Frequently Asked Industry Questions

American Portable Nuclear Gauge Association

The APNGA FAQ section is comprised of questions that are frequently asked on the “Ask A Question” section as well as emphasis items found to be lacking during regulatory inspections.

A standard block is used to give a quick check of the gauge’s ability to measure density and moisture. While in the standard mode the gauge will test the number of density and moisture counts received by the detector tubes. The number of counts are compared to previous standard counts. If the current count is acceptably close to previous counts it will pass. If the count is outside an acceptable range it will fail.

There could be several reasons why a standard count would fail. One could be the amount of time that has elapsed since the last standard count. Remember that the radioactive material in your gauge is always decaying. The amount of decay since the last test might be sufficient enough that your current counts are outside the acceptable range for a passing comparison. If you are concerned that the time interval could be the problem you can re-check the gauge by performing four new and separate counts. This will wipe out the old tests stored in the gauge or the tests that were manually recorded. Average the new counts, take a new standard count and compare it to the average of the new counts. You should find that the new test is now within an acceptable range of 1% density and 2% moisture. Check the gauge manual for proper standard count procedures.

If the counts are still not acceptable contact the manufacturer or your service provider for instructions.

Standard counts are also used to adjust for the decay that has occurred to the radioactive sources. Standard counts are very important and should be taken and recorded every day the gauge is used.

First off, check for any offsets that may still be entered in the gauge.

Make sure you took a standard count and it passed.

If working on soils you may have a situation where the gauge is viewing natural hydrogen as moisture. Soil with high amounts of mica is an example.

Working on frozen materials can fail due to the expansion of moisture. This will lead to higher moisture content and lower density readings.

Asphalt mats with significant surface voids can also be a problem. Use sand or fines to fill in these voids and dampen the effects of the voids.

Make sure to source rod is well seated in the appropriate depth rod notch.

A target value is entered into the gauge software and used to determine a percentage number indicating the compaction achieved by rolling the material. Each pass of the roller should result in a higher density. The gauge is used to measure and confirm roller actions. A reduction of compaction can occur if the material is over-compacted, which results in a break-down of the material.

The gauge will continue to give independent density and moisture readings.

The direct transmission method is generally only used while measuring soils. It requires the operator to hammer an access hole with the drill rod. The gauge is placed on the surface with the source rod opening situated over the drilled hole. The operator depresses the trigger mechanism on the gauge handle and extends the source rod down into ground to the desired depth.

Because destructive testing, such as the drilled hole used with direct transmission, is not acceptable on an asphalt mat, the backscatter method is typically used on asphalt measurements. The operator depresses the trigger mechanism on the gauge handle to release the source rod but only lowers it to the first depth setting. This places the bottom of the source at the surface of the asphalt mat. The radiation can measure density to a depth of 3-4 inches.

Although soil measurements are usually made using the direct transmission method, the backscatter method can be used for lifts of 3-4 inches. Backscatter is not effective below 3-4 inches below the surface. Soil backscatter is best used when the direct method is affecting the larger aggregate, and therefore the density, of the material.

ASTM D6938 and AASHTO T310 – In Place Density and Moisture Content of Soil and Soil-Aggregate by Nuclear Metholds

ASTM D2950 – Density of Bituminous Concrete in Place by Nuclear Methods