A measurement head for making X-ray measurements on drilling fluid includes an inner pipe (30) having a outlet (32) and an outer pipe (34) around the inner pipe. Drilling fluid is pumped through the outlet refreshing the fluid at the outlet. The pump is then stopped. A height sensor (42) is then used to measuring the height of a meniscus of drilling fluid at the outlet (32). An X-ray head (50) including an X-ray source (52) and an X-ray detector (54) is then moved into a reproducible position above the meniscus of fluid above the outlet. The height sensor (42) may be fixed to a movable cover (40), to the X-ray head (50) or to some other part of the measurement head.