Whitepaper "Radiometric Measurements – Accuracy, repeatability and errors"
Berthold has published the whitepaper “Radiometric Measurements – Accuracy, repeatability and errors”, wherein the radiometric measurements are explained and how your process can be performed with highest accuracy and reproducibility and the error sources can be minimized.The paper elaborates on how Berthold with its highly sophisticated radiometric measuring systems helps plant operators to maintain a reliable and repeatable measurement.
While for nuclear limit switches and density measurements the point-to-point connection, utilizing point source and point detector is the industry standard,
continuous level measurements usually utilize at least one rod type device. As indicated earlier, rod detectors are very vulnerable against background, and therefore the potential errors caused by background are by far dominating. The sensitivity of a radiometric measurement is not automatically an indicator for the quality of the measurement. It is obvious that background and its fluctuation play a major role and error caused by background can dominate the system. Typically, it is thought that cutting activity half is having the same effect than bisecting the sensitivity.
Halving sensitivity of the detector does not change the systems overall error assuming that there is no other major source of noise like electronical noise, which can only be held up if the measurement device is developed and produced on a high standard. In fact, calculations illustrates, that the signal to noise ratio is the far better lever and this ratio is what we positively influence by managing background.
Similar to the nuclear limit switch, density measurements are typically point-to-point measurements, with the same error modes. For density measurements it is important to carefully manage the potential causes for systematic errors. As already explained, sophisticated temperature and aging compensations preserve the high reproducibility of the measurement itself. Also, it is imperative to establish a proper calibration, where sample taking and laboratory measurements might affect the accuracy of the calibration value. In the measurement itself the statistical error is most dominant. In real life a density measurement has high demands on accuracy and reproducibility in a very limited calibration range. Typically, the count rate ratio at the highest and lowest measured density is low and therefore the acceptable statistical error is quite small, which prohibits the use of similar count rates to a limit switch. To handle the statistical error, it helps to have a high count-rate in combination with a long time constant.
A level switch is a point-to-point measurement, utilizing a point source and a point detector.Systematic errors can be reduced very well, by applying good workmanship during installation and setup of the measurement. This includes establishing the system with properly calculated switching limits.
The most important and dominant error of a nuclear limit switch, is the statistical error, which is handled by statistical methods, e.g. by setting an appropriate time constant and build a reasonable mean value. Designing the system with a high enough count rate and leaving a large enough sigma difference between empty and full counts, prevent from false switching. The figure illustrates that for safe switching operations a difference of at least 6σ is recommended. Count rates leaving the 6σ band due to statistical variations occur in 0.0000001973% of all measurements – only every 16 years when applying typical detector characteristics.
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 and level switch measurements 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 radiometric level measurement because the measuring material is recognised as a second radiation source.
Berthold with its LB 480 SENSseries now 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.