Here is some email I forgot to post regarding some specific questions that I haven't answered yet. Bob L. has asked about extending the effective period of a "baby" Benioff seismometer using either the equalization technique of Roberts or a VBB feedback method. An advantage of the "baby" Benioff 1 second seis is that it has a 14.402kg (for Z) mass. A disadvantage is that it is a variable reluctance transducer, so the inductance of the coils is a major problem. The seis has 8 coils of 125 ohms each, so it can be set up with the proper series/parallel combination to match almost anything, including the 80 ohm, 0.75 second galvanometers of the WWNSS system. For this a net DC resistance of 65 ohms is realized, which has an inductance of 5 henries. This puts the inductive pole (Rs+Rd)/L*2*pi at 4.5hz, or within the passband of the data. This inductance presents a major phase delay problem for a fedback system using the coils. For electronic recording the coils are connected all in series for 1000 ohms and overdamped with 1200 ohms in parallel. The inductance of this configuration is measured at 8 henries (mutual inductance of the individual coils makes the various combinations incalculatable). This moves the inductive pole to about 44 hz. For a feedback system, this is still a serious inductance, and the high coil resistance makes the transfer function unstable at higher frequencies. For this reason, I have not attempted to install a displacement transducer on one of my Benioffs so as to try a VBB feedback. But I see no reason why an equalization circuit wouldn't work to improve the long period response to 10 seconds or longer. To really make an effective improvement for teleseismic recording, 20 seconds or longer would be needed, which would really push the amplifier noise requirements, particularly the 1/f noise. _________________ On a previous topic regarding brass and stainless steel shim stock: Chris has suggested a readily solderable stainless steel alloy, EN58. There is possibly the rub, as it were. McMaster's shim stock is a 303 alloy, which McMaster says meets the specifications of AMS 5515, SAE 30302, Fed. Spec. QQS-766, and ASTM A177. It makes no mention of EN58 (which may actually be included in one of the given specifications.) The major difficulty of soldering thin/fragile assemblies and using a very corrosive flux is that one needs to use an electronic solder pencil to control the heat so as not to greatly modify the temper (as a micro torch would do), and I don't want to contaminate the tips I use for electronics with a corrosive flux, so they have to be carefully separated and cleaned. McMaster sells a variety of fluxes that I have tried, (and may try again, since I have managed to break a number of the box flexures that I made with 0.002" brass.) Also: George H. has suggested using a heat-treatable beryllium-copper alloy that can be heat treated to restore a hard temper after forming in a soft condition. This would make it a spring, which of course is needed in a vertical sensor, and I believe that some of the diaphram-style spring/flexures, such as in the L4-C, are made this way. But for an application as strictly a flexure in a fedback system, the ideal flexure will have NO restoring force, only positional stabiltiy, such as a pivot (which unfortunately have large micro-positional noise so they are not used). Heat treating might allow a thinner material to be used for the same strength as a thicker soft material with a minimum restoring force, so some calculations are needed. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>