The Effect of Helium Emissions by a Superconducting Gravimeter on the Rubidium Frequency Standards of Absolute Gravimeters

Recently, large offsets in the frequency of rubidium oscillators of absolute gravimeters have been reported, due to contamination by helium from a superconducting gravimeter that shares the same laboratory space. Such offsets can cause large errors in measured gravity if they go unnoticed and are not corrected for in the data processing. We give an account of the calibration histories of the rubidium oscillators of the FG5-221 (2003–2012) and of the FG5X-221 (2013–2014) of the FGI from the viewpoint of helium contamination. These gravimeters have been stationed (when not in field work) in the Metsahovi gravity laboratory, about 5 m from the superconducting gravimeter GWR TT020, in a separate room. The drift of the oscillator of the FG5-221 during the 10 years has been larger than specifications but reasonably linear. Evidence for possible helium effects is indirect: when the gravimeter is away from the laboratory some relaxation of the frequency offset is seen. This could be an indication that contamination by helium contributed to the large drift. The drift of the oscillator of the FG5X-221 has been much larger and correlates with known episodic helium emissions. During the installation of a new superconducting gravimeter GWR Dual OSG 073 in January-February 2014 and the ensuing large helium emissions we monitored the response of the FG5X-221 oscillator continuously. In 5 days, the clock frequency increased by 4.7 × 10−9. Such an offset, if not corrected for in the data processing would cause an offset of −9.2 μGal in observed gravity.