To minimize such changes, temperature and humidity should be regulated in some way. A degree of automation is offered by multiple sampling units, which switch between successive filter papers, mounted in cartridges, over a period of time (e.g. to make daily measurements over a period of a couple of weeks), but analysis of the weight gain must still be performed manually. A related technique is impaction, in which particles impact on a medium due to their aerodynamicdiameter-dependent inability to follow a bending airstream.