Humidity and Temperature Effects on LaCoste & Romberg Gravimeters.

LaCoste & Romberg (LCR) gravimeters are able to record small non tidal gravity variations for periods ranging from some days to several months if one corrects the instrumental effects due to air temperature and humidity. An experiment has been carried out in five stations using the same gravimeter (the LCR G906) and the temperature effect was established for short and medium periods. At Membach (Belgium), the LCR G906 was installed next to the superconducting gravimeter C021 in very stable conditions of humidity and temperature and after removing a long term drift from the LCR G906 signal, the gravity variations recorded by both instruments agree at the microgal level. In order to reach a similar result with LCR gravimeters in stations where the temperature and humidity are not stable, some experiments were performed in the fundamental station of Brussels in the Royal Observatory of Belgium (ROB) using the LCR G8 to study the meteorological influences. This gravimeter showed a correlation with the humidity signal for periods longer than 10 days. To a humidity step of 35%, it reacted quasi-instantaneously by going steadily up by more than 1.5 milligal in three months and going down similarly while the humidity was returning to normal. We showed that the LCR G8 behaves like an integrator of air humidity changes. With this experiment, we partially met the result of Bastien et al (1990) and we significantly improved the understanding of the behaviour of LCR gravimeters on what concerns the humidity effect. As the reaction of each gravimeter is different, a similar experiment should be performed for each instrument.