Terry

Terry
Site Admin

Joined: Apr 18, 2006
Posts: 109

Posted: Tue Apr 25, 2006 5:44 am

Here’s a little project that I have been working on lately, an inverted, horizontal seismometer. In the following photos and drawings I hope to explain what I have done. The first thing that I wanted to do is build a durable and sensitive yet easy to duplicate seismometer. I will start with a drawing to illustrate what I did.

Here are some pictures to show the unit disassembled on the floor. Here you can see the various parts that make up the unit. Everything is made from 3 inch PVC. The magnets are commonly used in a microware oven magnetron. The reason I like using these magnets are that they are polarized top to bottom. You might also super glue some flat magnets on top of the microwave magnets for additional sensitivity. A common source of these magnets is hard drives from PCs.

To start the actual construction of the seismometer I used a 12 inch piece of threaded rod that was 5/16” and 20 thread. Then I welded some small pieces of metal on the rod at odd angles to keep the rod secured when the concrete base is poured. Remember that where the rod goes threw the pvc into the concrete, it must be waterproof. I used a large washer on the bottom, then some silicone in the middle and another large washer on top. Then tighten the nuts very firmly to ensure that the seal is waterproof .

To make the concrete base form I used a flower pot and to keep it from sticking I put an old plastic shopping bag in the pot. After 24 hrs of drying the base slipped right out.

Next I used the bottom portion from an old antenna mount to screw onto 5/16” threaded rod that is mounted to the concrete base. Then I used an old cell phone antenna, make sure it is the old type with the coil in the middle exposed. The wire diameter of the antenna needs to be as small as possible to ensure the best possible sensitivity.

The antenna will need trimming and I found, through trial and error, that it is best to trim the bottom of the antenna. The red line in the second picture denotes where to cut the antenna. I trimmed the antenna down to a useable length and attached it to the mount in the concrete base.

At the top of the antenna is another antenna mount turned upside down. On this end use two washers and a bolt to secure the magnetron magnet to the antenna mount.

The coil I used was and old spool that had some small diameter wire on it. That spool was the same size as the magnetron magnets. After winding the spool it had a total resistance of approx. 1200 ohms.

Now here is were things get a little tricky. In the beginning I assembled the unit together with no glue of coarse and I found that once it was touched it took a very long time for the oscillations to die down. What it needed was more damping. At first I thought about using a fluid in the bottom chamber but then I remembered a smaller version of this style seismometer that I made from aluminum. It did not suffer the same problem, in fact it worked beautifully. The aluminum from the smaller seismometer was working to add to the damping so I took the original plastic coil mount out and made one from aluminum that was 3/16 of an inch in thickness and 3 inches in diameter with a 2 ½ by 2 ½ by ½ thick aluminum block bolted on top with a stainless steel bolt and nut.

Next was the adjustable damping magnet on top. The explanation of this is pretty straight forward. The top magnet lifts the bottom magnet and actually suspends it vertically. This is possible due to polarization of the magnets being north to south top to bottom. The bottom magnet needs to be as close to the coil as possible without actually touching it. Give enough room for the magnet to swing side to side without hitting the coil. The top magnet in mounted on a threaded rod with a knob on top. The adjustment of this top magnet by pulling it further away from the bottom magnet will give a controllable lessening of the damping function.

Once the bottom portion is finished the top portion above the adjustment knob is for the amplifier board and wires leaving the seismometer. I placed a piece of Styrofoam between the knob and amp board. In theory if I needed to adjust the damping I could pull the board out reach down and make the adjustment. The length of the top piece of pipe is variable depending on how deep you want to bury it. Remember were the wires come out of the unit use silicone to seal out water.

In a vacant area on my property I dug down approximately 4 feet to solid rock. Then I cleaned the rock very well and poured a bucket of wet cement into the hole. Then I placed the seismometer in the hole and continued to fill the area with two 80 lb and one 40 lb bags of cement. I then filled the remainder with dirt to the surface with just the top part above the ground were the wires come out the side. Then I made a cover to fit over it to keep it from being disturbed.

Last edited by Terry on Tue Jun 27, 2006 10:06 pm; edited 1 time in total

jonfr
Magnitude:2

Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland

Posted: Tue May 23, 2006 5:55 am

What length is your seismometer ? I plan to build this type of seismometer since I like the desgning of it, it's simple and appears to work great.

I need length in cm / meters. I also need a list of what you use in this seismometer. I also need to know how to tune it to work on 2Hz and lower frequancyes.

Terry
Site Admin

Joined: Apr 18, 2006
Posts: 109

Posted: Tue May 23, 2006 6:09 am

I will get that information to you ,give me a little time to compile it . I am in the process now of building a slightly larger version of the PVC seismometer. I will make better notes on this one.

Terry

jonfr
Magnitude:2

Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland

Posted: Tue May 23, 2006 6:15 am

I can wait, building this device doesn't happen over night.

Terry
Site Admin

Joined: Apr 18, 2006
Posts: 109

Posted: Tue May 23, 2006 10:34 pm

Jonfr I will start at the antenna I used for the pendulum . The length of the antenna was 10 in or 254 mm and the diameter is .062 in or 1.5748 mm. Please note where the antenna was cut near the bottom coil . Also note that the diameter of the antenna is critical in order to give the seismometer the correct response .

The magnets that I used was common magnetron magnets from a microwave .Two are required one for the top and one for the bottom.They are approximately 50mm in diameter .The size of those magnets will vary a little.

The coil that I used was an old spool of wire approximately 50mm in diameter and a 12mm thick . The diameter of the wire was .005 in or .127mm. The total resistance of the coil was about 1100 ohms.

These are the first measurements and the most critical . There will be more to come.

Terry

Terry
Site Admin

Joined: Apr 18, 2006
Posts: 109

Posted: Thu Jun 01, 2006 12:06 am

Jonfr I am still working on getting you those number but I have gotten side tracked with work .I will get them soon .

Terry

jonfr
Magnitude:2

Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland

Posted: Sat Jun 03, 2006 5:40 am

I have been busy too, so I haven't checked this website for past few days. I am going to collect the info on how to build this into one document.

jonfr
Magnitude:2

Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland

Posted: Fri Jun 09, 2006 11:09 pm

I am going to create a document with instruction on how to build this seismometer when all the info comes. I also have few ideas that I am going to test on this type of seismometer when I start to build it.

Terry
Site Admin

Joined: Apr 18, 2006
Posts: 109

Posted: Fri Jun 09, 2006 11:28 pm

Excellent !I hope to get a small break from work this weekend and I will get some more measurements for you .

Terry

jonfr
Magnitude:2

Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland

Posted: Sun Jun 11, 2006 2:44 pm

I also need to know the seconds this seismometer works on, 5 sec, 10 sec, 20 sec, and so on. Since seconds appearantly count how low frequancy the seismometer has. But I need to have it at 1Hz or lower as natrual frequancy.

Terry
Site Admin

Joined: Apr 18, 2006
Posts: 109

Posted: Tue Jun 13, 2006 5:38 am

I have been having internet problems:

I wanted to build a cost effective seismometer that an amateur seismologist could easily construct with readily available materials. If you have a hardware store and a good junk box, you should have no problems building this unit. The over all design is sensitive and quite robust.

Note: The magnet on the end of the pendulum will not support itself .It is this instability that gives the seismometer its sensitivity .The magnet on the top will balance the bottom magnet to give it stability and by adjusting the distance between the two it also acts as a damping adjustment.

Ok here goes we are starting at the bottom. The end cap is PVC and is 3 in. or 7.62cm in diameter. The next piece that surrounds the bottom pendulum is 13.1 in. or 33.274 cm long. The next piece is a standard pipe coupling and it is this piece that holds the coil in place. Then after that there is another short piece of pipe that is 5 in. or 12.7 cm. That is followed by another pipe coupling that holds the top magnet for damping .Then the top and last piece of pipe is cut to the length that you need determined by hole depth . Please remember that there needs to be room for the damping adjustment rod and knob along with the amplifier board in the top cavity .For my needs the top portion of pipe was cut to 18 in. or 45.72 cm. It is also possible to change the damping after installation to tailor the seismometer to your needs. Then to top it off use an end cap with a screw in plug. Also on the side right under the bottom of the end cap .I put a ½ in. or a 1.27 cm 90 degree elbow so power and sensor wires could easily enter the seismometer. Remember to use plenty of silicone sealant where the wires enter the unit and try to keep all moisture out of the unit. Make sure this unit is as water tight as possible !

Helpful hint remember to trial fit everything together and make sure the magnet on the pendulum does not come in contact with the sensing coil but at the same time it needs to be very close for maximum sensitivity .This can be difficult to determine since you can not see this area when it is assembled . Also I made a place in the corner to let this seismometer run for a few days before I did the final assembly. Then you need to choose a nice quite area to place it in the ground. Then if everything goes correctly this unit should give you many fine years of enjoyment .

Patience and trial fitting is your friend when assembling this unit .