Question 1012568
You can say that as long as the different conditions under which you measured the weight under represent 99.7% of the conditions you'll see during a possible measurement. 
As an example, if I measure a piece of nylon in the winter in Eau Claire, Wisconsin when the humidity is low (20%), I will have a fairly stable measurement. 
But if I then take my piece of nylon down to Savannah, Georgia in the summer when the temperatures is 90F and 100% humidity, I could potentially gain 2%-3% increase in weight because of water absorption from the atmosphere. 
Then you'll probably fall out of your 3 standard deviations for a measurement.