The invention relates to an arrangement for providing a constant contact pressure for a probe to a finger of a patient, including a housing having a top position and a side portion to support said top portion, said top portion having a thickness defined by a top surface and a base of said top portion, said top portion having a bore extending through, the top portion, said bore dimensioned to slidably receive a probe, said probe having a probe tip, a channel extending substantially perpendicular and on a same plane to said bore, said channel sized to receive a finger of a patient, wherein the probe further includes a collar positioned along the probe such that at rest, the collar is in contact with the top surface of the top portion of the housing, and the probe tip extends a distance from the base of the top portion, and into the channel.