A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert VanderPol II
Date: 2017 Dec 20, 06:01 -0800
I am a construction inspector. In my line of work I do some tests in the field on construction materials.For a number of tests the final value reported has higher precision than the underlying data measured. The most egregious example is unit weight.
In the field we generally use a 1/4 cf bucket accurately calibrated in the lab prior to the nearest 1/000th of a cf, let's say 0.249 cf is a normal calibrated value, 3 significant digits.
In the field we use balance scales accurate to 1/10 lb as required by ASTM.
With a 1/4 cf of concrete in bucket that works out to about 37 lb or so of concrete. So 37.2lb would be a normal recorded value, 3 significant digits.
Unit weight results are required to be reported to the nearest 0.1pcf. For normal weight concrete that's about 140.0 pcf.
A 1/10th lb change in recorded weight results in a 0.5lb/cf change in reported unit weight. A 0.001cf change in the calibrated volume of the measuring bucket results in a 0.4pcf change in the reported unit weight.
This isn't the news media screwing up math and precision, this is the ASTM folks that write testing standards for the materials that go into building the places we live and work.