Publications
Understanding and Operating Nuclear Gauges More Effectively
InstroTek, Inc.
Raleigh, NC
January 18, 1999
Introduction
This short paper is intended for the nuclear gauge managers users and maintenance technician. The recommendations in this paper are intended to help both new and experienced operators to better understand gauge measurements and to reduce gauge maintenance costs.
Objective
This paper was produced to help InstroTek management in training of its employees. The paper is written in a style suitable to both the new and experienced operators of nuclear gauges. By distributing this paper we hope to share our more than 25 years of experience in materials testing and nuclear gauge development with our valued customers. We believe better understanding of gauge operation and maintenance will help users with quality of measurements and will save significant amount in maintenance costs.
We have tried to stay away from theoretical details in order to reduce possible insomnia. However, for those interested in more detail, please call us. We love to hear from you.
Gauge Operation
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 which can be positioned at different depths in the material. The moisture source is fixed in the base of the gauge and can not 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 Q/C
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. can not 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.
Example: 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. Immediate effects; are gauges delivered to the end user with an inaccurate calibration.
» The gauge is not properly burned in.
We 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.
» Operator error.
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.
» Environmental effects.
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.
We recommend that calibration blocks be placed a minimum of three feet from any walls.
» Shipping effects.
Gauges can change during shipment due to bad and aggressive handling. It is important to verify the calibration accuracy after each shipment.
Long term effects; are gauges with variations that occur during field operations.
» Mechanical changes over time.
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 changes over time.
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, we 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.
» GM tube sensitivity changes.
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 (an important characteristic) 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.
» Inaccurate Standard Counts.
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 your 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.
» Moisture in the Gauge.
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. In our experience 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. We recommend that you simply loosen the gauge front panel during storage and allow air to circulate into the gauge. This can save you thousands of dollars in repair costs.
Moisture
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, we recommend that a standard count be collected every three hours during each operating day.
Gauge Verification
This paper briefly outlines some of the problems that can cause the gauge density calibration to be inaccurate. Without us trying to sell you on anything, I think it is clear that without an effective verification system most of these variations can not be detected. This is the reason we developed the ValiDator to provide gauge users with a simple and accurate method to verify the gauge calibration and operating status. Until now the only method of gauge verification has involved the shipment of the gauge back to a repair facility. With the technology employed in the design of the ValiDator you can perform verification and calibration at your own facility.
Our initial testing and development of a verification system consisted of multiple density verification points. Some of the earlier units sold contained two insertion holes for verification at different densities. After extensive field testing we realized that since the ValiDator is different in composition than calibration blocks, one point verification was just as accurate as two or more density point verification. Using the ValiDator, any inconsistency in calibration and manufacturing is detected by the pass/ fail limits provided. These limits are not determined theoretically but are based on actual measurements taken on over 200 gauges. Readings obtained on calibration metal blocks alone is not sufficient to design an effective verification system.
The following points should be considered, when selecting a verification system.
» The block material used for verification should be different from the materials used during calibration. Using the same material as the ones used during calibration could mask significant inconsistencies in the gauges. Generally, these inconsistencies are discovered by users in the field when the gauge in question is compared to other gauges.
» The block material should be heterogeneous to better simulate soil, asphalt and concrete.
» Pass/fail percentage for verification should be realistic to assure that gauges passing this limit are accurately calibrated and would comply with the intent of the ASTM standards.
Conclusion
Understanding the nuclear gauge operation can improve measurement quality. Taking a few simple steps recommended in this paper can help organizations save thousands of dollars in maintenance costs and down time. Our goal at InstroTek is to better educate customers on the use of nuclear density gauges. This has now prompted gauge manufacturers and others to follow our lead with classes and conferences on calibration theory and gauge operation information.
We hope this paper is helpful. For more details on any aspect of this paper, please call us at 919-875-8371.