1. 5. Possible reasons 1. FGE sensor adsorbs water vapors on it so that the centre-of-gravity position changes and FGE system leans, which could cause the change of based line derived from FGE. 2. The impurity outside electric circuits and units mixes with water vapors, and the produced electrolytic solution could cause the big noise. The Effect on FGE Fluxgate Magnetometer Caused by the Variation Room Humidity Fuxi Yang Wulumuqi Geomagnetic Observatory, Earthquake Administration of Xinjiang Uygur Autonomous Region Wulumuqi, Xinjiang, 830011 1. Magnetic record affected by Humidity It was found big noise appeared on the vertical component recorded by the DMI Suspended FGE Fluxgate Magnetometer (FGE) of Guiyang Observatory (GYX) from 16pm Nov. 20, 2008. However, the Z component recorded by the GM4 Fluxgate Magnetometer (GM4) did not show any unusual change, although the GM4 is installed in the same variation room with FGE. The observatory stuff installed a sealed glass box outside the FGE sensor, and put the silica-gel desiccant inside the box to reduce the humidity around the sensor. After that, the recording data became normal. Another observatory, Dedu Observatory (DED), also showed the same phenomena with GYX. Key words: Humidity, variation room, fluxgate magnetometer Fig.1 the Z minutely profile of GYX derived from FGE (20081120~20081124) Fig.3 the Z minutely profile of GYX derived from FGE (20081120~20081204) Fig.4 the Z minutely profile of GYX derived from GM4 (20081120~20081124) Fig.2 the Z second profile of GYX derived from FGE on Nov. 21, 2008 2. Base line affected by Humidity The base line derived from FGE Fluxgate Magnetometer of Wulumuqi Observatory (WMQ) showed big change after heavy rain on Mar. 7, Mar. 14 and Mar. 19, 2009, respectively. It was found that the rainwater had flown into the underground variation room. After the FGE sensor was sealed with a glass box and put the silica-gel desiccant inside it, the base line became normal again. The FGE noise did not show significant change during this period. Fig.5 the Z minutely profile of DED derived from FGE (20090215~20090216) Fig.7 the Z second profile of DED recorded by FGE (20090215~20090216) Fig.6 the Z minutely profile of DED derived from FGE (20090219~20090220) Fig.8 the Z second profile of DED recorded by FGE (20090219~20090220) Fig. 9 The base line of WMQ derived from Z component of FGE (20090101~ 20090418) Fig. 10 The difference between the Z minutely value derived from FGE and GM3 Fluxgate Magnetometer (20090301~20090418 ) Fig. 11 The F-P minutely profile of WMQ derived from FGE (20090308-20090322) Fig. 12 The original second profile of WMQ recorded by FGE from 09am to 10am on Mar. 8, 2009 3. Both noise and base line affected by humidity The FGE sensor was dried at Chengdu Observatory (CDP) on Dec. 26, 2008, big noise disappeared and base line recover to normal level after that. It is the same with both GYX and WMQ, the Z component recorded by the GM4 which installed in the same variation room with FGE did not show any unusual change. Fig.13 the Z minutely profile of CDP derived from FGE (20081223~20090106) Fig.14 the Z minutely profile of CDP derived from GM4 (20081223~20090106) 4. Observatories with FGE affected by humidity and dependence of the affected component on humidity Station code Affected component appearance of data GYXZ increased noise CDPZ increased noise WMQ D、H、ZD、H、Z Changed base line QIX D、H、ZD、H、Z Changed base line DEDZ increased noise QZH H、ZH、Z Changed base line Fig. 15 Distribution of observatories with FGE affected by humidity (red dot) Table 1 appearance of data affected by humidity Station code Com- ponent Dependence of the affected change value on humidity Changed value before and after drying the sensor days for data to recover CDPZ increased with decreasing humidity ↗ ≈200nT 10~15 WMQ D increased with decreasing humidity ↘ ≈0.3′ ↗≈1′↗≈1′ 20~25 H increased with decreasing humidity ↘ ≈1.5nT ↗ ≈2nT 20~25 Z decreased with decreasing humidity ↗ ≈4nT ↘ ≈3nT 20~25 Table 2 Dependence of the affected component on humidity Fig. 16 schematic diagram of tilted suspended FGE Fluxgate Magnetometer 6.Suggested solutions 1. Reducing the humidity inside the variation; 2. Sealing the FGE sensor with a glass box and put the silica-gel desiccant inside it. The silica-gel desiccant which could be discolored by water vapors is recommended. 3. It is recommended to use above mentioned two methods at the same time, because only to reduce the humidity inside the variation room does not take effect sometimes. 4. In addition to sensor, other electric units should also be sealed and keep dry. FGE-GM3