Whitepaper "Radiometric Measurements – Accuracy, repeatability and errors"
A whitepaper explaining how radiometric measurements work and the components, as well as how they can be optimized for the highest accuracy and reproducibility. The paper explains best practices and how proper design minimizes error.
While for nuclear limit switches and density measurements the point source and point detector is the industry standard, continuous level measurements usually utilize at least one rod type device, in order to cover an extended measuring range.
One option is to use a point source in a fan beam shield, creating an extended gamma field, and a rod detector. Due to their large scintillators, rod detectors and their subsequent outputs are more strongly influcenced by background radiation than point detectors. To reduce error caused by fluctuating background radation, sources must be sized larger to reduce the impact of brackground. In addition, we can even offer rod detector collimators to cut down background.
A second option is to use a custom rod source to create an extended gamma field, and a point detector located at the top of the measuring range. Rod sources are custom designed to fit the application, with Co-60, Cs-137, or Am-241 sources. Most commonly Co-60 is used in continuous level applications due it's ease of production in house by Berthold, the gamma's ability to penetrate thick walled vessels, and its cost effectiveness. The custom gamma field is sized so that cps/height is maintained across the measuring range. Rod sources are typically paired with point detectors to minimize background influence, which subsequently reduces the required activity. However, they can also be paired with rod detectors for the most difficult applications with the thickest vessel walls.
Similar to the nuclear limit switch, density measurements are typically point-to-point measurements, with statistical error still having more impact than background radiation fluctuations.
Berthold's sophisticated temperature and aging compensations preserve the high reproducibility of the measurement itself. Density measurements have high demands for accuracy and reproducibility in a very limited calibration range. A proper calibration, with representative samples and accurate laboratory density values, strongly affects the accuracy of the measurement. Typically, the count rate ratio between the highest and lowest densities is low, which prohibits the use of similar count rates to a limit switch. To decrease statistical error, it helps to have a high count-rate and a long time constant.
A level switch is a point-to-point measurement, with a point source and a point detector. Background error can be reduced by the use of a collimator and properly calculated switching limits.
As background influence is reduced with a point detector, the strongest cause of error for these measurements is statistical error, which is handled by statistical methods, e.g. by setting an appropriate time constant, and sizing for an appropriate number of cps. Designing the system with a high enough count rate and leaving a large enough sigma difference between empty and full counts prevents false switching. We recommend a difference of at least 6σ, as shown in the figure below, which makes the likelihood of a false measurement 0.0000001973% of all measurements – once every 16 years.
X-Ray Interference Protection (XIP)
In industrial plants, weld inspections on pipes are regularly examined for cracks. Gamma sources with very high activity are frequently used as test equipment. Radiometric level and density measurements can however be affected by this gamma radiation and thus simulate low readings. Influence areas of up to several 100 m distance are not uncommon. The range of influence essentially depends on the activity of the test source and whether any buildings or vessels situated between the testing point and the measuring point minimize or even shield the influence.
A falsification of the measured value by external radiation can be prevented with the function XIP from Berthold. If external radiation is detected, the measured value is frozen until no external radiation is present. As long as the measured value is frozen, the measurement then signals external radiation via a binary signal, which informs the control room about this operating state. It should be noted that all Berthold level switch and are equipped with XIP.
Gas Property Compensation (GPC)
Does the gas pressure in your vessel change?
This can falsify the measured value in the case of a radiometric measurement, unless you have a gas density compensation from Berthold. With the feature GPC (Gas Property Compensation), a second measurement determines the current gas density in the vessel and compensates for the connected level measurement. Thus, a level measurement is realized, which provides an unaltered measured value even with fluctuations in the gas density. Find more information on our product page level measurement with
Product Radiation Compensation (PRC)
Special applications require special solutions. It is not unusual that products where the level is measured contain natural radioactivity. For example, in the processing of uranium ores where the level of uranium sludge is measured. Likewise in the production of plastics, under certain circumstances radioactive isotopes can be present in the process gas. The radioactivity contained in the measured material may interfere with the radiometricbecause the measuring material is recognised as a second radiation source.
Berthold with itsnow offers a radiometric level measurement, which cannot be influenced by the radioactive material to be measured. The feature PRC (Product Radiation Compensation) ensures, by means of independent activity measurement and integrated compensation, for a reliable and accurate level measurement.
How does the PRC-function work?
The Product Radiation Compensation (PRC) feature provides reliable, accurate level measurement through an independent activity measurement and integral compensation. Here, two separate detector systems are used, which simultaneously measure the level and the radiation from the product. Even under difficult process conditions, a high measurement accuracy can thus be ensured.
In case the source must be exchanged …
Radiometric sources decay by time. So the signal to noise ratio to the natural background radiation is going worse. If you already have installed radiometric measurements and are faced with the decision to replace the meanwhile weak sources, a PRC system can initially prevent the replacement of a source. Because with the PRC measurement, the background effect, which interferes more and more in the case of a weak source, is simply subtracted.