Ambient light
Instruments such as the LISST and the ac-meters are sensitive to ambient light. One needs to make sure that no ambient light can reach the detector, e.g. by putting opaque black tape on plumbing entering/exiting the instrument and/or covering the setup with black felt. You can test whether ambient light is affecting your instrument by observing the data as you turn off/on the lights in the lab where the instruments are connected.