Radon monitoring now is taking a permanent step into the digital age – also for the home owner. It has become much easier to diagnose the health of buildings when it comes to radon radiation. Radon measurement procedures have become much easier, more flexible, and more accurate.
The use of Corentium does not require any knowledge about the monitor technology. For those eager to have more insight into the world of digital radon measurement, we will go into more detail here:
Measurement principles
A Corentium can be compared to a digital version of a traditional alpha track detector. The detection of radon is based on the principle that radon gas diffuses into a detection chamber. When the radon atoms decay they emit energetic alpha particles. The alpha particles are detected by a silicon photo diode. Every alpha particle generates a small signal current when it hits the photo diode. By the use of a low-power amplifier stage the signal current is converted into a larger voltage signal. The maximum amplitude of the voltage signal is detected and sampled by an analog to digital converter (ADC). The amplitude is proportional to the energy of the alpha particle that hit the photo diode. The brain of the monitor is a micro-controller that registers the time and energy of every detected particle. This information is used to calculate the mean radon gas concentration for daily, weekly and yearly periods. This is all realized in electronic circuitry that in total only consumes a few microwatts of power, making the Corentium able to operate for almost 3 years on a single set of small AAA alkaline batteries.
The calculation of radon gas concentration is complex since there are multiple error sources that have to be considered. When radon decays, several radioactive so-called daughters are produced - most notably Polonium-218 and Polonium-214, - which themselves emit alpha particles like radon does. These alpha particles could be mistaken for alpha particles from radon and give a false radon gas reading, but Corentium uses algorithms that are able to distinguish between the various alpha sources. The main input to these algorithms is the alpha particle energy in combination with the knowledge of the energy spectrum of radon and polonium isotopes. The Corentium has a very good energy measurement resolution, which is required when using such algorithms.
Accuracy and precision
The Corentium is not constructed to give instantaneous measurement results of the radon gas concentration, but rather averages over time. The reason is that instantaneous values are of little importance, since radon can vary a lot over a short time span, and since the health effect is linked to the average exposure over time. This is the reason why the three values that the Corentium presents are referred to as ‘averages’. The only difference in these three averages is the time span over which the averaging takes place. The day average (1 day) is taken over the last 24 hours, the week average (7 day) over the last 7 days and the long term average for the duration since the Corentium was first started (or last RESET). If the total measurement period is more than one year, the long term measurement is the average over the last year only.
The Corentium performs several levels of calibration. The first calibration takes place during the first 3 minutes after the batteries are inserted. After the Corentium has been running for several days it will ‘learn’ the local conditions and measurement accuracy will improve over time. As an example, the daily averages presented will be more precise after some weeks than during the first few days. The reason is that measurements are affected by aerosols and trace gases in the air, which are typically ‘stable’ for each house/environment and changes little over time, allowing the Corentium to ‘learn’ the local conditions. When the Corentium is first started there is no history for the current location, and the display shows a value of 0 Bq/m3 (15 Bq/m3 in the first sold version of the Corentium). As an example, if the unit is placed in a room with exactly 100 Bq/m3, one will see the measurement slowly ramp up from 0 (15 Bq/m3 in the first sold version of the Corentium) to 100 Bq/m3 during the first 24 hours. As mentioned earlier, the unit does not measure an instantaneous radon gas concentration, but rather an average over a time interval. Therefore, it is of little value to read the unit in the first few hours after it was first started - one should give it at least 24 hours. Before that, the values are likely to be an underestimation of the real value. However, in the case where a room has a very high radon gas concentration - say 1000 Bq/m3 as an example - even the underestimation you get after a few hours will give a hint that the unit may reach high values by the end of the first day.
There is a similar situation related to the weekly average. Until the Corentium has been measuring a full week, the weekly average value will not be as precise as when the unit has been measuring for more than a week.
At this point it is natural to bring forward the two concepts; measurement accuracy and measurement precision.
Accuracy is related to how well the Corentium measurement value matches the true radon gas concentration. This is verified by placing many Corentium units in a laboratory with a known radon gas concentration to check how well the Corentium results match those of professional high cost reference monitors (which is the proxy for the ‘true’ radon gas concentration in the laboratory). We have performed such measurements in renowned laboratories in Japan and the Czech Republic. The result of these tests shows that the Corentium accuracy is around 5%. This means that the results of such comparison measurements are within +/-5% of the result the reference monitors show. Accuracy is really related to systematic errors, for example due to a wrong calibration or other effects that systematically would produce too high or too low values.
Precision on the other hand is related to the spread of the measurement results. If we would place 100 Corentium units side by side in the same room (i.e. at the same radon gas concentration), how well do the results of the individual Corentium units match each other? Looking back at accuracy, this would be how well the average readings of these 100 units match the true radon gas concentration (or how it matches a professional well calibrated monitor). Since we have indicated that Corentium has 5% accuracy, it means we can assume that the average of the 100 monitors would be no more than 5% away from the true value.
For the Corentium the measurement precision (spread in measurement values for the various Corentium) depends on the radon level itself. The higher the radon gas concentration, the more precise the measurement will be. The precision is also different for the various time averages that are measured. The longer the measurement duration the more precise the result will be. These measurement precisions are illustrated in the tables below.
It is customary to give the measurement results (for radon) with a 20% precision. When a precision is indicated it will for most monitors mean that the measurement will follow a Gaussian distribution. A Gaussian distribution with a 20% standard deviation means that in about 68% of the cases (say you had 100 units) the result of a selected unit (pick one of the 100) will be inside a window of +/-20% of the average in 68% of the cases. If the average measurement of the 100 units were 100 Bq/m3, it means you can expect 68 of them to show a result from 80 to 120 Bq/m3. Further, the Gaussian distribution means that approximately 95% of the measurements will be within two times the standard deviation (i.e. 40%) of the average. This means for a 20% reported precision you can expect 95 of the 100 units to show a value between 60 and 140 Bq/m3 for a true radon concentration in the room of 100 Bq/m3.
The total measurement uncertainty is typically the root mean square of the individual contributions (where individual contributions are Gaussian distributed). If we operate with an accuracy of 5% and a precision of 20%, the fact that the accuracy is much better than the precision will only slightly increase the total uncertainty above the precision:
20%2+5%2≅21%
This means we are dominated by the uncertainty introduced by the lower precision.
In a large comparison study between various suppliers of alpha track detectors performed by NIRS (The Japanese Health Authorities) in 2009 it was found that 16 of the 26 contributors (62%) were within +/-20% of the reference value reported by NIRS. Comparing this result to the earlier discussions about precision, we see that this can be taken to indicate that alpha track detectors typically have a measurement precision of about 20%. This is the reason why our tables below show typical measurement durations and radon gas concentration which will allow the Corentium measurement precision to be 20% or better (than alpha track detectors).
The following tables show the total measurement of uncertainty for a Corentium for different radon gas concentrations and for the various averages (weekly and long term). The table does not present values for radon gas concentrations below 100 Bq/m3, since no countries use an action level below this value.
Measurement duration to get below the indicated uncertainty for the long term measurement.
Radon concentration
[Bq/m3] |
20% uncertainty |
10% uncertainty |
100 |
4 days |
3 weeks |
200 |
2 days |
10 days |
300 |
2 days |
7 days |
500 |
1 day |
4 days |
1000 |
1 day |
2 days |
2000 |
1 day |
1 day |
Measurement uncertainty (%) for the weekly average at different radon gas levels
Radon gas concentration
[Bq/m3] |
Measurement uncertainty
[%] |
100 |
16 |
200 |
12 |
300 |
10 |
500 |
8 |
1000 |
7 |
2000 |
6 |