20
24
Below 100mm (4 inches) the gauge is relatively unaffected by any change
in density. In fact, a large change in density below 75mm (3 inches) has very
little effect.
There is always a question of when to use the nomograph method. The
table on the next page presents some information as a guideline.
Since the best accuracy that one can expect for a backscatter density
measurement, even assuming corrections for chemistry, is about 2.0% then
attempting to correct for the bottom layer density when it causes less than
a 2% error is futile. The conditions noted within an * are ones where the
nomograph correction is recommended.
Top Layer
% Error with no correction for
% difference in density
mm
inch
2%
4%
6%
8%
10%
15%
20%
25.0
1.0
1.0
*2.1
*3.1
*4.1
*5.2
*7.8
*10.4
37.5
1.5
0.7
1.4
*2.1
*2.8
*3.5
*5.3
*7.0
50.0
2.0
0.5
0.9
1.4
1.8
*2.3
*3.4
*4.6
62.5
2.5
0.3
0.6
0.8
1.1
1.4
*2.1
*2.8
75.0
3.0
0.2
0.3
0.5
0.7
0.8
1.2
1.6
87.5
3.5
0.1
0.2
0.3
0.4
0.5
0.7
0.9
100.0
4.0
0.1
0.1
0.2
0.2
0.3
0.4
0.6
112.5
4.5
0.0
0.1
0.1
0.2
0.2
0.3
0.5
125.0
5.0
0.0
0.1
0.1
0.2
0.2
0.3
0.4
137.5
5.5
0.0
0.1
0.1
0.2
0.2
0.3
0.4
150.0
6.0
0.0
0.0
0.1
0.1
0.1
0.2
0.2
162.5
6.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
Errors Due To Mat Thickness
mm
inch
Relative
Response
0
0.0
0.000
25
1.0
0.490
50
2.0
0.778
75
3.0
0.912
100
4.0
0.960
125
5.0
0.985
150
6.0
0.998
162
6.5
1.000
Thickness
