Hello SHARPCO and Ken. SHARPCO this is an excellent question and very happy that you have brought it to the Exchange. Learning to interpret and apply measurement results correctly is as important to new users as the measurement data itself. Let us first put you at ease and then we will address the problem. Your PT50B will not yield 35 point differentials under identical measurement circumstances and, even more reassuring, the PT50B will not tell you that an imperfect edge is perfect.
We want to speak more generally here to all new users but will try to address some your specifics SHARPCO at the same time. As much as we would like to visualize edges as very homogenous cutting surfaces they typically are not. To the eye, perhaps so but on a microscopic scale, no. The microscopic scale is the measurement basis that all of our instruments operate on. Case in point; your delta (difference) reported here was 35 points. 35 points on the BESS represents a difference in apex radius of 35 nanometers. An almost infinitesimal measurement. While you may think that you are measuring in exactly the same spot it is very difficult to do so and location changes of only a few thousandths of an inch (a distance, perhaps, 50,000 times greater than the apex radius) can easily induce the sort of variability that you describe. Additionally, the PT50B is a manually operated device and some differences between measurements may be induced through variability in technique. These are typically reduced through experience. Experienced instrument users and sharpeners frequently report +/- 10 point differentials (and better) along the length of the blade. They often credit their PT50B's for having pointed out their previous sharpening inconsistencies.
Even DE razor blades do not hold perfectly homogenous apex radii. The DE blades work nearly perfectly as a sort of standard for the BESS scale though because their apex fluctuations are on a smaller scale than the diameter of BESS test media (.009"). Thus, the greater test media diameter averages most of the fluctuations out. If you had experienced 35 point differentials when measuring new DE razor blades (nearly impossible unless the error is induced intentionally) we simply could have told you to look to your measuring technique. In this case, we assume that you are measuring an edge that has seen some use. Under these circumstances we would not have been surprised if the differential you measured was much greater. Remember, even one partial cut through the paper wrapper of a DE razor blade can roll (dull) the DE edge 35 to 100 points.
The most important takeaway here for everyone is, please, don't shoot the messenger. A PT50B will tell you how it is, not how you think it should be. If a PT50B tells you that a given point on an edge is duller or sharper than another point or a previous measurement, it most very likely is. It now becomes your very pleasant and interesting, sometimes disconcerting, job of figuring out why.
EDIT REMARK: We see that Grepper stepped in and addressed some of our points before we could get this posted. Thank you Grepper for your help.
We want to speak more generally here to all new users but will try to address some your specifics SHARPCO at the same time. As much as we would like to visualize edges as very homogenous cutting surfaces they typically are not. To the eye, perhaps so but on a microscopic scale, no. The microscopic scale is the measurement basis that all of our instruments operate on. Case in point; your delta (difference) reported here was 35 points. 35 points on the BESS represents a difference in apex radius of 35 nanometers. An almost infinitesimal measurement. While you may think that you are measuring in exactly the same spot it is very difficult to do so and location changes of only a few thousandths of an inch (a distance, perhaps, 50,000 times greater than the apex radius) can easily induce the sort of variability that you describe. Additionally, the PT50B is a manually operated device and some differences between measurements may be induced through variability in technique. These are typically reduced through experience. Experienced instrument users and sharpeners frequently report +/- 10 point differentials (and better) along the length of the blade. They often credit their PT50B's for having pointed out their previous sharpening inconsistencies.
Even DE razor blades do not hold perfectly homogenous apex radii. The DE blades work nearly perfectly as a sort of standard for the BESS scale though because their apex fluctuations are on a smaller scale than the diameter of BESS test media (.009"). Thus, the greater test media diameter averages most of the fluctuations out. If you had experienced 35 point differentials when measuring new DE razor blades (nearly impossible unless the error is induced intentionally) we simply could have told you to look to your measuring technique. In this case, we assume that you are measuring an edge that has seen some use. Under these circumstances we would not have been surprised if the differential you measured was much greater. Remember, even one partial cut through the paper wrapper of a DE razor blade can roll (dull) the DE edge 35 to 100 points.
The most important takeaway here for everyone is, please, don't shoot the messenger. A PT50B will tell you how it is, not how you think it should be. If a PT50B tells you that a given point on an edge is duller or sharper than another point or a previous measurement, it most very likely is. It now becomes your very pleasant and interesting, sometimes disconcerting, job of figuring out why.
EDIT REMARK: We see that Grepper stepped in and addressed some of our points before we could get this posted. Thank you Grepper for your help.