Negative number on weighing scale if I move my hand above it? So, I wondered if my electrical weighing scale could detect the air pressure exerted by something flat like a plastic plate or even my hand. I noticed that if I move my hand with my arm vertical to the scale, 2-3 grams are detected at most, as expected since I move air toward the scale. If I try this with my arm parallel to the scale I can get it to detect -1 gram, if I do it quickly enough. I thought that maybe this happens because I "move away" some air from above it/decrease pressure or something?
Arm vertical: 

Arm parallel: 
 A: There are 3 effects. First is the orientation of the balance. Moving around a floor that flexes or deforms, even slightly as you move your weight around, is a no-no. (similarly if the balance isn't isolated from you putting or removing weight from the table it is on). Second is that YOU carry an electric charge. So could a plastic plate. Does the reading persist? Or rapidly diminish? Charge will depend a lot on humidity. Different readings with same motion but different humidity suggests it is not air motion. Should be easy enough to dramatically change humidity in a room by boiling some water (if the room allows hotplates or is a kitchen...) placing pans of hot water near the balance should also reduce the charge your carry. Hmmm. I guess if you took an aluminum foil "hand" the size of yours, grounded it, and moved it in the same way as your hand, that you'd see the effect of wind alone. Air currents will generally diminish more rapidly than electrical charge, but the only way to tell for sure is to use a grounded conductor to move the air with.
A: If you detect a reading of -1 gram when you move your hand above the scale, it's quite possible you did generate some negative pressure.
This is a simple consequence of Benoulli's law, which states that
$$\frac12 \rho v^2 + P + \rho \cdot g\cdot h = \mathrm{const}$$
so when you increase the velocity $v$ of the air, you lower the pressure. Now you have to take into account the direction of the air flow as well: if you have air directed towards the scale, it has to change direction - that exchange of momentum results in a positive pressure on the scale (and pressure times area = force). But at the trailing edge of your hand moving over the scale, you have a vortex with horizontal velocity and therefore just lower pressure.
It's quite possibly a real effect - especially if you have carefully ruled out vibration etc.
