I have no idea how far back in time the concept of the bubble level goes nor do I know when the development of the NAA tiltmeter began, but I first became aware of the NAA tiltmeter development program when the largest black granite surface plate I had ever seen anywhere appeared in our Instrument Test Laboratory. How could one not be curious about an object that large. It must have been at least six feet wide by 12 feet long and over one foot thick. In response to my questions, I was advised to talk to Bill Kohlenberger. I had already learned from experience that you did not talk to Bill; you listened to him. I had also learned that the wisest of his questioners remembered what he said. Bill was an expert on almost everything – he really was. (Bill always wore a gray suit from which he could produce almost any small tool you could imagine. Besides that he was a nice guy, but I never worked for him.) My questions concerning the huge surface plate in our lab prompted him to tell me all about the tiltmeter and its growing reputation as a wonder instrument. He told me he had, using the tiltmeter, observed our building tilt as the Sun went through its daily cycle, observed the local effects of storms at sea, measured the local tilting as a result of small earth quakes, recorded local tilting during large earthquakes, watched the tilt of the lab floor as people walked near the test site, and watched the floor tilt as the parking lot filed up in the morning. He added he could tell when people went to lunch. (Later the tiltmeter was used to detect the inflation of volcanoes as magma flowed into the volcanoes.) ( It was thought for awhile that pre-earthquake tilts could become the basis for prediction of earthquakes. This notion was later dropped for reasons not known to me. )
Over time I became more aware of the simplicity of the tiltmeter and how NAA became its developer. It IS simple. It is a small quartz, dishlike container, with a quartz lid. The lid is pierced by three electrodes placed in a triangular pattern. The inside surface of the lid has a spherical shape. The lid is sealed. The container is filed with an electrolyte fluid such that a small bubble surrounds the electrolytic fluid. Tilting of the instrument is detected by measuring the change in capacitances between the pins resulting from the tilt induced bubble movement. Detection of the changes in capacitance is done through the use of a bridge circuit. It is a two axis instrument.
The NAA tiltmeter was a part of the Minuteman I missile system. The concept of the Minuteman I included the means for the recovery of missile launch capability after the missile silo was disabled by a seismic event. Several tiltmeters were located on the IMU for the purpose of providing reference to local level during the recovery from the seismic event. The NAA tiltmeter proved to be very reliable in use and was used by many programs over many years. I would not be surprise if someone told me some are still in use today. I do not know if the tiltmeter was part of the Minuteman II or the Minuteman III systems.
I worked the nightshift in the instrument lab for a few years. This experience taught me that people who worked the night shift love to play pranks on the day shift people. I remember being told about a prank inflicted on the tiltmeter crew. After the granite surface plate was placed in our lab, someone used tape, or paint, to create a very realistic looking crack in the granite plate. I remember being told the effect on the tiltmeter people was as had been hoped for – short lived panic. I don’t know, I was not there.