to build this base were acquired at a local hardware store. However, it should be placed on a level surface to assure accurate readings. The actual base needs to be leveled as well. This can be achieved by placing thin shims of metal or wood under the three supports. Once this is done the base is complete.

We chose the Lehman base system because it has been proven to work numerous times. This provides us with the assurance that our system will work. The main reason we altered it was to fulfill the cost requirements for this project. Our model is quite a bit larger than the Lehman Model, so it was difficult to meet the cost requirements even with the elimination of a vertical support. However, This base is large enough and strong enough to support the entire system.

 

          The Boom.  The boom will be made of aluminum and will take the shape of a hollow square tube, which is missing the bottom side.  The length of the boom will be 4ft long and 1in tall.  The thickness of the walls of the tube will be 1/8th inch.  The reason for the use of the aluminum instead of the previously chosen steel or wrought iron is the weight (Table 2) [5].

The shape of the boom has a significant impact on the durability and the preciseness of the movement.  First of all, the lighter the boom is the better.  The precision of the movement depends on where the center of mass is.  The weight on the boom is theoretically the point where gravity acts upon the rod.  So if the rest of the boom is light it will have less impact on this. This is why it will be aluminum and hollow. The rectangular shape also is better for the design because it doesn't allow for as much bend between the knife-edge and the wire attachment, as a solid and or spherical boom would [6].  There will be a slit cut into the knife-edge side of the aluminum wide enough to fit the dull end of a razor blade.  Also 3ft 8in away from the knife-edge, there will be a screw drilled into the boom.  This will connect the chain to the boom, which will be attached to the base.  
 
There will be a weight placed one meter away from the knife-edge.  The weight to be added is two balloons filled with sand.  The dramatic increase of the weight will compensate for the weight of the boom itself and will not affect the periodicity of the pendulum swing [7].  Two rubber bands will stretch around the balloons and the boom keeping them in place.  This will keep the weight at the one meter mark while still allowing for adjustment if necessary, in order to acquire the correct period of the swing.  The flag is the instrument that will block the light signal to detect movement.  The flag attachment will be constructed of a wooden stick attached to the boom.  It will be perpendicular to the boom and attached with string.
 
There will be a block of wood attached to the end of the boom in order to accommodate the needs of the damping mechanism described in a following subsystem.
 
The attachment of the boom to the base structure has two components. The first is the knife-edge pivot point where the end of the boom touches the cross support plate of the base with a razor blade.  The blade will be inserted into a slit 1 mm wide on the end of the boom. The other component is the attachment of the wire that connects the top cross support of the base with the boom. The support wire will be chained steel and will connect just before the lead weight near the end of the boom.  In order to keep it in place, the square boom will have a screw drilled into the center of the top, and the chain will hook on to it.  It will attach to the top of the base with another screw. The construction of the boom in this fashion will yield the desired period if oriented properly.  We want a 20 second damped period. In order to achieve this we need to set the boom at an angle downward from the knife-edge point.  According to [6] the desired free period is not the same as the period you will get when the boom is damped.  Instead the desired free period is 12 seconds because when damped, the boom will go slower and the 20 second damped period will result.  (This relationship works with the one-meter length boom)
 
The formula for the period of a pendulum is:
 
Time (s) = 2P X
°(Length / gravity X a)
a = angle of orientation compared to level ground
 
For a one meter boom, the a is 0.6
° or 1/100 radians for a 20 second
free period.
 
What we want, however, is a 12 second period, so the a becomes .028
radians or 1.6
°
 
To get the seismometer to yield the results you are looking for, the boom must be oriented so the period of the boom is 12 seconds before damping and 20 seconds after. To achieve this, you must test it.  If the period is not correct, adjust the length of the steel chain.  This will change the period of the boom [2]. 

          The Light Sensor.  The light sensor will interface along the boom and detect the boom’s motion relative to the earth.  The sensor is the most complicated part of the seismometer because it needs to measure the minute measurements of the boom.

 

The sensor will be entirely contained within the system of the seismometer; it will be contained within a volume of 60 cm long by 20 cm high by 20 cm deep.  The sensor will consist of an infrared emitter, an infrared phototransistor, a resistor, an amplifier, an AD converter, and a power source for each.  An AC adapter will be external to the sensor, as it plugs into the wall and provides a voltage across the sensor.  The AC adapter will produce 3V and 500mA of current.  This will be plenty of energy since most photo diodes work in the range of 0-6V [8].

 

The infrared light source will be attached to the boom with a sensor on each side, equidistant from the boom.  As the boom moves back and forth, each sensor will detect more or less light.  The light source will be a 5mm diameter LED (Light Emitting Diode) that emits infrared light [9].  The detector is also a 5mm diode that will only respond to infrared light, so there will be no interference from ambient light.

 

The light sensor will rest on the base of the seismometer and any light that strikes it will induce a rise in current from the power source.  The sensor will have a diameter of 5mm [10].  The current through the phototransistor will either rise or fall depending on the amount of infrared light that falls on it.

 

The sensor will be connected to a resistor, which will refine our signal so that it can be amplified better.  A raw signal will have some degree of noise, so if we try to amplify that raw signal, the noise will be amplified as well, so the final signal will be inaccurate.  The amplifier, filter, and AD converter are being subcontracted to the Colorado School of Mines EPICS coordinator, Bob Knecht.

 

We are confident that the current design will be able to meet the specifications of the client.  The client has asked that the seismometer will detect motion of 20 micrometers.  The initial current can be regulated with a potentiometer so that the desired sensitivity is achieved.  Our sensor design will be calibrated to meet that demand.  The sensor poses no danger to the operators and the students observing its activities.  Infrared light is not dangerous to humans or plants.  The sensor uses very little electrical energy; there is small risk of shock.

 

FCC regulations state that a device must not emit excessive harmful interference, and must operate under external interference conditions.  The light source emits only infrared light, and is only sensitive to infrared light.  We believe that the sensor emits, and is subject to electromagnetic interference by other devices like remote controls and other IR sources.  The phototransistor has a filter that neglects ambient light; any device producing IR that strikes it directly will interfere.  The seismometer will need to be isolated from any strong sources or IR light.  Also, since the IR LED only operates and 7.368 milliwatts, it is not strong enough to interfere with any IR device.  The sensor only detects light of wavelengths of 940 nanometers, so devices that emit at this wavelength will need to be avoided when nearby the seismometer.

 

We are not aware of any OSHA regulations that the sensor violates.

 

The sensor will measure the change in position of the boom, and translate that into a change in electrical signal, which can be measured. 

 

The light source will be angled so that it shines light on the sensor.  As the boom moves back and forth, each sensor will receive more or less signal, which will increase or drop the current.  In the end we will measure the change in the electrical current induced by the changing light intensity on the phototransistor.  This current change will be very small, and not easily distinguished from current coming from the AC adapter.  We will use a resistor with a resistance very close to the current produced by the adapter so we can amplify and measure the signal.  The resistor will have a resistance of about 490 milliohms while the adapter produces 500 milliamps.

 

This system will need to use DC rather than AC.  This is because the current out of a wall socket comes at 60Hz and this will change the current too much to provide accurate measurements of the boom.  The boom has a frequency of 0.05Hz.  If AC were to be used, it would mask the current jump produced by the sensor.

 

The sensors will be attached to small poles that will be attached to the base.  They will be placed equidistant from the light source so that when the boom moves they will give the same reading.

 

The Damping Mechanism.  The damping mechanism consists of three main parts: the wood block, the piece of aluminum, and the magnets.  The wood block is 2 inches long.  The aluminum is a 2in by 1in rectangle.  The magnets are 1in diameter and 1/4in wide.

 

The damping mechanism complies with the requirements set by the client in that it is part of the Lehman model.  The Lehman model is already an approved model and is currently in use.  This component is composed of the same parts as the Lehman model.  Another requirement met by the damping mechanism is the cost.  The cost to produce the damping mechanism is well under $10.  The aluminum comes from a pop can, this can be found anywhere pop machines are or beverages in aluminum cans are found.  The magnets were found at Radio Shack for $1.99 for a package of five magnets [11].  The wood block was a piece of scrap wood at Home Depot.  The parts for the mechanism are inexpensive but they are durable so they won’t be easily destroyed once the system is built.  The damping mechanism also meets the safety requirement.  Any sharp edges will be taken care of and the parts are safe to handle.

 

The damping mechanism is assembled by first attaching the wood block to the boom.  The wood can be either made to tightly fit the boom or it can be glued on for easy assembly.  Then the piece of aluminum is glued on to the wood so that the top edge is centered over the end of the wood.  The magnets are then placed on either side of the aluminum so that the aluminum will swing through the two magnets when the boom is in motion.  The period must then be checked.  If the period is not correct then the magnets will have to be moved closer to the aluminum or farther away depending on whether the period needs to be shortened or lengthened.  To check the period, move the boom off center and let the aluminum swing through the magnets to generate a current.  If the aluminum swings past the center too far then the magnets will need to be adjusted.  Once the period is set then attach the magnets to the base so they cannot be easily moved to disrupt the set period.  Attached is a figure showing the steps to building the damping mechanism that are described above.

 

After the damping mechanism has been assembled and the period has been set, the damping mechanism will inhibit the natural pendulum motion of the boom.  When an earthquake occurs the boom will move back and forth causing the aluminum plate to move through the magnetic field.  The magnetic current created will try to inhibit the motion but the waves of the earthquake will drive the boom to continue moving.  The recording device will then only pick up the period and the arrival times of the seismic waves and not the natural pendulum swing of the boom.

 

Putting It All Together

 

Each subsystem can be viewed as a step in the building process of the entire system.  Refer to Figure 3.  Taking the information that was given in the previous section about the four subsystems: base, boom, light sensor, and damping mechanism; they can be made into one functioning seismometer.  First there is the base.  The base is made of the bottom platform (base) the rest of the system is attached to.  Part of the base is the frame, which is attached to one end of the platform.  The frame consists of one metal rod with a plastic cap fastened to the top.  Near the bottom of the frame the knife-edge of the boom rests on the lower crosspiece and acts as a hinge.  To the other end of the boom a steel chain is attached that runs to the top of the frame and is attached to the upper crosspiece.  Two balloons filled with sand act as weights and are placed on the boom approximately one meter from the frame end.  Along the boom after the weight is the next subsystem, the light sensor.  The emitter of the sensor is attached to the boom.  The boom will swing with seismic activity and light will pass from the source to the sensor.  The end of the boom connects the last subsystem, the damping mechanism.  The damping mechanism controls the natural pendulum swing of the boom so the only movement the boom should have will be of the seismic waves minus the pendulum’s natural swing.  This keeps the periods and arrival times of the waves from being lost [2].

 

Now that all the subsystems can be connected the seismometer can be built.

 

Assembly of the Seismometer

 

The first step in building the seismometer is to take the 5ft wooden board with holes drilled into it.  This is the base of the seismometer.  In the largest hole on the board goes the metal rod with the cap attached to the top.  Push the rod down into the hole.  Then take the flange (the piece of metal that looks like the letter ‘L’), the U-shaped screw that is threaded at each end, and the two nuts that fit the screw.  Also take two more screws that will fasten the flange to the board.  Take the flange, put it up against the rod so that the holes on the board match up with the holes in the flange.  Then take the U-shaped screw and put it around the rod and through the holes.  Tighten the bolts around it.  Then take the other screws and screw them into the holes that go into the board through the flange.

 

Take the short piece of wood and glue it with super glue to the end of the aluminum boom that does not have the slit in it.  Take an aluminum pop can and cut out about a 2in by 1in rectangle, fold the sides over so that no one can get cut.  Glue this to the end of the piece of wood.

 

Then take another screw and put one link of the chain on it then screw it into the hole in the aluminum boom.  Take the long screw and put one link of the other end of the chain on it and put it in the top of the rod where the plastic top is.  Take the razor blade and place it in the slit with the sharp side towards the rod.  Rest this on the side of the rod.

 

Then take two pieces of wooden dowel and glue them into the holes that are closest to the rod; these become the stoppers.  Take the other two wooden dowels and glue them into the other two holes.  The emitter is attached to the boom at the one-meter mark and detectors are attached to the top of each wood dowel.

 

Then take the two sand filled balloons and form one to fit on the underside of the boom around the one-meter mark.  Put a rubber band around this balloon so it will not fall off then take the other balloon, if more weight is needed and place it on the underside next to the first one; rubber band this balloon in place.  Take the two magnets and place them on either side of the piece of the pop can; within an inch is ideal.  Glue each magnet down so that the edge is on the board and the circle is facing the can.

 

The seismometer is now complete except for hooking it up to the amp/filter and the computer.

 

 

 

 

CONCLUSION

 

 

We have now built a seismometer.  The cost to build this seismometer is less than the cost constraint of $50 for the sensor and transducer.  The seismometer was built with a laminate wood board for the base and a metal rod for the upright support for the boom.  The boom is a 4ft aluminum square that is missing the bottom side.  A steel chain connects the boom to the top of the upright and the end of the aluminum rests on the rod with a razor blade.  Two sand filled balloons add the weight to the boom one meter from the razor blade. On one side of the boom there is two detectors each attached to the top of a wood dowel.  The emitter is attached to the top of the boom.  On the end of the boom there is a piece of wood with a piece of aluminum attached.  Magnets are attached to the base on both sides of the aluminum so that an electric field is created that dampens the natural period of the boom.

 

The setup of the seismometer looks like it will work but there are improvements that need to be made.  One improvement is when the weight is added to the boom the boom twists where the razor blade holds it in place against the rod.  The second improvement is that the base is not level so the boom swings to one side.  The stoppers keep the boom from swinging to far keeping the razor blade from moving off the rod.  When the improvements are made the seismometer should work correctly.

 

In the manufacturing and sale of this seismometer we would recommend the parts should be sold in a kit.  The kit would include every thing except for the pop can because those can be found at a school.  The board should already have the holes drilled in it and the aluminum rod should have the slit for the razor blade and a hole for the screw where the steel chain attaches.  An adult should assemble the seismometer since there are sharp pieces.  There should also be some sort of cover for the seismometer so it cannot be played with or touched but only viewed.

 

 

 

 

REFERENCES

 

[1]  “The Early History of Seismometry to 1900.”  History of seismometers.    

      http://inventors.about.com/library/inventors/blseismograph1.htm

      (12/01/02).

 

[2]  J. Walker.  “The American Scientist: How to build a seismograph to record

      earthquakes at home.”  Scientific American.  Vol. 241.  pp. 152-162.  1979.

 

[3]   “How to build a simple seismometer to record earthquake waves at home.” 

       Lehman seismometer.  psn.quake.net/Lehmntxt.html.  (9/26/02).

 

[4]  Photodiode.  http://www.tpub.com/neets/book7/26g.htm.  (10/10/02).

 

[5]  J.R. Davis.  Metals Handbook.  Materials Park, OH;  ASM International. 

     1998.

 

[6]  Conversation with Dr. Matt Young, Physics professor at the Colorado

      School of Mines.  October 25, 2002.

 

[7]  K. Cunningham.  “Construction Details of my Force-Balance Seismometer.” 

      http://www.keckec.com/seismo/indes.html#overall.  (10/29/02).

 

[8]  Optoisolators, Phototransistor Outputs. 

      http://www.nteinc.com/Web_pgs/phototransistor.html.  (12/2/02).

 

[9]  Kingbright T-1 Ύ (5mm) LED Lamp specifications. 

      http://www.kingbright-led.com/pdf/L7113VGC-H.pdf.  (10/28/02).

 

[10]  Lumex 5mm phototransistor specifications. 

       http://www.lumex.com/gallery/images/cad_files/OED-ST-1L1R2.pdf

       (10/28/02).

 

[11]  “Radio Shack.”  www.radioshack.com.  (10/24/02).

 

 

 

TABLES

 

Matrix 1.  Models of Seismometers

Model

Durability

Cost

Efficiency

Ease of Assembly

Total

Lehman

4

4

4

4

16

Hall-effect

3

5

1

3

12

Optical

3

3

2

3

11

Piezo-electric

3

1

3

2

9

Scale:  1-Poor to 5-Best

Lehman:  We weighted the above factors and found the Lehman model to be the model used most by others who built a seismometer and it was not difficult to assemble. The Hall-effect is the cheapest but it is the least efficient.  The optical is right in the middle except for it lacks efficiency.  The Piezo-electric is the most expensive and it is harder to assemble.

 

Matrix 2.  Different Damping Mechanisms

Damping

Short Run Cost

Calibration

Long Run Cost

Total

Magnetic

3

3

4

10

Fluid

4

2

2

8

Scale: 1-Poor to 5-Best

Magnetic:  Magnetic won because even though in the short run cost the fluid mechanism was better the long run is what we are really concerned with.  The calibration would also be the least troublesome on the magnetic mechanism because the fluid one would require calibration each time the fluid level changed and how to be corrected.

 

 

 

Table 1.  Items and Cost of the Base

Item

Cost

5 sq. ft. 1/2" laminated board

$5.00

3' of 3/8" metal angle and bolt

$1.50 

3 3/8" hex nuts

$0.54

3/8" pipe

$2.94

Total

$9.98  

 

 

 

Table 2.  Weights of Candidate Metals                                                                

 

g/cm^3

grams if used as boom (500 cm^3)

Approximate weight in lbs.

Aluminum

2.7

1350

3 lbs.

Wrought Iron

7.7

3850

8.5 lbs.

Steel

7.5 – 8

3750 – 4000

8.25 - 8.8 lbs.

 

 

 

APPENDIX

 

This report was written by the following people and the sections they wrote:

 

          Matt Hergert – The section on the base.

 

          Mike Landers – The section on the boom.

 

          Margo Rettig – Everything but what the others wrote.

 

          Eric Williams – The section on the sensor.

 

This report was compiled by Margo Rettig.

This report was edited by Mike Landers.

 

The members of Beaver Design have read the entire report and by initialing by their name approve of the report as being complete.

         

          Matt Hergert

          Mike Landers

          Margo Rettig

          Eric Williams