Nuclear Gauges USA

Frequently Asked Questions (FAQs)

Here are some answers to common questions about operating a portable gauge in the USA:

Do I have to have a “target” maximum density entered into the gauge (Proctor, Marshall, Voidless) for it to give me results?

No. The function of target value is simply to give you relative density (% compaction achieved in the field as compared to the lab standard). Target is not used by the gauge’s software/detectors for density, moisture, or air voids determination. The % compaction can be calculated later by hand (if required).

Example: (DD/Proctor) x 100 = % Proctor  

                   (WD/Marshall) x 100 = % Marshall

What does frost do to gauge readings?

Frozen ground translates into higher gauge moisture readings which means lower dry density “read” by the gauge. Tests typically fail the project specifications as a result. As water freezes, it pushes out and takes up a larger volume of the material under the gauge. Placing and compacting frozen soil often leads to settling of roads, pipes, and structures when the material later thaws.

Why don’t my sand-cone readings match my gauge density readings?

Typically, the results are different. Errors can quite easily be introduced into the sand-cone test (which shouldn’t be used for certain granular materials and highly-plastic clays) where you excavate and complete a manual moisture content. Portable gauges give a relative in-place result which is not “absolute” and not necessarily more accurate than other methods. As is the case for the gauge when testing asphalt, the best method is to take several 4-minute gauge direct transmission measurements at different test locations (6 or 8 inches depth) and a sand-cone at the same locations as the gauge density readings. Calculate the difference between test methods at each spot and average them to correlate.

How do I correlate my gauge density to asphalt cores on the control strip?

Always refer to the project specs/procedures, from the DOT for example. The following method may be used:

Take 2 gauge wet density readings at each marked test site on the control strip and average the result (1 reading parallel and 1 with the gauge turned 180 degrees). Take 2 core samples below the positioned gauge and average the density. Compare density from each method (gauge and cores) to establish correlation at each site. Average the correlation from as many individual test sites on the strip as required (7 for example) to establish the final wet density offset. Enter the offset into the gauge and use it.

Which method should I use on aggregate/base (4 to 6 “ thick) – direct transmission (DT) or backscatter (BS)?

Either method is acceptable. However, industry practice is to use direct transmission if the base layer is more than 4 inches, to ensure full and representative lift coverage (from the bottom of the source rod back up to the detectors). In theory, DT should give you more accuracy, but sometimes lowers density in the zone, because the drill rod disturbs the compacted, larger, in-place aggregate. You can try both methods in the same spot (backscatter first), then continue to use the test with the higher density. 

How accurate is the gauge on concrete slabs?

Portable gauges have proven to be useful in gauging the density of concrete structures and layers; however, due to the error-causing ingredients and chemical admixtures that make up concrete mixes (such as air-entraining agents, accelerators, fly ash and water-reducers), moisture readings won’t be accurate. Reinforcing steel will affect density numbers. If the mix is consistent throughout the slab, you might use the gauge to compare relative moisture in possible problem areas. For example, if most areas show 15% moisture, and one is 20% moisture, you can use the data as a guide to where cores should be drilled for lab analysis.

Why is my moisture reading with the gauge higher than from the oven-dry method or speedy-dry?

The oven-dry procedure is the “true” lab standard which precisely removes the water from the soil under “controlled” conditions at 110 ±5°C (230 ±°9F). The gauge measures the hydrogen present in the material (usually in the form of water). If the material consists of hydrogen in forms other than water (H₂O), the gauge will usually measure erroneously high. Typical hydrogenous materials include cement, fly ash, gypsum, coal, lime, mica, phosphates, and recycled products. Some less-common substances have the opposite effect by absorbing the gauge’s “slowed” neutrons. This happens when you find high salt and iron oxide, boron, lithium, or cadmium below the gauge. Either way, a moisture offset corrects for this problem.

How often do moisture/density gauges need to be calibrated?

It’s recommended each gauge be calibrated (or calibration-verified) every 12 months. Refer to ASTM (D2950 and D6938) and AASHTO (T 310 and 355) standards as well as state DOTs for compliance requirements.

What are the ASTM and AASHTO standards for density and moisture testing by nuclear gauge?

Density & Moisture Content Testing Standards (Portable Nuclear Gauge)

Does a nuclear gauge need an MSDS?

No. Radioactive sealed sources are exempt from OSHA regulations, as the only hazard they pose is radiological. Contact the gauge manufacturer for documentation on exemption.

One should drill the hole for the direct transmission measurement at least 2 inches deeper than the desired testing depth. What happens if you make the hole, say, a full 12 inches every time?

Nothing. As long as there is an extra 2 inches below the bottom of the extended source rod, there is no problem. It’s a minimum two inches for the drill rod. The extra 2 inches prevents you from “bottoming out” the source rod and simulates factory calibration. If you make a 12-inch hole for a 6-inch test, there is no effect on accuracy.

Can testing at high elevations, in the mountains for example, throw the gauge off, because of the higher level of background radiation caused by cosmic rays from space?

Potentially. There have been situations where this has occurred. While the standard count will generally account for the extra cosmic radiation, there have been jobs where a dummy gauge (with detector tubes only) has been brought in to measure only background radiation counts, to establish a correlation factor.

Can excessive humidity affect the gauge’s readings?

Yes. Thick, moist air can have an effect on the gauge’s counts. Generally, the daily standard count will adjust for the moisture to keep results in line. In southern, east coast states, and states near the Great Lakes, that see high fluctuations in humidity in the air, you will see operators run standard counts in the morning and the afternoon, to account for moisture ranges.

How dirty does the base of the gauge have to be to negatively impact readings?

A small amount of dirt on the sides isn’t a big deal. But, any material caked on the bottom will create air voids under the gauge (lowering density). Clumps of mud increase the moisture counts. The nuclear test requires positioning the gauge on a smooth and level material surface.

Does surface moisture on the asphalt mat make the density test inaccurate?

No. A small amount of water from the roller (or from the environment) won’t impact density readings of the placed hot mix. A heavy rain that soaks the voids in the mat might have some effect on gauge readings.

What would be considered enough of a defect on a standard block to have an effect on accuracy of standard counts?

Big chips and cracks obviously would be detrimental. Even smaller scratches can cause impacts. The block needs to be clean and dry for all standard counts. It is best not to “mix and match” blocks from other gauges, even if they are from the same model.

How often do gauge runovers occur?

Nuclear Regulatory Commission figures show there is a number of incidents each year in the United States where portable gauges are damaged by moving, heavy equipment on jobsites. The main reason is the gauge operator leaving the gauge unattended on the ground, i.e., not maintaining immediate control and constant surveillance while the device is removed from secure storage.

How far from the gauge do I have to be to be sure I’m receiving no radiation?

The radius of influence is 2 feet for a moisture/density gauge where you are receiving radiation and affecting its counts. For radiation safety purposes, minimum 4 feet is a guideline (while the gauge is measuring). A yellow-II labeled gauge with a transport index of 0.1 to 1.0 must be a minimum of 1 foot from everyone in the vehicle. While testing, 10 to 12 feet ALARA distance, keeps you (and others) out of the radiation field, and gives you time to maintain control of the device, and to react to unexpected situations, as per the regulations.

What can I do to keep my radiation exposure from nuclear gauges as low as possible?

You are required to following the guiding principles of radiation protection: time, distance, and shielding, anytime you encounter a portable gauge. This means reducing time near radioactive sources, increasing distance, and utilizing shielding, to keep your dose as low as reasonably achievable (ALARA). By following company procedures, regulations, and license conditions, you will be able to keep your occupational dose below what is considered natural “background” level each year.

How much radiation can I expect to receive from working with or near a portable nuclear gauge?

Depending on workload and job role, an operator (using ALARA), should receive less than 100 mrem in a year occupationally from the device.

For a typical moisture/density gauge, what is the pass/fail tolerance for the standard count?

For most gauges, to “pass,” the new count has to be within 1% for density and 2% for moisture, as compared to the average of the previous four standard counts stored in memory. Most models perform the pass/fail calculation for you.

How much radiation might I be exposed to when cleaning and lubricating the gauge?

Gauge manufacturers have estimated that 1 routine cleaning and lubrication of the tungsten sliding block and associated bottom plate cavity, results in 4 mrem dose (whole body equivalent), following ALARA practices. You could perform 125 cleaning/lubrications each year and remain within 10 % of the 5,000 mrem annual occupational dose limit (4 mrem x 125 = 500 mrem).

What are the training requirements for portable gauges?

Any person handling or using a gauge must receive nuclear gauge safety certification training. The initial gauge certification training is to include USDOT hazmat training. Training (including hands-on) must be completed before working with or handling the device, although you can perform certain hazmat functions such as preparing paperwork, for the initial 90 days, under the direct supervision of certified personnel. Recurrent (every 3 years) hazmat training is required for all authorized users that transport gauges. Many states also require annual refresher training for authorized users, covering operating, transport, emergency and security procedures.

Can a nuclear gauge operator get in trouble for irresponsible conduct?

Yes, the licensee and individual gauge users are required to conduct activities in a reasonable and responsible manner, to keep occupational doses and doses to members of the public ALARA. The person driving a company vehicle, as well as the company, can be sanctioned for DOT hazmat violations. Examples of serious offences would include: transporting or storing the gauge with an open shutter (causing excessive exposure), willful actions such as lying to an inspector, failing to notify the agency of an accident or incident involving a gauge, and, falsifying records (e.g., related to missed inventories and leak tests or training).

Do I need radioactive signs on my truck, when I’m transporting the gauge?

No. You must not “placard” the vehicle when carrying a typical Type A Package Radioactive Yellow-II category portable gauge (or gauges). Placards are reserved for higher-quantity Radioactive Yellow-III devices. Also, note that if your see a road sign indicating bridge or tunnel restrictions for “Class 7 Radioactive Materials,” this applies to higher-activity placarded vehicles, not portable gauges. Generally, you don’t have to follow the designated hazardous materials route or pull over for inspection with a moisture-density gauge.

If radiation is dangerous, why do we use it in nuclear moisture/density gauges?

Even though it can cause injury and death, radiation is like a sharp knife. If the knife is stored and used properly, it helps us prepare and eat food. Nuclear gauges use gamma ray interaction and scattering and neutron thermalization to quickly and reliably measure density and moisture of materials.