Gauges used in the construction industry generally contain two different nuclear sources. One source is used for wet density measurement and the other for moisture density measurement. Each gauge contains one or two Geiger Mueller (GM) tubes for detecting the counts from the density source (Cesium-137) and one Helium-3 (He3) tube to detect counts from the moisture source (Am-241:Be).
The density source is placed in the source rod that can be positioned at different depths in the material. The moisture source is fixed in the base of the gauge and cannot be positioned at different depths.
When the gauge is placed on the test site, each system produces a count. These counts are used to calculate the wet density (WD) and moisture (M) by using previously established calibration parameters. The wet density and moisture can be used to calculate other important practical parameters such as dry density (DD) and percent moisture (%M).
Gauge Calibration and Quality Control
All manufacturers calibrate gauges before shipment to the customers. The objective of the calibration is to generate gauge parameters that can convert the density counts (DC) and moisture counts (MC) to wet density (WD)and moisture content (M).
Each manufacturer uses a different number of blocks to calibrate the gauge. Presently, there are four different methods of calibration. The Five Block Calibration, the Three Block Calibration, the Two Block Calibration and a One Block Calibration. The Five Block Calibration is the oldest and the most comprehensive method of calibration using both metal and mineral blocks.
Regardless of the calibration method, all manufacturers use the same equation and the objective is to produce enough counts for known density blocks, either through historical data or actual measurements, to determine the gauge parameters A, B and C. These parameters are placed in the gauge after calibration and are used by the microprocessor to calculate WD and M.
Once the gauge parameters A, B and C are calculated and placed in the gauge memory, the gauge is placed on the same set of blocks for Quality Control (Q/C) checks. Because the Q/C is performed on the same blocks as the ones used during calibration and is not an independent check, major manufacturing problems such as missing filters, incorrect source height setting, base flatness, etc. cannot be detected.
The most effective Q/C should involve an independent check using a block of known density with a different composition than the blocks used during calibration of gauges. A block with heterogeneous quality similar to the quality of asphalt and soil is ideal. The Validator is used by many calibration facilities for this purpose.
Five gauges calibrated in the same bay with no operator errors but major manufacturing inconsistencies will pass the present Q/C checks. If these gauges are placed on a same asphalt site, variations of up to 4.0 lb/ft3 (56kg/m3) between gauges can be seen. This phenomena has been observed by every gauge user at one point or another and is due to manufacturing inconsistencies.
The present calibration Q/C procedures used by many facilities will fail to catch manufacturing inconsistencies. It is recommended that each gauge user employ a verification method to detect these problems, when the gauge is received from the manufacturer.
As stated above, a block of known density which is different in composition than the blocks used during calibration is ideal. Some customers have a marked spot on the concrete or asphalt floor for verifying their gauges. However, this method can only check the backscatter and not the direct transmission positions of the gauge.
Reasons for density system going out of calibration
Before describing some of the reasons for gauges going out of calibration, it is important to note that gauge measurements are independent at each depth. A gauge with an accurate calibration at one depth does not necessary indicate calibration accuracy at other depths. Of course, the opposite is also true. A bad calibration at one depth does not mean bad calibration at all depths.
The causes for gauges going out of calibration can be divided into two categories. Immediate and Long term effects.
A. Immediate effects; are gauges delivered to the end user with an inaccurate calibration.
It is recommend that after each repair, gauges be left on with the counting circuit active for at least 24 hours prior to calibration.
GM tubes require a minimum burn in time. GM tube manufacturers recommend a 24 hour burn in time before actual use. If the gauge is not properly burned in prior to calibration, there will be a shift in count rates after shipping and the end user will receive gauges with an inaccurate calibration.
Bad counts are collected on at least one of the gauge positions and gauge parameters are determined based on a bad count rate.
Without an independent and proper Q/C procedure at repair facilities, operator errors can be a routine occurrence.
Walls, surroundings of the calibration facility and radiation background can adversely effect the count rates. Manufacturers recommend a distance of at least three feet from any standing objects. This is especially critical when calibrating gauges with a plastic top shell.
It is recommend that calibration blocks be placed a minimum of three feet from any walls.
Gauges can change during shipment due to bad and aggressive handling. It is important to verify the calibration accuracy after each shipment.
B. Long term effects; are gauges with variations that occur during field operations.
Depending on the design and incorrect use of the gauge the mechanical components such as the base, the source rod and the sliding block spring can change in operation and physical shape. For example the base can warp and change from the time the gauge was calibrated. The sliding block spring can change in characteristics or the sliding block scraper plate can wear causing different radiation path to the detectors. These changes are unpredictable and can continue during the useful life of the gauge.
Electronic components can drift or can become unstable. These effects will cause density readings to drift or become unstable. Normally this condition will require maintenance and maybe a new calibration. Electronic repair in our experience does not necessarily mean that a new calibration is required. However, it is strongly recommend calibration verification after each repair.
Most electronic problems can be identified by the statistical and drift tests provided in the gauge software by all manufacturers.
Most gauges in the market employ two GM tubes. In our experience, GM tube variations is the most common cause for a gauge requiring a density re-calibration. When gauges are calibrated, the calibration is specific to those tubes, and the tubes are in effect “synchronized” to each other to read an accumulated count at a particular density.
Theoretically, GM tubes are expected to have an infinite operating life. However, in practice the expected life of a GM tube is on the order of 1 billion counts (approximately five years, under normal conditions).
Over time, the plateau length of the tube decreases resulting in a possible increase in the count rate. When this occurs the two tubes in the gauge will go out of “synchronization” causing the gauge calibration to become inaccurate. In this case a calibration is required to re-synchronize the tubes.
Standard counts are the most important measurement of the gauge. Density standard counts are taken with the source rod in the safe position. In this mode the source is surrounded by tungsten or lead shielding and a window in the shield allows a path between the source and the GM detectors. The counts in this mode are primarily from inside the gauge. Standard counts correct only for radioactive decay of the source and are employed in every calculation. A bad standard count can significantly effect gauge measurements. It is recommended that a standard count be taken every three hours during an operating day in the field.
The density standard counts should be within 1% and the moisture standard count within 2% of the previous standard counts collected, if the previous standard count were collected less than two months from the day of measurements.
If the time from the last gauge operation is more than two months, take four new standard counts and average them for your comparisons to your daily standard count. Also, on cold and very hot days allow the gauge to stabilize for fifteen minutes in the outside environment before taking the standard count.
Taking a standard count at one temperature and actual measurement counts at another can cause errors in the measurements.
Other than the obvious problems during strong rain storms, gauges can collect moisture inside and cause serious damage. Water in the gauge results in erratic readings in either the density or the moisture circuit of the gauge. Majority of the moisture problems are caused by bringing the gauge from hot and humid weather into an air conditioned facility. The hot air inside the gauge condenses creating corrosion and other serious problems. It is recommend to simply loosen the gauge front panel during storage and allow air to circulate into the gauge. This can save thousands of dollars in repair costs.
We have conducted long term variation studies on the moisture calibration and have found that moisture system of the gauge does not require re-calibration. This claim can be verified theoretically and practically. Since the moisture source and the detector are contained in the gauge, the source has a half life of 433 years and moisture response is linear, a good moisture standard count will correct for all variations in the gauge measurement.
Moisture standard count is the best indicator of moisture calibration variation over time.
Many users have used the standard block for years to determine gauge moisture verification in the field. However, other blocks like the validator can also be used for determination of moisture verification. Again, it is recommend that a standard count be collected every three hours during each operating day.
It is clear that without an effective verification system most of these variations can not be detected. Until now the only method of gauge verification has involved the shipment of the gauge back to a repair facility. With the technology employed, it is possible you can perform verification and calibration at your own facility.
Storage/Security of gauges
Permanent Storage Location
Soil moisture gauges may only be stored in the approved locations specified on your Radioactive Material Approval The gauge and storage location must meet the following conditions:
The gauge must be stored inside the manufacturer’s transport case.
The gauge and transport case must be locked in a closet or room that is not readily accessible to the public.
The storage area must be at least 10 feet from any location of high occupancy (e.g. desks, work stations or a residence)
The storage location must be under the exclusive control of the Approval Holder or users listed under that Approval.
The storage area must have the following items readily visible in the storage room
Field Site Storage
Gauges must be stored in a locked vehicle when at a field use site. If it is transported to the field site in a pickup truck, the gauge must be stored in the cab when not in use.
The giant lasers, X-ray machines and supercomputers called essential a decade ago for the upkeep of U.S. nuclear weapons have fallen behind schedule, yet even with those crippled or delayed capabilities, the weapons themselves are faring well, with little sign of falling apart.
The Federation of American Scientists, a group of scientists to advocate for arms control, argued that Congress needs to rethink some of the multibillion-dollar instruments promised to bomb scientists at the end of nuclear testing.
Topping the federation's target list is a stadium-size laser complex called the National Ignition Facility, or NIF, at Lawrence Livermore National Laboratory.
Livermore weapons scientists and federal nuclear-weapons managers have argued since the early 1990s that the NIF and its reach for thermonuclear fusion with 192 laser beams are critical to ensuring the operation of the U.S. nuclear arsenal.
Last year, for example, the head of weapons work for the U.S. Energy Department suggested that scientists might be unable to say whether the bombs would keep working if NIF's lasers fail to squeeze fusion energy out of a pea-size ball of hydrogen by 2010 or so.
Failure to achieve ignition in the 2010-2011 time frame may affect our continuing assessment and certification of low-margin systems such as the warheads that are the most numerous in the U.S. and U.K. arsenals, then-defense programs. Failure to achieve ignition in the long term could call into question our stockpile stewardship tools And, therefore, the premise that the stockpile can be maintained indefinitely without nuclear testing."
Likewise for purchases of the world's fastest supercomputers and construction of a huge X-ray machine to peer inside imploding bomb cores -- all were needed to say whether bombs would work.
The decline of bomb reliability without testing -- have not come to pass. Yet these enormously expensive programs persist.
The big fusion laser at Livermore originally was priced at less than $400 million but had risen to $1 billion by the time Congress agreed to build it. Even then, supporters low-balled the billion- dollar price tag because of a calculation that lawmakers otherwise never would pay for it. Livermore officials were forced to admit in 1999 that the laser was over budget and would not be completed by 2002 as promised. The General Accountability Office projects its cost at about $4 billion, with completion next year.
NIF should have been operating years and a billion dollars ago, and it's fair to ask whether we should go forward with this machine when the whole context around it has changed. We need to re-evaluate, and it is not yet done.
Energy Department officials said they had not seen the federation's report but took issue with the observations about billion-dollar cost overruns and schedule breakdowns. It is important to keep in mind that all three of these facilities are unique, one-of-a-kind -- some that have never before been built in the world.