Something strange with the BESS sharpness tester

[WiSpi]

Good morning,


I am a new member of this group and recently got the BESS sharpness tester (Edge-On-Up PT50A). 
I notice something strange with the BESS sharpness tester

If I put a block of wood on a digital accurate scale, it weighs (for example) 46.5 grams. If I press tara and weigh that same block again, it again weighs 46.5 grams. That's the way it's supposed to be.

But with my BESS sharpness tester (Edge-On-Up PT50A), I measure a much higher value for the same block of wood: 57 grams.

If I repeat the same measurement as described above on the BESS scale, the results vary: 57 grams, 60 grams, 58 grams 74 grams, and so on.

That there is a difference between scales, I can understand, but the difference is substantial. But that the same cube on the BESS scale gives different measurement results, I find strange.

I do have an idea that difference occurs with putting the block of wood on it. If I put it on more forcefully, I get a higher value than if I put it on more subtly.
Maybe I shouldn't think of my BESS sharpness tester as a "scale.
Who can tell me a little more about this?

Thanks in advance for the responses!


Wim

[grepper]

When sharpness is tested, force is applied to the test media by the edge of the blade.  The force increases until the test media severs.  When the media severs the force decreases.  The BESS tester displays the amount of force applied to the media at the moment it severs.  In other words, the maximum amount of force required to sever the test media.

The BESS tester is extremely accurate.   Force of impact of a falling object exceeds the resting weight of the impacting object.  If an object is dropped onto the pressure plate of a BESS tester, the force of impact is displayed, not the resting weight of the object.  Attempting to place a weight on the tester by hand without influencing the tester is almost impossible.  If the object is released above the pressure plate, even by a tiny distance, so that it falls onto the pressure plate the displayed force will vary from the resting weight of the object.  If the object is pressed onto the pressure plate, the displayed weight will vary from the resting weight of the object due to the influence of your hand.

Attempting to place a weight on the tester by hand without influencing the tester is almost impossible.  Your observations are normal and expected.  Sharpness testing is different from simply weighing an object.

Let me know if that does not make sense or if you have further questions.

[WiSpi]

Thanks!

Now I have repeated test and when I very, very carefully put a block of wood on the BESS tester it practically matches the weight measured on an accurate kitchen scale.

If I repeat that on the BESS tester, so very carefully placing that block of wood, the measured values are not much different.

In itself, this is a good exercise to see for yourself how carefully you have to handle the knife on the BESS tester for a good measurement. I did look up at that. Instructive for me.

Thanks for thinking with me!
Wim

[grepper]

“In itself, this is a good exercise to see for yourself how carefully you have to handle the knife on the BESS tester for a good measurement.”

In light of your observations and experiments placing a block of wood on the tester, I understand what and why you are saying that you must be extremely careful with the knife to get an accurate reading when doing a sharpness test.  However, it ends up that extreme care is really not necessary to get an accurate reading.  Only ordinary care and even slightly sloppy care will provide an accurate reading.  The point is that there is really no need to over-think it.  Taking an accurate sharpness reading is actually easy.

I’ll attempt to explain and clarify.  Hopefully it will be helpful.

To take a reading, simply place the blade in the fulcrum and slowly press the blade into the media until the media severs.  You don’t need to over-think the word “slowly” either.  No need to creep along.  Just don’t hack at it like chopping a carrot.  Simply press down until the media severs.  That’s it!

Let’s take a closer look at what’s going on.  

Each time force increases on the pressure plate of the tester, that force value is stored and displayed on the tester display.  If force upon the pressure plate is lessened, the last highest force value remains displayed on the tester.  This is where the sharpness tester differs from a regular scale.  A regular scale will continuously display the current force applied to the pressure plate whereas the sharpness tester will display the last greatest force applied.  

During a sharpness test, force increases on the test media until it severs.  The force value at the time the media severs remains displayed on the tester while on a regular scale the displayed value would quickly drop to zero when the test media severs.  Using a regular scale to measure sharpness is difficult and inaccurate because the displayed force value must be carefully observed until it suddenly drops off when the media severs.   Very difficult and inaccurate with a regular scale.

So what does this mean when taking a sharpness reading?  

Let’s consider a 150 sharpness edge.  Imagine that while taking a sharpness reading you get bumped causing the tester display to jump to 25g.  Do you need to tare the scale and start over?  No!  Because with a 150 sharpness edge, 150g of force is needed to sever the test media.  Simply continue the test until the media severs.  In this instance as force is applied to the blade the tester will display 25g until the applied force exceeds 25g at which time the tester will display the new greater force.

In other words you could start a test, get half way through, go to lunch, come back and continue the test.  The only thing that matters is the amount of force applied to the test media by the knife edge when the media severs.  That force value is the sharpness reading and what will be displayed on the tester.  

I don’t remember the exact specs, but the PT50A reads force many times per second, so as long as you simply apply pressure to the blade at any reasonable rate an accurate sharpness reading is achieved.  Of course you don’t want to swing the blade at the test media like using a hatchet, but I’m sure you get the idea.

Getting an accurate sharpness test is easy.   No need to over-think it or use extreme care.  Just push the blade at any reasonable rate and you’ll be fine.

That was a good question! Please post if you would like further clarification or have additional questions.

[WiSpi]

Thank you for this clear explanation. It explains the method of operation very well.
It reassures me that there is nothing wrong with my BESS sharpenig tester.

Greetings, Wim