Humboldt H-2795 Roll-a-Meter - Manual (Page 4)

• there must also be accurate batching and accurate recording of all
ingredients (including water) in the batch.
• thorough mixing of all ingredients.
• accurate measurement of the sample taken for an entrained air test
How reliable is the unit weight method? It is possible for two well-qualified
laboratory technicians to run parallel tests on all the above details and to come
out with results differing materially in apparent entrained air. Here is some
convenient information in regard to the extreme accuracy required in
determination of percent of air by other methods. Slightly incorrect factors
may make errors as great or greater than listed below:
Cement brands may differ .10 to .15 in specific gravity, or using kerosene instead
of water for obtaining specific gravity of cement may cause .6%. A sample taken
from a poorly mixed batch may weigh as much as 3 lbs. per cubic foot more or
less than average. This would lead to an error of 2% or so in apparent air
entrainment. Thus, accuracy by other meters or methods are dependent on:
• highly refined accuracy in determining all of the factors listed above;
• on getting truly average representative samples of all materials used
for making the above determinations;
• on getting all materials uniformly mixed;
• on getting a sample for air content test, which has all the ingredients
contained in the main batch, in practically the same proportions as
the main batch.
Satisfactory accuracy of all of these operations is very difficult to obtain
in the field.
Specifications
Total weight. Including accessories and carton:
22Ibs.
Height: 22 Inches
Outside diameter at center: 8 Inches
Volume of base: 130 Cu. In.
An error of: in Material= % Air Error
.03 Specific Gravity Sand .3%
.03 Specific Gravity Gravel .7%
.1 Specific Gravity Cement .6%
2% Free Water Sand .5%
1% Free Water Gravel .5%
1% Absorption Sand & Gravel .7%