For a recent project at a medical research university we were called in to investigate a vibration issue. The lab is located on the fourth floor of a newly constructed building. The research team was having problems with samples in Petri dishes clustering into groups. For their work the samples needed to be evenly distributed across the dish. The clustering of samples occurred when a dish was placed into an incubator, however the clustering was not consistent as to time of day or other obvious parameters. As their test, a Petri dish filled with water was placed on top of the incubator and visible wave patterns (similar in pattern to the clusters) could be seen in the dish. Our first step when we arrived at the facility was to conduct measurements with a portable vibration meter. This meter allowed us to view measured vibration levels in one-third octave bands. We quickly found that when we attached the accelerometer sensor via its magnetic base to the incubator there were two frequency bands that stood out higher than the others. These fingerprints were found at the 50 Hz and the 1,000 Hz one-third octave bands.
Measurements were made on the incubator and on the floor next to the incubator as a control point. We also measured the vibration levels on the wet sink next to the incubator, the subzero freezer on the other side of the wall from the wet sink, and on the structural floor adjacent to building mounts for the equipment in the penthouse above the lab. The vibration frequencies observed on the incubator did not appear in the control point on the floor next to the incubator. Occasionally another tone cycled into our measurements on the surface of the wet sink and was also seen on the incubator. This frequency corresponded to the operational cycle of the subzero freezer in the lab.
Because of the location of the mechanical equipment in the penthouse above the lab we also conducted measurements in various locations in the penthouse. The Energy System Re-circulating pump produced feel-able vibration in the floor but was outside of the apparent driving frequency of the vibration source for the incubator. While the compressed air system did produce a tonal component in the same fingerprint as on the incubator, the vibrations at the incubator did not correspond to the operation of this system. The other equipment in the penthouse did not have the characteristic fingerprints.
Based on our investigation, we determined the primary cause of the vibration within the incubator was the incubator itself (the circulating fan motor). The first step will be for the manufacturer to confirm that the fan is installed and working properly. We also provided recommendations to isolate the unit from the building as well as address a few other issues we noticed. Investigative projects like these are always a fun challenge.