The only measurements of instrument precision are those provided by the ASTM assessment. It is reasonable to assume the within patient measurement variation will be greater than the measurements from the controlled testing. A sample size of 40 patients is, from a statistical standpoint, a reasonable number and this sample size provides acceptable power for a mean difference of .1 for a ten-fold increase in the estimate of minimum test variability. This sample size will also provide enough data for the Bland-Altman test for agreement. Because this is a repeated measures test and even one missing measurement will impact the results, the investigators plan to over-sample in order to assure the study has the power to answer the questions posed.