Electronic stylus force gauge question

I always have similar but not exactly identical problem with most of my cartridges. If I turn on digital scale on first and let it zero first, as soon as I move my cartridge over the scale, it gave negative reading. However the number usually does not fluctuate wildly. The reading is very much reproducible every time. I assume it was just magnetic interference from cartridge or something along that line. Usually I just move my cartridge above the scale first then turn the scale on and let it zero itself then. This happened with my airtight koetsu dynavetor and Lyra with 2 different scale (same made though) so I thought it was normal.

Just to add to the fun, I have three arms mounted on two tables, and in each case, my cheap digital scale tells me that the VTF varies depending on where I put the scale on the respective turntables. That is, it measures slightly differently depending on whether the scale is placed where the lead-in grooves are, or half-way in, or where the end grooves are. I find this curious but not really important. Certainly trying to pinpoint 1.75 g -- as opposed to l.72 or 1.78 or whatever -- is not in the cards for me :-)

Here is another thought as a way to check the condition of your tonearm bearings in the vertical direction. After placing the stylus on your VTF gage and getting a reading, apply a light pressure downward on your counterweight to bring the VTF to near zero and then let go. If the scale has a new reading, that difference is called hysteresis. e.g. let's say your scale reads 1.5g and then you apply a pressure to your counterweight until the scale reads near zero, let go and now the scale reads 1.49g. The hysteresis is the percent difference or in this case 0.67%. For mechanical systems, up to 1% hysteresis is very good. Up to 2% is ok, but considering the quality of our tonearms and bearings used, I would expect it to be less than 1%. Try it in the other direction too. Apply a slight upward pressure on the counterweight to raise VTF by 0.5g or so and then release.
So why is hysteresis a factor? Because as the tonearm/cartridge tracks over a record, any waviness in the record causes the tonearm to move up and down. If the bearings have too much hysteresis then the VTF will be changing slightly- going higher as the tonearm is pushed upward and then being a little lighter as the tonearm comes back down. Aside from inertia, 0% hysteresis means the VTF would stay constant as the stylus tracks over a wavy record.
The scale itself should have extremely low hysteresis since it is a strain gage. Good quality scales that I have used change maybe one digit out to the third decimal place when I have added/subtracted the load slightly.

Suteetat
I always have similar but not exactly identical problem with most of my cartridges. If I turn on digital scale on first and let it zero first, as soon as I move my cartridge over the scale, it gave negative reading. However the number usually does not fluctuate wildly. The reading is very much reproducible every time. I assume it was just magnetic interference from cartridge or something along that line. Usually I just move my cartridge above the scale first then turn the scale on and let it zero itself then. This happened with my airtight koetsu dynavetor and Lyra with 2 different scale (same made though) so I thought it was normal.

Suteetat (and others),
Do you have any concerns that the magnetic interference might negatively affect the cartridge long term? Beyond inconsistent readings, the icing on the cake would be to damage a few $K cartridge with a cheap scale while trying to dial in the VTF!

Hmmm I thought it was the cartridge that interfered with the scale. The
amount of time spent on the scale is very minor in comparison to the hours
playing record so I never even thought about it. please don't give me
another thing to worry about :) already nervous enough when playing
records as is :(