I have person weighing spring scale. I live in Japan where the house has wood floors which deflect significantly under weight. I weighed my self in the bathroom and in the tiled bath area. The difference was 2.5 kilograms greater in the bath area but it could still be affected by possibly an angled floor or deflection. So how you get as true a result as possible with that spring scale?
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The elasticity does not influence the measurement as long as the scale stands perfectly level and you stand perfectly still.
The angle of the floor however can greatly influence the result. Your gravitational force won't compress the spring in full strength, it will rather split into normal force and downhill force. The spring will only "see" the normal force.
Also, the downhill force could exert shearing pressure on the spring, effectively changing its suspension rate due to frictional forces on the spring's mounting.
To make sure you get the correct result, you should use a water level and place sheets of paper beneath the lower edges until it is perfectly level.
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$\begingroup$ The floor could be deflecting unevenly so it has a slope simply due the weight on my heels. $\endgroup$ Commented Aug 8, 2014 at 10:15
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$\begingroup$ To check that, you could take the level with you onto the scale when measuring. $\endgroup$ Commented Aug 8, 2014 at 10:20
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$\begingroup$ I can't fit anything else on the scale when I'm standing on it. $\endgroup$ Commented Aug 8, 2014 at 10:44
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$\begingroup$ maybe put a light, stiff and straight plate on top of your scale, which effectively enlarges its area? You would not need to read the scale, just check whether it is level. $\endgroup$ Commented Aug 8, 2014 at 10:51