Another vote for the Heiland TRD2 - a remarkable easy-to-use device that, like Ralph, has never been necessary to re-calibrate. I've been using mine in conjunction with a Stouffer 31-step wedge for determining the characteristic curves of expansions/contractions for my chosen film/dev combinations. The 21-step wedge would also work, but I opted to pay the extra $ to get more data points along each curve (just part of my analytical nature).