Methods and devices for measuring chemical or biological properties of a liquid from a subsurface position are disclosed, wherein a closed casing is submerged in the liquid to be sampled. The flow of the liquid for the sampling is driven by a hydrostatic pressure and is thus independent of pumps or the like when placed at the correct subsurface position. The flow rate is in a first aspect of the present invention controlled by providing a constant hydrostatic pressure during the sampling period. Alternatively, the inlet is provided with a cartridge comprising a tracer material, which is partly soluble by the liquid and is released proportional to the flow rate.