Let us consider a completely sealed weighing scale such that the air pressure above and below the pan of the scale are equal and is equal to 1 atm. pressure. The scale initially reads zero. Now if this weighing scale is placed in a completely evacuated chamber, will there be any change in the reading? I am guessing that as the force due to atmospheric pressure above the pan of the scale is now absent, but the air pressure below the pan is still present, the scale will show a negative reading as it experiences a net upward force which is equal in magnitude to the force exerted by the atmosphere on the pan initially. Is this reasoning correct? If not, what is the actual answer?
You must consider the effect of air pressure on each side of the pan when reading the weight, because the scale measures the exerted force, and this force can be exerted from the air pressure as well as any other object on the scale.
So, if the pressure below the pan is 1atm more than the pressure above, your explanation is correct and there will be a negative number.